Michael A. Pikos, DDS*

The posterior edentulous mandible presents unique challenges for implant reconstruction because of deficiencies in bone quality and quantity. Autogenous mandibular block grafts can be used in a predictable manner to enhance bone volume and density, allowing for placement of maximum diameter implants to facilitate stress distribution for long-term implant survival. This article will feature the importance of staging and recipient site preparation using mandibular block autografts for posterior mandibular edentulous reconstruction. (Implant Dent 2000; 9:67-75)

Key words: recipient site preparation, decortication, rigid fixation, staging

A necessary factor for implant-supported prostheses is the availability of bone in the edentulous region. The posterior mandibular edentulous area of the mouth often presents unique challenges for implant reconstruction because of deficiencies in bone quality and quantity. Several treatment options have been suggested to address these challenges. These include nerve repositioning, guided bone regeneration, the unilateral subperiosteal implant, and bone augmentation with mandibular block autografts. Nerve repositioning for posterior mandibular reconstruction has inherent morbidity. Although not common, there is a risk of long-term paresthesia that may include hyperestheia and pain.^1-3 There are reports in the literature that include dysesthesia and mandibular fracture after nerve repositioning and implant placement.^{4, 5} Furthermore, many times an unfavorable crown implant ratio results because of placement of relatively small-length implants that are intended to avoid vital anatomical structures.

Guided bone regeneration using expanded polytetrafluoroethylene (e-PTFE) membranes is a treatment option that has been used with varying degrees of success for posterior mandibular reconstruction.^6-11 It seems that smaller particle size autographs resorb more quickly than block grafts, so that barrier membranes need to be used to contain particulate grafts and minimize bone resorption.^{12, 13}

The timing of membrane resorption or removal in relation to graft incorporation is not totally predictable and requires further investigation.⁹

The unilateral subperiosteal implant is still another treatment option for posterior mandibular edentulous reconstruction. Changes in implant design concepts by Misch and Dietsch,¹⁴ addition of hydroxyapatite on the implant surface, and specific prostheses and occlusal schemes have resulted in improved clinical success of this implant. Although Misch¹⁵ has reported on 60 consecutively placed unilateral subperiosteal implants over 17 years with a 100% success rate, there are earlier reports indicating poorer success rates.

A final treatment option includes autogenous bone augmentation as a prerequisite for implant placement in atrophic posterior mandibular edentulous areas of the mouth. A proven modality of treatment includes autogenous block grafting^16-26 for lateral and vertical ridge augmentation. Autogenous bone grafts have been used for many years for ridge augmentation and are still considered to be the “gold standard” of jaw reconstruction.^{27, 28} The use of autogenous bone grafts with osseointegrated implants was originally discussed by Branemark et al,²⁹ who used the iliaccrest as a donor site. For repair of most localized alveolar defects, however, bone grafts from the mandible offer advantages over iliac crest grafts.^{20, 30} These include the proximity of donor and recipient sites, convenient surgical access, decreased donor site morbidity, and decreased cost (because this surgery can be performed as an outpatient procedure).

Typically, there is loss of alveolar bone height in the posterior mandible secondary to periodontal disease and after tooth removal. Tooth loss results in buccal plate compromise and a reduction in alveolar width. This bone resorption process continues in a medial direction until a knife-edged ridge forms. This may well result in a deficiency of alveolar height that would preclude implant placement. The cortical plate may be minimal or absent, further complicating implant placement. Finally, occlusal forces are greater in the posterior than in the anterior area of the mouth, necessitating appropriate surgical and prosthetic treatment planning for long-term success.

Stress Factors

Although esthetics is a concern with posterior mandibular implant reconstruction, the primary goal is to create a biomechanically sound support for the prosthetic complex of the implant.

A variety of stress elements that affect the ultimate success of posterior mandibular implants need to be addressed. Biting forces are increased in the posterior mandible. These forces are primarily directed perpendicular to the occlusal plane and are usually of short duration. Recent studies have indicated that normal maximal vertical biting forces on teeth or implants in the posterior mandible range from 35 to 50 psi; in the molar area, they vary from 125 to 250 psi. Finally, the opposing arch is a consideration.

Often there is either natural dentition or implant or tooth-supported fixed prostheses, thus allowing maximum occlusal force transfer unlike that of an opposing full denture.

