Clinical and scientific background of tissue regeneration via alveolar callus distraction
Augmentation procedures require a highly skilled surgeon for a successful regeneration to be performed, especially if there is an existing lack of bone in the vertical dimension.6,35,37 Patients with excessive vertical bone loss can generally be classified into two groups: those with a highly atrophic edentulous jaw,10,36 and those with severe vertical defects alongside the residual dentition. Some reasons for the occurrence of vertical bone defects in edentulous patients are long-term loading with a removable prosthesis and post-tumor resection.5,39 It is not always easy to reconstruct these extended bony defects with an autologous graft and the procedure is limited by poor soft tissue quality and quantity,34 by the regeneration potential and vascularization of the recipient site, and by the difficulty of protecting the graft from loading during the healing period.21 Vertical bone augmentation adjacent to the residual dentition with severe loss of attachment is also difficult and risky due to the persistence of the deep pocket on the neighboring teeth, which can serve as an open door for possible infection. Another problem is the necessity of a second, mostly extraoral (e.g. iliac crest) surgical site to obtain an adequate volume of bone for the grafting procedure.36 Some authors have reported on the high morbidity of the donor site, with sometimes long periods of hospitalization for the patient or a high resorption rate of the graft,14,30,31 depending on the technique and the protocol of harvesting bone blocks from the iliac crest.
6.2 History of the callus distraction
Intraoral distraction devices have been in use for the past 25 years for maxillofacial and craniofacial indications.7,29 For the vertical distraction of the alveolar bone to vertically augment and improve the implant site, TRACK (tissue regeneration by alveolar callus distraction by Köln) was developed in 1997. Since then, it has become established as a low-procedure system without the need for patient hospitalization.15
Ideas about lengthening the bone by surgical intervention were already being discussed and evaluated at the beginning of the 20th century. However, the technique of a one-stage extrusion of the osteotomy gap and fixation with a splint showed a high complication rate, without reossification of the osteotomy and pseudarthrosis of the femur. The continuous procedure by extrusion of the callus was described by Abbott in 1920,1 but the high complication rate due to improper mechanical stabilization, the lack of knowledge on biologic principles, and poor surgical hygiene led to the abandonment of the procedure.
The Russian surgeon, Gavriil Abramovich Iliazarov, is known as the father of modern distraction osteogenesis, which is based on the adverse use of an external fixator for the treatment of a complex fracture. Instead of applying a compression force on the fracture, the segments of the fracture are diverted.17 As a result of the slow movement, the tissue becomes reorganized. By performing a corticotomy at a selected site, Iliazarov was able to increase the length of the extremity bones and their surrounding tissue by 40 cm, without the need of an implantation of spongy bone. The fragments are stabilized by an external device that moves the segments 1 mm per day. The callus, which builds up during the latency period of 3 to 7 days, expands at the same rate in the direction of the tension. This ‘distracted’ area remodels during the consolidation phase, which takes about a month.17-19 Iliazarov’s research and clinical reports of this technique for the rehabilitation of large defects to increase or reconstruct bone volume resulted in the procedure becoming well accepted internationally.
The indication for this technique in maxillofacial and craniofacial surgery at the end of the last century was for the reconstruction of congenital defects,20,27,28 e.g. the lengthening of the hypoplastic ramus in mandibular condylar hypoplasia could be performed using this technique with good functional and esthetic results. Further developments led to the introduction of the intraoral distractor, with different functions, which is placed under the mucogingival flap to treat and correct various disorders.23,33
Based on this knowledge, around 1996 different centers began to utilize distraction devices that were designed for enlarging the mandibular ramus in the treatment of alveolar crest defects15 or for moving the remaining teeth into the correct position. As opposed to the external devices that were used in orthopedic surgery, the maxillofacial devices were designed to be implanted in the soft tissue, without coetaneous perforations.16 In 1996, Chin and Toth7 published a report about the successfully performed distraction of an edentulous segment of the anterior mandible with a transgingival screw activated over a rectangular wire.7 The effectiveness of such a technique has been additionally confirmed by experimental studies on animals with individually manufactured implant-supported distractors, where the hard and soft tissue of the alveolar crest is reconstructed in one surgery with a low complication rate.8 The soft tissue volume follows the bone distraction without an additional surgery having to be performed.4
6.3 Principles of the callus distraction
The primary principle of a callus distraction is the same as that for a vertical distraction of the alveolar bone. Clinical and experimental studies have confirmed that through the osteotomy surface, a set length and width of the regenerated part will still be stable, independent of the distraction route. Each stage of the callus tissue formation during the distraction and the following consolidation phase have been histologically confirmed, as they have for vertical distraction.3 There is controversy about whether the soft callus becomes mineralized during the distraction, with or without an intermediate cartilage. After a consolidation period of 3 months, the tissue is transformed into hard callus though osteoblast activity. The remodeling stage occurs between 10 and 12 weeks after the end of the distraction procedure.
