Craig M. Misch DDS, MDS*
Carl E. Misch DDS, MDS**

Severe alveolar deficiencies can prevent ideal implant placement. Management of osseous defects often necessitates autogenous bone grafting. The mandibular symphysis graft technique offers ease of access, good bone quantity for localized repair, a corticocancellous block graft morphology, low morbidity, and minimal graft resorption. An improved bone density results along with a shorter healing time as compared with other methods for bone repair. An understanding of graft management and implant placement is essential for clinical success. (Implant Dent 1995;4:261-267)

The management of alveolar ridge deficiencies poses a challenge for tooth replacement with dental implants. Osseous defects resulting from trauma, congenital anomalies, and pathology are more severe and often require more complex treatment planning as compared with tooth extraction as a result of dental disease.^{1, 2} In contrast to the anterior mandible (Fig.1), the premaxilla often requires reconstruction for implant support due to esthetic and phonetic demands.³

The replacement of missing hard and soft tissues is critical not only to the esthetic outcome but also to the biomechanical support of the prosthesis. ^{4, 5} The development of anterior guidance may contribute to the generation of unfavorable moment forces on maxillary anterior implants (Fig. 2). ^{4, 6} This is especially true when the volume of the alveolar deficiency is replaced by the prosthesis instead of considering ridge augmentation. Factors to consider when contemplating augmentation procedures include crown-implant ratio and incisal edge position in relation to the implant body. Prosthodontic solutions to the deficient ridge (ie, ridge lap design) encourage food impaction and plaque accumulation that can result in inflammation of the peri-implant tissues. ^{1, 4}

Ridge augmentation techniques available for implant placement in the anterior maxilla include nasal floor elevation, bone spreading, bone grafting, guided bone regeneration, and/or combinations of these procedures. The morphology of an osseous defect is an important consideration in the selection of an augmentation method.⁴ Fewer remaining bony walls indicate a greater need for osteopromotive techniques. ⁷

Guided bone regeneration (GBR) techniques have been used during implant placement or staged with implant placement after bone formation. ⁹ When implants are placed simultaneously with guided bone regeneration, the best results are obtained when treating circumferential or vertical dehiscent type defects, ^9-11 with less predictable osseous gains found in a horizontal dimension. ¹²

Recent research has questioned the consistency of obtaining actual bone fill, ¹¹ the degree of bone-implant contact with the simultaneous technique, ¹³ and the stability of the newly regenerated bone following loading.¹⁴

The staged technique of implant placement following bone regeneration has the advantages of (1) a larger available osseous surface contributing to bone formation; (2) allowing improved implant alignment; (3) permitting better initial stability of the implant; (4) increased maturation of the new bone with probable improved apposition to the implant surface.¹⁵ Buser et al1⁵ have suggested that the staged approach be used for treating large bone defects and the simultaneous approach used for smaller defects. The staged technique has the disadvantage of a long healing period before implant placement (9 months) and a poorer bone quality of the regenerated tissue, unless corticocancellous autologous bone grafts are used beneath the membrane.^{8, 11, 15-18}

The predictability of GBR is significantly compromised by infection of a membrane site.^{9, 12, 16, 19, 20} Early membrane removal is associated with a smaller gain in bone volume.^{20, 21} The risk of premature exposure of expanded polytetrafluoroethylene membranes (e-PTFE) is a disadvantage. Dahlin et al²² have written that “guided bone regeneration techniques should preferably be used in a situations where the prognosis could be enhanced by, but not dependent on, the use of a membrane.”

The functional remodeling of augmented hard tissue in response to implant loading may differ from normal untreated sites. The biomechanical distribution of stress occurs primarily where bone is in contact with the implant.²³ A denser interfacial bone quality provides better distribution and transmission of stresses.²⁴ As the greatest stresses are located around the neck of a loaded implant at the ridge crest,^{25, 26} bone augmentation methods should ideally provide crestal bone with adequate density to withstand implant loading. When a large volume of bone must be replaced, the most predictable technique is autogenous bone grafting.

