Mandibular bone block grafts
Tooth loss leads to a remodeling of the alveolar bone, especially at the buccal bone wall. This process is usually the reason for the inevitable extensive resorption during the first 3 months after tooth loss. The resorption becomes moderate after 6 months, reaching some stability after 1 to 2 years. However, bone atrophy can increase with time through inactivity or the compression of a removable prosthesis, reducing the chances for an implant insertion in an adequate situation without any augmentation procedure. Especially in the esthetic zone, correct implant positioning is essential for long-term esthetic success. Therefore, when the height and width of the bone volume is insufficient, bone reconstructive treatment is essential.11,12,48,49,51,73 The morphology of the bony defect is the main criterion for the selection of a grafting technique. The larger the bony defect, the more important the need for techniques that promote osseointegration.5,9,81,82
During the last 30 years, different techniques and materials have been recommended for the reconstruction of alveolar defects, including autogenous, allogenic or alloplastic bone grafts. Although the actual evolution of alloplastic33 and allogenic16,83 materials and guided tissue regeneration techniques15,84 seems to be promising, information on the healing process involved, the reproducibility, and the predictable prognosis is still lacking compared with autogenous bone.73 The superiority of autogenous bone has been demonstrated with respect to other bone substitutes on a biologic, immunologic, and even medico-legal basis. Autogenous bone has additional mechanical (cortical) and osteogenic (cancellous) properties because of graft morphology, allowing early revascularization and functional remodeling, with low complication rates that are unequalled by any allograft, xenograft or alloplastic material.46,99,100
The main problems of xenografts and allografts, especially in block form, is their poor ability for revascularization, offering to the material low resistance against oral flora. This leads to several complications and failures.2,13,88,102
Several donor sites have been described in the literature such as the cranium (parietal bone),29 tibia,9 ribs,80 maxillary tuberosity,76 palatal bone, torus zygomatic arch,109 iliac crest,34,39,67,80,99 and mandibular sites.49,54,58 Resorption observed in the mandibular sites seems to be lower than for the other sites.89
This chapter details the different mandibular donor sites for bone block graft techniques. In particular, bone harvesting techniques and instruments57,58 are presented, and their indications, advantages, and complications discussed. In addition, biologic methods for the reconstruction of atrophic sites with mandibular bone block grafts are presented and discussed.
4.2 Biologic procedure for mandibular bone grafting
From a pathophysiologic perspective, all types of autografts, whether osseous, gingival or dental, follow a similar regenerative process. However, the success and long-term stability of the grafting procedure depend mainly on the amount of revascularization. The prognosis for the graft is determined by the quality and intensity of revascularization.68,79 This is fundamental for the survival of an important amount of the graft, for the physiologic remodeling, and for the quality of the regenerated and grafted area, which is important for the long-term stability of the grafted area.
The goal of a bone-grafting procedure is the reconstruction of good, vascularized hard tissue with a form, shape, and quality similar to the original bone, with all its anatomical and structural parts: compact and cancellous. This also includes the haversian system, which is the microscopic structural unit of compact bone. The bone unit or osteone contains the haversian canal with blood and lymphatic vessels, nerves, and osteocytes, which are the primary cells of mature bone, maintaining the mineral concentration of the matrix via the secretion of enzymes and by communicating with each other, and receiving nutrients through canaliculi (Fig 4-1a). Bone also contains osteoblasts, which are the bone cells responsible for forming new bone. Osteoblasts are similar to osteocytes but do not divide; they synthesize the surrounding collagen matrix, and after its calcification, the collagen matrix changes the osteoblasts to osteocytes (Fig 4-1b). Other very important cells contained in bone are the osteogenic cells, which are found in their immature form in the deep layers of the periosteum and the bone marrow. They possess a high mitotic activity and are the only bone cells able to divide. They differentiate into osteoblasts. The cells responsible for bone renewal and remodeling are the osteoclasts, which are found on bone surfaces and originate from monocytes and macrophages but not from osteogenic cells. Osteoclasts continually remove and break down injured, old, dead or unneeded bone (Fig 4-1c), while osteoblasts are continually forming new bone. All the above-mentioned parts of this newly reconstructed bone are important for the long-term stability of the regenerated area because they are linked to the new bone’s resistance to oral flora. In contrast to bone grafting procedures in other areas of the body, where, after the grafting procedure, the transplanted bone becomes completely covered by muscles and soft tissue, intraoral grafting occurs in a highly contaminated area and, after the insertion and restoration of the implants, will continue a continuous communication with the flora of the oral cavity.
Graft revascularization normally starts within the first hours after surgery.68 Osteoblasts, osteocytes, and osteogenic cells can survive for up to 4 days on their own reserves and with nutritive and fluid support via diffusion.19 They can also survive for longer if vascular supply is available through early revascularization within the first 3 to 4 days.19 If this does not occur, the biologic part of the osseous cells will die.
While soft tissue cells are repelled by necrosis, the process in osteocytes is somewhat different. Here, only the biologic part of the osteocyte dies, whereas the mineral part remains. Figuratively speaking, this phenomenon is similar to what happens with a snail: If its biologic component dies, the housing remains, and can serve as a skeleton or guide rail for other new cells.
Blood vessels that penetrate the transplant carry specific cells from their origin, i.e. the recipient bed. When these blood vessels originate from the bone bed, they bring with them new osteoblasts. Skoglund et al97 demonstrated, on the basis of a microangiographic study, the vascular changes in replanted and autotransplanted teeth with incomplete root growth (open apical foramen). After 10 days, the revascularization reached halfway up, and after 30 days, it reached the top of the pulp cavity. After 180 days, despite the continued good vascularization, a significant reduction in volume of the pulp cavity occurred, a phenomenon that is also known clinically after successful tooth transplantation (Fig 4-1d to m). This is shown by a gradual radiographic obliteration of the pulp and, in the course of time, applies to the entire pulp and root canal area (Fig 4-1n and o). Both clinically and histologically, it was shown that this obliteration is caused mostly by bone, and received the name osteodentin (Fig 4-1p and q). While it is easier for tooth transplants to be revascularized through the open apical foramen, the situation is a bit different in the case of a bone block, where there are no holes to guide the revascularization. The perforation of the recipient site and of the bone block to facilitate and improve the revascularization of autogenous bone blocks in experimental animal studies did not confirm the advantage of such procedures.1,114 The conclusion was: “Recipient cortical bed perforation offered no advantage over non-perforated bed regarding healing and integration of a bone graft.”
The revascularization of a bone block occurs through what is called the cutting cone. Osteoclasts drill a tunnel into the bone block to open a biologic route for the growing blood vessels carrying osteoblasts, which lay down new bone (osteoid) parallel to the revascularization of the bone graft.31
During the first postoperative days after autogenous bone grafting, all types of blood vessels, regardless of their origin, easily enter here. This ensures the survival of a maximum number of bone cells. Blood vessels from the periosteum and even from the adjusting soft tissue play an important role in the revascularization of bone areas that are distant from the bone bed. If a bone block graft is covered with a membrane, this form of revascularization is mostly inhibited, and the parts of the transplant lying further from the bone graft recipient site are involved much later in the remodeling process. This model was confirmed in a comparative study developed by DeMarco et al17 that investigated the revascularization of autogenous bone blocks with or without coverage through an expanded polytetrafluoroethylene (e-PTFE) membrane in rats. In the group of block grafts without membrane application, revascularization was performed earlier and more intensively and extensively in all treatment periods.
Provided there are no healing complications and that early exposure does not occur, nonresorbable membranes reduce the resorption of bone transplants. At the beginning, they have a positive effect on the volume stability of the bone block through reducing revascularization and remodeling. The graft surface under the membrane is poorly remodeled and revascularized. This phenomenon is clinically seen after the removal of the membranes (Fig 4-1r to t), following which a later remodeling occurs and leads to increased resorption of the graft due to a reactive osteoclast activity that removes most of the non-revascularized and thus dead areas of the graft. Resorbable membranes may have a worse effect on this phenomenon because resorption is an inflammatory reaction that involves many macrophages and osteoclasts. The activated macrophages do not only select and resorb the membrane, they also attack every resorbable material available, e.g. the grafted bone (Fig 4-1u to w).
