The last article discussed the first five keys to bone grafting. The keys to bone grafting are local factors that affect the prognosis of the procedure and include: defect size, absence of infection, soft tissue closure, space maintenance, graft immobilization, regional acceleratory phenomenon, bone vascularity, growth factors, healing regeneration time, graft materials and a transitional prostheses. The management of defect size was covered as it relates to the number of walls present. The importance of surgical asepsis was discussed, as well as the importance of parenteral forms of antibiotics when infection is present. Soft tissue closure is vital in keeping graft material in place. This can also be accomplished with a collagen plug sutured in place. The significance of graft immobilization was also highlighted. Graft stabilization is paramount to obtain a predictable bone augmentation. This ensures initial blood clot adhesion with its associated growth factors. The purpose of this article is to discuss the remaining keys to bone grafting.
Regional Acceleratory Phenomenon
The RAP is the local response to a noxious stimulus and describes a process by which tissue forms faster than the normal regional regeneration process.3-8 By enhancing the various healing stages, this phenomenon makes the healing process occur two to 10 times faster than normal physiologic healing. The RAP begins within a few days of injury, typically peaks at one to two months, usually lasts four months in bone, and may take six to more than 24 months to subside.1-3 The duration and intensity of the RAP are directly proportional to the type and amount of stimulus and the site where it was produced. For bone injuries, the degree of remodeling activity varies depending on the extent of bone injury, the quantity of soft tissue involved in the injury, and the configuration of the bone fracture or trauma.
Growth factors (e.g. BMPs) are released by a RAP from the periodontal complex and walls of bone during the extraction process. As a result, bone grows in the site even without initial soft tissue closure over the graft site. It has been suggested that socket grafting should be delayed for as long as six weeks after extraction to decrease the risk of infection. However, this delayed method is not indicated since the RAP process induced by the extraction is lost if the graft is delayed.
Noxious stimuli of sufficient magnitude, such as fractures, mechanical abuses, and noninfectious inflammatory injuries (including dental implant procedures) can evoke RAP. The tooth extraction process creates a RAP in the extraction site. No further bone preparation is necessary. Bone grafting surgery and internal fixation procedures also produce RAP.3 When injury to the bone is due to a pathologic process (e.g., arthrofibrosis, neuropathic soft tissue problems, rheumatoid phenomena, secondary osteoporosis, excessive heat), the RAP is either delayed or not initiated, and a complete healing process may not occur. When the RAP is inadequate, the result is a slow callus formation that is replaced by lamellar bone. The increased rate of new formation of bone caused by the RAP does not result in a change in bone volume. In other words, the RAP in and of itself is usually restricted to bone remodeling.9 In addition, the RAP is more evident in cortical bone because the normal turnover of bone cells is 2% compared with 18% for trabecular bone. Biochemical agents, such as prostaglandin E1 and bisphosphonate, also appear to facilitate the RAP.10
The local RAP is usually accompanied by a systemic response, defined as the systemic acceleratory phenomenon (SAP), that demonstrates a metabolic response similar to the local response.11 Inadequate RAP is associated with several systemic medical conditions, including diabetes mellitus, peripheral neuropathies, regional sensory denervation, severe radiation damage, and severe malnutrition.
Host Bone Blood Vessels
Blood vessels must provide nourishment to an autologous bone graft to keep the transplanted cells alive. These vessels may arise from two primary sources. The host cortical bone contains very few arterioles, whereas cancellous bone and the PDL has an intensely vascular network.
The host bone blood vessels that grow into a bone graft are of primary importance for predictable bone augmentation.12 These arteries can grow rather rapidly, compared with other tissues. Fibrous tissue may grow 1 mm each day, whereas woven bone grows at a rate of 60 microns each day. It would appear then that fibrous tissue would always win the race to fill a bony void. Yet, bone forms in an extraction socket when surrounded by walls of bone. One primary reason is that the blood vessels from the surrounding walls grow rapidly into the void and determine what type of tissue will form in the extraction site.
