Alloplastic Bone Grafting Materials

Abstract

Bone grafting has become a valuable mainstream clinical procedure in today's dentistry in a variety of reconstructive applications. Various materials and formulations have been developed for this purpose. The synthetic bone graft substitutes as yet offer only a part solution to the management of localized bone loss. Ideally a synthetic bone graft substitute should mimic the native bone in both mechanical and osteogenic properties. They possess some of the desired mechanical qualities of bone as well as osteointegrative/conductive properties. It has given the clinicians the ability to offer patients alternative treatment modalities that can serve to restore the missing bone anatomy. Present review deals with the various alloplastic bone graft materials. Various advantages and disadvantages with these materials are discussed.Keywords: Reconstruction, Bone grafts, Osteointegration , Osteoconduction, Osteogenesis Introduction Bone grafting has become a valuable mainstream clinical procedure in today's dentistry in a variety of reconstructive applications. Ridge preservation after tooth extraction is very important for making any kind of prosthesis, be it dentures, fixed bridge or implants.1 Various materials and formulations have been developed for this purpose. It has given the clinicians the ability to offer patients alternative treatment modalities that can serve to restore the missing bone anatomy for ridge maintenance or preservation, ridge augmentation and for enhanced bone foundation for endosseous implant placement and to fill in any bony defects or voids such as caused by removal of a cyst or tumor. 2 There are four characteristics that an ideal bone graft material should exhibit which include:6 * Osteointegration, the ability to chemically bond to the surface of bone without an intervening layer of fibrous tissue. * Osteoconduction, the ability to support the growth of bone over its surface. * Osteoinduction, the ability to induce differentiation of pluripotential stem cells from surrounding tissue to an osteoblastic phenotype. * Osteogenesis, the formation of new bone by osteoblastic cells present within the graft material. Synthetic bone graft substitutes currently possess only osteointegrative and osteoconductive properties. The synthetic bone graft substitutes as yet offer only a part solution to the management of localized bone loss. They possess some of the desired mechanical qualities of bone as well as osteointegrative / conductive properties but are largely reliant on viable periosteum/bone for their success. Ideally a synthetic bone graft substitute should mimic the native bone in both mechanical and osteogenic properties Classification of Bone Graft Materals3 (i) Autogenous bone grafts They are osteogenic, osteoconductive and osteo inductive. There is no risk of host rejection or disease transmission. But its major disadvantages are procurement morbidity, limited availability and high cost.4 (ii) Allograft It is nonvital , osseous tissue taken from one individual and transferred to another of the same species. They are osteoconductive and weakly osteoinductive. Its advantages include greater availability of banked bone than autograft and no additional surgical procedure needed. Its disadvantages include risk of disease transfer, not osteogenic, immunogenic, variable clinical results and expensive.5 (iii) Xenogenic bone grafts It is an osseous tissue that is harvested from one species, processed and then transferred to a recipient site of a different species. If properly prepared it is well tolerated by the tissues. It involves the risk of autoimmune disease. For oral applications, it generally comes in a granular or powder form, making it somewhat difficult to handle. It may also require some form of retentive structure, such as a membrane to hold the material in the desired location, because of an increased potential for migration of the material. (iv) Synthetic/Alloplastic bone grafts They are osteoconductive only. They eliminate the risk of disease transfer and procurement morbidity and it is abundantly available. (v) Composite grafts 7 It offers the advantages of autografts and allograft to the synthetic materials, a composite graft may be considered. Such a graft can combine the synthetic scaffold with biologic elements to stimulate cell infiltration and new bone formation. Synthetic Bone Graft Materals Cermics8 Its major advantages include1. Potentially limitless availability, 2. Do not require a second operative site, 3. No risk of disease transmission or immunogenic response, 4. The osteoconductive scaffold provides an appropriate environment in which bone cells and bone morphogenic proteins can adhere and proliferate. Although initially it lacks compressive and tensile strength, subsequently, it attains the mechanical strength similar to cancellous bone. 9 Its disadvantages include 1. The main disadvantage of using pure ceramic as a bone graft material is the minimal immediate structural support, 2. Absence of osteoinductive or osteogenic properties. Slowly resorbing ceramics (Calcitite)Hydroxyapatite exhibits brittleness and resorbs slowly; implants of this material can become a source of mechanical stress. It is often modified and combined with other materials for improved functionality and faster resorption. According to a study by Taylor et al, synthetic HA materials allowed osteoclast attachment but exhibited limited surface etching, which is consistent with limited osteoclast resorptive activity.2 Coralline Hydroxyapatite (e.g. Prosteon) It is a coral based bone graft substitute. It is derived from the hydrothermal chemical exchange of the calcium carbonate exoskeleton of a South Pacific coral to a crystalline HA replica. It has a consistent pore size and improved interconnectivity. Mechanically coralline HA is only slightly greater in compressive strength than cancellous bone. Like other HA preparations it is weak in tension, brittle and difficult to shape. Its main advantage is that its interporous structure. Allows complete ingrowth of fibro-osseous tissue 50-80% of the void is filled within 3 months. 