Autogenous bone grafts are still the gold standard in reconstructive surgery. Gold, however, exists in different qualities, from 8 to 24 carat. In comparison with bone grafts from other parts of the body, the quality of bone grafts harvested from the iliac crest is, from a regenerative point of view, the best in terms of healing – they are 24 carat gold. This is due to their morphologic structure (Fig 4-2a to h), including a large proportion of cancellous bone.39,64,80 Bone marrow has a high capacity for revascularization, promoting the survival of a great number of osseous cells.68 Although an iliac graft is considered to be of excellent quality from a regenerative point of view, this is not so when it comes to implant osseointegration. Biopsies from iliac grafted bone 4 months postoperatively showed low-quality bone from a density point of view (D4 bone with wide areas of marrow fibrosis; Fig 4-2i). Since optimal osseointegration of implants depends directly on the bone quality, implants inserted in grafted iliac bone present a low bone–implant contact (BIC), which means poor osseointegration at the beginning. Uncontrolled loading during this time will lead to micro-fractures of the poor BIC, causing looseness during osseointegration. Over time, the quality of the bone graft adapts to the recipient site due to genetic influence and functional loading: It takes an average of 2 years until biopsies from this grafted bone demonstrate density of a better quality (Fig 4-2j).
On the other hand, mandibular bone grafts, which consist mostly of thick cortical bone, are more resistant to revascularization and consequently have poor regeneration potential. To continue the gold analogy, the quality of these grafts is about 12 carats. Although the mandible is considered to have excellent bone quality and density for optimal osseointegration of implants, the same is not true from the regeneration point of view.24 A high number of dead osteocytes can still be found for as long as up to 3 years in large areas of grafted, thick cortical bone blocks, which can compromise osseointegration after implant placement (Fig 4-2k).
Graft revascularization depends not only on the quality of the donor site, but also on the regenerative potential of the recipient site,19 which is generally unknown prior to surgery and healing results. This explains why two grafts harvested from a similar donor site respond differently when grafted into two different areas. The first one can have excellent revascularization, even with less contact with the recipient site. This has been confirmed clinically, with a reddish color and heavy bleeding occurring during drilling 4 months after the grafting procedure (Fig 4-3a). During the same period, the second bone block graft can have poor revascularization, even with very close contact with the recipient site over a large area. This poor regeneration potential of the recipient site is confirmed by the white color of the grafted bone, representing poor bleeding during implant bed preparation (Fig 4-3b and c). Biopsies taken from this graft demonstrate the presence of a large number of dead osteocytes and poor vascularization (Fig 4-3d and e). The insertion of implants in a bone site such as this, with a poor vascular supply and a large number of dead bone cells, presents a high risk for the long-term stability of the graft and the osseointegration of the implant. It will be very difficult for bone with poor vascularization to resist oral flora after the exposure and restoration of the implant (Fig 4-3f to h), presenting similar outcomes to those when biomaterials or allografts are used. The goal of bone grafting is to create a new bone crest similar in shape and quality to the original native bone.
For these reasons, a modification of the standard grafting technique for mandibular cortical bone blocks is necessary to improve graft regeneration and revascularization and to ensure that grafting becomes reproducible and predictable, independent of recipient site quality or by chance, while maintaining their density and osseointegrative properties.
Several theories concerning the healing and regeneration mechanisms of grafted bone have been discussed. Already in 1892, the orthopedic surgeon Wolff111 mentioned the possible healing of grafted bone by osteogenesis, or what he referred to as the osteoblast theory. Wolff stated that a great number of osteoblasts and osteocytes survive the grafting procedure and are responsible for bone regeneration. In a microscopic study a year later in 1893, Barth3 reported that the number of surviving osteocytes and osteoblasts is not sufficient for regeneration. He introduced the framework theory, which addresses regeneration through osteoconduction and explains how the mineral part of the graft that remains after the death of the biologic part serves as a scaffold for recipient site osteoblasts, which are mainly responsible for bone regeneration. Current studies by authors such as Boyne (1997)5 discuss a third mode for regeneration of autogenous bone grafts, called osteoinduction. During bone regeneration by osteoinduction, pluripotent stem cells differentiate under the influence of humeral and bone morphogenetic proteins (BMPs) into osteogenic cells such as osteoblasts, which will then produce osteoid layers over the bone surfaces, to be mineralized later to osteocytes. BMPs are already present in native bone. According to the body’s natural reparation process, after the death of the bone cells, an additional expression of BMP occurs for the regeneration process. However, this kind of osteoinduction is limited to bony recipient sites, probably through the influence of genetics. In addition, the cell death activates the osteoclasts to remove dead materials, and also activates a neoangiogenesis in the form of an inflammation for the bone remodeling (Fig 4-3i and j).
The main advantage of autogenous bone grafts – the gold standard – is represented by these three different possibilities for healing, which give them the best regenerative capacity compared with all other grafts, including biomaterials, which can only heal through osteoconduction.
In summary, the regeneration of free autogenous bone graft sites follows three different modes: 1) osteogenesis through part of the osteocytes/osteoblasts/osteogenic cells that survive the surgical procedure; 2) osteoconduction through the presence of the original bone mineral functioning as a scaffold for the attachment of the osteoblasts from the recipient site; and 3) osteoinduction through the formation of new, additional osteoblasts under the influence of BMPs parallel to the neoangiogenesis. The percentage rate for each mode of healing mainly depends on the origin and quality of the bone graft. A free autogenous bone graft from the hip, for example, contains a large volume of cancellous bone with a high concentration of active osteogenic cells and BMPs. The structure of the spongy bone scaffold makes quick revascularization easier, which is essential for the survival of bone cells. This will not only increase the potential of regeneration by osteogenesis, but also the remodeling. However, this is not the case with free mandibular bone grafts.
From the clinical and practical points of view, bone grafts harvested from intraoral sites are more appropriate for daily work than those harvested extraorally because they offer many advantages for the patient and the surgeon, including a less-invasive approach and lower morbidity, and without the need for general anesthesia or hospitalization. However, the mandible is formed primarily of thick cortical bone, which contains a limited number of active osteoproductive cells and BMPs. The structure of this bone, with its thick cortex, makes revascularization and regeneration of this type of free graft difficult. Revascularization/regeneration occurs with the aforementioned cutting cone. The cutting cone is a unit containing osteoclasts, blood vessels, and osteogenic cells. Osteoclasts, which are positioned on the front of the cone, drill a tunnel into the bone block through resorbing bone material. This opens a biologic route for the growing blood vessels coming from the recipient site and carrying osteoblasts. The trailing osteoblasts lay down new bone.21
Bone cells of the free graft can survive up to 4 days without direct vascular supply due to their own reserves and through diffusions from the surrounding tissue. If the blood vessels reach these cells during this period they will have a chance to survive and continue to work normally; if not, these bone cells will die. A dead osteocyte is seen under the microscope as an empty mineral lacuna in the bone unit (osteone). In fact, it will be more difficult and take more time for the cutting cone to go through a thick cortical bone wall than through the softer bone of the hip. In the case of a mandibular bone block graft this can take a long time, with the result that many bone cells will not survive due to a lack of vascular supply. The percentage of surviving osteocytes found in biopsies taken from thick cortical bone grafts is not more than 20%, and this is the maximum part of the bony regeneration by osteogenesis. Attempts were made to improve the capacity of revascularization through perforation of both the cortical recipient site and the graft. However, experimental and clinical studies showed that there were no advantages to these procedures.1,114 Other attempts have focused on improving the angiogenesis by using specific growth factors/proteins, but they have not achieved a clinical breakthrough to date. In summary, there is no method at present of influencing and improving the revascularization of thick free cortical bone grafts.
Another possibility for improving the healing of thick cortical bone grafts is the addition of BMPs to the grafted area to enhance the production of osteoblasts. The protein part of the bone material is composed of soluble and non-soluble proteins; the latter contain few BMPs, and their extraction from xenogenic or allogenic bone (native BMPs) for use in regenerative treatment is risky due to possible allergic reaction and contamination. At the same time, poor results have been seen due to the low concentration of the extracted material.105 Today, recombinant BMPs (rhBMPs) can be produced safely in the laboratory with cell cultures and are available as a mono-substance, with rhBMP-2 and rhBMP-7 being the most frequently used in experimental and clinical studies, with good results.18,74,75
Although many studies have reported good results with BMPs for various indications, their use in Europe for dental implants is not permitted due to the many possible complications and adverse effects of these materials such as ectopic mineralization; severe pain, swelling, and hematoma; osteoclast-mediated bone; adipogenesis; neuralgia; and unclear tumor genesis. The very high cost of these materials also makes them unrealistic for use in dental implantology. In summary, the use of BMPs to improve bone regeneration in routine treatment is still not an option today.
