Review of Bone-Grafting Materials
Although alveolar bone can be a contraindication for dental implants, bone grafting can provide the structural or functional support necessary in such cases. Grafts can provide scaffolding (Fig 2-1) for bone regeneration1 and augmentation for bony defects resulting from trauma, pathology, or surgery. They can also be used to restore bone loss resulting from dental disease; to fill extraction sites; and to preserve the height and width of the alveolar ridge through augmentation and reconstruction. Autogenous bone remains the best grafting material because of its osteogenic properties, which allow bone to form more rapidly in conditions that require significant bone augmentation or repair. The allografts most commonly used for restoring osseous defects are mineralized or demineralized freeze-dried bone allografts (FDBA). The primary alloplasts are hydroxyapatite, bioactive glasses, tricalcium phosphate (TCP) particulates, and synthetic polymers. The primary xenograft material is purified anorganic bone, either alone or enhanced with tissue-engineered molecules. These augmentation materials can be incorporated in the modeling, remodeling, or healing processes of bone to assist or to stimulate bone growth in areas where resorption has occurred and implants are needed.
Mechanisms of Bone Regeneration and Augmentation
Three different processes are associated with successful bone grafting: osteogenesis, osteoinduction, and osteoconduction. 2, 3, 4, 5 Osteogenesis is the formation and development of bone. An osteogenic graft is derived from or composed of tissue involved in the natural growth or repair of bone. Osteogenic cells can encourage bone formation in soft tissues or activate more rapid bone growth in bone sites. Osteoinduction is the process of stimulating osteogenesis. Osteoinductive grafts can be used to enhance bone regeneration and may even cause bone to grow or extend into an area where it is not normally found. Osteoconduction provides a physical matrix or scaffolding suitable for the deposition of new bone. Osteoconductive grafts are conducive to bone growth and allow bone apposition from existing bone, but they do not produce bone formation themselves when placed within soft tissue. To encourage bone growth across its surface, an osteoconductive graft requires the presence of existing bone or differentiated mesenchymal cells. All bone-grafting materials possess at least one of these three modes of action.
Types of Graft Material
As noted above, the three primary types of bone graft material are autogenous bone; allografts; and alloplasts, of which commercially available xenografts are generally considered a subgroup. The mechanism by which these graft materials work normally depends on the origin and composition of the material.3, 6 Autogenous bone, an organic material harvested from the patient, forms new bone by osteogenesis, osteoinduction, and osteoconduction. Harvested from cadavers, allografts, which may be cortical or trabecular, have osteoconductive and possibly osteoinductive properties, but they are not osteogenic. Alloplasts, which may be composed of natural or synthetic material, are typically only osteoconductive.
In determining what type of graft material to use, the clinician must consider the characteristics of the bony defect to be restored. 3 In general, the larger the defect, the greater the amount of autogenous bone required. For small defects and for those with three to five bony walls still intact, alloplasts may be used alone or with allografts. For relatively large defects or those with only one to three bony walls intact, autogenous bone must be added to any other type of graft material being considered. Soft tissue ingrowth can be a complication during augmentation procedures with any grafting materials, so guided bone regeneration (GBR) using resorbable or nonresorbable membranes is often employed.7
Autogenous bone, long considered the gold standard of grafting materials, is currently the only osteogenic graft material available to clinical practitioners. Grafted autogenous bone heals into growing bone through all three modes of bone formation; these stages are not separate and distinct, but rather overlap each other.3 Common areas from which autogenous bone can be harvested include extraoral sites such as the iliac crest or tibial plateau and intraoral sites such as the mandibular symphysis, maxillary tuberosity, ramus, or exostoses.3, 8, 9 Less resorption has been associated with the use of mandibular bone grafts than with iliac crest grafts.8 Resorption may be reduced during healing by the use of expanded polytetrafluoroethylene (e-PTFE) membranes or slowly resorbable collagen membranes.10 Bone grafts obtained intraorally generally result in less morbidity; however, intraoral donor sites provide a significantly smaller volume of bone than do extraoral sites such as the iliac crest or tibial plateau.