Although the iliac crest is the most common donor site in maxillofacial reconstruction, the cost is high due to the need for hospitalization and general anesthesia. For the repair of localized severe alveolar defects, alternative donor sites such as the mandibular symphysis are advantageous.^{3, 27-30} In addition to the procedural advantages, bone harvested from the chin appears to have inherent biological benefits attributed to its embryologic origin.

Studies suggest that membranous bone grafts show less resorption than endochondral bone grafts.^{31, 32} Although cancellous grafts revascularize more rapidly than cortical grafts,³³ cortical membranous grafts revascularize more rapidly than endochondral bone grafts with a thicker cancellous component.³⁴ The early revascularization of membranous bone grafts is a possible explanation for the improved maintenance of graft volume³⁴ and suggests why chin grafts (primarily cortical bone with few osteogenic cells) exhibit little volume loss and show good incorporation with shorter healing times.^{3, 30, 35-37} Another hypothesis is that bone of ectomesenchymal origin, such as the mandibular symphysis, has a better potential for incorporation in the maxillofacial region because of the biochemical similarity of donor and recipient bone.³⁶

In most cases sufficient bone can be harvested from the symphysis for deficiencies in width involving up to four teeth or sites of one or two teeth that require increases in both vertical height and width (Figs. 2 and 3).³ A diagnostic wax up or denture tooth setup of the reconstructed defect is helpful to determine graft requirements (Fig. 4) and facilitates the fabrication of a surgical template for the placement of the graft and implant(s) (Fig. 5). A panoramic radiograph is necessary to evaluate the donor site and a lateral cephalometric radiograph to determine the anteroposterior dimension of the anterior mandible. Periapical films are required for precise measurements of the lower tooth roots. Tomography is a useful adjunct for surgical planning and mapping of the ridge defect.

Fig. 1.  Mandibular fixed detachable prosthesis replacing teeth and associated structures lost as a result of trauma.
Fig. 2. Case 1 – Occlusal view of maxillary arch. The maxillary central incisors were traumatically avulsed in a motor vehicle accident
Fig. 3. Lateral view of the maxillary bone deficiency of patient illustrated in Figure 2. Note the large discrepancy between the ridge and lower incisal edge position.
Fig. 4. Four-month postoperative view of the maxillary anterior region reconstructed with a symphysis bone graft. Implants were placed in ideal prosthodontic positions.
Fig. 5. Surgical template fabricated from a diagnostic wax up used for block autograft positioning and eventual implant placement.
Fig. 6. Small diameter (1.2 and 1.6 mm) titanium alloy screws with screwdriver used for graft fixation (OsteoMed, Dallas, TX).
Fig. 7. Exposure of a mandibular symphysis donor site by a sulcular incision approach. The size of the maxillary defect requires extension of the osteotomy outline below the canine roots.
Fig. 8. Exposure of a mandibular symphysis donor site by a vestibular incision approach. The distal extent of the incision is limited to the canine region. The maxillary defect is limited so the osteotomy outline is between the canine roots.

BONE GRAFTING

Block grafts harvested from the mandibular symphysis have been used successfully to manage localized severe alveolar defects for the placement of dental implants. The bone graft surgical technique has been described by Misch et al.³

Graft Incorporation

The use of a surgical template will insure proper graft positioning for future implant placement (Fig. 5). The recipient site is completely prepared for grafting before harvesting the donor bone. Perforation of the underlying host bone with a small round bur increases the availability of osteogenic cells, expedites revascularization, and improves graft union.³⁸ The host site is also recontoured to improve bone to graft contact.

