Autogenous Chin Block Grafts for Implant-Supported Rehabilitation: A 20-Year Follow-Up Case Series

Currently, osseointegrated implants are safely used to rehabilitate edentulous patients. However, bone reconstructive surgeries are often necessary to guarantee long-term prosthetically driven implant success. Success is based on both aesthetic and functional considerations.^1–3  Bone augmentation techniques have advanced, but the treatment of atrophic alveolar ridges persists as a major challenge in oral surgery.^1,4  The use of autologous bone grafts is a well-documented technique for the rehabilitation of congenital or acquired alveolar defects. To obtain a good clinical outcome, it is mandatory to plan and place implants as determined by the prosthetic needs, according to the concepts of restoration-driven implant placement and prosthetically guided regeneration.^5–7  Depending on the size of the osseous defect, bone reconstruction can be performed either before implant placement or simultaneously with implant placement.⁸ 

Autologous bone represents the “gold standard” for bone augmentation, because of its optimal biological features and biocompatibility.⁹  Indeed, different from other bone graft materials, autologous bone is the only osteoconductive, osteoinductive, and osteogenic material.¹⁰ 

Intraoral bone harvesting has many advantages when compared with extraoral approaches. The first benefit is that intraoral donor sites, such as the mandibular symphysis and ramus, have a membranous embryological origin. This is correlated to higher success, lower resorption rate, and better and faster revascularization compared with extraoral donor sites with endochondral origin, such as the iliac crest and tibial plateau.^11,12  The second advantage of intraoral harvesting is that general anesthesia is not required, thus surgery can be performed under local anesthesia. Furthermore, the intraoral approach generally shows a lower complication rate with an easier and faster postoperative recovery.¹³  Nevertheless, intraoral harvesting also presents some drawbacks, such as an extended surgical site and the risk of complications.¹⁴ 

Common donor sites in the oral region include the following: (1) mandibular symphysis, (2) retromolar area, and (3) maxillary tuberosity.^13,15  Autogenous bone grafts harvested from the mandibular symphysis provide some advantages in the reconstruction of atrophic ridges. This symphysis approach permits the harvesting of a greater than 50% volume with an easier surgical access in comparison with mandibular ramus grafting. Moreover, the bone harvested from the mandibular symphysis presents a faster integration, thanks to its cortico-cancellous nature. The composition of mandibular symphysis grafts (on average) is 65% cortical bone and 35% cancellous bone, which positively affects the vascularization of bone blocks placed in the recipient site, as opposed to mandibular ramus grafts, composed of almost 100% cortical bone. Conversely, mandibular ramus harvesting provides lower postoperative patient morbidity and lower incidence of incision dehiscence.¹³ 

The resorption of appositional grafts can be reduced by covering autogenous bone blocks with bone substitutes with low turnover rates.¹⁶  To this aim, the use of deproteinized bovine bone (DBB) particles stabilized by a resorbable membrane reduce the resorption rate of autogenous onlay blocks by almost 50% compared with the traditional technique.^17,18  The role of bone substitutes is both to fill the gaps between the block graft and the recipient site and to create a scaffold to support bone regeneration.¹⁷  In contrast to nonresorbable devices, the main advantages of using resorbable membranes are the following: (1) ease of handling, (2) ease of fixation, and (3) lower risk of wound dehiscence.^16,18 

To the best of our knowledge, no studies are currently available in the existing literature reporting on the survival rates of dental implants placed in sites augmented with mandibular symphysis onlay grafts with a follow-up of 20 years. In view of the above, the aim of the present case series was to analyze the survival and success rates of dental implants placed in resorbed alveolar ridges reconstructed with mandibular symphysis autogenous onlay bone grafts.

In the present study, 5 patients referred to the authors' department between 2000 and 2001 seeking implant-supported fixed rehabilitation were evaluated. All the patients included in the study were periodontally and systemically healthy (ASA Class I according to the American Society of Anesthesiologists Physical Status Classification), with no smoking habits (≤10 cigarettes per day).

During the first visit, patients underwent both clinical and a radiographic evaluation. At the clinical examination, it was possible to analyze the edentulous site and the soft tissue condition. At that time, the radiographic exam was performed with orthopantomography to assess the bone defect, giving particular attention to the mesial and distal bone peaks.

After informing patients regarding treatment alternatives, bone augmentation procedures by means of mandibular symphysis autogenous bone blocks and delayed implant placements were planned. All patients provided signed informed consent. All surgical procedures were conducted by the same clinical team in accordance with the 1964 Declaration of Helsinki guidelines for investigations involving human subjects and its later amendments.

