6

Bone Grafting: Standards and Surgical Technique

Unlike most other tissues, bone can be freely avascularly grafted even in larger volumes and layer thicknesses. In the case of bone, unlike many soft tissues, survival of all cells of the graft is not necessarily required, because bone morphogenetic protein (BMP)-driven remodeling can also restore avascular grafts to vital functional bone. The natural regenerative potency of autogenous bone can be therapeutically stimulated and controlled in terms of form and speed by augmentation materials.

6.1 Conditions for Bone Grafting

Nonsterile environment and defensins

Although the oral cavity is not a sterile surgical field, bone wounds in the mouth heal with astonishing speed compared to other body regions, and bone can even be freely grafted via intraoral approaches. This is due in part to β-defensins.¹ Defensins are a group of small proteins with a high proportion of cationic and hydrophobic amino acids that have a high affinity for cell membranes not containing cholesterol, as found only in bacteria. There they form membrane pores that lead to the death of the microorganism. Defensins are part of the genetically ancient innate nonadaptive immune system and make up much of the content of the granules of neutrophil granulocytes, in which they serve to kill bacteria. They are also expressed in high concentrations by cells of the oral mucosa and jawbone.^2,3 Despite this special defense situation in the oral cavity, careful preoperative bacterial count reduction and sterile instrumentation are prerequisites for the clinical success of an augmentation.

In the case of bone grafts, care must be taken to ensure complete fill of the defects with coagulum of the defects, which can be supplemented by admixing venous blood to the bone grafts in case of doubt. The cells of the medullary cavity may need to be allowed to connect to the defect by perforating the cortical bone of the recipient bed. In a small human study, perforation of the recipient bone during augmentations resulted in greater and faster graft vascularization and better bone formation.⁴

Storage of bone grafts

Bone proteins are stable up to about 60°C, above which they denature, especially the BMPs. This, in addition to cell vitality, is the reason why drilling on bone should only be done under abundant water cooling. Normal sterile physiologic saline solution is sufficient for cooling.

Fig 6-1 Three types of osteosynthesis for intraoral bone grafts: Lag screw, positioning screw, and plate fixation.

Experimental data are available on the proper storage conditions for autogenous bone grafts after harvesting. The vitality of bone cells of bone grafts decreases significantly by dry storage compared to storage in physiologic saline or covering with compresses moistened in physiologic saline. In contrast, storage in more elaborate media such as cell culture medium did not provide a significant advantage.⁵ Platelet-poor plasma also did not provide an advantage over saline.⁶ Since cell viability decreased significantly as early as 2 hours after harvesting, bone grafts should be harvested immediately prior to placement if possible.⁷ Ice cooling (without freezing) increased cell viability compared to room temperature storage.⁸ In an experimental study, bone grafts with vital cells resulted in 30% more bone growth than grafts without vital cells.⁹

Mechanical rest

Mechanical stability during the healing phase should be ensured by good fixation of the bone grafts with screws and by avoiding soft tissue pressure.

Internal resorption of the bone graft is desired and necessary in the sense of “creeping substitution” (see chapter 2). Beyond the healing phase, desired internal resorption occurs as part of the functional remodeling of the bone, starting from the recipient side of the bone graft. After about 3 to 4 years, free bone grafts will be almost completely internally resorbed and replaced by regrown new autologous bone. The bone cutting cones are responsible for the remodeling process. To ensure that the bone cutting cones can advance from the recipient bone into the graft without interference, a form-fitting adaptation of a bone block to the recipient bone or at least relining with autogenous chips is helpful. An intermediate layer of bone graft substitute should therefore be avoided under bone blocks, and when using the shell technique with autogenous compact blocks, autogenous chips should be used to backfill the shell.

Fig 6-2 Commercially available microscrew osteosynthesis set in a sterilizable container (1.5 mm Centre Drive System, KLS Martin).

Fixation of bone block grafts

Mechanical stability is essential for bone healing. There is almost constant unrest in the oral cavity due to chewing, tongue movements, and swallowing. Therefore, it is important to reliably secure bone grafts against movement during augmentations. This is done by means of lag screws, set screws, or plate fixation (Fig 6-1). Suitable screw sizes (eg, 1.5 mm Micro System, KLS Martin) are available from osteosynthesis material manufacturers (Fig 6-2).

Fig 6-3 Two types of fixation of an intraoral bone block graft. Bone shells should not be backfilled with bone substitute material, only with autogenous bone chips to allow rapid remodeling by bone cutting cones.

The screw is applied as a lag screw, by drilling the screw access channel in the graft larger than the largest screw diameter. Then the screw head pulls the graft against the bone base when it is screwed in, wedging it. If tightening is not desired, a positioning screw is indicated. These are needed for shell techniques, for example (Fig 6-3). For this purpose, the screw channel is not overdrilled in the graft. When the positioning screw is screwed in, the distance between the graft and the base is adjusted first. The distance does not change even with the strongest tightening of the screw. If there is no space to fix a lag screw, eg, due to existing implants or tooth roots, the graft can also be fixed with osteosynthesis plates with slightly increased materials use and cost (Fig 6-4).

As a rule, at least two screws are placed per block to secure it against rotational loading. Smaller grafts and bone substitute material should be placed around the block as fillers. Small grafts that cannot support screws are somewhat stabilized by the blood coagulum and the tension of the uninjured periosteum (Fig 6-5).

