April 1, 2010
by Michael Dib, DMD, MSD, FRCD(C)
Bone grafting is often considered one of the most challenging procedures to undertake in a dental office. Numerous factors have to first be considered to ensure a predictable outcome, devoid of post-operative complications. Fortunately, the success rate of such procedures can be greatly improved with a good basic understanding of the different materials and techniques. Furthermore, diagnosis is probably the most important factor to assess when determining the predictability of a case. Alveolar ridge deficiencies, whether the result of bone lost during tooth extraction or post-extraction bone resorption, may preclude implant placement. To correct such deficiencies, a clinician has to utilize ridge augmentation techniques such as guided bone regeneration, ridge splitting, distraction osteogenesis, block grafts, and sinus lift procedures.
Guided bone regeneration (GBR) involves the use of a membrane that physically separates the osseous tissue from the soft tissue. Ridge augmentation is based on the principle that different cellular components in the tissue have varying rates of migration into a wound area during healing. By using a membrane as a mechanical barrier, fibroblasts and epithelial cells are excluded from the osseous tissue, so that only osseous cells are available to repopulate the healing area. There are several important elements to take into account when grafting a site, such as use of membranes, type of graft, space maintenance, suturing, flap release and design, site preparation, and post-operative medication.
Without a membrane, competing cells from the soft tissue may populate the treatment area and prevent osteogenesis. Barrier membranes used in guided bone regeneration procedures can be either resorbable or non-resorbable. Non-resorbable membranes provide a long-term barrier effect but are subject to soft tissue opening and membrane exposure, which can lead to infection, early membrane removal, and compromised results. Resorbable membranes are less prone to soft tissue opening and becoming infected but some may not function as an intact barrier for extended periods of time. The efficacy of the membrane technique to prevent graft resorption may be due to a combination of different factors, including:
1) Prevention of fibroblast mass action;
2) prevention of contact inhibition by heterotypic cell interaction;
3) exclusion of cell-derived soluble inhibitory factors; and
4) concentration, locally, of growth stimulatory factor.
An osseous graft material may stabilize the blood clot, and serve as an osteogenic, osteoinductive or osteoconductive agent. Osteogenic implies that the graft contains vital cells, which will contribute to new bone growth. Osteoinductive implies that the graft will stimulate the differentiation of osteoprogenitor cells into osteoblasts. This is said to be due to the bone morphogenetic proteins (BMPs). Osteoconductive means that the graft will serve as a scaffold for new bone formation. There are four major types of graft materials: autograft, allograft, xenograft, and alloplast. Autograft bone is obtained from the same individual. Allograft means the bone was obtained from a different individual, but from the same species, whereas xenograft bone is obtained from different species. Finally, an alloplast graft is made of synthetic materials.
Titanium screws may be used to establish desired ridge dimensions and to ensure adequate space and proper shape for the graft particles. Space maintenance is the most important factor to consider when doing ridge augmentation. Titanium screws should be used whenever there is doubt about the stability and space maintenance effect of a bone graft. The created space gives time for the osteoblastic cells to come in and colonize the site, allowing for bone growth. Furthermore, tenting screws can also be used to stabilize a block graft, whether autogenous or allogeneic.
Suturing can “make or break” a bone grafting case. When suturing, the clinician should never impede the soft tissue vascularization. The sutures have to help achieve a tension-free flap closure. If too much tension is created at the flap margins, the soft tissue can undergo necrosis. The end result is an open margin over the grafted area. Numerous studies have shown that once the graft is exposed, the success rate decreases greatly. A good way of preventing this is by using deep horizontal everting sutures at the base of the flap. By doing so, all tension is removed from the flap margins. Once this is done, the clinician can go about suturing the flap margins together to achieve a tension-free closure. The type of sutures used will depend on the type of graft used and the time needed for flap stability and closure. If lateral ridge augmentation is performed, the clinician might want to use a non-resorbing and/or slow resorbing suture material. If bone grafting is done in an area where minimal flap manipulation is done (flapless approach for ridge preservation for example), then a fast resorbing suture such as chromic gut might be best. Sutures should be removed when they start becoming loose, because at that point they do not serve their purpose anymore, and can in fact be a path of entry for bacteria.
As vital as the suturing is in a case, a clinician should never solely rely on the sutures to keep the grafted area closed. Flap design and release play an equally important role in helping prevent soft tissue dehiscence. The flap should be designed to prevent tension once the sutures are placed. A good way of doing so is to extend the incisions 1-2 teeth mesial and distal to the grafted area. In some cases, extending the flap will alleviate the need for releasing incisions. Moreover, the clinician should utilize a combination of full and split-thickness design. The flap should be elevated full-thickness until the mucogingival junction is reached, then split-thickness from then on. For severely atrophied areas needing extensive bone grafting, a superficial split-thickness flap should be utilized. This technique was pioneered by Dr. Henry Greenwell and allows for additional flap release.
Cortical perforations are an essential component of guided bone regeneration. The theory behind the intramarrow penetration is to allow for osteogenic cells to come and colonize the grafted area. The ridge is perforated over the whole area where bone graft is to be placed. A small round carbide bur is used to drill through the cortical plate until the marrow spaces are reached. Bleeding is a sign that communication has been made. Clinicians should be careful not to go through all the way to the lingual/palatal, where they risk perforating the flap in those areas. Another consideration is the location of vital structure such as the inferior alveolar nerve. If no computed tomography (CT scan) was done previously, a good periapical radiograph should be taken prior to decortification to ensure a safe distance from anatomical landmarks.
The idea behind prescribing medications is to reduce the post-operative discomfort and prevent infections. Different drug regimens are given, all depending on clinician preferences and complexity of the procedure. A good basic regimen to follow is one that includes an anti-inflammatory, an antibiotic, and an anti-bacterial mouthwash. Corticosteroids are often given after extensive bone grafting procedures to avoid excessive swelling that could compromise the integrity of flap closure. Regardless of the medication given
, nothing will prevent the case from failing if inadequate techniques or materials are used. When faced with a case where insufficient bone is available for implant placement, a clinician should first assess the extent of ridge deficiency. This can be done by bone sounding and/or with radiographs. Once decision has been made to proceed with bone grafting, the clinician should utilize all the principles of guided bone regeneration to help establish a stable, tension-free area that will remain intact during the entire healing process. Flap design should preclude stretching of the tissues, and intramarrow penetration will help supply the necessary osteogenic cells. Membranes should always be used to cover the grafted area, and if necessary, tenting screws should be placed to maintain space during the whole healing phase. Finally, suturing techniques that allow for uninterrupted healing are privileged. OH
Dr. Dib is Diplomate of the American Board of Periodontology. He maintains a private practice in Montreal, Canada. He can be reached at firstname.lastname@example.org
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