One week prior to surgery, all patients received a professional oral hygiene session, and 0.2% chlorhexidine mouthwashes were prescribed twice daily. All procedures were performed on an outpatient basis under local anesthesia. Premedication with diazepam 0.2 mg/kg was administered orally 30 minutes before surgery. Local anesthesia was administered by local infiltration, using mepivacaine 2% with epinephrine.

Surgical step 1: The recipient site was exposed raising a full-thickness flap. Any soft tissue remnants were carefully debrided, and cortical perforations were performed with a 0.10-mm diameter round carbide bur mounted on a surgical handpiece under copious irrigation with sterile saline to expose the medullary compartment and favor the nourishment of the graft.

Surgical step 2: Then a full-thickness mucoperiosteal flap was reflected also in the donor site, extending from 1 cm beyond the mucogingival junction to pogonion and to each distal region of the canines.

Surgical step 3: Dependent upon to the size of the bone defect, single or multiple monocortical bone blocks were obtained from the mandibular symphysis, by means of osteotomies performed with a surgical fissure bur in a surgical handpiece, under copious irrigation with sterile saline. The osteotomies were ideally placed 5 mm mesial from each mental foramen, 5 mm apical from the apex of the anterior teeth, and 5 mm coronal from the inferior border of the mandible. A bone chisel was tapped along the osteotomies to deliver the graft.

Surgical step 4: Following the harvesting of the bone, a hemostatic collagen dressing was placed into the symphysis area.

Surgical step 5: Then the block graft was fixed at the buccal aspect of the bone defect by means of 1.6-mm diameter osteosynthesis screws (KLS Martin, Tuttlingen, Germany) with variable lengths, ranging from 7 to 11 mm depending on the thickness of the grafted block. The block graft was subsequently covered with DBB particles (Bio-Oss, Geistlich Biomaterials, Wolhusen, Switzerland), protected and stabilized by a resorbable collagen membrane (Bio-Gide, Geistlich Biomaterials) (Figures 1–5).

Surgical step 6: Primary tension-free closure of the surgical flaps was obtained by performing horizontal mattresses and multiple detached stitches, using polyamide sutures in the receiving site and silk sutures in the donor site. Before suturing the donor site, the soft tissue superior to the initial access incision was elevated a few millimeters to reduce tension on the flap from edema and lip movement.

Postoperative medications included antibiotic therapy consisting of amoxicillin 1 g twice daily for 6 days starting on the day of surgery, anti-inflammatory/analgesic therapy with naproxen sodium 550 mg tablets as required for pain control every 6 hours, and starting the day after surgery, antiseptic mouth rinsing with 0.2% chlorhexidine mouthwashes twice daily for 2 weeks. After 14 days, sutures were removed and an orthopantomography was performed.

Surgical step 7: Following a healing period of 4 months, the second surgical phase was carried out. During this step, fixation screws were removed, and implants were placed in a prosthetically guided position with the aid of a surgical stent based on a preliminary wax-up performed on stone casts, according to the prosthetic needs (Figure 6).

Surgical step 8: After an additional 5 months, the implants were uncovered and healing abutments were placed. When the soft tissues reached a proper maturation, the prosthetic phase started.

Prosthetic step 1: Impressions were taken with custom impression trays.

Prosthetic step 2: Temporary implant-supported screw-retained acrylic resin prostheses were constructed in the laboratory.

Prosthetic step 3: The temporary prostheses were screw retained to the implants to initially load the implants and conditioning the soft tissues.

Prosthetic step 4: After 6 months of soft tissue conditioning, definitive implant-supported cemented-retained metal-ceramic prostheses were delivered.

Gingival revision surgery as needed: When necessary, a buccal peri-implant surgical gingival plastic procedure was performed with a diamond bur to improve the aesthetic of the soft tissues.

All patients were enrolled in a maintenance program that included routine follow-up recalls and professional prophylaxis every 6 months. The intraoral radiographs and orthopantomography collected at the 20-year follow-up visit were scanned to obtain standardized digital images with a resolution of 1.200 dpi (Figures 7–11).

At the 20-year follow-up visit, the following parameters were evaluated: implant survival, implant success, marginal bone loss, and peri-implant health. Implant survival was defined as the percentage of implants remained in place and in function at the latest follow-up recall. Each implant was also classified as successful or failing according to the following criteria, proposed by Buser et al¹⁹: (1) absence of persistent subjective complaints, such as pain, foreign body sensation and/or dysesthesia; (2) absence of a recurrent peri-implant infection with suppuration; (3) absence of mobility; (4) absence of a continuous radiolucency around the implant; and (5) possibility for restoration. Marginal bone loss was measured as the distance between the most apical bone-to-implant contact and the implant-abutment connection level at the mesial and distal aspects of each implant, using specialized computer software (ImageJ 1.53a, National Institutes of Health, Bethesda, MD). The calibration of the pixel/millimeter ratio was performed on the basis of a known distance, namely, the length of the implants. The evaluation of peri-implant health was based on the criteria defined in the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions²⁰: (1) visual inspection demonstrating the absence of peri-implant signs of inflammation; (2) lack of profuse bleeding on probing; (3) stable values of probing pocket depths, in general ≤ 5 mm; (4) bone loss ≤ 2 mm during the first year; and (5) absence of further bone loss following initial healing.

The study population consisted of 5 patients, of which 2 were females and 3 were males. Demographic data of patients and characteristics of surgical procedures are reported in the Table. Healing was uneventful in all cases and prosthetic restoration was always possible. One of the included patients (4-LG) moved to another city and decided to finalize the definitive implant-supported prosthetic rehabilitation in a local clinic.

In total, 10 implants placed in 5 patients were evaluated with a follow-up of 20 years. Since no implant was removed after 20 years from the prosthetic loading, implant survival rate was 100%. After 20 years, the clinical examinations revealed healthy soft tissues, with no mobility, no signs of suppuration, no bleeding on probing and stable values for peri-implant probing depths. None of the patients reported persistent complaints. Since all the implants respected the above-mentioned criteria, implant success rate was 100%. Marginal bone levels were evaluated in 8 implants placed in 4 patients. In 1 patient (1-CM) it was not possible to obtain reliable data due to the low quality of orthopantomography. The mean marginal bone loss at 20 years of loading was 0.32 ± 0.39 mm (range, 0–1.3 mm). Considering the conditions of peri-implant hard and soft tissues, the diagnosis of peri-implant health was made for all the implants included.

Ridge augmentations can be performed successfully by harvesting intraoral bone grafts from the mandibular symphysis, mandibular ramus, and maxillary tuberosity.²¹  Because of these techniques, both harvesting and grafting procedures are performed on an outpatient basis, reducing costs and avoiding inpatient care.²²  In cases with extended defects, the retromolar area and symphysis are good options as donor sites. However, according to some authors, the mandibular symphysis approach allows for easier surgical access, when compared with the mandibular ramus.¹³  The replacement of autogenous bone by means of bone substitutes is still under investigation, due to the osteoinductive, osteoconductive, and osteogenic properties that only autogenous tissue provides. Accordingly, many authors have proven the benefits obtained by using autogenous bone in the reconstruction of atrophic alveolar ridges prior to the placement of dental implants.²³ 

The aim of the present study was to evaluate the 20-year survival and success rates of dental implants placed in atrophic alveolar ridges reconstructed with autogenous bone grafts harvested from the mandibular symphysis. The rationale was to demonstrate that alveolar bone augmentation with autogenous onlay grafts restored with dental implant placement has a long-term success rate and thus, a reliable technique. Neither surgical or healing complications were observed during the 20-year follow-up. According to the literature, the most frequent postsurgical complication associated with bone block harvesting is temporary impaired sensitivity in the soft tissues of the chin.^24–27  Morbidity after chin bone harvesting may also include persistent pain and teeth disturbances, such as lamina dura increase or apical pathology.^28,29 

The surgical access to the symphysis was obtained via a vestibular incision, limiting the distal extent of the incision to the canine area to avoid mental nerve paresthesia. The main advantages of this approach are the following: (1) the creation of good access to the chin, (2) preserving the periodontal apparatus of the proximal teeth, and (3) reducing the risk of gingival recession. Clearly, the experience of the operator is an important factor for creating a tension-free 2-layer suture closure. Indeed, the risk of dehiscence at the suture line is higher for a crestal incision when compared with a sulcular incision. Moreover, the sulcular incision may provide easier visualization of the lower anterior teeth; therefore, preventing damage to the apices of the teeth. Examination for, and localization of, the mandibular incisor canal should be performed to define specific anatomical considerations for bone harvesting. Nevertheless, physiologic variations make it impossible for the surgeon to completely avoid the risk of having any neurosensory disturbances. It is therefore fundamental to preoperatively evaluate the specific characteristics of every case, by means of orthopantomography, cone-beam computed tomography (CBCT), 3D scan, and stereolithographic models.

More than on fixation screw at the recipient site is usually necessary to limit adverse rotational movements of the graft. However, in some cases, such as a single tooth or small defect as that reported in the present case, the block graft might be secured with a single screw. The principle reasons for this choice were the following: (1) the presence of a contiguous bone defect, limited by adjacent teeth's root prominences, and (2) the risk of fracture of small bone block by placement of multiple screws in close proximity to each other. In general, the surgeon should fix the block graft in the receiving site using the number of screws necessary to immobilize the bone block. In the present report, the use of a single screw yielded sufficient stability to the block, making the use of an additional screw unnecessary.

Previous studies revealed that combining block grafts with deproteinized bovine bone (DBB) particles covered by a resorbable membrane reduce bone graft resorption during healing.^30,31  A previous study reported a resorption rate of 7.7% at a follow-up of 10 years after lateral ridge augmentation using this technique.³²  In the present study, postoperative 3D radiographs were not performed to prevent the unnecessary X-ray radiations dose for patients; therefore, bone graft resorption rate was not measured. However, since marginal tissue recessions and peri-implant pockets were not observed upon probing, it can be assumed that vestibular marginal bone level remained stable during the 20-year follow-up. Some authors studied a correlation between the location of the donor site of the bone block graft and resorption rate. In fact, block grafts harvested from the mandible reveal higher stability, due to a thicker cortex and higher density, when compared with block grafts obtained from the iliac crest. On the other hand, the cancellous portion of block grafts stimulates the revascularization and the formation of new bone. The use of cortico-cancellous block grafts is therefore indicated to combine both advantages.^23,33,34  The decortication of the recipient site may have contributed to the favorable results of the present study. Many studies reported that perforation of the recipient site favor the following: (1) vascularization, (2) bone remodeling process, and (3) bone apposition; leading to bone graft incorporation.^35,36  This phenomenon was first described by Frost³⁷  as a regional acceleratory phenomenon. Regional acceleratory phenomenon is a concept that describes an increase in the healing capacities of the bone tissue when affected by an intentional surgical injury that enhances bone turnover and de novo bone formation.³⁷  According to a recent systematic review that evaluated the role of cortical perforations in bone regeneration, there was weak evidence that decortications did increase bone formation.³⁸  However, when the regenerative surgeries included in the present study were performed, the perforation of cortical bone was considered the preferred technique. More controlled studies in humans are required to determine the potential benefits of decortication. Survival and success rates of implants included in the present study were 100%. According to a recent systematic review involving implants placed in atrophic ridges augmented with intraoral block bone grafts, implant survival rate in the literature varied between 96.9% and 100% while success rate ranged from 89.5% to 100%, with a mean follow-up period of 12–24 months.²³  Peri-implant bone loss is considered a crucial parameter to evaluate implant success. In this study the mean peri-implant bone levels remained stable, showing a mean marginal bone loss of 0.32 ± 0.39 mm at 20 years of loading. These findings are supported by recent studies, which reported mean values of marginal bone loss around implants placed in reconstructed ridges between 0.09 mm and 0.48 mm after a 10-year follow-up.^32,39 

The main limitations of the present investigation include the following: (1) the reduced extent of the sample, (2) the nonhomogeneity of the prosthetic restoration, and (3) the poor accuracy of the radiograph analysis. Indeed, postoperative 3D imaging would have been necessary to evaluate the success of buccal bone grafting. However, the added value of the present study should be noted, which evaluates patients with a 20-year follow-up. This long-term follow-up is poorly represented in the literature, while most studies have a follow-up of only 1–2 years.⁴⁰  To the best of our knowledge, in the literature there is only 1 case report evaluating the survival rates of dental implants placed in sites augmented with mandibular symphysis onlay grafts that has a follow-up of 20 years. That study was published by this same research group.⁴¹ 

In conclusion, the present study provides evidence that implants placed in alveolar ridges augmented by means of autogenous chin block grafts demonstrate the following: (1) long-term survival, (2) high success rates, and (3) low marginal bone loss. Correct management of peri-implant soft tissues and an accurate prosthetic rehabilitation are also fundamental to obtain the durable aesthetic success of the treatment. However, postoperative 3D imaging would be necessary to evaluate the success of buccal bone grafting.