Following harvest, the graft should be stored in a suitable medium (ie, sterile saline) and minimal time should elapse before graft placement.^{39, 40}Decreased resorption and increased vascularization of fixated membranous grafts has been reported.⁴¹ Although small diameter titanium alloy screws are most often used for fixation (Fig. 6), wire may be necessary for additional stability. The graft is mortised in place and cancellous bone harvested from the donor site can be used to fill small discrepancies. As the corticocancellous block graft generally completely occupies the defect, the additional use of a membrane is unnecessary and may lead to complications if early exposure occurs. ^{9, 12, 16, 19, 20}

Although barrier membranes can help minimize autograft resorption,¹⁵ they may not be necessary with membranous bone grafts that exhibit minimal resorption. ^{3, 30-32, 34-37} If the size of the block is inadequate or the harvest is particulate in nature, a membrane can be used to help contain and stabilize the graft and enhance bony regeneration of any deficient areas.^{15, 42}

Esthetic areas mandate provisional tooth replacement. Removable soft tissue-supported provisional prostheses areadjusted to prevent graft loading. The patient is instructed to use the provisional restoration only for esthetics. More favorable provisional solutions are tooth-supported removable partial dentures, resin-bonded prostheses, or denture teeth bonded to the adjacent dentition.

Soft Tissue Management

The recipient site must be completely healed before graft placement. Removal of foreign bodies, soft tissue surgery, and/or tooth extractions are completed at least 8 weeks before grafting. Prudent soft tissue handling will minimize trauma of the recipient site flap. Beveled incisions slightly distant from the ridge crest (facially for the lower jaw, palatally for the upper jaw) and divergent releasing incisions remote to the defect facilitate closure and maintain an adequate blood supply. Incisions extended too far palatally are avoided due to possible postoperative epithelial sloughing.

Full-thickness reflection in the anterior maxilla is extended to the anterior nasal spine, inferior and lateral rim of the nasal cavity, and the canine fossa regions. Complete flap coverage and tension-free wound closure are essential for successful graft incorporation. Before graft placement, the periosteum at the base of the facial flap is carefully incised with tissue scissors and/or a scalpel blade to allow stretching of the mucosa and tension-free adaptation of the wound margins.

Frenectomies may also be considered to reduce flap tension. Incision line opening and exposure of the graft has been associated with smoking, which should be discouraged. Procedures to enhance graft coverage will usually result in a reduction in keratinized mucosa over the ridge crest and a loss of vestibular depth. Occasionally tissue grafts may be necessary. The attached mucosa may be repositioned facially at second-stage surgery.

Donor Site Considerations

The most common patient concern is postoperative appearance of the chin. Although the original research on young cleft palate patients did not consider augmentation of the donor site,^35-37 radiographic evidence of incomplete bony regeneration has been reported for older patients.²⁸ However, the reported incomplete bone fill did not result in any discernible profile changes. Misch et al³ advocated grafting of the donor area, which does not result in alteration of chin contour.³⁰

The recipient site may be augmented with demineralized freeze-dried bone and/or resorbable hydroxyapatite (OsteoGraf N; CeraMed Dental Products, Lakewood, CO). It is not necessary to leave a midline strip of intact bone. Ptosis of the chin has not occurred and can be prevented by avoiding “degloving” of the mandible.⁴³ The inferior borderof the mandible should be left intact, and although perforation of the lingual cortex may inadvertently occur, it is not encouraged.

Incision designs vary according to the local musculature and periodontal status of the mandibular anterior teeth. In patients with a shallow vestibule or tense mentalis posture, a sulcular incision along the mandibular anterior teeth is used (Fig. 7).

The presence of marginal gingival inflammation or alveolar bone loss around the lower incisors usually warrants a vestibular incision. A vestibular approach beyond the mucogingival junction permits easier access but produces more soft tissue bleeding and intraoral scar formation. (Fig. 8). Limiting the distal extent of the vestibular incision to the canine area has reduced the incidence of temporary mental nerve paresthesia to less than 10 percent,³⁰ and to date, no cases of less than complete recovery have been reported.

Bone wax (Ethicon Inc., Somerville, NJ) can be placed in areas of heavy osseous bleeding. Postoperative pressure dressings reduce the development of hematoma formation, incision line dehiscence, and infection. The use of glucocorticoids is helpful in diminishing post operative edema.⁴⁴ The average preoperative dose is 9 mg of dexamethasone with additional tapering doses for the following 2 days. Postoperative pain of the donor area is minimal to moderate.