After an osteotomy, wound healing follows the same principles as a healing fracture.11–13 Depending on the space between the two bone plates, contact healing or distance healing (also called callus healing) occurs. For callus healing to take place, the blood clot that is generated between the fragments shows signs of an inflammation for the first 3 days. Within the blood clot, a mesenchymal cell recruitment with an endocrine and paracrine reaction occurs.9 The macrophages resorb the blood clot, and fibroblasts organize the granulation tissue with the ingrowth of blood vessels. The further differentiation of the granulation tissue leads to the callus formation. The osteoblasts and fibroblasts are organized, the transforming growth factors (TGFs) are released, and the blood vessels spread in an intensive network of capillary structure.2 The fibroblasts then develop into osteoid tissue. After 6 days, vessel maturation and matrix calcification occur with the stabilization of the callus. Following the initial calcification, remodeling takes place that involves the orientation of the collagen fibers and secondary osteons. Mineralization and stabilization of the callus can only be achieved if the fracture segments are immobilized. Mobilization during fracture healing leads to a pseudoarthrosis due to the dissection of the callus if a large movement occurs, or a lack of mineralization in the event of a continuous movement.
The principle of distraction osteogenesis is callus movement prior to complete mineralization, so that the vessel network can follow the movement by further growth. The movement has to be performed continuously so that the mineralization phase is prolonged. After the final bone length is reached, a stabilization period is necessary to achieve full mineralization and remodeling of the callus (Fig 6-1).
The newly generated bone shows a longitudinal direction according to the direction of the distraction movement. It has been reported that vessel ingrowths occur tenfold faster than with regular fracture healing.32 At the osteotomy lines, two zones of cylindrically oriented primary osteons are covered with osteoblasts, which build the base for further bone growth. The first radiologic signs of ossification are visible at the border of the osteotomy lines. Major reossification of the space between the osteotomy lines is observed after 3 months. Various studies regarding the modification of the parameters were carried out by Iliazarov and coworkers. The optimal speed for the movement of the segments for bone and soft tissue healing is 1 mm per day. The quality and quantity of the newly generated bone depend on the stabilization during the fixation period, the local vascular supply, and the dynamics of movement during the distraction period.
The technique is currently used in orthopedic surgery mainly for bone lengthening to correct congenital deformations and for reconstruction after trauma or tumor treatment.
There are different types of devices on the market for distraction osteogenesis, the most popular being the TRACK distractors (KLS Martin, Tuttlingen, Germany) that are available in different lengths and allow for a distraction osteogenesis from between 6 and 15 mm (Fig 6-2). The design principle for all these devices is the same, in that osteosynthesis plates are laserwelded to a cardanic system. The lower plates, called basal plates, are utilized to fix the device to the basal, nonmobile bone. The upper plate, called the transport plate, is mounted onto the transported segment. One screw in the center of the cardanic system is responsible for the movement. On the top of the system, a hexagon is used to transfer the rotational force, which moves the upper plate. All components are fabricated from titanium.
6.5 Surgical technique
The surgical approach for all patients follows the same treatment concept.15,16,24 Depending on the anatomical region and the degree of atrophy involved, the procedure is performed under local or general anesthesia. A perioperative prophylaxis antibiotic is administered (one administration of 1 g amoxicillin). For local hemostasis, the regional application of local anesthesia is performed with articaine 4% and epinephrine 1:100,000. A mucoperiosteal flap is prepared after an incision is made deep in the vestibule to obtain appropriate access for the placement of the device and for good coverage of the osteotomy line.
Soft tissue preparation is performed in this way so that the anatomical structures are protected and the access offers an overview for the mounting of the device (Fig 6-3a to d). Vertical releases of the flap should be avoided in order to guarantee good nutrition of the transported segment. The transported bone should only have the periosteum removed in the area where the distractor is to be placed on the cortical plate. Most of the periosteum should remain on the bone to support the nutrition supply to the distracted segment. After soft tissue preparation is complete, the distractor is adjusted to the bone and fixed with at least four screws for each bone plate (see Fig 6-3d). The initial screw length should not be longer than 4 mm in order to avoid a large opening of the preceptor sides.
The osteotomy line is marked with a small round head bur for subsequent separation of the bone. If possible, the height of the transported segment should be at least 4 mm. The distractor is than removed, and the final osteotomy is performed using a micro oscillating saw (Fig 6-3e and f) or a Lindemann bur, while protecting the supporting tissue to guarantee subsequent nutrition. The osteotomy must be performed in such a way as to allow for open movement of the transported part in a trapezoid/rounded shape. The osteotomy should not be performed with sharp lines because this can set an initial point for a fracture. Finally, the distracted part is mobilized using chisels to generate a greenstick fracture with the protection of the stabilization tissue (Fig 6-3g). If the osteotomy is performed with an oscillating saw through the complete jaw, the lingual soft tissue should be protected by digital control. Depending on the thickness of the soft tissue flap, especially in the maxilla, the palatal periosteum should be released for free movement of the segment (Fig 6-3h). The distractor is mounted according to the first adjustment with further screws (Fig 6-3i). Especially in the transported segment, longer screws can then be used to stabilize the segment. Prior to wound closure (Fig 6-3j