6 Rapidly absorbing ceramics (Tricalcium phosphate) 10 It contains 39% calcium and 20% phosphorous by weight, which is similar to natural bone. The materials calcium phosphate rich surface layers seem to enhance bonding with adjacent host bone. This stimulates osteoclast resorption and osteoblastic new bone formation within the resorbed implant. Injectable ceramic cements Injectable calcium phosphates (e.g. Norian SRS) represent a class of ceramics that combines some of the qualities of cement with those of bone void filler. Norian cement contains alpha tricalcium phosphate mixed with calcium carbonate and monocalcium phosphate monohydrate. The initial compressive strength of the hardened material is similar to that of cancellous bone. The calcium phosphate implant undergoes long term remodeling, and is completely replaced by host tissue. A potential drawback of the liquid injectable cement is accidental extraosseous extrusion. Difficulties in controlling final placement may result in soft-tissue or intra-articular deposits.4 Ultraporous beta-tricalcium phosphate (e.g. Orthograft) A highly porous bone void filler that is composed of 90% interconnected void space with a broad range of pore sizes that is similar to the natural trabecular pattern of cancellous bone. Small pores allows the wicking of phagocytic cells for resorption and bone forming cells, nutrients and growth factors for bone recovery through capillary refill. The larger size pores encourage vascularization and bone ingrowth. The material can be manipulated during placement, sculpted as blocks, or packed. After it is packed to conform to the shape of the bone defect, its high porosity remains intact. The resorption rate of beta-tricalcium phosphate scaffold is intended to match the course of natural bone healing after implantation. 5 Calcium Sulphate It is actually Plaster of Paris. It is a safe biocompatible, osteoconductive bone graft substitute that acts well as space filler preventing the ingrowth of soft tissue allowing osseous ingrowth in bone defects. For this to occur it is very essential that the calcium sulphate be placed in close proximity to the viable periosteum or endosteum. Its rate of resorption corresponds to the rate of new bone formation in graft site. Calcium sulphate in its set form has a compressive strength greater than cancellous bone and a tensile strength slightly less than cancellous bone. Calcium sulphate however requires a dry environment to set and if it is re-exposed to moisture it tends to soften and fragment. For this reason it has no reliable mechanical properties in vivo. The primary use of calcium sulphate is as a bone void filler.6 Bioglass Bioglass are particulate materials, slowly resorbing and when mixed with fluids form an adherent surface layer of silicon, calcium, fluoride and sodium, which binds the graft to the bone. They are not osteoinductive but conduct bone mineralization by promoting absorption and concentration of proteins used by osteoblasts to form the extracellular matrix of bone. In a study done by Oonish et al13 they compared bioactive glass to hydroxyapatite and found that bioactive glass was easy to manipulate and haemostatic and allowed full restoration of bone in 2 weeks compared to 12 weeks needed for HA to produce a comparable response. The Bioactive material is used up in the process and any problems associated with the production of a composite of bone and biomaterials are avoided in the fully restored bone. Biocompatible oteoconductive polymers (e.g. HTR) It is a nonresorbable, particulate of calcium layered with polymethylmethacrylate and hydroxyethylmethacrylate.11 The HTR is reported to act as its own barrier and prevent gingival soft tissue migration ingrowth. Histologically it is osteoconductive and biocompatible and can be used both as bone substitute and as a barrier for guided tissue regeneration in implant therapy.12 No complications caused by infection, inflammation or rejection of the implanted graft material were observed in a study done by Harris AG in 224 patients over a period of 5 years.13 References * Predictable synthetic bone grafting procedures for implant reconstruction. GanzSD, Valen M. J Oral Implantol 2002; 28(4):178-83 * In vitro osteoclast resorption of bone substitute biomaterials used for implant site augmentation: a pilot study by Taylor JC, Cuff SE, Leger JP, Morra A, Anderson GI. Int J Oral Maxillofac Implants 2002 May-June 17(3):321-30 * Vaccaro RA: The role of osteoconductive scaffold in synthetic bone graft, Orthopedics 2002 May; 25(5 Suppl):571-80 * Betz RR: Limitations of autograft and allograft: new synthetic solutions, Orthopedics 2002 May;25(5 Suppl):561-70 * Bucholz RW: Non allograft osteoconductive bone graft substitute, Clin Orthop 2002 Feb(395):44-52 * Moore WR, Grave SE, Bain GI: Synthetic bone graft substitutes, ANZ Surg 2001 Jun;17(6):354-61 * Al Ruhaimi KA: Bone graft substitutes: a comparative qualitative histologic review of current osteoconductive grafting materials, Int J Oral Maxillofac Implants 2001 Jan-Feb;16(1):105-14 * Rosen PS, Reynolds MA, Bowers GA: Treatment of intrabony pockets with bone grafts, Periodontol 2000, 2000 Feb;22:88-103 * Gazdag AR, Lane JM, Glasser D, Forester RA: Alternatives to autogenous bone graft efficacy and indication. J Am Acad Orthop Surg 1995;3:1-8 * Bauer TW, