Bone Augmentation Using a Mandibular Symphysis Graft-a Case Report

INTRODUCTION

The upper central incisor region is a zone of high aesthetic value with a limited area available for a precise implant insertion.¹ It may result in alveolar bone and buccal plate defects. The repair of acquired alveolar defects with autogenous bone grafts for implant placement is one of the most common surgical techniques in implant surgery. Once adequate quantity of bone has been augmented, implant placement plays a pivotal role in the rehabilitation of these patients.²

Bone reconstruction techniques have been improved in order to optimize the aesthetic and functional result.^3,4 The extension of the bone defect determines whether bone augmentation procedures can be performed simultaneously with the placement of implants or as a separate procedure.⁵

Among the various bone augmentation materials available, only autologous bone combines osteoconductive, osteoinductive and osteogenic features when compared to any other bone substitute.⁶ Owing to its biological properties and absence of immunological reactions, autologous bone graft has been set as the “gold standard” for bone regenerative procedures.⁷

Intraoral harvesting may lead to donor site morbidity and possible complications that could occur during the bone harvesting procedure.⁸

Intraoral donor sites such as the mandibular symphysis and ramus show reduced resorption and higher revascularization rates than endochondral bone grafts, like the tibial plateau and iliac crest.^9,10 Bone harvesting can be performed with local anesthesia without resorting to general anesthesia.

The mandibular symphysis provides a greater graft volume with a simpler surgical access. The corticocancellous composition of the bone harvested from the mandibular symphysis region aids in rapid vascularization and integration after it has been positioned at the recipient site. Better postoperative patient morbidity and low rate of wound dehiscence make the ramus a preferred donor site.^11,12

The use of bone substitutes with low turnover rates to cover the graft reduces the resorption rate of the bone block.¹³ Some authors found that deproteinized bovine bone (DBB) particles stabilized by resorbable membranes covering onlay block grafts reduced resorption by almost 50% in comparison to noncovered grafts.^1,14 Bone substitutes contribute to the creation of a smooth connection between the block graft and the recipient bone and can provide a scaffold promoting bone regeneration.^13,14 Another advantage of using resorbable rather than nonresorbable membranes is the elimination of a second surgical phase.

Based on the above rationale, the main aim of the presented case report was to evaluate the placement of dental implants in resorbed alveolar ridges augmented with autogenous onlay bone grafts obtained from the mandibular symphysis.

CASE REPORT

A 17 year old female reported to the Department of Prosthodontics in Dr.D.Y.Patil University School of Dentistry with a chief complaint of missing tooth in the upper right region. The anatomy of the upper jaw was evaluated by panoramic radiography and clinical examination. A unilateral bone defect in correspondence with the upper right central incisor region was observed. The patient had a thin gingival biotype. (Figure 1)

A bone augmentation procedure was planned using an intraoral autogenous bone harvested from the mandibular symphysis and delayed implant insertion. (Figure 2)

Several injections were required as the anterior symphyseal region of the mandible is innervated by the mandibular branch of the fifth cranial nerve (V3) and cervical nerves from C-3 and C-4. Bilateral mandibular block with 2% lidocaine HCL (1:100,000 epinephrine) accomplish anesthesia for V3 innervation.¹⁵

Assessment of the periodontal status of lower anteriors, periodontal risk of root fenestration, amount of bone loss, subgingival margins of the restoration, amount of keratinized gingiva and local musculature should be done to determine the ideal incision design. Depending on the anatomy, surgical access to the symphysis region can be obtained via a crestal incision, mid-keratinized tissue incision, vestibular incision or sulcular approach.

Submarginal/attached gingiva incision is the horizontal or parasulcular, scalloped incision^15,16,17 that should be given in the attached gingiva at least 1 mm above the mucogingival junction. If >3 mm of keratinized gingiva is present, beveled incision in apical direction is given. If >3 mm of keratinized gingiva is present, perpendicular incision to the underlying bone gives a butt joint for tissue adaptation during the suturing.¹⁷ The vertical incision is given either at the midline or between the canine and the first premolar, keeping a safe distance from a local neurovascular bundle. The midline vertical release can be given when the harvest is for 3 teeth or less and extends up to 5 mm of the inferior aspect of the mandible. (Figure 3)

Misch in 1992, proposed the Rule of 5’s surgical technique to harvest a bone block graft from symphysis with bone cuts perpendicular to the cortex in a right angle to the vestibular plain of the symphysis. The superior cut should be atleast 5 mm below root apices to prevent any injury to tooth roots. The inferior cut should be 5 mm above the lower border of the mandible. Vertical cuts should be minimum 5 mm away from the mental foramen. To obtain a monocortical graft, depth of the cut should extend from the outer cortex to the opposite cortical plate. Lingual cortex should not be perforated.¹⁸