The most important mode of regeneration of autogenous bone grafts is osteoconduction, which contributes to more than 50% of the healing process.24 Thus, to improve the regenerative capacity of a mandibular bone graft, focus should be placed on osteoconduction, which is defined as the colonization by osteoblasts from the recipient site of the mineral part or graft skeleton that serves as a scaffold.24 Osteoblast penetration occurs through two pathways: 1) through neovascularization involving the newly formed blood vessels originating from the bone from the recipient site (cutting cone); and 2) through the capacity of the graft surface to attract osteoblasts derived from neighboring bone.24 The formation of osteoid layers with these osteoblasts, which mature later to become lamellar bone, is possible on every free graft surface that is close to vital bone (Fig 4-4a). However, and parallel to these phenomena, osteoclasts are also active in trying to eliminate and transport the parts of the graft containing dead cells, and this sometimes occurs before osteoblasts complete the process of colonizing the graft (Fig 4-4b). This leads to the resorption of a large part of the graft before its colonization and regeneration by new osseous cells. As a consequence, there is a great interest in rapid revascularization and regeneration with a maximum number of vital osteocytes in order to limit the osteoclastic reaction. Clinically, this resorption normally starts in the area of the graft that is more distant from the recipient site, which is the last to be reached by the new bone vessels and still contains a high number of dead osteocytes. For example, the most vulnerable zone for resorption in a lateral bone block graft is the area of the occlusobuccal angle.
The fact that every free surface of the grafted bone is able to attract new osteoblasts from the recipient site makes healing possible with osteoconduction, which is responsible for more than 50% of the regeneration of mandibular bone grafts, depending on the amount of graft surface involved. A larger free surface area automatically means more regeneration through osteoconduction; therefore, increasing the surface size of grafted bone has a positive influence on new bone formation and regeneration through osteoconduction. A one-piece bone block graft has a limited surface. If the same block is crushed, milled or scraped into small pieces or chips, the total surface area of the grafted bone increases exponentially, which leads to better regeneration potential and new bone formation (Fig 4-4c to e). In a histologic and stereologic study, Pallesen et al86 showed that the volume of grafted particulate bone influences the regeneration rate and speed. The study results showed that the total volume of newly formed bone in defects filled with small particles (0.5 to 2 mm3) was larger and more mature with better regeneration compared with defects filled with bone particles (10 mm3) after 2 and 4 weeks.
However, the use of particulate bone grafts alone to reconstruct an alveolar ridge is unstable. Stabilizing the graft using membranes is possible,109 but it is riskier and could lead to complications such as exposure of the membrane and infection. The biologic concept developed by the author as a solution is based on the use of a combination of a thin bone block graft and small pieces of particulate cancellous and cortical bone.45,48,56 The unequal particulation is recommended because small volumes of transplants with a size of 0.5 to 2.0 mm3 show better regeneration but also more resorption than particles of 10 mm3.86 The thin cortical block acts as an autogenous biologic membrane for stabilization of the small pieces of particulate bone.
In practical terms, the grafting procedure is performed as follows: A thin bone block that recreates the alveolar crest shape, giving it form and volume, is fixed with screws. This thin bone block is screwed in at a distance from the recipient site rather than in close contact with it, leaving a free space between the block graft and the atrophic alveolar crest. The definitive form and thickness of the new alveolar crest is then fixed (Fig 4-5a to e). The space between the bone block graft and the recipient site is then filled with mandibular cancellous bone and small pieces of cortical bone, which is scraped from the surface of the bone block and possess high revascularization and regeneration potential. It is important to note that this space should be well packed with particles to prevent the migration of fibroblasts. Once the alveolar crest reconstruction is complete, it takes on the appearance of an iliac graft, with thick internal cancellous bone (cortical and cancellous particulate bone) and a thin external cortical layer (Fig 4-5f and g). This leads to an ideal graft, with a high regeneration capacity comparable with iliac bone, and an osseointegration potential close to that of the mandible. In addition, after 3 to 4 months, the regeneration mode of this form of the graft resembles that of an iliac graft. The area filled with particulate and spongy bone, where the implants will be placed, is well vascularized and is reddish in color, while the external cortical bone is a bit whiter and provides shape, protection, and stability to the osseous particles (Fig 4-5h to o). The inserted implants will experience ideal conditions for osseointegration, with the original lingual or palatal bone on one side and the well-regenerated and vascularized bone graft on the other (Fig 4-6a to s).
The same principle applies to vertical bone defects. In these cases, three-dimensional (3D) bone grafting is performed with not only one but two bone block grafts, e.g. the missing vestibular and the palatal or lingual cortical walls, respectively, are reconstructed with two thin bone blocks. The space between the bone blocks is, in turn, filled with particulate bone (Fig 4-7a to e).
This modification of the grafting procedure not only improves healing through osteoconduction, it also increases the amount of revascularization that takes place. Therefore, it is logical that it is much easier and quicker for the cutting cone to traverse such a unit of thin bone block and bone chips than to transverse a thick cortical bone block (Fig 4-7f).
The practical and clinical aspects of this technique are described in detail later in this chapter.
Bone resorption of the alveolar crest and bony defects of the jaw caused by various factors lead to unfavorable anatomical conditions that make implant prosthetic rehabilitation procedures more difficult and sometimes impossible. To achieve a satisfactory esthetic result and a favorable implant-to-crown ratio, extensive augmentation procedures are required. The gold standard for the bony reconstruction of the implant bed is still autogenous bone.
Bone grafts harvested from intraoral sites are convenient for the reconstruction of alveolar defects. This technique is frequently used prior to, or in combination with, implant placement due to the relatively simple procedure compared with bone harvesting from extraoral sites.48,49,56,67,79,80
The intraoral donor sites include:
This bone can be harvested in different forms and with different instruments. For the reconstruction of small bony defects, the bone graft is harvested in the form of particulate bone chips and/or bone core. This is performed using various tools such as a bone trephine, bone scraper, and the standard burs for implant bed preparation.
Grafts in the form of bone blocks are required for the reconstruction of large bone atrophies and defects. The main areas for obtaining bone block grafts of a large size are the retromolar and chin area (Fig 4-8).
The proximity of the donor and grafted sites reduces the surgical and anesthetic time, leading to ideal conditions for implant surgery in outpatients. Moreover, patients report less postoperative discomfort than when extraoral sites are used.68
4.3.2 Preoperative clinical examination and radiography
A bone grafting procedure for dental implant rehabilitation is a facultative and not an emergency surgery. Before every surgery, special attention should be given to the possible presence of general and/or local contraindications (see Chapter 2). Patients are identified to be under general risk if they have diabetes; are under immune suppressive and corticosteroid treatment; are under all kinds of bisphosphonate therapy; have recently experienced radiation; are under anticoagulation medication for different reasons; have systemic bone disease; are smokers; and/or are multimorbid. Local risks factors include periodontally compromised patients; those with complications and failures after grafting, especially with biomaterials or allografts; the presence of large areas of scar tissue; the presence of a thin gingival biotype; and patients with significant bony defects. These local risks should be taken into consideration and an individual risk profile discussed with the patient.59
All patients need to be properly informed of both the advantages and risks of a grafting procedure. A possible second intervention may be necessary, and intra- and postoperative complications might occur.56 The anatomical variation of donor sites creates grafts with morphologic differences. Clinical evaluation and the comparison of donor sites are essential.25,54 Visual examination and digital palpation25,54,56,58,108 allow for a preliminary estimation of the morphologic contours and dimensions of the donor site such as symphyseal protuberance, the volume of the edentulous crest, and the thickness and extent of the external oblique line (linea oblique externa). This clinical examination provides information on the shape of the available bone at the donor site. Radiography should be used to supplement the information on the donor site and the relationship of the site to important neighboring anatomical structures. Radiography can include:
The main diagnostic radiograph for bone harvesting from the retromolar area is the panoramic radiograph, which gives primary information about the bony defect and the presence and extent of the external oblique line as well as its density and distance from the mandibular canal (Fig 4-9a). An estimation of the bone quality can sometimes be obtained.70 Depending on the volume required for the bone grafts, additional diagnostic CBCT images might be necessary in cases where the mandibular canal appears to be close to the external oblique line on the panoramic radiograph. Precise information about the correct position of the alveolar nerve is essential to prevent nerve complications during the harvesting procedure (Fig 4-9b to d).58
In addition to the panoramic radiograph, profile teleradiography, as an alternative to CBCT, determines the presence of sufficient bone volume at the chin area and its relation to the neighboring teeth (Fig 4-9e). Knowledge of the 3D location of the roots of the anterior mandibular teeth is essential to ensure the presence of a sufficient distance to safely harvest chin grafts.
Further details about the bone defect morphology at the recipient site and its relation to the neighboring teeth can be obtained with retroalveolar radiographs (Fig 4-9f), whereas quantitative radiography and tomography are needed when the presented techniques do not provide sufficient information.
4.3.3 Patient preparation
As bone harvesting and bone augmentation for implant treatment are facultative and not emergency surgeries, patients should follow all preparation modalities to ensure their success, including systematic periodontal treatment with the elimination of tooth pocketing, the extraction of destroyed teeth, and the preparation of a functional and esthetic temporary restoration. The temporary restoration must always be fixed on natural teeth or on osseointegrated or temporary implants (Fig 4-10a to f). An uncontrolled, removable temporary restoration can load and later injure the augmented area, exposing the grafted bone. This can lead to a total failure of the augmentation procedure.
