August 3, 2016
by Carl E. Misch, DDS, MDS, PhD (hc); Vivian A. Roknian, DMD
To satisfy the ideal goals of implant dentistry in a predictable manner, hard and soft tissues should present in ideal volumes and quality. The alveolar process is affected so often after tooth loss that socket grafting is usually indicated to achieve optimal results. Reasons for socket grafting include an improved esthetic appearance particularly in the anterior alveolar ridge, preservation of remaining alveolar bone for bone or soft tissue reconstruction and fewer post-operative complications relative to implant position.
Prior to the introduction of various bone grafting materials and membranes, the socket historically was allowed to heal by secondary intention. It has been well documented in the literature that following extraction of a tooth and healing by secondary intention, the alveolus loses both bone volume and height.1,2 Depending on the dimensions of the extraction site and remaining alveolar walls, the majority of bone loss occurs in both a horizontal and vertical dimension. This bone loss becomes particularly visible in the anterior region where labial resorption may leave an unattractive esthetic condition for tooth replacement, whether this is by conventional means or by a dental implant.
Bone grafting alveolar sockets after tooth extraction has been a popular subject.3,5,6,8 However, given this is a highly useful procedure that takes only minutes to perform, any dentist who routinely extract teeth should learn the keys to predictable socket grafting in order to increase the success rate of restoring a full, ideal alveolus and soft tissue drape. Performing this procedure is of great benefit to the patient.
From an economic perspective, the overhead cost of grafting materials and related products can be considered minimum. *Based on ADA reported numbers, the average cost for 0.5cc (which is enough grafting material for one socket) averages around $80. Some sites require a barrier membrane, which may cost an additional $110. If a barrier membrane cannot be secured under soft tissue via primary closure, a low-cost collagen plug for about $8 is useful for keeping the graft material in place and for healing of the soft tissue. Sutures are typically another few dollars. Therefore, the cost to the patient of the extraction and graft may only amount to a few hundred dollars. However, when the cost of grafting is added to the average fee for a routine extraction, some patients may not be convinced of the value of the procedure (*Salvin Dental Specialties, Charlotte, N.C. USA).
Herein is an excellent opportunity to educate the patient about bone loss after tooth loss. In the anterior maxilla, the alveolar bone is rapidly lost, or remodeled, after the loss of natural teeth. The majority of bone loss occurs in both a horizontal and vertical dimension. There is a 25% decrease in horizontal bone volume during the first year and a 40% to 60% decrease within the first three years after tooth loss.1,2 As a consequence, socket grafting is routinely suggested after anterior tooth extractions, despite the marginal added cost. In fact, investing in grafting at this point may reduce the need for extensive additional grafting when the patient is ready for implants. Bone-replacement grafts in the aesthetic, anterior region can also improve ridge topography even when fixed bridgework is treatment planned.
Socket grafting can greatly improve the aesthetics and function of crowns, implants, and dentures, as well as eliminate future problems that are common to bone loss situations.
Surgical Keys to Socket Grafting
Consistent bone grafting results have typically been difficult to achieve. Often, this has been the case because similar techniques have been used across patients, regardless of the patient’s existing implant site conditions, the volume of bone desired and the region of the augmentation. Instead, specific elements (keys) need to be present for successful bone grafting.5,6
The keys to socket grafting are factors that affect the prognosis of the procedure. These include: absence of infection, soft tissue closure, space maintenance, graft immobilization, regional acceleratory phenomenon (RAP), host bone vascularization, growth factors, healing time, defect size and topography, graft materials and the transitional prostheses (Table 1).
Several of these keys are interrelated; often, one key affects another and may form a cascade toward failure or success. The surgeon should attempt to align procedures to address all these elements, but especially those factors missing in the bone augmentation site.
1. Defect Size and Topography (Walls of Bone)
In the periodontal literature, it is well documented that a three-wall bony defect next to a tooth root can be restored to an ideal condition more predictably than a one wall, two-wall bony defect. Likewise, a three-bony wall defect in an edentulous site can be augmented more favorably than a two-bony wall defect. Most often, two- to four-wall defects in implant dentistry include a lack of facial bone. Bone is often present on the lingual, mesial, distal, and apical regions (four wall bone defect). However, the apical region is too often narrow or compromised (three-wall bone defect), or a bone defect is present next to a tooth root.
As the number of boney walls is reduced, soft tissue closure, space maintenance, and graft immobilization become more critical. The graft material in a one- or two-wall bone defect more often requires an autograft as a major component. A barrier membrane and longer healing time may also be necessary. Use of a barrier can prevent ingrowth of fibrous tissue. Hence, the number of bony walls remaining around the defect is a key component to the method and predictable aspect of the augmentation. Thus, addressing an extraction site with a socket graft can decrease the likelihood of more complicated bone grafting techniques.
The topography of the graft site is also a key for predictable bone augmentation, as it affects soft tissue closure, space maintenance, graft immobilization, growth factors, and BMPs. In other words, almost every aspect of the equation. The size and topography of the defect after tooth loss/extraction factors in the healing time, the vascularization, and the transitional prosthetic options. It is also a factor in graft material selection.
When the vital bone of an extraction socket is greater than 1.5 mm to 2 mm thick on the facial, lingual, mesial, distal, and apical regions, a five-bony wall defect is present. This is an ideal environment for bone growth, as most all the keys for bone regeneration are already present (Fig.1). The space will be maintained by the surrounding walls of bone and the graft is immobilized by the bony walls. In other words, the number of walls of host bone and the size of the defect combine to affect almost all aspects of the socket graft process.
Fig 1. A five-wall boney defect may be an extraction socket with walls of bone on the buccal, palatal, mesial, distal and facial. Typically no bone is found on the occlusal aspect of the socket.
2. Surgical Asepsis/Absence of Infection
Resorption of a graft primarily occurs through two different mechanisms: solution-mediated and cell-mediated. More predictable implant site development occurs when a cell-mediated resorption replaces the graft material with osteoblasts. This “creeping substitution” uses osteoclasts to resorb the graft material and osteoblasts to replace it with bone. The osteoclasts form from monocytes in the blood which aggregate at the trauma site (i.e., extraction). Osteoblasts come from bone and periodontal ligament (PDL) blood vessels.
Solution-mediated resorption of a material is a consequence of the pH of the surrounding media. As the pH decreases, mineralized materials such as HA dissolve. It should be noted that HA resorbs at these low pH levels at a similar, rapid rate whether calcium phosphate, porous HA, or formulations of dense HA are used.8 Solution-mediated resorption occurs too quickly to maintain a space and participate in bone formation. Therefore this category of resorption is to be avoided in most all augmentation sites. To decrease the risk of infection or low PH, it is prudent to have scaling and root planning prior to tooth extraction and augmentation. Sulcular scrubbing with chlorohexidine, pre and post-operative antibiotics are also suggested.
In cell-mediated resorption, cells surrounding the grafted material resorb the material by phagocytosis and then intracellular degradation. Bone and graft materials resorb at different rates under normal pH conditions, based upon porosity, size, and crystallization. However, all graft materials rapidly resorb through solution-mediated resorption as compared to cell mediated-resorption in conditions of low pH. The hydroxylapatite crystal of bone (or enamel) is dissolved into calcium and phosphate components at a pH of 5.5 or less. For example, the enamel of a tooth is composed of 95% dense, crystalline hydroxylapatite. Lactobacillus acidophilus bacteria produce a pH of 5.5. When bacterial plaque and bacteria remain on enamel for more than five days, a solution-mediated resorption occurs, which is seen as a radiolucent zone on the radiograph. Infections within the bone often create a pH of less than two. As a result, when a tooth becomes non-vital, a solution-mediated resorption occurs at the apex and a radiolucency at the apex of the tooth with an endodontic lesion may occur within a few days. Thus, bone grafting in the presence of infection, or infected bone grafts after surgery, increases the risk of insufficient volumes of bone formation and may even cause recipient bone loss. As a result, methods should encourage cell-mediated resorption and eliminate solution-mediated resorption.
Before bone grafting, all evidence or potential causes of infection should be eliminated. A blood supply within the graft is required for the normal distribution of an antibiotic to the site. Because no blood supply is present early on in the graft material, when bacterial contamination is a greater risk (such as in sinus grafting), antibiotics may be added to the alloplastic material and autograft (Fig. 2). Although tetracycline is often used in periodontal bone grafting to improve collagen formation, it chelates calcium and arrests the bone formation process.4 Instead, parenteral penicillin, cephalosporin, or clindamycin may be mixed into the graft material, as these antibiotics do not affect the bone regeneration process. Tablets or capsules for oral administration of antibiotics are not used in the graft site, as these often contain fillers of no benefit to the graft site.