Two rectangular monocortical block grafts were harvested from the mandibular symphysis region. Two osteosynthesis screws were used to fix them at the buccal aspect of the bone defects (Figure 3 and 4). Sutures were given to obtain a first-intention seal of the flaps (Figure 5). The sutures were removed after 14 days and an orthopantomograph was obtained. Six months post grafting, fixation screws were removed and an implant was placed with the aid of a surgical stent (Figure 6,7 and 8). The insertion torque was >35 Ncm. At this point, DBB particles (Bio-Oss®, Geistlich Biomaterials, Wolhusen, Switzerland) and a Type- I resorbable collagen membranes from bovine origin (Cologenesis Colo Gide GTR Membrane) were used to cover the block graft at each site. (Figure 9 and 10)

After five months, implants were uncovered to connect the healing abutments (Figure 11). After proper maturation of the soft tissues, impressions were taken to start the prosthetic phase (Figure 12). Temporary implant-supported acrylic resin prostheses were connected to the implants for soft tissue conditioning and initial load. Definitive implant-supported cemented-retained metal-ceramic prosthesis was delivered (Figure 13 and 14). The clinical and radiological assessments showed the presence of adequate marginal bone levels circumferentially around the dental implant.

DISCUSSION

The present case was reported to document clinically and radiographically the survival of dental implants placed in atrophic alveolar ridges augmented with mandibular symphysis autogenous onlay grafts. Implant rehabilitations in bone augmented with autologous mandibular symphysis block grafts are a reliable treatment modality in patients with bone defects.

In a recent retrospective study,¹⁹ it has been claimed that intraoral bone grafts harvested from the mandibular symphysis, mandibular ramus, and maxillary tuberosity provide a good treatment modality for ridge augmentation. Bone availability from these regions is adequate for anatomical bone defects spanning up to the width of three teeth.²⁰ The surgical access to the mandibular symphysis is easier than that of the mandibular ramus.¹¹ Both the procedures can be undertaken on an outpatient basis, while harvesting of bone from distant sites or extraorally requires inpatient care along with increased costs.²¹ Both the harvesting and grafting procedures are usually performed in the same surgical field.²² The use of autologous bone in the present case showed excellent survival and success rates. The aesthetic aspect of the treatment is a success when the gingival scalloping has been preserved.^23,24 Bone augmentation resulted in stable lamellar-bone conditions along with a low risk of mucosal recession.

Loss of bone architecture and volume, can be solved using narrower sized implants and careful management of soft tissue. Soft tissue augmentation techniques have demonstrated good aesthetic results so that more invasive bone augmentation procedures may be avoided.²⁵ At the time of surgery, the patient being only 17 years old, an alternative approach with an additional waiting period of three to four years was contemplated to prevent the intrusion of the peri-implant tissues that could occur due to premature implant insertion.²⁶ The patient also refused presurgical orthodontic treatment aimed at optimizing the alignment of the anterior sector.

The hypothesis that bone substitutes could be effectively used for the replacement of autogenous bone with its osteoconductive, osteoinductive and osteogenic properties is still being investigated. Studies have also proved benefits and appropriateness of autogenous tissue for an ridge reconstructive procedures of atrophied ridges before implant surgery.²⁷

Regarding surgical complications, the postoperative morbidity and flap dehiscence with or without exposure of the grafts or membrane.²⁸ The peri-implant mucosa needs to be supported by an adequate three-dimensional volume of alveolar bone, including an intact buccal plate of sufficient height and thickness.^4,29,30 Deficiency of the buccal bone anatomy has a negative impact on the aesthetic outcome and is therefore considered a critical causative factor for implant complications and failures.^4,30,31

Postoperative morbidity after mandibular bone harvesting procedures was reported to be mainly related to temporary or permanent neural disturbances involving the inferior alveolar nerve and its branches.²⁷ Although the mandibular incisal canal has defined anatomical limits, harvesting bone from the mandibular symphysis, may still lead to neurosensory alterations. Physiological changes in the course of the mandibular incisal canal may be the causative factors for such occurences. It is therefore advisable to subjectively evaluate the feasibility of the technique on a case-by-case basis through orthopantomographs and second-level investigations such as dental scan, computed tomography and stereolithographic prototypes. No complications related to neurosensory function were reported in this case.

Marginal bone loss measurement on the mesial and distal aspects may be a limitation to this case owing to 2D imaging. The loss of buccal and lingual bone can only be measured using 3D imaging modalities. However, panoramic radiographs are frequently used in clinical settings for the evaluation of bone peak stability.^2,32

In the given case report, loss of the upper central incisor can be treated with bone grafting from the mandibular symphysis and delayed implant placement leading to an enhanced functional and aesthetic results. Rate of resorption of the graft may be reduced with correct management of the peri-implant soft tissues and the prosthesis.

CONCLUSION

The mandibular symphysis is a convenient intramembranous source that supplies a dense quality transplant. The thick cortical layer of the graft reduces the amount and rate of resorption, while the cancellous part fastens the regenerative process. This transplant does not cause any immune reactions and incorporates osteoclastic resorption with a shorter healing period compared with other methods of osseous repair. Precise case selection and a comprehensive surgical planning is the prerequisite for successfully harvesting a graft for bone augmentation procedures. Application of safety guidelines in mandibular symphysis harvesting could reduce the risk of altered post-operative neurosensations due to injury of the mandibular incisal nerve.

Conflict of Interests

The authors declare no conflict of interest.

REFERENCES