Directly before the surgery, an additional cleaning of the mouth is required, as is mouth rinsing with a chlorhexidine 0.02% solution for 2 min. Chlorhexidine solution is also prescribed to support mouth hygiene for 1 week postoperatively.
Preoperative antibiotic administration is obligatory, either intravenous (penicillin G, 1×106 IU)16 directly before local anesthesia is injected (it is important to do this before vasoconstriction occurs) or by way of the mouth (penicillin V, 1×106 IU/day) at least 1 h prior to surgery. Antibiotics are continued for 7 to 10 days postoperatively at 3×106 IU/day. As an alternative, amoxicillin52 (2 g per day) is administrated in cases where an additional sinus floor graft is planned. In case of a penicillin allergy, clindamycin 300/600 mg54,72 is administered at 1.2g/day, but in this situation more postoperative complications were registered due to a growing resistance to clindamycin.62 Other alternative antibiotics, e.g. cyprofloxacin or moxifloxacin, are associated with many negative side effects and have not shown any significant advantages to date, so that patients with a penicillin allergy are today classified as risk patients.
Bone graft procedures with intraoral harvested bone are usually performed under local anesthesia in conjunction with oral or intravenous sedation. Intravenous sedation under monitoring has the advantage of being more secure and comfortable since the dose can be adapted to the patient situation and the demands of the surgery. General anesthesia is normally indicated for large reconstructions involving multiple donor sites and a surgery time exceeding an average of 4 h.
4.3.4 Instruments for bone harvesting and bone augmentation
Bone harvesting, bone augmentation, and implant insertion are performed with various assorted instruments that are specific for such interventions for easier fulfillment of the treatment outcome by the surgeon (Fig 4-11a to d). Depending on the indication and extent of the interventions, other instruments can be added to the basic sets.
The basic instruments include a near standard surgery set and osteotomes (BoneCondenser/BoneExpander, Dentsply Sirona, Bensheim, Germany), which allow implant site preparation through expanding, extending, and condensing (Fig 4-11e). Osteotomes can also be used for intra-alveolar sinus floor elevation as part of an internal sinus lift as well as to bolt small bone vessels for hemostasis. Additional standard tools are two-piece trephine drills of different external diameters as well as the newly developed pre-trephine drill in a special kit, which allows for local bone harvesting from the future implant site or from other areas in a simple and safe manner (Fig 4-11f to j). The pre-trephine preparation of the platform for the trephine drill bit of the same diameter simplifies the harvesting procedure. A bone scraper (Safescraper; Divisione Medicale Meta, Reggio Emilia, Italy) can remove small bone particles and bone chips from various regions, i.e. from the facial sinus wall during the preparation of the maxillary sinus window for external sinus floor elevation or from edentulous bone areas. This instrument is also used for the thinning of cortical bone blocks harvested from the mandible (Fig 4-11k to n). Bone mills are used to mill bone blocks into small bone particles (Fig 4-11o). Particularly in bone mills, the sharpness of the grinding mechanism must be ensured because if this is blunt, grinding of cortical bone might lead to its contamination with metallic parts of the cutting mill due to abrasion (Fig 4-11p). Bone chips can also be gained using different bone forceps (rongeurs) while shaping the bone graft and smoothing it to eliminate sharp edges. Another possibility for collecting bone chips is the use of implant site preparation drills during implant bed preparation, which should be used at a low speed (80 rpm) and without cooling with external fluid in wet and bloody areas (Fig 4-11q). A further possibility for collecting drilling dust or small particles of bone is the use of bone filters of various types (Fig 4-11r). However, it has to be ensured that the vacuum in which the bone filter is connected only absorbs bone, to avoid contamination of the collected drilling dust with foreign materials or food debris.106
For the harvesting of intraoral bone blocks, the MicroSaw (Dentsply Sirona) has for decades proven to be very effective and useful for such surgeries.49 The MicroSaw was originally developed in 1984 by the author41,43,49,50,51 for osteoplastic surgery and to prepare a bony lid for the apicectomy of mandibular molars (Fig 4-12a). It consists of a thin (0.25-mm) diamond disc with a diameter of 8 mm that is mounted on an angle handpiece or a handpiece, with a disc protector to prevent any soft tissue injuries (Fig 4-12b and c). The MicroSaw kit also includes a straight and contra-angle handpiece with diamond discs and corresponding disc protectors, drill burs, and various straight and curved chisels (Fig 4-12d).
Nowadays, most screws for stabilizing bone blocks are manufactured from titanium. Titanium mini screws mainly have their origin in children’s trauma surgery where, generally, they are not removed due to their biocompatibility after the healing of the fracture. These mini screws are also used for reconstructive bone surgery in dental implantology. For stability reasons, most titanium screws are used with a diameter from 1.3 to 2 mm. However, such screws are difficult to use to stabilize the described thin and small blocks in bone augmentation procedures. Titanium micro screws with a diameter below 1.3 mm have a high risk of fracture during screw removal due to osseointegration of the titanium (Fig 4-13a and b). As an alternative, micro screws made of a medical stainless steel with a diameter of 1 mm are therefore a better choice for the stabilization of thin and small blocks in order to prevent osseointegration and the danger of fracture. Their reliability has been confirmed both experimentally and clinically. Simple tools with reliable screw mounts (Fig 4-13c to e) allow an easy and safe insertion of screws into different regions (Fig 4-14a to h). Today, these screws are produced with a special, strong medical stainless steel (Meisinger, Hager & Meisinger, Neuss, Germany), which allows for the avoidance of certain fractures as well as some of the negative side effects that were observed with the old types of screws.
Minor defects can be reconstructed by using techniques and tools to collect bone in the area of the planned implantation. This approach can reduce the operational costs as well as patient morbidity. One possibility is to use a bone filter to collect bone chips incurred during implant site preparation. Ideally, two suctions should be present, namely a suction for saliva, blood, and cooling water, and another for collecting bone chips. If only one surgical aspirator is used, the collected bone is flushed out and contaminated. Alternatively, a trephine drill can be used to collect bone during implant bed preparation (Fig 4-15a to l).61 Sufficient cooling water and drilling without pressure is required to prevent overheating of the bone. Damage to the bone through overheating can already occur at temperatures around 52°C, which is not very high considering that normal body temperature is 37°C. The selected trephine bur should have a slightly lower diameter than the planned implant diameter so as to achieve good primary stabilization of the inserted implant. Therefore, for the insertion of an implant with a diameter of 3.8 mm, the outside diameter of the ideal trephine bur should not be larger than 3.5 mm.
Bone cores collected with trephines can sometimes break and remain inside the canal of the trephine, especially when trephines have a narrow diameter. The removal of these cores can be difficult and complicated. Two-piece trephines were developed to allow for the removal of the bone core more easily from the trephine canal using specific tools (Fig 4-16a to c). Pre-trephines, used to prepare a platform for the trephines (see above), help to achieve a secure and precise harvesting of bone cores (Fig 4-16d to n).
With their special design, some burs and drills used for implant bed preparation are able to more easily collect a large amount of bone chips at low speed and without water cooling. Also, during this procedure it is important to prevent the heating of the bone through drilling in a good bleeding socket. In very dense bone structure without good bleeding, it is advisable to work without cooling only until the second last drill bur. The last drill bur is then used with good cooling to remove the last layer of bone from the socket, which had probably suffered from overheating.
If insufficient bone has been collected in the area of the implant bed, additional bone can be harvested from neighboring sites. Thus, bone can be removed with the Luer bone rongeur, for example, from the anterior nasal spine or from the maxillary tuberosity. The MicroSaw and the fine chisel allow for the ablation of exostoses from the maxilla and mandible in all dimensions and forms (Fig 4-17a to f). Torus palatinus sometimes offers sufficient bone grafts; this is also the case for a 3D bone reconstruction (Fig 4-18a to d). An extreme case with a giant exostosis in the mandible is presented in Figure 4-19a to m.
During the sinus window preparation, small grafts can be obtained with a bone scraper from the facial sinus bone wall. The bony lid from the facial sinus bone wall, opening the window for external sinus lifting, can sometimes be used as a bone block graft (Fig 4-20a to e).
4.3.6 Intraoral bone harvesting techniques for reconstruction of large defects
In severe bony defects, large bone block grafts are required for 2D and 3D reconstructions, respectively. These grafts can be obtained from extra- or intraoral sites.
