Distraction osteogenesis for alveolar reconstruction

Introduction

Osseointegrated implants are now an essential treatment modality in restorative dentistry. Having an adequate volume of bone and soft tissue is a basic prerequisite to successful treatment with dental implants. Early experience with implants focused on rehabilitation of the edentulous mandible. In these cases the bone volume was usually adequate and the esthetic requirements limited. Augmentation of the bone volume was rarely an issue in these cases. The need to reconstruct implant sites developed in response to the extension of implant dentistry into partially edentulous patients. Partially edentulous cases often combine compromised implant sites with increased esthetic demands. Deficiency in the volume or position of an implant recipient site is one of the most common problems encountered by the implant surgeon. Reconstruction of an alveolar defect is challenging because it involves replacement of both bone and soft tissue. Whether an alveolar deformity results from congenital, developmental, or acquired pathosis, the health of the prospective implant site is often compromised. Traumatic loss of teeth may leave the underlying bone and soft tissue damaged. Periodontal disease often persists in the teeth adjacent to the site being reconstructed. Congenital absence of teeth is often associated with hypoplasia of the alveolus and soft tissue.

Combined reconstruction of alveolar defects with surgery, implants, and prosthetics must meet structural and esthetic requirements. The site must be resistant to existing and future pathosis of adjacent teeth. The quality of the reconstruction impacts the short term mechanical and esthetic result as well as the long term stability of the tissue-integrated prosthesis and health of the adjacent teeth. Placement of implants in children poses a special challenge because of the potentially long service period.

Distraction osteogenesis in orthopedic surgery

Gavriel Ilizarov, a Russian orthopedic surgeon, is credited with defining the basic science of distraction osteogenesis (^{Ilizarov, 1991}). His treatment and research center in Kurgan, Siberia, produced the armamentarium and clinical experience to apply this technique to a wide variety of orthopedic limb deformities. Ilizarov developed the ring fixation device which is associated with the technique. A series of rings were joined by threaded rods. The bone segments were purchased by transosseous wires connected to the rings. The bone segments could be manipulated in their position and alignment by adjusting the frame components and wires. Forces to effect bone movement could be introduced by adjusting the nuts of the threaded rods or by repositioning the wires. Following the work of Ilizarov, distraction osteogenesis (^{Block et al., 1996} ; ^{Block et al., 1998}) as applied in orthopedic surgery has depended on these external frames with trancutaneous wires to effect the bone transport process and fixate the segment during maturation of the new bone. The confines of the oral cavity prohibit the use of external devices. The development of miniature devices has made oral distraction osteogenesis possible (^{Chin and Toth, 1995} ; ^{Chin and Toth, 1996} ; ^{Ueda, 1997}).

Many problems encountered by dental implant surgeons are common to those encountered by orthopedic surgeons. Although the size of alveolar defects is small compared to orthopedic deformities, the basic pathosis is the same. The model in ^{figure 1} illustrates a typical application of the Ilizarov ring fixator to close a discontinuity defect in the distal tibia. In this hypothetical case, an orthopedic surgeon may be confronted with a nonhealing, discontinuity defect in the distal tibia. These defects may be compromised by (1) lack of bone volume, (2) lack of soft tissue, (3) impaired blood supply, and (4) unfavorable microflora. Following the principles of conventional surgery, the surgeon most often approaches this clinical problem by (1) bone grafting, (2) moving soft tissue into the site, and (3) antibiotics. Distraction osteogenesis is an alternative treatment method. The distraction treatment would begin by excising the margin of the damaged site and discarding the tissue. An osteotomy is created at a proximal, healthy site. The distraction device is used to slowly transport the mobilized bone segment (^{fig. 2}). In response to the process, new structural bone fills the osteotomy gap and the transport process closes the defect. Because the soft tissue is transported with the bone segment, the soft tissue defect is also closed. The primary site of bone regeneration is at the distant osteotomy which is more capable of predictable healing.

This example illustrates important principles of distraction osteogenesis. First, slow displacement of a surgical fracture results in the formation of new structural bone. Soft tissue displaced by slowly transported bone responds by enlarging. Bone at the primary site of pathosis is damaged and should be discarded. The osteotomy should be placed at a site with good healing potential. By planning the osteotomy within a zone of healthy tissue, the primary site of pathosis is not involved in the regeneration of additional bone volume. In this manner, the site of pathosis which is resistant to healing is exchanged for a distant site with better regenerative capability.

Clinical example

A twenty-seven year old woman presented with the failure of both abutment teeth supporting a three unit fixed prosthesis. The distal abutment had a vertical fracture and was removed. The mesial abutment fractured at the level of the alveolar bone and would require crown lengthening to be restored. Because of prior bone loss, the retainers and pontic of the failed bridge were vertically longer than the corresponding natural teeth on the right side (^{fig. 3} and ⁴). The patient wished to resolve this discrepancy in crown length and replace the fixed bridge with implants and crowns. The alveolar deficiency included : (1) vertical bone loss at the alveolar crest, (2) labial bone resorption, (3) bone loss on the mesial aspect of the second premolar, (4) soft tissue deficiency, and (5) a lateral incisor requiring crown lengthening (^{fig. 5}). Distraction osteogenesis was used to transport the alveolar ridge inferiorly and laterally. The lateral incisor root was included in the transport segment to improve its position for crown lengthening.