Depending on the volume requirements, the osteotomy may be between the canine roots, or below them if a greater graft size is necessary (Figs. 7 and 8). A 5 mm border is indicated between the most superior bone cut and the apices of the tooth roots.^{3, 45} Even with this boundary, altered sensation of the lower teeth is a common temporary postoperative symptom.³⁰

Vitality tests on anterior teeth in patients following genioplasty have found no response in up to 20 percent of teeth tested.⁴⁶ Hoppenreijs et al⁴⁵ studied the pulpal response of the lower anterior teeth in children with alveolar clefts treated with mandibular bone grafts and reported a high frequency of pulp canal obliteration (12 percent) and a smaller incidence of negative pulp reaction (4 percent). Although the need for endodontic therapy has not arisen, patients should be aware of the potential for pathologic changes.

Fig. 9. Case 2- Symphysis corticocancellous block autograft fixated to the host bone with screws (patient shown in Figure 5).
Fig. 10. Four-month healing of the graft. Note the incorporation of the graft and lack of significant resorption. Two 13 mm implants were placed.
Fig. 11. The implant was uncovered 4 months after placement.
Fig. 12. Wide diameter (4.5mm) threaded implant (middle placed into graft site. Implant placement occurred at the graft-host union and caused a slight fracture of the block 2 mm anterior to the implant. A fixation screw was replaced to insure stability during healing.

Implant Placement

As the grafts exhibit minimal resorption (0 to 25 percent), sufficient bone volume can be gained to allow implant placement (Figs. 5 and 9 to 11). Implants are placed after a healing period of 4 to 6 months.^{3, 27, 30} The maxillary onlay graft maintains the bony architecture and dense quality of the anterior mandible.^{3, 30} The improved bone density may offer biomechanical advantages over other reparative techniques.^{23, 24}

Implant insertion during chin graft placement has revealed graft fracture, wound dehiscence with exposure of the implants and graft, and a higher implant failure rate as compared with a staged approach.^{3, 28-30, 47} Diminished bone contact has also been found around implants placed simultaneously with iliac bone grafts.⁴⁸

A staged surgery permits implant placement for satisfactory prosthodontic alignment without the concern of graft fixation or remodeling. As gains in volume are often impressive, the use of wider diameter implants can be considered. However, care should be taken during bone preparation and implant placement as this site is often at the graft-host union and separation of the graft can occur (Fig. 12).

Slightly oversizing the osteotomy and tapping the site for dense bone may be necessary for atraumatic implant placement.²⁴ Staged implant placement allows for initial graft resorption and should provide a stable foundation. ⁴⁹ Additional graft resorption following implant insertion had not been noted radiographically on cases followed for up to 5 years by the authors. The osteotomy during placement activates bone formation50 and implants can induce interface remodeling with bone maintenance, even in unloaded conditions.^{3, 49, 51}

CONCLUSION

The mandibular symphysis graft technique offers advantages in the repair of localized severe alveolar bone defects before implant surgery, including ease of donor site access, increased quantities of donor bone as compared with other intraoral sites, and low morbidity. An improved bone density results with a shorter healing time as compared with guided tissue regeneration techniques. Disadvantages are the limited amount of donor bone and the potential for damage to nerve branches and the lower anterior teeth. Symphysis grafts offer a predictable alternative for the reconstruction of ridge deficiencies for implant placement.

ACKNOWLEDGMENTS

The authors acknowledge Drs. Ashraf Bessada, Niki Hakime, Francisco Herrero, Barry Marshak, and Othon Pipis who participated in the treatment of the cases illustrated during their oral implantology residency at the University of Pittsburgh.

*Co-Director, Oral Implantology Center; Clinical Assistant Professor, Division of Periodontics.
**Director, Oral Implantology Center; Associate Professor, Division of Restorative Dentistry. University of Pittsburgh School of Dental Medicine 3501 Terrace Street Pittsburgh, PA 15261

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Reprint requests to:
Dr. Craig M. Misch
St. Frances Medical Center
Department of Oral and Maxillofacial Surgery
400 45th Street
Pittsburgh, PA 1521-1198

ISSN 1056-6163/95/0404-261$3.00 Implant Dentistry Volume 4 ~ Number 4 Copyright © 1995 by Williams & Wilkins