Extraoral donor bone block grafts for implant treatment can be harvested from the calvaria (parietal bone), the tibia head, and the iliac crest (anterior or posterior). Calvaria bone harvesting is mostly not well accepted by patients. When obtaining bone grafts from the hip, a monocortical block is usually harvested from the inner part of the crista iliaca anterior superior, simultaneously maintaining the correct contour of the hip. The procedure is often performed under general anesthesia, with several days of hospitalization. Some mobility problems in the first postoperative days or weeks can occur, with possible sensory disturbance in the thigh and an additional scar in the area of the bone removal (see Chapter 5 on extraoral bone harvesting).
Compared with extraoral donor sites for harvesting bone blocks, intraoral sites have the advantage that no second surgical field outside the oral cavity is needed. Usually, voluminous bone block grafts can be harvested intraorally from the mandible, and mostly from the retromolar area. Different instruments and tools have been described for this surgery such as specific burs, oscillating and rotating saws, and piezosurgical devices.
One of the oldest and most effective instruments for such surgeries is the MicroSaw, developed by Khoury in 1984.43,49 This tool is also used to prepare and remove a bony lid, which can be replaced back at the end of the procedure, thus avoiding additional bony defects in different surgeries. This technique was described as an alternative to destroying bone by using different burs during bone surgeries, which leaves large holes in the bony area at the end of the procedure and compromises safe and biologic healing, especially in cases of surgical endodontics in mandibular molars. The MicroSaw was later described as a practical and safe tool for harvesting bone blocks from all intraoral areas.25,54 In the last 30 years, it has been confirmed that the MicroSaw is an effective instrument with a clear protocol, not only for harvesting bone blocks but also for all kinds of osteoplastic surgeries, preventing bony defects and conserving the bony contours.57 The instrument is regularly used for the preparation of bony lids; for conserving the bony contours in the removal of various kinds of cysts; for the removal of impacted or broken teeth; for the explantation or removal of foreign bodies from all intraoral areas, including the maxillary sinus; and for the lateralization of the alveolar inferior nerve.
Bone blocks harvested with the MicroSaw are suitable as onlay, lateral or inlay bone grafts. The following sub-sections describe in detail the method for safe and reproducible bone block graft harvesting from the mandible with the MicroSaw.
22.214.171.124 Harvesting bone from the mandibular retromolar area
An inferior alveolar nerve block anesthesia is usually avoided. Local vestibular and lingual infiltration with 4% articaine and 1:100,000 epinephrine (Ultracain DS forte; Sanofi Aventis, Frankfurt, Germany) is sufficient in most cases and decreases the risk of injury to the inferior alveolar nerve: A patient who is not very deeply anesthetized can inform the surgeon in case of accidental approach to the mandibular canal. The fact that all anesthesia solution is only injected locally leads to excellent vasoconstriction, giving the surgeon a good view of the donor site.
A trapezoid-like incision is performed, starting distal of the second molar, with a 2 cm vestibular incision over the ramous bone, continuing parallel and lateral to the second molar, and then going back in the vestibular direction on the distal border of the fist molar. A full-thickness mucoperiosteal flap (similar to that used for the removal of impacted wisdom teeth) is then elevated and used to expose the bone at the level of the external oblique ridge, to a length of 3 to 4 cm and a depth of 2 cm. This incision is kept paramarginal to the neighboring teeth if no additional wisdom tooth removal is planned. The volume of the bone block to be harvested depends on the size and extent of the external oblique ridge and the bone quantity needed for the grafting procedure (Fig 4-21a).
The graft is harvested with abundant saline irrigation according to a precise protocol of the MicroSaw technique, as described below:
The MicroSaw consists of a diamond disc (8-mm diameter; 0.25-mm width) and is mounted onto a contra-angle handpiece or handpiece with a soft tissue protector. The harvesting protocol includes three osteotomies performed with the diamond disc: two that are proximo-vertical and made with the MicroSaw handpiece (Fig 4-21b and c), and one that is baso-horizontal and made with the contra-angle handpiece (Fig 4-21d). Depending on the extent of the external oblique line, the first vertical incision is performed mesially with the MicroSaw handpiece at the mesial border of the external oblique line and with a length about 1 to 1.5 cm. The posterior vertical incision is then made at the lateral border of the ramus (vertical part of the posterior mandible), perpendicular to the external oblique ridge. The graft size and the position of the inferior alveolar nerve determine the length of this osteotomy. The diamond disc should possibly be set perpendicular to the wall of the jaw, so that no undercuts or sectional areas generated will hinder the dislocation of the block. The maximum osteotomy depth that can be reached with the MicroSaw is approximately 3.2 mm. This should take into consideration the position of the mandibular canal.
The baso-horizontal osteotomy is made slightly overlapping both vertical osteotomies basally. The disc protector allows for the extension of the osteotomy lines, as needed, to the basal edge of the mandible without injuring the adjacent soft tissue.54,56,58
This incision with the diamond disc should have a controlled depth (width of the diamond layer on the disc is 1 mm) in cases where the mandibular nerve is close to the osteotomy area, depending on the measurement on the CBCT scan. Normally, in the first and second molar area, the mandibular nerve has a lingual position, with an average distance of 4.5 to 6 mm from the surface of the vestibular bone (Fig 4-21e to i), so that if the protocol is followed, nerve injury with the diamond disc in this region is highly unlikely. The diameter of the diamond disc is 8 mm, offering a maximum penetration of 3.2 mm, which is normally the thickness of the cortical vestibular bone. The nerve moves to a more superficial position when the osteotomies move in a distal direction into the ramus. The nerve can be damaged with the harvesting tools in cases of incorrect diagnosis and/or inadequate surgeon performance.
The final osteotomy, on the occlusal crestal side parallel to the external oblique ridge, is achieved with a thin, 1-mm drill bur. Small perforations of 3 to 4 mm in depth, parallel to the buccal bone wall, are made with the drill bur at the level of the crestal platform of the external oblique ridge, at a distance of about 4 mm from the external border of the external oblique line, and between the two vertical incisions (Fig 4-21j). These perforations are interconnected by using a special small chisel (Fig 4-21k). The chisel has to be held parallel to the outer surface of the mandibular body, allowing for a relatively unforced dislocation of the block in the vestibular direction. This chisel, with its form similar to that of a splitting axe used for chopping wood (Fig 4-21l and m), will produce tension in the cortical bone, creating an ‘explosive’ effect in the area of the crestal perforations, leading to an easy lateral dislocation of the bone block (Fig 4-21n to p).
The use of such a chisel to connect the perforations, and without any occlusal incision, ensures higher margins of safety and prevents an uncontrolled intrusion of the chisel in the mandibular canal area.
If, after the third hammer blow, no fracture line connecting to the perforation is seen, care should be taken to control all the osteotomy lines and ensure that they completely overlap (Fig 4-21q). It often turns out that, mainly in the mesial area, the horizontal osteotomy line does not overlap the vertical incision deeply enough. In this case, an extension of the cuts is necessary. A smooth and unforced dislocation of the bone block is often hindered by a thick cortical density of the bone marrow area. In this case, the bone block should be divided vertically into two to prevent an intraoperative fracture of the mandible. After the division, it is easier to first remove the smaller block, followed by the second one (Fig 4-21r).
Cadaver studies have highlighted the issue of the position on the mandibular nerve in the molar, retromolar, and ramus areas as well as its distance from the vestibular and lingual bone. One study found that the distance of the inferior alveolar nerve from the buccal wall of the mandible in the retromolar area was 3.8 to 5.7 mm (mean 4.7 mm) (see Fig 4-21e to g).37 In the area of the ascending branch (ramus mandibulae), however, the nerve runs much closer to the vestibular surface due to the reduced width of the ramus, compared with that of the retromolar area. Today, a CBCT scan is always recommended to provide more information about the position of the mandibular nerve in cases where the harvesting of a large bone block graft is planned that reaches the alveolar canal distally in the ramus area and/or basally.
In some situations, nerve exposure may occur if the linea obliqua externa is weak and the osteotomies of bone block preparation are performed below the level of the nerve course. Nerve exposure can also occur when the distal vertical osteotomy is positioned in the area of the ascending ramus, as the alveolar nerve in this area is still close to the buccal cortex before it extends lingually into the body of the mandible. In these cases, it is advisable not to sink the diamond disc to its full cutting depth, or alternatively to harvest a first bone block above the nerve course, which can be seen on a panoramic radiograph. In this case, if needed, a second, more apically located bone block can be removed after a better assessment of the bony structures (see Fig 4-21h and i).
These days, and for more security, it is recommended to obtain a preoperative CBCT in cases where the harvesting of a large bone block close to the mandibular canal is planned.