Materials and method

The procedure was performed as an office surgery. Anesthesia consisted of lidocaine and intravenous midazolam, 3 mg. An horizontal incision in the vestibule exposed the lateral maxilla. Subperiosteal dissection preserved the mucosa covering the crest, palate, and periodontal apparatus of the teeth adjacent to the defect. The horizontal osteotomy was started with a 703 side-cutting bur and the vertical osteotomies were started with a 701 side-cutting bur. Extension of the osteotomy through the palatal cortex was accomplished with a sagittal saw with a 2 mm wide blade. The width of a bur shank will not allow extension of the osteotomy through the palatal cortex on the vertical components without removing excessive bone. In this case, the lateral incisor root was included in the osteotomy block. A vertical hole was drilled transmucosally through the mobilized bone segment. A Leibinger Endosseous Alveolar Distractor (LEAD) was used to transport the bone segment (^{fig. 6}). The threaded rod was placed into the vertical hole until the tip could be seen in the horizontal osteotomy. The threaded plate of the LEAD device was fitted over the tip of the threaded rod. Rotating the threaded rod engaged the threaded plate. The base plate of the system was then fitted to the tip of the threaded rod (^{fig. 7}). Rotation of the rod resulted in the separation of the two plates. The threaded plate traveling along the threaded rod caused coronal displacement of the mobilized bone fragment. The plates were stabilized with miniature bone screws. Before closing the incision, a wrench was used to tighten the device and verify that the transport process was satisfactory (^{fig. 8}). The vector of transport will influence the shape and position of the distracted alveolar ridge. If the axis of transport will not meet the structural and esthetic criteria of the reconstruction, the position of the device is modified by altering the location of the base plate. The position of the transported bone segment can also be modified by traction using splints, a provisional prosthesis, or orthodontic appliances as anchorage. No grafting materials or membranes were used.

The device was left in a minimally distracted position and the mucosa closed with 3-0 chromic gut sutures. Penicillin VK 500 mg tablets four times daily for five days was prescribed. The patient returned to the office in five days to begin activation of the device. One revolution results in 0.4 mm of transport. The threaded rod is turned two revolutions daily until the desired transport is achieved (^{fig. 9}). The transport objective is significantly beyond the anatomical objective. Regeneration of additional bone and soft tissue will accommodate for contouring of the ridge, crown lengthening on adjacent teeth, and decrease in the volume due to remodeling. In the case of knife-edged alveolar ridges, the thin, primarily cortical ridge should be removed to allow a more viable ossicle to form the new ridge crest.

The active distraction process takes place over one to two weeks depending on the desired amount of augmentation. This patient underwent active distraction for ten days which resulted in the transport of 8 mm (^{fig. 10}). As a result, the ridge crest was transported coronally and laterally. The removable provisional prosthesis required periodic adjustment to accommodate for the changing alveolar ridge contour (^{fig. 11}). A hole drilled through the prosthesis allowed for placement over the threaded rod. The fracture lateral incisor was transported until the composite provisional restoration was in occlusion with the mandibular teeth. After the transport process was completed, the threaded rod was left in place for four weeks. This is the fixation period during which a union is established across the vertical components of the osteotomy. Bone begins to form in the widened horizontal osteotomy that represents the regeneration chamber. After four weeks of fixation, the patient returned to the office for removal of the threaded rod. By rotating the rod counter-clockwise, it was removed. The device plates were left in place. No anesthesia is necessary to remove the rod. The patient was then scheduled to return for implant placement in six weeks.

Placement of dental implants

Implants are placed by a standard protocol. A crest incision exposes the healed segment of transported bone. Reflection of the crest incision laterally exposes the threaded plate which is removed (^{fig. 12}). The stabilizing plate is left in place. Endosseous implants are positioned with the aid of a splint. Substantial counter-sinking is necessary to position the fixtures in an appropriate vertical relationship with the adjacent teeth. The distal fixture is countersunk to position the machined collar at the level of the interproximal periodontal attachment of the premolar. The objective is to adjust the fixture height so the periodontal attachments of the tooth and fixture are at the same position. Anteriorly, the fixture is placed to anticipate the crown lengthening procedure necessary to restore the lateral incisor. After five months in the submerged state, the implants are uncovered for abutment connection. The ridge crest is now contoured to establish the best periodontal attachments to the teeth and fixtures (^{fig. 13}). Crown lengthening is performed on the lateral incisor root. Healing abutments are placed and after uneventful healing, the prosthodontist began the restorative treatment.

