Use of a modified shell technique for three-dimensional bone grafting: description of a technique

Authors



Dr Michael Stimmelmayr
Josef-Heilingbrunnerstr 2
93413 Cham
Germany
Email: michael.stimmelmayr@med.uni-muenchen.de

Abstract

Background:  Defects of the alveolar crest often lead to three-dimensional bone loss after tooth extraction. Therefore, hard tissue grafting is required prior to implant placement. Different techniques have been described in the literature.

Methods:  In this case report three-dimensional hard tissue grafting was performed with a modified shell technique and autogenous bone harvested from the mandibular ramus. The shells were trimmed to a thickness of 1 mm and placed to recontour the ideal shape of the alveolar ridge. The shells were then fixed with micro titanium screws, and the gap between the shells and the alveolar ridge was filled with autogenous bone chips.

Results:  Wound healing was uneventful. Consolidation of the bone graft showed almost no resorption and the implant was placed into vital bone.

Conclusions:  The described shell technique for rebuilding three-dimensional alveolar defects showed promising results and could be an alternative treatment to other hard tissue grafting techniques.

Abbreviations and acronyms:
FDP

fixed dental prosthesis

RDP

removable dental prosthesis

Introduction

Although inadequate bone volume was thought to be an absolute contraindication for implants in the late 1980s, guided bone regeneration substantially expanded the range of indications for implants in the early 1990s.1,2 Initially, tenting screws and membranes were used for volume maintenance,3 however bone substitutes or bone grafts have shown better clinical results.4

Horizontal defects of the alveolar crest can be reconstructed predictably with these techniques,5 but predictability decreased with the size of the defects.6–8 Localized bone defects in the alveolar crest can be treated with different augmentation techniques and grafting protocols depending on the morphology of the defect.7 Slight horizontal defects can be solved with a single-stage augmentation procedure. Bone-splitting and bone-spreading techniques with or without filling the space is one option.6 Alternatively, a buccal augmentation of dehiscence-type and fenestrations-type defects can be treated with particulate autogenous bone, allograft (FDBA, DFDBA), xenograft, alloplast or a combination. In addition, a resorbable or non-resorbable membrane can be placed to prevent resorption.7 In an alveolar ridge with insufficient height or width to accommodate an implant with the desired dimensions, a two-stage procedure is indicated. These three-dimensional crestal defects showed unpredictable results when reconstructed with bone substitutes.9,10 Hence, reconstruction of large defects requires horizontal and/or vertical augmentation of autogenous bone grafts.11–13 Therefore, particulate autogenous bone or autogenous bone blocks in combination with resorbable or non-resorbable membranes can be used.7 Alternatively, a vertical augmentation can be done in combination with distraction osteogenesis, sandwich or interpositional techniques.6–8 However, additional bone grafting at the time of the re-entry may be needed.12,14,15

The chin or retromolar region was used to harvest intraoral bone grafts. Pikos described the inlay technique; placing cortico-cancellous block grafts with accurate fit to the residual alveolar bone.16 However, the mandibular cortical block graft requires a long period of time for vascularization and remodelling, and can be sequestrated years after the augmentation procedure.17 Therefore, Khoury described the shell technique for three-dimensional hard tissue grafting.17–19 Thin cortical bone shells, harvested with a special cutting wheel from the retromolar region, were placed to reshape the alveolar crest and to protect the particular bone (placed in the cavity between the shells), from resorption. Harvesting the bone shells and extraoral trimming with a cutting wheel is very technique-sensitive.17–19 Additional harvesting of bone chips is also necessary. Therefore, the authors modified and simplified this technique.

The technique described in this article used one donor site to harvest the bone graft. The cortico-cancellous bone graft was thinned out using a bone mill. Hereby the thickness of the cortical bone shell could be easily controlled. In addition, the shape of the shell was slightly convex, corresponding to the shape of the alveolar crest because of the bone mill’s circular rasp. Bone chips were also harvested simultaneously. The following case report details the surgical procedure step-by-step.

Case Description and Results

The requirements of the Helsinki Declaration were observed and the patient gave informed consent for all surgical procedures. A healthy 20-year-old male sustained an injury to the left upper central incisor 10 years earlier and the tooth had to be extracted. In June 2010, examination of the patient revealed a three-dimensional defect of the alveolar crest in region 21 (FDI tooth numbering system) (Fig. 1).

Figure 1.

 Clinical situation of the three-dimensional bony defect in region 21.

Surgical planning

A single X-ray (Gendex Expert DC) of region 21, a panoramic radiograph (Gendex OrthOralix 9200) and bone mapping were performed to plan the surgical procedure. The desired position of the implant was determined preoperatively using a positioning guide, fabricated by copying the wax-up. The patient was instructed to take antibiotics (amoxicillin, 1000 mg t.i.d.), Ibuprofen 400 mg and Decortin® 50 mg to prevent inflammation and swelling one hour before ridge augmentation. Ibuprofen was continued for three days t.i.d. and the antibiotics for six days t.i.d post-augmentation. Immediately before surgery, the patient rinsed his mouth with a 0.2% chlorhexidine solution for three minutes.

Graft site preparation

A palatal shifted crestal incision in region 21 was followed by a sulcular incision from tooth 11 to tooth 23, with one relieving incision at the distal line angle of tooth 23. A full-thickness flap was elevated, and all inflammatory and granulation tissue were debrided with a curette. The incisive nerve was also eliminated (Fig. 2). To ensure tension-free wound closure, immediately before surgery the periosteum was slit basal of the flap to prevent bleeding at the time of membrane placement. The bony defect was measured using a periodontal probe to determine the size of the bone shells.

Figure 2.

 Intrasurgical situation of the bone defect, combined with a large incisive foramen.

Graft harvesting

Bone was harvested at the oblique line at the left retromolar region. Using an ultrasonic knife (Piezosurgery, Mectron, Cologne, Germany), a cortico-cancellous bone block was obtained. The bone block was harvested on the lateral side of the ramus and had a thickness of about 3 mm (Fig. 3). In a bone mill (Bull Bone Mill, Mondeal, Mühlheim an der Donau, Germany) (Fig. 4), the block was reduced to a thickness of about 1 mm (Fig. 5), split into two pieces with a cutting wheel and used as the shells. The milled bone chips were mixed with autogenous blood and placed between the shells after fixation (Fig. 6).

Figure 3.

 Bone block of 3 mm thickness, harvested from the retromolar area.

Figure 4.

 Bone block placed in the bone mill.

Figure 5.

 Bone block of 1 mm thickness after the milling process.

Figure 6.

 Bone chips mixed with autogenous blood and the bone shell.

The donor site was filled with a collagen fleece (Resorba, Nueremberg, Germany) and sutured with a row of single sutures (Supramid 5-0, Stoma, Emmingen-Liptingen, Germany).

Graft placement

The bone shells were trimmed and adjusted with a round bur and anchored in the host bone with titanium microscrews (Q-Bone-Grafting Set, Trinon, Karlsruhe, Germany) (Fig. 7). Space between the shells and the alveolar bone and the canal of the incisive nerve was filled with a mix of milled bone chips and autogenous blood (Fig. 8). The bone graft was covered with a resorbable collagen membrane (BioGide, Geistlich, Wolhusen, Switzerland) and the wound was closed with a combination of a deep horizontal mattress (Gore-Tex 6-0, W.L. Gore, Flagstaff, Arizona, USA), vertical mattress sutures on the mesial and distal line angle of the bounded space, and interproximal and interrupted sutures (Trofilene 6-0, Stoma).

Figure 7.

 Bone shells fixed at the buccal and palatal alveolar crest with titanium microscrews.

Figure 8.

 Grafting of the bony defect with autogenous bone chips.

Wound healing

Wound healing was uneventful. The single sutures on the retromolar donor site and the grafted site were removed on day 10 after surgery and the mattress sutures on day 14 after surgery. While there was some superficial epithelial sloughing, re-epithelialization was completed after two weeks. After four weeks, the wound had primary closure without any signs of inflammation. The patient was provided with a removable dental prosthesis (RDP). Four weeks after surgery, a resin bonded fixed dental prosthesis (FDP) was seated by the referring dentist. Particular care was taken not to apply pressure to the graft.

Implant placement

The patient was instructed to take antibiotics (amoxicillin, 1000 mg t.i.d.) and Ibuprofen 400 mg one hour before implant placement. Ibuprofen was continued for three days t.i.d. and the antibiotics for six days t.i.d. post-surgery. Mouth disinfection was performed as described above.

Five months and 11 days were allowed for the bone graft to integrate. A palatal shifted crestal incision in region 21 was followed by a sulcular incision to tooth 11 and tooth 22 without any relieving incision. Palatal a full-thickness flap, buccal a combined full-split-thickness flap was elevated and the microscrews were removed (Fig. 9). Due to optimal incorporation of the bone graft, an implant (Bonelevel SLA Ø4.1/12 mm, Straumann AG, Basel, Switzerland) was placed with a positioning guide at the desired position. Additionally, a connective tissue graft harvested from the left hard palate was placed to thicken the soft tissue buccally and occlusally. The connective tissue graft was secured with a palatal horizontal mattress suture. The wound was closed with a deep horizontal mattress and interrupted sutures (Trofilene 6-0, Stoma). Wound healing was uneventful. The single sutures on the palatal donor site and in region 21 were removed on day 8 after implant placement and the mattress sutures on day 14 after implant placement.

Figure 9.

 Re-entry five month post-augmentation with ideal consolidation of the bone graft and reconstruction of the alveolar crest.

The patient was provided with a RPD after implant placement and the resin bonded FDP was seated four weeks after implant placement. To support wound healing, the palatal donor site was covered with a palatal stent (Erkodent 1.5 mm, Erkodent-Germany) for three days.

Discussion

From March 2009 until today, we have performed 24 augmentation procedures using the modified shell technique. All cases ended with primary wound closure without any soft tissue dehiscence and no healing problems during consolidation of the graft. All planned implants could be placed at second stage about five months after ridge augmentation. However, a slight horizontal resorption of the hard tissue graft was noticeable in some cases. The titanium microscrews (Q-Bone-Grafting Set) acted as an indicator for resorption. In cases of horizontal resorption, we adjusted this by thickening the soft tissue with a connective tissue graft. When the resorption proceeded vertically, the implant was placed slightly deeper.

The major advantage of this technique, in comparison to a bone block augmentation placed as an inlay graft, is the regeneration of vital bone.17–19 Shells of about 1 mm thickness prevent resorption of the bone chips and provide the shape of the graft. Blood supply from the host bone ensures survival of the bone chips. Normal bleeding during drilling of the implant bed seemed to indicate vital bone.

The modified technique described above is relatively simple. Intraoral harvesting and extraoral trimming of bone shells with the cutting wheel, as described by Khoury, appears to be very technique-sensitive.17–19 Thinning the thick bone block with the bone mill is comfortable and the thickness can be easily controlled. Additionally, the shape of the bone shell is slightly convex, like the alveolar crest, according to the bone mill’s circular rasp. Another advantage is the simultaneous harvesting of the particular bone chips by thinning the bone block. Hence, only one donor site is necessary. Future clinical studies are required to evaluate the amount of bone regeneration quantitatively.

Conclusions

The modified shell technique for hard tissue grafting is a good reconstruction method of the alveolar crest with vital bone. This is a key factor for successful implant placement. The resultant three-dimensional volume reconstruction facilitates aesthetic implant-supported restorations. For most intraoral hard tissue augmentations, only one donor site is necessary, which reduces post-surgical morbidity and complications. The technique described above provides reliable three-dimensional guided bone regeneration, regeneration of vital bone, autogenous graft material without any risk of external infection, and trimming of the shell and harvesting of bone chips in a one-step procedure.

Ancillary