The blood vessels in an extraction site arise from the PDL; with the extraction, these vessels are broken and create a RAP. Blood vessels from bone that enter the graft site provide pluripotential perivascular cells that have the capability to become osteoblasts. Monocytes in the blood form osteoclasts, which precede the blood vessel into the bone graft site by forming cutting cones, which resorb devital bone and graft material. As the osteoclasts resorb the graft material, the blood vessel can grow into the site. As importantly, the sides of the blood vessel that comes from bone carry osteoblasts, but only when the vessel comes from the host bone. Not only is the blood vessel needed to help the autograft maintain vitality, it is also needed to repopulate the area with osteoblasts to grow new bone. It has been postulated that the surface fibroblasts, if left to migrate within the graft, not only invade the space, but also may inhibit osteogenesis by contact inhibition.3
A tooth extraction socket fills with bone because the blood vessels from bone form granulation tissue in the site and prevent the epithelial cells from migrating into the site. Four to six months are then needed for the socket to replace the area filled with the blood clot (initially) and granulation tissue (later) with bone.13
When an alloplast is placed into an extraction site and covered with soft tissue (without a barrier membrane), the bone below the bone graft grows four times faster than the fibrous tissue grows down into the graft. This is because the blood vessels from bone grow rapidly into the region, and once they invade the site, bone follows.
An alloplast placed upon host cortical bone with no preparation or RAP forms fibrous tissue around the graft, as angiogenesis will primarily be from soft tissue so no osseous blood vessels are able to grow into the graft material. The key to whether bone or fibrous tissue forms in the bone graft site is the source of the blood vessels.
Growth Factors
Bone growth factors can enhance formation and mineralization of bone, induce undifferentiated mesenchymal cells to differentiate into bone cells, and trigger a cascade of intracellular reactions and the release of a number of additional bone growth factors and cell-enhancing factors. These growth factors bind to specific receptors on the surface of target cells. More than 50 known growth factors have been identified and categorized, with some specific to the functions in bone healing. These constitute a separate group of proteins because of the way they can be produced and their mode of action. They are, however, part of the large superfamily of TGF-b.14 Bone growth factors are primarily present in bone matrix and released during remodeling or after trauma. They act on the local osteoprogenitor differentiated cells and therefore have limited areas of action.15 In contrast, BMPs, although also found in extracellular bone matrix, are osteoinductive and can trigger the differentiation of mesenchymal cells into osteoblasts.16
Transforming Growth Factor Beta
TGF-b is a super family of growth factors with more than 47 known varieties. TFG-b includes cytokines that contribute to connective tissue repair and bone regeneration. Bone is the body’s most abundant storage site for TGF-b, which acts as a weak mitogen for human osteoblastic cells.17 TGF-b also induces chemotaxis and stimulates extracellular matrix formation in osteoblastic cells and may inhibit osteoclast formation.17 TGF-b1 activates fibroblasts to form procollagen, which deposits type 1 collagen within the wound18,19 and is therefore credited with the enhancement of soft and hard tissue repair. PDGF and TGF-b1 assist in soft tissue healing and bone mineralization; therefore they can be mixed with grafting materials into the bone graft site and applied to the top layer of the graft.
Bone Morphogenetic Proteins
Bone morphogenetic proteins are distinct from other growth factors in that they can be found in extracellular bone matrix and can induce mesenchymal cell differentiation into chondroblasts or osteoblasts.16 However, they do not have mitogenic properties.20 BMP represents a collective term that now includes more than 15 proteins (BMP-1 through BMP-15), many of which have been purified and cloned. For example, recombinant (concentrated) human BMP-2 (rhBMP-2) and others have been produced and have been shown to induce a complete sequence of endochondral ossification in decreased time, even for large defects.21
Recombinant BMP-7 (OP-1) has been developed in the orthopedic field for very large bone defects. Most of the literature has concentrated on long bones of endo-chondral origins and dental applications appear remote.
In conclusion, there are four methods to increase bone and tissue growth factors during bone augmentation:
1. The PRP collected from the patient’s blood may benefit the bone formation process and/or may be laid on top of the graft and barrier membrane to promote soft tissue healing.
2. The use of autologous bone in the graft site can increase PDGF, FGF, TGF-b, IGF, and BMPs, which are stored in the bone and released during the augmentation process. The BMP in an autograft primarily has an effect to provide growth factors at two weeks and with a peak at six weeks.
3. A third method to introduce growth factors at a bone graft site is to use an allograft in the graft site.19 The FDB from cortical bone contains a higher percent of BMP than trabecular bone, and therefore is the material of choice. The amount of BMP in commercial bone bank allografts is very small (0.001 mg) and is not a very significant factor.
4. A fourth method to increase the growth factors in the graft site is by the RAP process, which triggers a release of growth factors into the site.
All four of these methods are typically used in the bone augmentation process, especially when other key elements are scarce.