After the dislocation, the bone block is gently and slowly elevated to the buccal site. This is important because in some rare situations, the lamina dura of the mandibular canal with the alveolar nerve might be stuck to the buccal cortical bone in cases of a weak external oblique ridge. An aggressive removal of the bone block could then stretch and irritate the nerve unnecessarily. Especially in the area of the distal osteotomy, the risk of nerve exposure is increased, since this area is thinner in the bony structures.
A bone block harvested from the cortical external oblique ridge generally also has a small cancellous osseous layer adherent to the internal side (Fig 4-21s); consequently, it is appropriate to consider it as a ‘corticocancellous block graft.’9,42 The cortical bone in the retromolar area has an average thickness of 3 mm (Fig 4-21t), whereby the mesial cortical bone is much thicker than at the distal site. For this reason, to achieve a penetration depth of about 3 mm, the diameter of the diamond disc is chosen accordingly (Fig 4-21u), cutting only the cortical part of the bone. The spongy bone part is then detached gently through dislocation. Additional cancellous bone can be harvested from the donor site with a surgical curette (Fig 4-21v), but respecting the trajectory of the mandibular canal47,72,108 and the lingual cortical plate. If indicated, the harvesting procedure can be combined with the removal of the inferior wisdom tooth.
In general, the donor site is dressed with collagen fleece35 (Resorba, Nürnberg, Germany), which has hemostatic properties that allow for clot stabilization and better healing (Fig 4-21w). In addition, the collagen fleece seals the opened bone marrow cavity after bone block removal and reduces bleeding. The wound is closed with interrupted monofilament 5-0 or 6-0 resorbable sutures (Fig 4-21x). Filling the donor site with biomaterials is usually unnecessary. Even in cases in which reconstruction of the donor site is planned, it is very important to keep the biomaterial, especially bovine xenografts, far away from cancellous bone close to the mandibular nerve, and to always place collagen in between to prevent the migration of the biomaterial to the mandibular canal, leading to degenerative neural lesions (Fig 4-22a to d).54
In the case of a large external oblique line, it is possible to harvest blocks of large dimensions up to 5 cm (Fig 4-23a and b). In some cases, it is possible to combine bone block graft harvesting with lateralization of the inferior alveolar nerve (Fig 4-24a to e). In this situation, a preoperative CBCT scan is essential. The horizontal incisions should be made at a lower position, with one above (crestal to) and the other below (apical to) the mandibular canal. However, both vertical incisions should remain superficial (maximum depth of 2 to 2.5 mm), although this also depends on the information obtained from the CBCT scan. The dislocation of the bone block should be made carefully with the fine bone chisel only at the level of the superior horizontal incision to avoid inferior alveolar nerve injury.
As previously described, the bone block is initially split lengthwise into two blocks using first the MicroSaw and then a large diamond disc. This is followed by scraping the blocks to collect bone chips (Fig 4-25a to e). Additional bone graft can also be harvested from the recipient site bone area (Fig 4-26a to c). According to the protocol described, one of the two bone blocks is stabilized with screws at the planned distance from the recipient site, giving the form of the future crest. The gap between the block and the recipient site is filled with particulate bone and bone chips. The precise osteotomies of the MicroSaw allow users to reset a remaining bone block half in the donor site without additional fixation (Fig 4-26d to j), offering the best way to reconstruct and regenerate the donor site.57,58 This is important, especially in young patients, as it gives them back the external oblique line for possible reharvesting in the future, which can already be done after 6 months of healing.
A soft diet is advised for patients during the first 6 postoperative weeks to avoid a potential postoperative mandibular fracture. The greatest risk of fracture occurs about 2 to 3 weeks postoperatively, when the pain and swelling dissipate and the patient potentially starts to load the mandible at a time when the coagulum is organizing. The sutures are removed 10 days postoperatively. Osseous regeneration in this region is similar to that observed after the osteotomy of impacted wisdom teeth. Radiographically, surgical scars disappear within 6 to 12 months, depending on the regenerative potential of the donor site.
126.96.36.199 Harvesting bone from the chin
Locoregional anesthesia is given as a mental block on both sides, with local vestibular and lingual infiltration in the anterior mandibular region using 4% articaine and 1:100,000 epinephrine (Ultracain DS forte). In cases where the anterior mandibular teeth are still present, access to the chin area is achieved through a buccal incision between the two inferior right and left canines, following a circular line about 0.3 to 0.5 cm below the mucogingival border. This incision is not perpendicular to the bone surface, but follows a slightly obtuse angle to the bone in order to obtain a large surface of soft tissue in the wound site to allow for two-layer closure at the end of the surgery. In the case of an edentulous mandible, the incision is made on the top of the crest, allowing good exposure of the bony crest and, at the same time, the insertion of implants, if necessary. A mucoperiosteal flap is then reflected toward the base of the chin.
As soon as the chin bone is exposed, the graft can be harvested with the MicroSaw. Graft dimensions are determined by the extent of the bony defect to be reconstructed, but always leaving a 3- to 5-mm security margin with respect to the apex of the mandibular incisors.47,71,72,99 A profile teleradiograph gives clear information, including the bone volume, position of the apex, and root angulation of the anterior mandibular teeth (see Fig 4-9e). The inferior limit of the harvested area should preserve the inferior mandible border, respecting the 5-mm security margin from the basal border for muscle insertion and esthetics. Both horizontal incisions are made using the MicroSaw contra-angle handpiece with the maximum depth of the diamond disc (Fig 4-27a). In the case of short roots of the canine teeth, these incisions can extend up to both mental foramina. Connection of the two horizontal incisions is performed vertically with the MicroSaw handpiece, also at the maximum depth of the diamond disc (Fig 4-27b). Normally, the maximum depth of the diamond disc incision (3.0 to 3.2 mm, depending on the angulations of the incision) passes the vestibular cortical bone wall (Fig 4-27c). A drill bur is then used to deepen the osteotomy incisions using stamp-like carving through the cortical plate (Fig 4-27d and e). This drill provides information on the bone quality, cortical thickness, and torque needed to release the graft. Graft removal is achieved with the thin bone chisel (Fig 4-27f to h). Supplementary cancellous bone can be obtained with the rongeur or with a bone chisel up to the lingual cortical plate (Fig 4-27i and j).
In contrast to the mandibular retromolar region, a symphyseal bone defect must be filled partly with a biomaterial and stabilized with a membrane prior to placement of two-plane sutures (Fig 4-27k to o).46,47,54 These facts are based on the results of a retrospective study by the author on 134 chin grafts harvested between 1989 and 1996, where the donor site was treated in different ways.47 The study showed the following results:
Sometimes, a third vertical incision made at the median line can be useful to facilitate the luxation of the blocks (now two) (Figs 4-30a to f and 4-31a to o). Bone block harvesting from the chin area can be easily combined with simultaneous implant insertion.
After the removal of bone blocks of a large volume, very often tearing and injury of the fragile attached vascular and nervous bundles supplying the anterior mandibular teeth is observed (rami dentales inferiores of the inferior alveolar nerve) (Fig 4-32a and b).
Chin grafts are especially indicated for reconstruction of sites in the anterior mandible, where only one access is needed for the donor and recipient sites (Fig 4-33a to n). The use of membranes over the grafted bone block is not necessary and is mostly counterproductive, especially when using resorbable (collagen) membranes without a lining xenogenic material: the activated osteoclasts are not selective and will resorb part of the resorbable autogenous material (Fig 4-33o to q).
188.8.131.52 Harvesting bone from edentulous areas
Mostly, bone blocks harvested from edentulous areas are removed from the mandible.
Local anesthesia is performed in a similar way to the other areas with 4% articaine and 1:100,000 epinephrine (Ultracain DS forte) on the vestibular and lingual sides. A trapeze-like paramarginal incision is made, similar to the one in the retromolar area. After the elevation of the mucoperiosteal flap, the osteotomy is also performed here with the MicroSaw. The superior horizontal incision with the contra-angle handpiece of the MicroSaw should start 5 mm under the alveolar crest to preserve the superior contour for possible future implant treatment (Fig 4-34a to f). The position of the remaining incisions depends on the neighboring anatomical structures, e.g. the position of the neighboring roots, the mental foramina, and the mandibular canal. The luxation of the bone block is performed with a thin chisel, similar to the removal of a bony lid. The resulting cavity is filled with collagen fleece to stop bleeding and stabilize the clot. The wound is closed with the same 6-0 monofilament sutures, which can be removed after 7 to 10 days.47
Small bone blocks can be harvested in some situations from the apical area of the site to be grafted (Fig 4-35a to k). The harvested bone block is then split lengthwise into two thin blocks, and the unused half is replanted into the donor site where it provides a complete restoration of the shape of the jaw (Fig 4-36a to o).