Treatment planning in the posterior mandible must include solutions to reduce stress. A primary plan is one that includes increasing the number of implants. Pontics are not to be used; so that one implant per buccal root is the treatment of choice for each case. In addition, no cantilevers are allowed. The splinting of crowns is also indicated for biomechanical force distribution. Occlusal considerations include eliminating lateral interferences during any excursive movements and decreasing the occlusal table relative to the implant diameter to decrease occlusal forces per unit area. The final factors involved in decreasing undesirable stresses to the implants are interrelated. These include increasing the bone density and maximizing the diameter of implants. These two goals are accomplished with mandibular block grafts. The quality of bone from the ramus buccal shelf is typically Type I, and the symphysis normally exhibits Type II and occasionally Type I quality bone. These mandibular block grafts create areas for the use of larger diameter implants that increase the surface area over which the stresses of occlusal forces are distributed.

Block grafts for alveolar ridge augmentation have been used extensively with great success and include (primarily) the symphysis and ramus buccal shelf as donor sites.^16-26 Many studies suggest that membranous bone grafts exhibit less resorption than endochondral bone grafts.^34-35 A possible explanation for this improved maintenance of graft volume is more rapid vascularization of the membranous block graft, as evidenced by the excellent incorporation with surrounding bone. This article focuses on posterior mandibular reconstruction using mandibular block autografts in a staged manner before implant placement. Implants are placed in a submerged or nonsubmerged mode after appropriate healing time with the block autografts.

Fig. 1. Missing mandibular left canine, first and second bicuspid, first and second molars. Note relative thin alveolar width. Fig. 2. Panorex radiograph indicating adequate alveolar bony height from crest of ridge to inferior alveolar neurovascular canal. Four root-form implants. Fig. 3. Left posterior edentulous ridge. Note relative narrow crestal bone. Fig. 4. Superior view of left ramus buccal shelf osteotomies. Fig. 5. Exposed symphysis with osteotomies completed for symphyseal block grafts .Fig. 6. Two symphyseal block grafts (superior) and one left ramus buccal shelf block graft (inferior). Fig. 7. Same as Figure 6 except grafts are turned 90 degrees. Note relative cortical thickness differential.

Fig. 8. Decortication and perforation of left posterior lateral alveolar ridge. Fig. 9. Block grafts shaped, positioned, and fixated. Note anterior two blocks harvested from chin and posterior block from ramus buccal shelf. Fig. 10. Eighteen weeks postgraft incorporation. Fig. 11. Shaping of block graft augmentation. Fig. 12. Trephine (2 mm) core bone biopsy from lateral alveolar ridge. Fig. 13. Representative histology of core biopsy (toluidine blue stain). Note trapezoidal-shaped grafted bone (right upper corner) well integrated into the newly formed lamellar bone (lower section). Fig. 14. Stage I surgery completed, four cylinder Spline implants, 4.0 diameter.

Case Presentations

Case 1

A healthy 61-year-old white woman was referred for implant evaluation. The patient’s chief complaint was her unhappiness with the mandibular unilateral partial denture. The patient stated that she had not worn her partial denture for a “number of months.” Clinical and radiographic examination revealed missing mandibular left canine, first and second premolars, and first and second molars (Fig. 1). Clinical examination and articulated study models revealed a deficiency in alveolar width of the left posterior edentulous span. A panoramic radiograph revealed adequate alveolar bone measured from crest of ridge to inferior alveolar neurovascular canal that would accommodate root-form implants (Fig. 2). The treatment plan would require lateral ridge augmentation before implant placement could be accomplished with subsequent restoration with an implant-borne five unit fixed splint.

Block graft surgery must be done separately, without implant placement, to ensure success of a case such as this. It is important to build adequate bone quality and quantity in a staged manner before implant placement to avoid the potential complications that have been reported with simultaneous implant graft placement. These include wound dehiscence with exposure of the block graft and implants, block graft fracture, and a higher rate of implant failure than occurs with a staged approach.^36-38

The augmentation aspect of the treatment plan included harvesting block grafts from the symphysis and left ramus buccal shelf to produce adequate bone volume for the edentulous span (Figs. 3, 4, and 5).

