By: Marcus J. Blue, DDS and Charles M. Cobb, DDS, PhD
Marcus J. Blue, DDS: Private Practice of Periodontics, Basalt, CO and Charles M. Cobb, DDS, PhD: Professor Emeritus, Department of Periodontics, School of Dentistry, University of Missouri-Kansas City, Kansas City, MI
Background: Reduced function due to edentulation is related to skeletal change such as residual ridge resorption and loss of cortical bone thickness. Even with adequate cortical plate thickness, the ridge itself may present inadequate buccal-lingual dimensions, thereby requiring lateral expansion to facilitate successful dental implant placement.
Methods: A single case report is presented that involves lateral expansion of an edentulous atrophic posterior mandibular alveolar ridge to facilitate dental implant placement. The surgery consisted of a detached bone segment technique, stabilized by bone screws, and the adjunctive use of a particulate bone graft covered by a resorbable barrier membrane. Primary wound closure was achieved, healing was uneventful, patient morbidity was minimal, and a dental implant was successfully positioned and restored.
Results: The initial 3 mm atrophic ridge was expanded to 10 mm following the surgery that utilized a detached bone segment graft stabilized with bone screws and an adjunctive particulate bone graft. A 4.8 x 10mm bone level implant was successfully placed following 4 months of post-graft healing. The implant site appeared well vascularized and exhibited Type II bone density.
Conclusions: This single case report demonstrates that the detached bone segment technique can achieve substantial gains in horizontal ridge width of the edentulous posterior mandible. The increase in lateral dimension and maintenance of viable cortical bone allowed successful positioning of a large diameter dental implant.
Reduced function due to long-standing edentulation is known to induced skeletal change such as residual ridge resorption and loss of cortical bone thickness.1 Even with adequate cortical plate thickness, the ridge itself may present inadequate buccal-lingual dimensions; a scenario commonly encountered in the edentulous anterior mandible and less so in mandibular molar regions. The placement of dental implants in an alveolus with inadequate horizontal dimensions is likely to compromise long-term stability and prognosis. Consequently, a variety of surgical approaches have evolved to increase the horizontal dimension of the deficient mandibular alveolus,2,3 including, among other procedures, guided bone regeneration,4,5 autogenic, allogenic, and xenogenic block grafts,6-10 distraction osteogenesis, 11,12 and ridge-splitting techniques.13-17
Based on the work of Miyamoto et al.,18 it appears that cortical bone thickness is important to initial implant stability, more so than implant length. Assuming this observation to be true then the dimensions of available cortical bone should be considered when selecting the preferred implant site.19 In cases of severe alveolar resorption, the posterior mandible is considered a difficult region for reconstruction with the ultimate goal of implant placement.20 Localized bone defects in the posterior mandible are frequently reconstructed with autogenous mono-cortical bone blocks prior to the placement of dental implants.6-8 In the current case, the authors present a single case report detailing the technique first presented by Basa et al.20 in which the posterior mandibular buccal plate is completely detached, moved laterally to increase horizontal alveolar ridge dimensions, and stabilized until appropriate healing occurred to allow insertion of a dental implant. The characteristic biomechanics of cortical bone and how such features can dictate the choice of surgical technique for the posterior mandible are discussed.
The patient, a 44 year old male, presented to the University of Missouri-Kansas City (UMKC) undergraduate dental clinic with a chief complaint of a toothache associated with the mandibular left first molar (#19) of 2-weeks duration. A diagnosis of chronic irreversible pulpitis was determined. The patient’s medical history was essentially negative, i.e., no allergies, no use of tobacco, and no current medical conditions. Past medical history included a back surgery and removal of a colon polyp. The patient was classified as ASA 1.
Tooth #19 was treated by endodontic therapy. In the process of preparing the tooth for an endodontic post and crown build-up the mesial root developed a longitudinal fracture leading to difficult extraction with loss of buccal bone. The patient was subsequently seen 18 months later in the UMKC Graduate Periodontics Department for evaluation of a dental implant to replace tooth #19.
The periodontal examination noted slight plaque induced gingivitis, 12/132 sites exhibited bleeding on probing (9%), and there were no probing depths greater than 3 mm. At presentation the patient was missing all molar teeth except for #14 and #30 (Fig. 1). Occlusal analysis revealed bilateral group function with obvious occlusal wear on all remaining teeth. The patient admitted to a clenching habit. Evaluation of the #19 area edentulous ridge revealed an atrophic ridge with a Seibert class III contour (Figs. 2 & 3). A CBCT showed the ridge measured 3 mm in the horizontal dimension and featured a severe slant to the buccal of approximately 25-30 degrees, indicating loss of the buccal cortical plate during the extraction. In addition, the CBCT revealed that the crestal buccal and lingual cortical plates were fused with no intervening cancellous bone to a depth approaching 4 mm. The patient was informed that to achieve implant placement a horizontal ridge expansion would be necessary.