The optimal donor site depends on the volume and type of regenerated bone needed for the specific case. The posterior iliac crest provides the greatest amount of bone—up to 140 mL (Table 2-1 and Fig 2-2). This compares to up to 70 mL from the anterior iliac crest, 20 to 40 mL from the tibial plateau (Fig 2-3), 5 to 10 mL from the ascending ramus, up to 5 mL from the anterior mandible (Fig 2-4), up to 2 mL from the tuberosity, and varying amounts from bone shavings (Fig 2-5) or exostoses or through the use of suction traps (Fig 2-6). Autogenous bone is highly osteogenic and best fulfills the dental grafting requirements of providing a scaffold for bone regeneration. 11 The disadvantages associated with the use of autogenous bone are the need for a second operative site, resultant patient morbidity, and in some cases the difficulty of obtaining a sufficient amount of graft material (especially from intraoral sites). These limitations led to the development of allografts and alloplasts as alternative grafting materials.2, 11 ,12
|Donor site||Form available||Maximum volume (mL)|
|Posterior iliac crest||Block and/or particulate||140|
|Anterior iliac crest||Block and/or particulate||70|
|Tibia||Particulate||20 to 40|
|Cranium||Dense cortical block||40|
|Ascending ramus||Block||5 to 10|
|Anterior mandible||Block and/or particulate||5|
|Miscellaneous (eg, bone shavings, suction traps)||Particulate||Varies|
Bone allografts are obtained from cadavers (Fig 2-7) or from patients’ living relatives or nonrelatives. Those obtained from cadavers are available through tissue banks that are accredited by the American Association of Tissue Banks, which process and store the allografts under complete sterility (Fig 2-8). The advantages of allografts include ready availability, elimination of the need for a patient donor site, reduced anesthesia and surgical time, decreased blood loss, and fewer complications.3 Disadvantages are primarily associated with the antigenicity of tissues harvested from another individual; transplanted bone may induce a host immune response. Cadaveric bone also may be rejected, as occurs with other transplanted tissues or organs.2, 3, 13
The most commonly used forms of allografts are frozen, freeze-dried (lyophilized), demineralized freeze-dried, and irradiated. Fresh allografts are the most antigenic; freezing or freeze-drying the bone significantly reduces the antigenicity.6 Because allografts are not osteogenic, bone formation takes longer and results in less volume than can be achieved with autogenous grafts.3 Concerns have been raised regarding the possible transmission of HIV through a bone allograft; however, when proper precautions and adequate laboratory studies are employed, the risk of using or receiving an allograft from an unrecognized early HIV–infected donor is approximately 1:1,600,000.14
FDBA can be used in either a mineralized or a demineralized (DFDBA) form. Demineralization removes the mineral phase of the graft material and purportedly exposes the underlying bone collagen and possibly some growth factors, particularly bone morphogenetic proteins (BMPs), which may increase its osteoinductive capabilities. 2, 15, 16 FDBA may form bone by osteoinduction and osteoconduction.3 Because it is mineralized, it hardens faster than DFDBA. Clinical experience has shown that grafting of the sinuses with DFDBA alone results in the presence of dense connective tissue after 6 months, whereas grafting with FDBA results in the presence of new bone formation.17 Bone is essential when treating defects in preparation for implant placement. The clinical and histologic findings of one study demonstrated that sites grafted with FDBA and complemented with an e-PTFE barrier can yield predictable results when augmenting alveolar ridges prior to the placement of implants.18
MTF (Dentsply Friadent CeraMed, Lakewood, CO) is an allogeneic freeze-dried bone that is available in both mineralized and demineralized forms. FDBA is more effective than DFDBA in the following situations:
- Repair and restoration of fenestrations
- Minor ridge augmentation
- Fresh extraction sites (used as a fill)
- Sinus lift cases (used as a graft)
- Repair of dehiscences and failing implants
This particulate material is available in a variety of sizes, which should be selected according to the intended application. Similar graft results have been shown from the use of particles ranging from 200 to 1,000 µm for various cases as needed. Indications for DFDBA are limited to periodontal defects.