Healing Time
The larger the bone defect in width and height, the longer the period of bone maturation before implant insertion. Adequate time must be provided for the graft to resorb and regenerate new bone volume. The amount of time required is variable and depends on local factors such as the number of remaining walls of bone, the amount of autogenous bone in the graft, and the size of the defect. Larger grafts, less autogenous bone in the graft, and fewer bony walls surrounding the site increase the amount of healing time. In addition, systemic diseases such as diabetes, hyperparathyroidism, thyrotoxicosis, osteomalacia, osteoporosis, and Paget’s disease may all affect the healing response. It is usually best to err on the side of safety. As a general rule, four to six months are recommended when graft volumes are less than 5 mm in dimension. Graft volumes more than 5 mm in dimension often require six to 10 months. Time frames may be shortened when additional keys to bone grafting are incorporated. As a general rule, the surgeon should provide enough keys to bone grafting to permit reentry in less than eight months. If more than half the graft is from autogenous origin, a prolonged healing period of more than one-year is often not beneficial and may result in bone resorption of the newly grafted site. On the other hand, some dense bovine calcium phosphate materials may require more than two years to be resorbed and replaced by bone.
Bone Graft Materials
The larger the bone defect, the more an autologous component is needed in the graft. This relates to the creeping resorption and vascularization requirement for a larger graft site. In addition to the keys needed to develop a predictable bone augmentation site, there are materials necessary to augment the location. Bone graft materials and their mechanism of action are not all the same. The materials most often used in implant dentistry to aid in bone augmentation include: (1) collagen, (2) human freeze-dried allograft (FDB), (3) xenograft bone, and (4) autogenous bone. Demineralized bone is not suggested since it resorbs too rapidly and doesn’t maintain the space.
Calcium Phosphate Minerals and Osteoconduction
It is postulated that the inorganic matrix of HA, which forms a scaffold in the autogenous graft, contributes to the osteoconductive effect of bone formation as new bone forms by creeping substitution. This may be considered a third phase of bone formation by autogenous bone.22
Osteoconduction, which characterizes bone growth by resorption or apposition from the surrounding bone, has been called creeping substitution. Therefore this process must occur in the presence of bone or differentiated mesenchymal cells. Osteoconductive materials do not grow bone when placed into subcutaneous tissues, muscles, or fibrous tissue. Instead, the material remains relatively unchanged or resorbs. Osteoconductive materials are biocompatible, and bone or soft tissue can grow adjacent to them by apposition without evidence of a toxic reaction. The most common osteoconductive bone grafting materials used in implant dentistry are allografts, alloplasts, and xenografts.
Osteoconductive materials for hard tissue maintenance or augmentation may be characterized as nonresorbable or resorbable, dense or porous, or crystalline or amorphous materials. Dense HA has become a popular bone substitute when living bone is not a requirement for the augmentation. This material has been described as nonresorbable when it is a highly dense crystalline structure.23,24 In the presence of bone, a direct bone-HA interface may be observed.23 This finding is more common when dense HA is placed within the bone and fibrous tissue is not in direct contact during healing. Fibrous tissue proliferates at a rate of 1 mm daily, compared with bone that forms much slower (approximately 50 µm per day). Thus the soft tissue has the capacity to reach and encapsulate the HA when it is placed on top of cortical bone. The contacting layer of HA may develop a bony interface, but the majority of material is surrounded by fibrous tissue.25 The purpose of this type of augmentation is to serve as a denture support region or tissue augmentation to improve soft tissue contours around implants or teeth. When the material is placed into a bone preparation, tooth socket, or other cavity, such as the maxillary sinus, or covered with a barrier membrane (which prevents fibrous tissue from reaching the HA), the tissue developing at the interface is more likely to be bone. However, the material does not resorb to allow creeping substitution and new bone to replace the HA.
The porosity of the material has a primary effect on the resorption time. Dense HA particles exhibit little to no porosity. A macroporous HA exhibits a larger, usually visible porous architecture with 15% holes or more by volume.26 This type of topography may be produced by a hydrothermal exchange reaction with CaCO3, found in the natural particles of the coral reef. A microporous HA (usually obtained from the inorganic portion of bone of xenografts or freeze-dried cadaver bone) exhibits in excess of 30% holes by volume with cortical bone and up to 70% pores when trabecular bone is used, thus leaving a large intraparticle volume of grafted area available for the regeneration of bone. Microporous HA will resorb by cellular activity faster than macroporous HA, and dense HA is the slowest to resorb, if all other factors are similar.
Does the product need to resorb and serve as a scaffold for new formation, or does it need to simply preserve the anatomy? If the product needs to resorb for bone growth, what time frame is appropriate for the type of procedure?
Short healing periods need a more readily resorbable product, which should be more porous to facilitate cell-mediated resorption. However, if too easily removed by the recipient site, it may not maintain sufficient space to allow adequate bone formation. In contrast, a more crystalline, less porous product will maintain the space longer but will also take longer to disappear and form bone. Therefore, the choice of product should not necessarily always be from one family or another but is based on the application and the macromolecular and biochemical profile of the product.
Collagen
Several types of collagen are found in the human body. Type I collagen is among the first products synthesized by the body when bone formation occurs.27 The irregular pattern of initial bone formation (woven bone) is a result of the rapid, unorganized response of the body to lay down collagen, which is then invaded and mineralized with HA along the direction of the collagen fibers by osteoblasts. The haphazard organization of collagen results in unorganized bone formation, called woven bone, which forms first and at a faster rate and is less strong. The most common source of collagen in implant dentistry is bovine collagen from the “Achilles” tendons in the leg. Another source of type I collagen is DFDB, which can provide the space necessary for blood vessel ingrowth into the graft and may contribute to the bone formation process. However, DFDB alone has not proven an effective graft material and should be combined with other categories of bone-grafting materials.
Collagen also is an integral part of the soft tissues with chemotactic and hemostatic properties. It can bond and activate platelets to form a platelet plug within the vessel. It may also act as a scaffold for migrating cells of the epithelium. In the application of bone regeneration, collagen may be used at the level of the soft tissue to accelerate healing over an extraction site or to promote coagulation of a bleeding surgical site.
The extraction site is first evaluated, including the soft tissue drape. When the interdental papilla are ideal, they are not reflected during the extraction or socket graft procedure. Papilla saving incisions are made when the facial plate is partially or completely missing. Sulcular incision made to cut the connective tissue fibers into the tissues above the crest of bone are made prior to a traumatic extraction. After the extraction, the labial plate of bone is evaluated. The interproximal bone height on the adjacent tooth roots is also evaluated. When depressed and the tooth root is not covered with bone, the papilla is often depressed. The cementum above the bone is often contaminated. Therefore, root planning this surface is indicated. When the facial plate is present in the socket, the graft site is more predictable. The socket is filled with freeze-dried bone or HA bovine bone sources. Collagen is then placed over the graft site.
Transitional Prostheses
The major concerns of the transitional restoration is to protect soft tissue healing, prevent movement of the soft tissue or graft in the missing tooth site and to improve esthetics and function. The use of soft tissue-borne provisional prostheses is discouraged for all augmentation procedures because the graft size may be modified and the graft may become mobile. When possible, soft tissue-bone removable restorations should have rest seats and clasps to prevent loading the soft tissue.
A removable restoration may be fabricated over the bone graft but should be designed to not load the soft tissue over the graft site. When teeth are present, a cast metal framework with direct and indirect retainers and rest seats may be fabricated. Before the removable partial denture framework design, a stone cast of the patient’s mouth is augmented with clay or wax in the laboratory in the sites of the future bone graft. In addition, the framework does not have metal mesh over the bone graft site. Otherwise, the surgeon most often must remove the chrome cobalt mesh, as the laboratory often does not provide enough relief between the augmented site and the metal framework.
Summary
The keys of bone grafting are the necessary ingredients to predictable bone augmentation. Because bone modeling is more difficult than remodeling, more keys are required. Bone graft materials help provide several of these keys but are not the entire picture. Instead, the materials and keys are blended for an optimum result. A layered approach to bone augmentation has been developed by Misch. The first layer is the host bone, which has an absence of infection and a surgical RAP before the placement of the bone graft. The defect size and topography is a factor for subsequent consideration. The autograft and RAP encourage host bone blood vessels to grow into the graft site. Many host growth factors are presented to the graft site as a consequence. The next layer is a mixture of DFDB (30%) and mineralized bone (70%), mixed with PRP. This provides induction, growth factors, and longer space maintenance. A collagen barrier membrane or plug is the next layer. Wound closure to immobilize the graft material is the next consideration.
A transitional prosthesis, off the soft tissue, is the next step in the process. The last consideration is the undisturbed healing time. When in doubt, “wait longer” is a most important consideration for a successful graft. As a consequence, as many keys and bone graft materials are incorporated into the process as possible, all of which increase the predictable nature of the bone augmentation process.
The clinician can successfully graft dental extraction sites to improve the aesthetics and function of the final restoration. When an extraction site receives a graft, ridge preservation is enhanced, pontic form can improve, dental implants can be placed in the correct position, and the prosthetic outcome will be enhanced. The technique is especially important in areas of the mouth where bone and soft tissue shrinkage will not allow adequate implant placement or would necessitate placing an unsightly pontic if an implant is not placed. OH
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Carl E. Misch, Past Director, Oral Implant Dentistry Pittsburgh Dental School, Director Misch Institute.
Vivian Roknian, Faculty, Misch Implant Institute, Diplomate, ICOI.
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