Local anesthesia was achieved by using 2.5 carpules of Septocaine® (articaine HCl 4%, 40 mg/mL) with 1:100,000 epinephrine. The anesthetic was delivered by inferior alveolar field block and lingual infiltration to anesthetize any aberrant branches extending from the superior root o the ansa cervicalis.
Following anesthesia, buccal and lingual intrasulcular incisions were made, starting at the distal of tooth #22 which joined at the distal of tooth #20 to become a single crestal incision extending distally up the anterior border of the mandibular ramus. This incision design allowed for a relaxed full-thickness mucoperiosteal flap reflection and exposure of the underlying bony ridge, confirming the Seibert Class III defect, the horizontal dimension of approximately 3 mm, and the buccal slant. Given the ridge dimension and architecture a horizontal expansion of the ridge was undertaken using a combination of in situ autogenous bone block supplemented with a graft of allogenic bone particles, all covered by a barrier membrane.
The initial step was to place two pilot-hole indentations in the buccal cortical bone for future positioning of titanium bone screws (Fig. 4). The pilot-holes were centrally placed, approximately 8 mm apart, within an area measuring 10 mm x 20 mm, representing the size of the intended in situ block of bone. The next step utilized a NSK VarioSurg ® piezo-electric unit (NSK America Corp., Schaumburg, IL) fitted with a SG1 titanium nitride coated 0.5 mm blade and copious amounts of sterile water to place a 20 mm cut along the crestal ridge. Vertical cuts of 10 mm length were placed, one from each end of the initial crestal incision, and extending downward to the buccal. The final cut was then made with a SG2R blade, connecting the two vertical incisions (Fig. 5). All bony cuts passed through the cortical plate thereby allowing for eventual free separation of the bone block. At this point, the pilot-holes were extended through the buccal cortical place using a #4 carbide round surgical length bur. Immediately prior to freeing the bone block, shallow indentations were made in the buccal surface of the lingual cortical plate with a #4 carbide round bur to allow insertion of fixation screws in their proper position. OsteoMed™ (OsteoMed, Addison, TX) Auto-Drive® self-drilling screws (2 mm diameter x 14 mm length) were then placed in the bone block and gently screwed to place until they engaged the lingual cortical plate (Fig. 6). This action, aided with osteotomes, effectively lifted the bone block from the underlying cancellous bone while insuring proper positioning and stabilization and allowing lateral expansion of the atrophic ridge. Following fixation of the bone, a 5-6 mm gap remained between the in situ bone block and the lingual cortical plate (Fig. 7). Sharp edges and corners of the in situ block were smoothed and rounded slightly to insure a lack of irritation during healing.
The next step consisted of compacting Puros Allograft® (Zimmer Dental, Carlsbad, CA), hydrated in sterile water, within the space beneath the stabilized bone and included covering the entire in situ bone block and ridge (Fig. 8). The grafted area was then covered with Puros Copios® pericardium membrane (Zimmer Dental, Carlsbad, CA). The membrane was tucked under both the lingual and buccal mucoperiosteal flaps (Fig. 9). The buccal flap was released slightly by two periosteal releasing incisions without vertical components to insure blood supply.
Primary closure was obtained and stabilized with Vicryl™ 4-0 suture (Ethicon, Inc., Somerville, NJ) using a continuous interlocking technique (Fig. 10).
The patient was placed on amoxicillin 500 mg, t.i.d., for 10 days and instructed to use an alcohol-free chlorhexidine oral rinse twice a day for three weeks. Sutures were removed at 2-weeks post-surgery.
Following a 4-month healing period, the site was re-entered and measurements taken prior to implant placement. The initial measurement of a 3 mm atrophic ridge was expanded to 10 mm following the surgery (Figs. 11-12). The titanium bone screws were removed and osteotomy prepared and a 4.8 x 10 mm Straumann® bone level implant (Straumann USA, LLC, Andover, MA) was positioned and a healing abutment placed (Fig. 13). During placement of the implant the bone appeared to be well vascularized and to exhibit a Type II bone density. At 4 months post-implant insertion the quality of healing was considered to be excellent (Fig. 14). The final restoration of the implant was achieved at 4 months post-insertion and the patient was evaluated at 1 month and then 3 and 6 months post-restoration (Figs. 15 & 16). Following restoration of the implant the patient was fitted with a mouth guard to counter the biomechanical stresses from the clenching habit.