Incision design included an oblique releasing incision distal to the mandibular right second bicuspid and continuing in an intrasulcular direction anteriorly to the most distal tooth (mandibular left lateral incisor). This incision then continued midcrestal through the edentulous span and retromolar pad with a distal oblique releasing incision into the buccinator muscle (Fig. 3).

A full-thickness mucoperiosteal flap was then reflected, allowing for visualization of the right and left mental neurovascular bundles. The left ramus buccal shelf block graft and symphyseal block graft were then harvested according to conventional protocol (Figs. 4 and 5)(Pikos, submitted, 2000).^22-24

The block grafts were then shaped appropriately with a 4-mm diameter round fissure bur (Fig. 6) [H71-104-050] (Brasseler USA, Inc., Savannah, GA). The range of cortical thickness of the symphysis graft was between 6 mm and 11 mm versus a relative uniform cortical thickness of the ramus buccal shelf block of approximately 4 mm (Fig. 7).

Next, the recipient site was decorticated and perforated (Fig. 8). The cortical plate was found to be relatively thin and thus collapsed into a marrow space. The decortication resulted in a step defect allowing for creation of additional anterior, posterior and inferior bony walls. The previously shaped block grafts were then positioned at the recipient site and rigidly fixated with 1.6-mm diameter OsteoMed screws using two screws per block segment (Fig. 9)(OsteoMed, Dallas, TX).

After 18 weeks of healing time, the surgical site was exposed, and the block grafts were found to be well incorporated (Fig. 10).

Screw fixation was released, and appropriate bone contouring of the graft site was done (Fig. 11).

Next, a 2-mm trephine wasused to obtain a core biopsy through the buccal cortical plate penetrating both recipient and donor bone (Fig. 12).

A representative histological photomicrograph is indicated in the Figure 13.

Stage I surgery was then completed with 4.0 diameter cylinder Spline implants (Sulzer Calcitek, Carlsbad, CA) placed into type I and II quality bone. Two 18-mm implants were placed into the mandibular right canine and first bicuspid tooth sites along with two 13-mm length implants into the second bicuspid and first molar tooth sites. Finally, a 10-mm length implant was placed into the No. 18 tooth site (Fig. 14).

Four months of time elapsed between Stage I and Stage II surgery. The restoration was completed ~2.5 months after Stage II surgery.

Figures 15 and 16 reveal the five-unit implant supported fixed splint at 1-year postprosthetic loading. In addition, the panoramic radiograph in Figure 17 also reveals the completed case 1 year in function.

	Fig. 15. Completed five-unit fixed splint. Fig. 16. Completed five-unit fixed splint with full occlusion. Fig. 17. Final Panorex radiograph at 1 year postprosthetic loading. Fig. 18. Edentulous left posterior mandible. Fig. 19. Monocortical block graft from left posterior ramus to alveolar crest of left posterior mandible. Fig. 20. Radiographic view of block graft with screw fixation at crest of ridge. Fig. 21. Screw fixation released at 4.5 months post-graft fixation.

Case 2

A healthy 44-year-old white woman was referred for implant evaluation of the edentulous left posterior mandible. This patient requested a fixed prosthesis for this edentulous span. Clinical examination revealed missing mandibular left second bicuspid tooth and first and second molars along with moderate/severe atrophy of the left posterior mandible (Fig 18). Radiographic examination also revealed a vertical deficiency precluding root-form implant placement.

The treatment plan included a mandibular block graft for vertical ridge augmentation followed in a staged manner by implant placement for a three-unit implant-supported fixed splint. The crest of the ridge was then decorticated and perforated. A left ramus buccal shelf block graft was harvested, contoured, and fixated into position at the crest of the ridge (Figs. 19, 20, and 21).

Approximately 18 weeks elapsed before Stage I surgery. At that time the surgical site revealed excellent incorporation of the block graft such that the recipient/donor bone interface was difficult to visualize (Fig. 21). Three 10-mm length X 4.0 diameter hydroxyapatite-coated cylinder Spline implants (Sulzer Calcitek) were placed into the mandibular left second bicuspid, first and second molar areas (Fig. 22). Bone density was type II. The patient was then restored approximately 3 months later with a three-unit implant supported fixed splint (Figs.23 and 24).

Fig. 22. Stage I surgery using three 10-mm length X 4.0 diameter cylinder Spline (Sulzer Calcitek) implants. Fig. 23. Radiograph of completed prosthetics at 1 year postprosthetic loading. Fig. 24. Completed three-unit implant supported fixed splint at 1 year postprosthetic loading.