Results

The distraction process lowers the alveolar ridge to manage the problem of excessively tall crowns (^{fig. 14}, ¹⁵ and ¹⁶). The soft tissue enlarges in response to the distraction process. The lateral incisor tooth is repositioned coronally. This allows for crown lengthening without compromising the bone between the central incisor and the root which may damage the interdental papilla. In the transported position, the bone removed on the distal aspect of the lateral incisor root does not compromise the augmentation of the crest. Bone is increased at the mesial of the second premolar which may be important periodontally in the future. Moving the ridge inferiorly and laterally allows for placement of implants in the best esthetic and biomechanical position. The crown-to-root ratio is improved and there is no need of ridge-lapping or distortion of the natural anatomic crown contour. The procedure was performed as office surgery using local anesthesia and intravenous sedation. No graft materials or membranes were used.

Reconstruction of the alveolar process using distraction osteogenesis has been utilized in fifty cases. Establishment of a site for placement of osseointegrated implants was the objective in most applications. Two cases were performed to augment the alveolus to improve the appearance of a prosthesis pontic. Satisfactory reconstruction of the ridge can be achieved through this method in most cases. Technical or treatment planning errors may compromise the result. Two cases failed and did not result in regeneration of new bone. In both cases, extensive prior attempts to augment the site with alloplastic materials probably compromised the healing potential of the transport segment. Two cases failed to generate an adequate quantity of bone. In one of this cases, the transport segment consisted to a bone graft to an alveolar cleft. As with the previously mentioned failures, the healing potential of the graft was probably not adequate to regenerate new bone.

In general, the alveolar distraction process results predictable and stable augmentation of the alveolus. Application of the technique requires adherence to the basic principles of distraction osteogenesis. The osteotomies must be planned to place the regeneration chamber in a site capable of generating new bone. Mobilization of the transport segment and placement of the distraction device must be done without compromising the circulation to the bone or soft tissue.

Discussion

Distraction osteogenesis is a new method to rehabilitate deformities of the alveolar process (^{Chin, 1997} ; ^{Chin, 1998}). Its unique ability to regenerate bone and soft tissue simultaneously makes is particularly useful to the implant surgeon. The process positions mature bone at the crest of the ridge. As the bone segment is transported to the ridge crest, it carries soft tissue that exists in a stable equilibrium with the underlying bone. The site of bone regeneration is distant from the ridge crest. By placing the primary site of regeneration distant from the ridge, two objectives are accomplished. First, the maturation of new bone takes place independent of the most important site for implant osseointegration, the ridge crest. Second, since the site of regeneration is distant from the primary site of pathosis, healing is more predictable. Positioning the regeneration chamber in the center of the functional matrix surrounded by healthy tissue promotes the process of healing with new structural bone. Because mature bone is transported into the implant recipient site, there is no need to wait for maturation of a graft or guided tissue regenerate prior to implant placement.

Ideally, the contour of the osseous ridge supports the periodontal attachment of the teeth and fixtures while maintaining the esthetics of the interdental papilla. Compromise is generally necessary because of the anatomic difference between the gingival attachment mechanism of the tooth and fixture. Both tooth and fixture must establish a minimal biologic width. The periodontal attachment on the premolar tooth, for example, is more coronal interproximally than facially. The level of the soft tissue attachment on the fixture is radially symmetrical because of the manufacturing process common to most endosseous implants. Establishing a mutually supportive periodontal attachment mechanism between teeth and implants is important to the long term stability of the rehabilitation. Future application of distraction osteogenesis will show improvements in the devices and modifications of the surgical techniques. New techniques will offer more precise three dimensional control of the distraction process. Recombinant bone growth factors will decrease the treatment times and improve the effect of distraction on periodontal regeneration.

Leibinger Endosseous Alveolar Distractor and LEAD are registered trademarks of the Howmedica Leibinger division of Pfizer.

BIBLIOGRAPHIE

• BLOCK MS, ALMERICO B, CRAWFORD C, GARDINER D, CHANG A. Bone response to functioning implants in dog mandibular ridges augmented with distraction osteogenesis. Int J Oral Maxillofac Implants 1998;13:342-351.
• BLOCK MS, CHANG A, CRAWFORD CH. Mandibular alveolar ridge augmentation in the dog using distraction osteogenesis. J Oral Maxillofac Surg 1996;54:309-314.
• CHIN M, TOTH BA. Distraction osteogenesis in maxillofacial surgery using internal devices : report of five cases. J Oral Maxillofac Surg 1996;54:45-53.
• CHIN M, TOTH BA. Distraction osteogenesis in craniofacial surgery using internal devices. Proceedings of the sixth international congress of the international society of cranio-facial surgery. Daniel Marchac, ed. 1995.
• CHIN M. Distraction osteogenesis in maxillofacial surgery. Tissue engineering in periodontics and oral surgery. Samuel Lynch, ed. Quintessence, 1998.
• CHIN M. Alveolar distraction osteogenesis. Proceedings of the international congress of cranial and facial bone distraction processes. Paris : Patrick Diner, ed., 1997.
• ILIZAROV G. Transosseous osteosynthesis. New York, Springer - Verlag, 1991.
• UDEA M. Maxillofacial bone distraction processes using osseointegrated implants and an intraoral device. Proceedings of the international congress of cranial and facial bone distraction processes. Paris : Patrick Diner, ed., 1997.