The literature is replete with reports of effective bone augmentation using a variety of techniques. 3,14 Regardless of anatomical location or surgical approach, successful bone augmentation requires atraumatic manipulation of host bone, stabilization of grafted sites, preservation of adequate blood supply, space maintenance allowing ingrowth of osteogenic cells, prevention of connective tissue invasion that would encapsulate interpositional bone grafts, and avoiding tension on soft tissues when achieving primary wound closure.2,13,14,17,22 In cases involving expansion of atrophic alveolar ridges, stability of dental implants is primarily dependent on cortical bone.18,19,23
The alveolar process of the posterior mandible, assuming the presence of teeth, typically exhibits a well developed buccal and lingual cortical plate comprised of a relatively thin outer layer of lamellar bone and a thickened subjacent layer of Haversian bone that transitions into cancellous bone. Various reports have estimated the thickness of cortical bone in the dentate mandibular 1st molar area to range from 0.6 mm to 2 mm near the crest 24-27 or 0.8 mm to 3.4 mm measured at 9 mm apical of the CEJ.25 However, with extraction of teeth the histologic and macroscopic anatomy of the alveolar ridge undergoes dramatic change.
It has been determined that cortical bone from all regions of the facial skeleton of edentulous individuals is thinner than in dentate skulls.1 Thus, depending on individual variation in bone turn-over and duration of edentulism, the cortical plates of the posterior mandible may or may not be of sufficient thickness to offer adequate support to implant placement. In the present case, several issues were considered that ultimately determined the surgical approach. First, although the time from extraction to the initial evaluation for implant placement was only 15 months, there was significant resorption mandibular posterior alveolus. Second, expansion of the alveolus by a simple ridge splitting osteotomy was contraindicated due to the horizontal crestal width of 3 mm and, as noted on the CBCT, fusion of the crestal buccal and lingual cortical plates with no intervening cancellous bone to a depth of 4 mm. Third, the lack of elasticity associated with cortical bone in the posterior mandible would require either an apical hinge cut or a more aggressive osteotomy through the entire thickness of the cortex, thereby allowing lateral repositioning and avoidance of an adverse fracture.15,28-30
Cortical bone exhibits a higher modulus of elasticity than cancellous bone, is stronger and more resistant to deformation, and will bear more load in clinical situations than cancellous bone.31 Indeed, it has been demonstrated that the edentulous mandible exhibits greater inelasticity than the dentate mandible in the retromolar region.32 As Bravi et al.33 noted, the inelasticity of mandibular cortical bone generally dictates a two-stage delivery of dental implants. Thus, it is not surprising that the absolute amount of cortical bone has more influence on implant stability than does cancellous bone. Based on the work of Baumgaertel and Hans,24 thickness of the buccal cortical bone in area of implant placement in the present case was estimated to range from 1.83 to 2.49 mm – a thickness that certainly would not allow horizontal repositioning of the buccal plate without an apical osteotomy. Furthermore, as Flanagan26 based on clinical experience, the mandibular lingual cortex is generally thicker than the buccal cortex. This observation appears to be supported by the fact that clinicians often utilize the lingual cortex for bracing osteotomes without inducing fractures. Thus, the lingual cortical plate can be used to anchor bone screws for stabilization of a detached bone segment.20
The rate of ridge atrophy in this case was relatively rapid and severe. The rate of atrophy is highly unpredictable as it can vary greatly between patients and even within the same person at different times or in different regions within the jaw.34 Alveolar atrophy is greatest during the first year post-extraction and without functional stimulation35 can become a life-long process.34 Given these observations, the rate of alveolar atrophy in the present case appears to be within normal boundaries.
It has been suggested that success of implants placed in pristine bone should be validated after 5 years of function.36 One could argue that a similar standard be used for validation of implants placed in sites following a lateral expansion procedure. In this regard, it should be noted that the most common complication during a lateral ridge expansion surgery is fracture of the buccal cortical plate.3 In the current case this possibility was avoided by using the detached bone segment approach.
Other reported complications observed during ridge expansion include loosening or fracture of bone screws, prolonged morbidity, paraesthesia, and membrane exposure with resulting loss of graft.3 Interestingly, lack of osseointegration has been reported in a relatively small number of cases.3 None of the reported complications was encountered in the current case. Thus, after six months of implant function the long-term success of the current case would appear favorable.
This single case report demonstrates that the detached bone segment technique first reported by Basa et al.20 can achieve substantial gains in horizontal ridge width of the edentulous posterior mandible without the morbidity associated with a secondary donor surgical site. The increase in lateral dimension and maintenance of viable cortical bone allowed successful positioning of a large diameter dental implant