Discussion Site Recipient Preparation

The regional acceleratory phenomenon is a process by which tissue forms 2 to 10 times faster than the normal regional regeneration process in response to a noxious stimulus.^39-42 This process is more evident in cortical bone than in trabecular bone because of the high turnover rate of cortical bone. In addition, growth factors present in cortical bone play an important role in bone formation. Thus, recipient site preparation should include decortication, especially in the posterior mandible as was found in these case examples. It is more important to decorticate the mandibular cortical plate as opposed to the maxillary cortical plate because of the relative deficient blood supply of mandibular cortical bone. The typical maxilla has abundant vascularity so that decortication becomes a convenience for mechanical stability of the block graft. Decortication also increases the number of walls of the defect so as to further facilitate the incorporation of the block graft.

The drilling of holes in recipient cortical bone is found to induce increased vascularization and an increased influx of growth factors and platelets. Platelet-derived growth factor and transformed growth factor 5 are released from damaged vessels, resulting in an increase of osteogenic cells. Rigid fixation is imperative to immobilize the graft preventing microrotation resulting in a nonunion or fibrous union of the block graft.

Note the histological section (Fig. 13) from case 1 that reveals intimate contact between the recipient site (woven bone) and the donor block graft (lamellar bone). In this toluidine blue-stained section, in the upper right corner of the biopsy, a small trapezoidal-shaped particle of grafted bone is well integrated into the newly formed lamellar bone present in the lower part of the biopsy. This bone has the aspect of a “primary spongiosa,” where the scaffold of woven bone by the closure of the intertrabecular spaces through the formation of new primary osteons. The bone is also forming an endosteal spongiosa in the lower part, thus differentiating a cortical layer (Fig. 13, upper) from the endosteal side (Fig. 13, lower). Because of the precise recipient site preparation includingdecortication and perforation along with rigid fixation, there is no need for a membrane to be placed to preserve bone volume and to minimize resorption as has been indicated by some authors.^{7, 8, 12} Recipient site preparation is critical to having enhanced bone incorporation.

Staging

Staging of the mandibular block graft allows increased bone volume and quality to be created before implant placement, ensuring better initial implant stability. Ideal implant alignment is also facilitated with increased bone maturation at the bone/implant interface.⁴³ Finally, increased bone density is obtained using symphyseal bone (Type II or I) as well as ramus buccal shelf bone (Type I). Because the greatest stresses of a loaded implant are located around the neck and ridge crest,^{44, 45} the crestal bone with increased density can withstand implant loading in a more favorable biomechanical manner.

Complications

Complications seen from posterior mandibular reconstruction include those inherent with block grafting from both the symphysis and ramus buccal shelf. These are minimal (Pikos, submitted, 2000). The overall block graft success rate during this 5-year study was 99% with only one failed graft of 115. Neurosensory deficits were minimal, with only one patient exhibiting permanent altered sensations of the lower lip/chin secondary to the chin graft. In addition, only one patient exhibited permanent altered sensation of the lower incisor teeth. The infection rate was <1%. Wound dehiscence at the donor site was not seen with either the ramus graft or symphysis graft; especially in light of the use of the intrasulcular incision. Wound dehiscence of the posterior mandible should also be nonexistent as long as tension-free closure is obtained over the block graft.

Conclusions

Mandibular autogenous block grafts for posterior mandibular reconstruction offer several advantages. All block grafts maintain the bony architecture and original bone density of the mandible regardless of existent quality of the recipient posterior mandibular site. Thus, bone density is typically improved, in that symphyseal block grafts exhibit Type II and I density and ramus buccal shelf grafts exhibit Type I bone density. Also, bone volume is increased in a predictable manner with minimal resorption (0-1mm) with no use of membranes. This allows wider diameter implants to be placed, further facilitating stress distribution of the implant bodies. Disadvantages of the block grafts include potential for incurring nerve injury (although minimal). In addition, there is a limit to the amount of bone volume that can be obtained. In summary, autogenous mandibular block grafts can be used in a predictable manner for the reconstruction of alveolar ridge deficiencies before implant placement.

Acknowledgement

Acknowledgement for histology is extended to Paolo Trisi, DDS, PhD, Scientific Director, Biomaterials Clinical Research Association, Pescara, Italy.