Additionally, contamination of the bone graft may occur from endogenous bacteria, lack of aseptic surgical technique, or failure of primary soft tissue closure. Graft materials that fall into the oral cavity may be contaminated by saliva and should be thoroughly irrigated before use or discarded. The lack of primary soft tissue closure or incision line opening places the graft at significant risk. Barrier membranes or fixation screws that become exposed often become contaminated by bacteria. The bacteria invade the graft site and cause local inflammation with resultant decrease in bone formation. In these instances, the use of a collagen plug is especially useful (Fig. 3).
Fig 2. An intravenous form of antibiotic may be added to freeze-dried bone when infection is more likely.
Fig 3. A piece of collagen is placed over the socket graft to promote soft tissue healing over the site, without primary closure.
3. Soft Tissue Coverage
Primary soft tissue closure is a mandatory condition for the success of grafting procedures in width and height (one or two wall defects). On the other hand, socket grafting does not require primary soft tissue closure for regeneration of bone in a socket at the time of extraction. In fact, in a clinical study comparing primary closure to secondary marginal healing over an extraction site and bone graft, there was no clinical difference in the volume of bone at re-entry.
Even when socket grafting is performed without primary closure after extraction of a tooth, the epithelium covers the healing socket before bone formation in the crestal region. In addition, collagen may be used over the tooth socket after grafting in order to encourage epithelium to form more rapidly over the socket (Figs. 4 & 5).
Fig 4. The collagen is sutured over the socket graft with x-sutures to keep it in position.
Fig 5. The suture removal two to three weeks later has epithelium over the graft site.
4. Space Maintenance
Space maintenance in the area of the bone graft site is paramount to the bone formation process. The space of the graft site refers to the anatomical size and contour of the desired augmentation. Maintenance refers to the fact a space must exist long enough for bone to fill the desired region. For example, collapse of the space under a barrier membrane may impair the desired size and contour of the graft.
The space for bone regeneration may also be provided by a graft material, as an autograft or alloplast “in excess”.6 The “barrier by bulk” concept by Misch applies to situations in which the graft site is over-contoured by several millimeters with a resorbable alloplast.3 As bone grows below the alloplast, the invading fibrous tissue only invades the superficial alloplast layer. When the soft tissue is reflected to insert the implants, the top layer of fibrous tissue and alloplast is removed, and the new regenerated bone underneath remains. This technique works best when larger graft volumes still allow primary soft tissue closure and in the absence of pressure on the soft tissue during regeneration.
Space maintenance is often provided by graft materials such as collagen, autogenous bone, demineralized freeze-dried allograft (DFDB), and calcium phosphate materials and are also necessary element for bone grafting procedures.
5. Graft Immobilization/Stability/Fixation
Changing the volume of bone represents bone modeling and requires a more rigid interface during the bone formation process. Micro-movement as low as 20 microns may be too much for bone modeling and may result in a non-fixated graft or fibrous encapsulation.
Graft stabilization is paramount to obtaining predictable bone augmentation. This ensures initial blood clot adhesion with its associated growth factors – cytokines such as interleukin-1, interleukin-8, tumor necrosis factor, and growth factors such as platelet-derived growth factor [PDGF], insulin-like growth factor, and fibroblast growth factor [FGF].
The granulation tissue that develops after blood clot stabilization is the initial mechanism for bone modeling and remodeling. If pieces of a particulate graft material or block bone grafts are mobile, they cannot develop a blood supply for new bone formation. Instead, the graft becomes encapsulated in fibrous tissue and often sequestrates. Likewise, when barrier membranes or fixation screws become loose or mobile, fibrous tissue will encapsulate them. Therefore, for particulate graft materials to work most effectively, no loads should be placed on the soft tissue over the graft, as these may cause movement of the graft.
The keys to bone grafting are necessary for more predictable bone grafting. Defect size is important in deciding an appropriate approach to socket grafting. An aseptic surgical site is crucial in creating optimal circumstances in healing and bone growth. Space maintenance without micro-movement is essential. Finally, graft immobility aids in optimal healing. However, there are more keys to socket grafting for more predictable results. OH
Carl E. Misch, Past Director, Oral Implant Dentistry Pittsburgh Dental School and Temple Dental School, Director Misch Implant Institute.
Vivian Roknian, Faculty, Misch Implant Institute, Diplomate, ICOI.
Oral Health welcomes this original article.
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7. Okubo Y, Bessho K, Fujimura K et al: Expression of bone morphogenetic protein in the course of osteoinduction by recombinant human bone morphogenetic protein-2, Clin Oral Implants Res 13:80-85, 2002. ?
8. Thompson DM, Rohrer MD, Prasad HS: Comparison of bone grafting materials in human extraction sockets: clinical, histologic, and histomorphometric evaluations. Implant Dent 15:89-96, 2006.
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