A barrier membrane performs well for positional fixation of smaller grafts. However, the block graft provides much more stability after osteosynthesis than all membrane techniques. When handled correctly, it is stable as a rock in the surf. This allows more frequent single-stage implant placement, even if there is insufficient support in the local bone volume for the implants. In addition, the regeneration potential and resorption stability of the block graft are higher than those of particulate materials in guided bone regeneration (GBR), even in critical cases. Osteosynthesis material removal after 4 months should be performed minimally invasively via stab incisions, if possible, because deperiostation of the block can jeopardize it and triggers unnecessary surface resorption (Fig 6-6).

Bone graft healing time

For the practice workflow, it makes sense to set the healing times of the two-stage bone grafts uniformly to about 4 months prior to implant placement. This period is based on the healing time required for ridge augmentation with an autogenous block graft from the external oblique ridge. If less than 4 months is allowed for autogenous block grafts to heal, the block may detach from the recipient bone during implant drilling due to lack of a strong connection. However, with longer healing periods, the surface absorption progresses too much.

The sinus elevation healing period can also be established as 4 months, as it frequently has to be performed together with block grafts in the edentulous maxilla. With autogenous iliac cancellous bone, a sinus elevation heals in as little as 4 weeks; only bone substitute requires 8 months. The mean value of 4 months is achieved with a mixed bone graft (25%/75%).

Fig 6-4 Long-term follow-up of a bone block graft. a. Tooth malformation in the maxillary right central incisor area with apical radiolucency in a 17-year-old patient. b. Clinical image of the right central incisor malformation combined with agenesis of both lateral incisors. c. Panoramic radiograph after gap opening. d. Panoramic radiograph after placement of dental implants in the maxillary lateral incisor sites immediately after gap opening. e. Defect after removal of the right central incisor filled by a contoured autogenous bone block graft from the external oblique ridge using an osteosynthesis plate. The block was multiply perforated to accelerate healing. f. Four months after grafting, removal of the osteosynthesis material and placement of the dental implant. Bleeding from the perforation holes is a sign of the beginning of vital healing of the block.

Fig 6-4 Long-term follow-up of a bone block graft. g. Radiograph after implant placement in the block graft. h Peri-implant bone resorption as a sign of remodeling with formation of a peri-implant soft tissue cuff (biologic width). i. Mirror image of the maxilla after prosthetic restoration. j. Radiograph 7 years later, nearly identical to that in h, with no further marginal bone resorption. Remodeling is complete, and the bone is now functionally defined. k. Intraoral situation at 24 years of age. Slight recession is evident at the implants in the agenesis sites. Stable tissue conditions at the site of bone transplantation. l. Intraoral situation at age 33. Stable tissue conditions at the site of bone block grafting.

Fig 6-5 Bone block grafting for a large maxillary defect. a. Dehiscence defect (1/4 according to the quarter rule; see Fig 1-11) at both central incisor the left lateral incisor implants with single-stage implant placement. b. Two autogenous bone blocks from the external oblique ridge. The block in the right central incisor region was regularly fixed with two screws to secure it against rotational movement. The block in in the left central incisor site fit tightly to the bone surface so that one screw was sufficient. c. Autogenous bone chips from the bone filter were used for contour filling. d. Covering with a collagen membrane to stabilize the position of the bone trap and to protect the block grafts against resorption.

Sandwich interposition in edentulous segments and jaws also requires 4 months, as does GBR with a mixed graft and ridge splitting.

6.2 Mixed Bone Grafts

Autogenous cancellous bone is rich in cells and BMPs and is osteoinductive. The material heals quickly, within about 4 weeks, but also tends to resorb rapidly. Mineral bone substitute material is devoid of cells and BMPs and therefore takes many months to heal from the defect walls by osteoconduction, if it ossifies fully at all. On the other hand, xenogeneic bone mineral, for example, is very stable to resorption resistant, so that a selected augmentation level is maintained until the implants are osseointegrated and from then on can contribute to the maintenance of the augmentation by functional loading. The best strategy is therefore to use autogenous chips with the resorption-stable bone substitute material. The mixing ratio is a compromise between good healing and good resorption stability. According to animal data on sinus elevation, this compromise is best achieved with a ratio of 25% autogenous chips to 75% bone substitute.¹⁰ Ridge augmentations are more challenging as a defect type than sinus elevations; here, osteoinduction is even more important. In a clinical study on ridge augmentation, the autogenous bone in a 90:10 mixture was too low in dose; a 60:40 mixture performed better.¹¹

Fig 6-6 Minimally invasive osteosynthesis material removal. a. Implant exposure after single-stage bone block grafting. The position of the osteosynthesis screw can be identified by the protrusion and anemia in the vestibule after palpation. b. To avoid compromising the block by re-entry and deperiostation, the screw is minimally invasively located by stab incision. c. The square head of the Centre Drive System attachment automatically engages in the screw head at depth, even without direct vision. This allows the screw to be removed without raising a flap.

Autogenous chips have the disadvantage that they were usually obtained in the contaminated oral cavity and are therefore themselves bacterially contaminated. If porous bone substitute material is inoculated with bacteria, there is a risk of biofilm formation and thus infection of the augmentation. This can be minimized by first mixing the porous bone graft material with sterile venous blood in a sterile dish. In this way, all cavities of the bone graft substitute are sealed with sterile liquid and all surfaces with sterile blood protein, so that bacteria do not find an interface on which to settle. However, the blood does not yet coagulate after this mixing, so that the particles of the bone substitute material do not hold together and are difficult to apply.

Tissue thrombokinase (tissue factor) is required for coagulation to activate the extrinsic pathway. It makes sense in nature that in an extraction wound the bleeding stops at the level of the alveolar opening. This is ensured by saliva, which is a rich source of tissue thrombokinase.¹²

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