*Private Practice, Palm Harbor Florida

References

1. Krogh PH, Worthington P, Davis WH, et al. Does the risk of complication make transpositioning the inferior alveolar nerve in conjunction with implant placement a “last resort” surgical procedure? Int J Oral Maxillofac Implants. 1994;9: 249-254.
2. Friberg B, Ivanoff CJ, Lekholm U. Inferior alveolar nerve transposition in combination with Branemark implant treatment. Int J Periodontics Restorative Dent. 1992;12:441-449.
3. Kan JY, Lozada JL, Goodcare CJ, et al. Endosseous implant placement in conjunction with inferior alveolar nerve transposition: An evaluation of neurosensory disturbance. Int J Oral Maxillofac Implants. 1997; 12:463-471.
4. Kan JY, Lozada JL, Boyne PJ, et al. Mandibular fracture after endosseous implant placement in conjunction with inferior alveolar nerve transposition: A patient treatment report. Int J Oral Maxillofac Implants. 1997; 12:655-659.
5. Fonseca RJ, Davis HW. Reconstructive Preprosthetic Oral and Maxillofacial Surgery, 2nd ed. Philidelphia: WB Saunders; 1995:357-362.
6. Buser D, Bragger U, Lang NP, et al. Regeneration and enlargement of jaw bone using guided tissue regeneration. Clin Oral Implants Res. 1990; 1:22-32.
7. Buser D, Dula K, Hirt HP, et al. Lateral ridge augmentation using autografts and barrier membranes: A clinical study with 40 partially edentulous patients. J Oral Maxillofac Surg. 1996; 54:420-432
8. Simon M, Baldoni M, Rossi P, et al. A comparative study of the effectiveness of e-PTFE membranes with and without early exposure during the healing period. Int J Periodontics Restorative Dent. 1994; 14:167-180
9. Simon M, Misitano U, Gionso L, et al. Treatment of dehiscences and fenestrations around dental implants using resorbable and nonresorbable membranes associated with bone autografts:A comparative study. Int J Oral Maxillofac Implants. 1997; 12:159-167
10. Jovanovic S, Spiekermann H, Richter EJ. Bone regeneration around titanium dental implants in dehisced defect sites: A clinical study. Int J Oral Maxillofac Implants. 1992; 7:233-245.
11. Jovanovic SA. Bone rehabilitation to achieve optimal aesthetics. Pract Periodontics Aesthet Dent. 1997; 9:41-52
12. Jensen OT. Guided bone graft augmentation. In: Buser D, Dahlin C, Schenk RK, eds. Guided Bone Regeneration in Implant Dentistry. Chicago: Quintesscence; 1994:235-264.
13. Jensen OT, Greer RO, Johnson L, et al. Vertical guided bone-graft augmentation in a new canine mandibular model. Int J Oral Maxillofac Implants. 1995; 10:335-344.
14. Misch CE, Dietsh F. The unilateral mandibular subperiosteal implant. Indications and technique. Int J Oral Implantol. 1991; 8:21-27
15. Misch CE. Contemporary Implant Dentistry, 2nd ed., St. Louis, MO: Mosby; 1999:443-444
16. Misch CM, Misch CE, Resnik R, et al. Reconstruction of maxillary alveolar defects with mandibular symphysis grafts for dental implants: A preliminary procedural report. Int J Oral Maxillofac Implants. 1992; 7:360-366.
17. Misch CM, Misch CE. The repair of localized severe ridge defects for implant placement using mandibular bone grafts. Implant Dent.1995; 4:261-267
18. Misch CM. Comparison of intraoral donor sites for onlay grafting prior to implant placement. Int J Oral Maxillofac Implants. 1997; 12:767-776.
19. Collins TA, Nunn W. Autogenous veneer grafting for improved esthetics with dental implants. Compend Contin Educ Dent. 1994; 15:370-376.
20. Sindet-Pedersen S, Enemark H. Reconstruction of alveolar clefts with mandibular or iliac crest bone grafts: A comparative study. J Oral Maxillofac Surg. 1990; 48: 554-558.
21. Pikos, MA. Buccolingual expansion of the maxillary ridge. Dent Implantol Update. 1992; 3:85-87.
22. Pikos, MA. Facilitating implant placement with chin grafts as donor sites for maxillary none augmentation; Part I. Dent Implantol Update. 1995; 6:89-92.
23. Pikos, MA.Facilitating implant placement with chin grafts as donor sites for maxillary bone augmentation: Part II. Dent Implantol Update. 1996; 7:1-4.
24. Pikos, MA. Alveolar ridge augmentation with ramus buccal shelf autografts and impacted third molar removal. Dent Implantol Update. 1999; 4:27-31.
25. Misch CM. Ridge augmentation using mandibular ramus bone grafts for the placement of dental implants: Presentation of a technique. Pract Periodontics Aesthet Dent. 1996; 8:127-135.
26. Perry T. Ascending ramus offered as alternate harvest site for onlay bone grafting. Dent Implantol Update. 1997; 3:21-24.
27. Burchardt H. Biology of bone transplantation. Orthop Clin North Am.1987; 18:187-195.
28. Marx RE. Biology of bone grafts. In: Kelly JPW, ed. OMS Knowledge Update, Vol 1. Rosemont, IL: American Association of Oral and Maxillofacial Surgeons. 1994; 1:RCN3-RCN17.
29. Branemark P-I, Lindstrom J, Hallen O, et al. Reconstruction of the defective mandible. Scand J Plast Reconstr Surg. 1975; 9:116-128.
30. Braun TW, Sotereanos GC. Autogenous regional bone grafting as an adjunct in orthognathic surgery. J Oral Maxillofac Surg. 1984; 42:43-48.
31. Helkimo E, Carlsson GE, Helkimo M. Bite force and state of dentition. Acta Odontol Scand. 1977; 35:297-303.
32. Haraldson T, Carlsson GE. Bite force and oral function in patients with osseointegrated implants. Scand J Dent Res. 1977; 85:200-208.
33. Braun S. Bantleon HP, Hnat WP, et al. A study of bite force: Part I. Relationship to various physical characteristics. Angle Orthodontist. 1995; 65:367-372.
34. Smith JD, Abramson M. Membranous vs. endochondral bone autografts. Arch Otolaryngol. 1974; 99:203-210.
35. Zins JE, Whitaker LA. Membranous vs. endochondral bone autografts: Implications for craniofacial reconstruction. Plast Reconstr Surg. 1983; 72:778-786.
36. Jensen J, Sindet-Pedersen S. Autogenous mandibular bone grafts and osseointefrated implants for reconstruction of the severely atrophied maxilla: A preliminary report. J Oral Maxillofac Surg. 1991; 49:1277-1287.
37. Jensen J, Sindet-Pedersen S, Oliver AJ. Varying treatment strategies for reconstruction of maxillary atrophy with implants: Results in 98 patients. J Oral Maxillofac Surg. 1994; 52:210-216.
38. Collins TA. Onlay Bone grafting in combination with Branemark implants. Oral Maxillofac Surg Clin North Am. 1991; 3:893-902
39. Frost H. The regional acceleratory phenomenon: A review. Henry Ford Hosp Med J. 1983; 31:3-9.
40. Frost H. The biology of a fracture healing: an overview for clinicians: Part I. Clin Orthop.1989; 248:283-292.
41. Frost H. The biology of a fracture healing: an overview for clinicians: Part II. Clin Orthop.1989; 248:294-309.
42. Shih MS, Norrdin RW. Regional acceleration of remodeling during healing of bone defects in beagles of various ages. Bone. 1985; 6:377-379.
43. Misch CE. Density of bone: Effect on treatment plans, surgical approach, healing, and progressive bone loading. Int J Oral Implantol. 1990; 6:23-31.
44. Clelland NL, Ismail YH, Zaki HS, et al. Three dimensional finite element stress analysis in and around the Screw-Vent implant. Int J Oral Maxillofac Implants. 1991; 6:391-398.
45. Kummer BKF. Biomechanics of bone; mechanical properties, functional structure, functional adaptation. In: Fung YC, Perrone H, Anliker M, eds. Biomechanics: Foundations and Objectives. Englewood Cliffs, NJ: Prentice-Hall; 1972:237-245

ISSN 1056-6163/00/0901-067

Implant Dentistry

Volume 9 ~ Number 1

Copyright © 2000 by Lippincott Williams & Wilkins, Inc.