Puros (Zimmer Dental, Carlsbad, CA) is an allogeneic graft material that has undergone a well-tested processing method to reduce antigenicity and to minimize any risk of viral cross-contamination from donor material.19 This type of allograft, which is solvent-preserved (as opposed to freeze-drying to extract the water component), has been shown to osseointegrate as effectively as cryopreserved material and to be equally biotolerable.20 Animal and human studies of this material have shown good bone formation and repair results.21, 22, 23, 24 Moreover, because the water component is removed by solvents rather than by freeze-drying, which can potentially alter the mineral as a result of the volume expansion that occurs during the transition from the liquid to the solid phase, the mineral matrix is purported to remain more intact.20 This material also has both the mineral and collagen phases of allogeneic tissues (Fig 2-9).
The use of DFDBA as a graft material has been questioned because of some reports that it is unpredictable in regenerating new bone. In one study in humans, for instance, the DFDBA particles were found to be surrounded by uninflamed connective tissue.25 A later study showed positive results with the use of DFDBA and a cell-occlusive membrane. Incorporation of the DFDBA particles was observed in new bone that contained lacunae with osteocytes. 25 The results of this study might have been improved by the use of FDBA instead of DFDBA. The benefits seen in this study could also have been from the barrier membrane rather than the graft material, as it was DFDBA and not FDBA.
It is generally believed that BMPs and other noncollagenous proteins in the exposed matrix are responsible for the osteoinductivity of DFDBA. This osteoinductivity, however, depends on the quality and quantity of the bone matrix in the graft material.26 Studies have shown that the osteoinductive activity of DFDBA may vary considerably among bone banks as well as among different samples from the same bone bank.27 There are no widely accepted tests or guarantees to ensure that DFDBA material meets any minimum standards for osteoinductive properties. As a result, this graft material has fallen out of favor with many surgeons. In vitro and in vivo assays have been used to a limited extent to assess the osteoconductivity of DFDBA.26
DFDBA has been combined with other materials that have the potential to enhance bone growth. For example, the use of tetracycline with a DFDBA allograft has been studied. No significant benefit was shown to be derived from reconstituting the DFDBA particles in tetracycline hydrochloride during grafting of osseous defects. 28 Osteogenin, a bone-inductive protein isolated from human long bones, has been combined with DFDBA and studied in the regeneration of intrabony periodontal defects. While regeneration of new attachment apparatus and component tissues was significantly enhanced with this combination, new bone regeneration was not.29
A study using athymic rats has shown that two commercially prepared and available DFDBA preparations with gel carriers—Osteofil (Regeneration Technologies, Alachula, FL) and Grafton (Osteotech, Eatontown, NJ)—yielded similar results in bone formation via osteoconductivity at 28 days after implantation. However, significantly more bone was produced by Grafton than by Osteofil, suggesting that graft processing methods could represent a greater source of variability than do differences among donors.30
Irradiated cancellous bone (Rocky Mountain Tissue Bank, Denver, CO) has also been used as a substitute graft material for autogenous bone.31, 32 This bone allograft is trabecular bone obtained from the spinal column and treated with between 2.5 and 3.8 Megarads of radiation. Some authors have reported that among all available allografts, irradiated bone is most similar to autogenous bone in terms of demonstrating rapid replacement and consistent establishment of a reasonable ratio of new bone with less expense and morbidity than that associated with autogenous material.31, 32 However, because of a lack of published scientific documentation, use of this material is not recommended.
Alloplasts, Xenografts, and Tissue-Engineered Materials
The most commonly used bone substitutes are ceramic materials, including deorganified bovine bone, synthetic calcium phosphate ceramics (eg, hydroxyapatite, TCP), and calcium carbonate (eg, coralline). The mechanism of action in these ceramics is strictly osteoconduction,3, 33 with new bone formation taking place along their surfaces.13, 34 These materials are used to reconstruct bony defects and to augment resorbed alveolar ridges by providing a scaffold for enhanced bone tissue repair and growth. They can also improve probing de/>