A sharp lower ridge in edentulous patients can be removed with the MicroSaw crestally for about 5 to 6 mm, but only if the ridge is clearly overlooking the floor of the mouth. Otherwise, significant soft tissue problems can occur if the remaining alveolar crest becomes lower than the soft tissue of the floor of the mouth. Thus, on the one hand, a larger crest is created, and on the other, bone blocks are obtained for lateral augmentation, stabilizing the weakly spread vestibular bone wall and at the same time augmenting the bone volume of the alveolar crest (Fig 4-37a to l).
Small bone blocks can also be obtained from the maxilla. Thin bone blocks can be harvested from the facial sinus bone wall and the basal zygomatic region (in combination with sinus floor elevation), from the palatal tubera area, from the anterior palatal bone,23 and from the edentulous maxillary regions (Fig 4-38a to p). On the vestibular side, the maxillary tubera area normally presents soft bone with a very thin cortex that contains an important amount of fat. For this reason, it is mostly harvested as bone chips. On the palatal side, this bone can be harder than that found on the vestibular side, allowing it to be used as a small bone block. In cases where the panoramic radiograph shows an important volume of bone in the maxillary tubera area (Fig 4-38q), this can be harvested as a bone block with the MicroSaw and a thin chisel. In this case, it is important to harvest this block with its own periosteum, which gives it more stability and rigidity (Fig 4-38r to z).
A 10-year prospective study for harvesting mandibular bone blocks with the MicroSaw technique described in detail the results of 3874 bone blocks removed from the retromolar area.58 The aim of this prospective study was to evaluate the outcome of bone block harvesting from the external oblique ridge with the MicroSaw and to identify possible morbidity, complications, and volume of the harvested block.
A total of 3874 bone blocks were harvested from the external oblique line of the mandible in 3328 patients within a 10-year period. Of these patients, 419 (12.59%) underwent bilateral bone block harvesting, and 127 (3.82%) had more than one block harvested from the same area during the study time. The blocks were harvested according to the MicroSaw protocol, with the performance of two proximo-vertical and one baso-horizontal osteotomy with the diamond disc, and small perforations on the occlusal crestal site with a thin drill bur. A special chisel was used to dislocate the bone block. The harvested bone blocks were split into two thin bone blocks with the diamond disc according to the split bone block (SBB) technique for biologic grafting procedures. In 431 cases (11.12%), only one bone block was required; the second was repositioned to reconstruct its donor site.
The average harvesting time was 6.5 ± 2.5 min, with a mean graft volume of 1.9 ± 0.9 cm3 (maximum 4.4 cm3). The volume of the bone block graft was measured by the Archimedes law while maintaining aseptic conditions. The harvested graft was placed inside a graduated tube filled with a physiologic serum. Graft volume is obtained by subtracting the volume of serum remaining once the graft is removed from the total volume (graft + physiologic serum).54
An exposition of the alveolar nerve occurred in 168 (4.33%) cases, leading to transient sensory problems lasting for a maximum of 6 months. In 20 cases (0.5%), minor nerve injury occurred: eight patients (0.2%) demonstrated a hypoesthesia, and 12 (0.31%) a paresthesia that remained for up to 1 year. In four patients (0.1%), the paresthesia was present for more than 1 year. No major nerve lesion with permanent anesthesia was observed in any case. A total of 61 (1.58%) donor sites showed primary healing complications, most of which occurred in smokers (80.4%). In 16 of the 431 cases with reimplantation of half of the bone block, a reentry of the harvested area was performed between 6 and 40 months later with another augmentation procedure. In all the cases, a well-regenerated and healed external oblique ridge was found. The results of this 10-year prospective study demonstrated the ability to harvest a relatively large-volume bone block graft from the mandible with a low complication rate. The reimplantation of half of the split bone block offered the possibility for complete regeneration of the donor site.
A retrospective study was carried out that involved a total of 716 grafts harvested from the symphysis area, with a mean graft volume of 2.7 cm3 and a maximum value of 4.8 cm3. This area often has a thinner cortical plate but more cancellous bone than the retromolar region. A total of 20 (2.8%) postoperative complications were observed, in addition to problems of sensitivity of the anterior mandibular teeth. Of these, 17 cases showed primary healing complications, with dehiscence and inflammation of the donor site. After local treatment with several applications of H2O2 irrigation, all of the 17 cases healed well up to 6 weeks postoperatively. During this period, it was necessary to remove the nonresorbable membranes in eight patients, where there was also some lost biomaterial. Two cases revealed apical changes on the mandibular canines (where crowns had already been present for many years) after a few months and received endodontic treatment. In one case (this was the third harvesting from the same area within 5 years), a fracture of the basal cortical border of the chin occurred during the harvesting procedure and was successfully treated with osteosynthesis screws. In all the other cases, the postoperative situation (swelling, hematoma, pain) was similar, but was less intense compared with the retromolar area. Changes in physiognomy (profile and chin prominence) were not observed in any of the cases (see Fig 4-28a).
The major complication in harvesting bone blocks from the chin area is the disturbance of the sensitivity of the anterior mandibular teeth for several months postoperatively.47 This complication, which represents the main morbidity in this region, was observed as a primary complaint in 38.8% of the patients for a period of up to 12 weeks. These symptoms were present especially after the removal of large bone blocks, and were due to lesions of mesial ramifications of the mandibular nerve (rami dentalis anterior) (see Fig 4-32a and b). These sensitivity disturbances of one or more anterior mandibular teeth were still present for more than 12 months in 6.84% of patients. Osseous regeneration in this region was confirmed by panoramic and profile teleradiography. The most favorable results were obtained in those sites filled with a resorbable bone substitute material in combination with coverage with a nonresorbable membrane.47
What follows are the results of grafting procedures with mandibular bone block grafts performed between 1994 and 2002 according to the biologic procedure described at the beginning of this chapter. Among 1229 lateral grafts, 14 cases showed primary healing complications associated with partial flap necrosis and exposure of part of the graft after lateral ridge augmentation. The reason for the graft exposure in four patients was the pressure of the removable prosthesis on the grafted area. A poor flap preparation with a non-atraumatic surgical procedure was the reason for failure in three other patients. A sharp edge on the block was the reason for a small graft exposure in two patients. After local treatment, with volume reduction of the grafted bone and reclosure of the wound performed 6 to 8 weeks after graft exposure, it was possible in all these cases to insert implants in an acceptable position without new grafting. The remaining five patients with graft exposure were heavy smokers. Local treatment of the exposed bone failed, and the grafts were completely lost. A new graft was performed 3 months after removal of the necrotic bone using the tunnel technique.
In all the other cases, the healing and regeneration of the grafted area were good enough to allow for the insertion of implants 3 to 4 months after surgery. The bone thickness of the recipient site, which was an average of 2.1 mm preoperatively, was increased 3 to 4 months postoperatively to an average of 6.8 mm. After a follow-up of up to 10 years, these implants demonstrated similar results to those observed for implants placed in non-grafted bone. The augmented area showed a stable situation with no significant changes in the peri-implant bone level.
Between 1995 and 2002, a total of 209 patients were treated with a vertical 3D reconstruction using mandibular bone blocks. The autogenous bone was harvested from the retromolar region (n = 184) and from the chin area (n = 25). After 3 to 4 months, 389 implants were placed. Overall, complications were registered in seven patients. In four cases, graft exposures occurred, and in the remaining three cases poor bony regeneration was observed with soft tissue migration into the graft. The healing went smoothly for all the other patients. In a period of 8 to 9 years postoperatively, only three implants were lost. All the remaining 386 implants with stable clinical conditions were still in function.
The histologic analysis of 119 biopsies from different grafted areas followed by a histomorphometric analysis of 96 biopsies were performed in four university centers: Münster and Hamburg (Germany), Toulouse (France), and Vienna (Austria). Histomorphometric analysis was performed on bone biopsies removed from the laterally or vertically grafted maxilla (Fig 4-39a to j) and mandible following the presented biologic concept describing the SBB technique. The results showed a slightly better density of the area grafted laterally than that grafted vertically. Differences were also present between the maxilla and mandible (Fig 4-40a).
Another analysis compared the percentage of vital osteocytes present in areas grafted according to the classical approach, with thick mandibular bone blocks harvested from the retromolar area or chin region, and in areas grafted according to the SBB technique. For both lateral and vertical bone grafting, there were significant differences in the percentage of vital osteocytes between biopsies removed from classically grafted areas and those grafted using the biologic approach (Fig 4-40b and c).
The external oblique ridge is favorable for obtaining large mandibular bone block grafts and is the first choice for intraoral bone block harvesting. The proximity of the donor and grafted sites reduces the surgical and anesthetic periods, leading to ideal conditions for implant surgery using autogenous bone grafts.
The anatomical variation of donor sites creates grafts with morphologic differences. Visual examination and digital palpation allow for a preliminary estimation of the morphologic contours and dimensions of the donor site such as the thickness and extent of the external oblique ridge. This clinical examination provides information on the shape of the available bone at the donor site. Radiography should be used to supplement information on the donor site and the position/relation of important neighboring anatomical structures. The location of the mandibular canal and the mental foramen can be traced on a panoramic radiograph, which also shows the density of the external oblique ridge. In the period between 2000 and 2008, the panoramic radiograph was the only radiologic diagnostic means for assessment before harvesting bone from the external oblique ridge using the MicroSaw. The maximum cutting depth of the MicroSaw of 3.2 mm seems to be anatomically appropriate for safe bone block harvesting in the retromolar and paramolar areas. However, in the area of the ascending branch (ramus mandibulae), the nerve runs much closer to the surface. Nerve exposure may occur if the linea obliqua externa is weak and the osteotomies of bone block preparation are made below the level of the nerve course. Nerve exposure can also occur when the distal vertical osteotomy is positioned in the area of the ascending ramus, as the alveolar nerve in this area is still close to the buccal cortex, before it extends lingually into the body of the mandible. It is recommended not to use the MicroSaw to full cutting depth at the distal incision behind the second molar when the osteotomies are located below the level of the nerve. A secure margin not extending to a depth of more than 2 mm of the osteotomy cutting in this area is recommended (the diamond layer of the disc is 1-mm wide). According to the low complication rate, the described clinical and radiologic diagnostic methods seem to be sufficient for safe bone block harvesting following the presented protocol. The use of diagnostic CBCT today provides more anatomical information such as the thickness of the cortical wall and the position of the mandibular nerve. It therefore gives helpful and important information before the surgery, especially if the osteotomies are planned to cross over the course of the mandibular canal.
The results demonstrate the ability to successfully obtain a relatively large volume of bone block graft harvested from the mandible with a low complication rate using a particular technique and protocol with specified instruments. In a study of 50 patients, Misch72 found a mean symphysis graft volume of 1.74 cm3, as opposed to 0.9 cm3 for ramus grafts. Compared with the aforementioned results,54,58 this difference might be explained by the small graft volume, the different harvesting technique and instrumentation, and the difference between harvesting from an area with neighboring teeth compared with one that is edentulous (especially the chin region). In fact, in his harvesting technique, Misch used fissure burs to make the osteotomy cuts, which led to significantly more bone loss compared with cuts made with the thin diamond discs of the MicroSaw.47,51 On the other hand, the MicroSaw offers the possibility of making the required incisions along the ramus and obtaining a large bone block graft without the risk of damaging the soft tissue.
Another study using a piezoelectric surgical device showed a mean graft size of 1.15 cm3 with a maximum of 2.4 cm3.28 Compared with the results of the aforementioned study58 – with 3874 bone grafts harvested from the external oblique ridge with a mean volume of 1.9 cm3, a maximum value of 4.4 cm3, and a graft thickness of up to 6.5 mm – this difference might also be explained by the different harvesting technique and instrumentation used.
A randomized split-mouth prospective clinical trial comparing bone block harvesting from the retromolar area with the MicroSaw and with piezosurgery27 demonstrated a significant difference in the osteotomy time, but without any difference in postoperative pain and swelling or complications. The average osteotomy time for harvesting, including luxating a bone block, was 5.63 (± 1.37) min using the MicroSaw, and 16.47 (± 2.74) min using the piezoelectric surgical device (P < 0.05). A mean graft volume of 1.62 cm3 (± 0.27) was measured with the MicroSaw, and 1.26 cm3 (± 0.27) with piezosurgery (P < 0.05).
Currently, there are other types of instruments used to obtain intraoral grafts, most of them being trephine burs of different forms and diameters. However, a trephine bur can only remove small bone pieces in bone core form, providing only particulate bone rather than a bone block. Saws that can cut a bone block from the mandible are rare, and do not provide satisfactory results in the retromolar area, such as in the chin region. The use of such instruments in this area is complicated and risky due to the poor access and uncontrolled depth of the horizontal and vertical sections.
The MicroSaw allows the surgeon to obtain large grafts in a short time with low risk, which allows for total reconstructions of the maxilla and mandible, if the protocol is followed as presented (Fig 4-41a to s). This is documented by the results presented with a low complication rate, especially in the retromolar area, where no severe lesion of the mandibular nerve was observed. In addition, the MicroSaw offers the possibility of obtaining different graft forms, e.g. by cutting a single block graft longitudinally with the thin diamond disc to yield two blocks with the same surface area but with half the thickness. These thin blocks harvested from one site can be used individually for the 3D reconstruction of extensive vertical bony defects. If only one block is required, the second one can be reset back into its donor site and stabilized with a small screw, if necessary, to completely restore the contours of the jaw and to allow for better regeneration (Fig 4-42).
The postoperative situation after bone harvesting from the retromolar area is similar to that observed after the osteotomy of impacted wisdom teeth: edema, hematoma, and pain. However, the infection rate in the study by the author and colleagues (< 1%) was less than infections from the extraction of wisdom teeth (6% to 8%). This could be related to the presence, peri-coronally and around the root, of a lamina dura, which can have a negative influence on vascular support, bleeding capacity, and the healing process. On the other hand, graft harvesting exposes a large surface of cancellous bone, which causes bleeding. Collagen stops the bleeding and stabilizes the clot to enhance wound healing. In a parallel randomized clinical study of two techniques for lateral bone grafting, Korsch et al65 found no significant differences in pain and swelling between a lateral bone graft procedure using a bone substitute material combined with bone chips harvested with a bone scraper (the shell technique using the SonicWeld Rx system; KLS Martin Group) and the SBB technique using the MicroSaw protocol with exclusively autogenous bone. The study could not prove that patients had fewer and less-severe postoperative complaints as a result of a smaller amount of autogenous bone being harvested at the donor site.
Despite harvesting a large part of the external oblique ridge, no esthetic or functional deficiencies resulted.
The main complication involved with harvesting bone blocks in the chin region is anesthesia or paresthesia of the anterior mandibular teeth for a prolonged period in many patients. This neurosensitivity problem is caused by laceration of the anterior ramifications of the mandibular nerve during chin graft harvesting in patients with vital anterior teeth. Without clinical signs of pulpal necrosis, the loss of sensation in one or several teeth does not indicate the need for endodontic therapy.54
Misch72 equates paresthesia of the mental nerve with anesthesia of the skin of the chin in 9.6% of cases and paresthesia of anterior teeth in 29% of cases. In another study, Von Arx and Beat108 did not observe dysesthesia of the mental nerve after bone graft harvesting from the chin, except for two reactions at the apex of the mandibular canines, for which graft drilling could not be held accountable. Despite the fact that Misch reported a security distance of 5 mm from the apex of the anterior teeth, the question arises as to whether the 3-mm security distance described by other authors is sufficient.54,89,101 It could be speculated that the risk of lacerating the mental nerve is inversely proportional to the proximity of the harvesting preparation of the corticocancellous graft to the apex of the teeth. Since the mental nerve curves anteriorly before it emerges from the foramen, this procedure presents a non-negligible risk. It is possible that the reason for some cases of mental nerve paresthesia as described, especially lesions with rapid remission of approximately 2 months, is the traumatic elongation of nerve fibers during the important mobilization of the flap in the apical direction.99
As this morbidity can disturb some patients, harvesting of chin grafts should be done primarily in patients presenting an edentulous anterior mandible or a large chin protuberance with short roots of the anterior mandibular teeth.
There was no alteration of the profile in over 716 patients who had a block graft harvested from the chin. The soft tissue profile remained unchanged and mandibular lip function was regained. Previous studies45,54,99 also demonstrated the preservation of the chin contour; however, when the insertion of the chin muscle cannot be maintained, muscle ptosis can result.92
The biologic procedure for grafting mandibular bone blocks is logical and leads to a high percentage of vital osteocytes in the grafted area within a short time (see Fig 4-40b), which is important for optimal osseointegration of the implants and for the prevention of graft resorption. Compared with blocks grafted according to the classical approach, biopsies 4 months after the surgery demonstrate approximately double the percentage of vital bone cells in the area grafted according to the presented biologic procedure (see Fig 4-40b). This is also important for the long-term prognosis of the restoration through the maintenance of a stable bone situation of the grafted/regenerated area and stable osseointegration of the implant. This is demonstrated in the clinical results presented. After a follow-up of up to 10 years, the implants demonstrated similar results to those observed for implants placed in non-grafted bone. The augmented area showed a stable situation, with no significant changes in the peri-implant bone level.
The presented modification of the bone block grafting procedure, published under different names as the SBB technique or Khoury’s shell technique,90,103,107 offers the biologic improvement of the healing of cortical bone grafts and the following advantages: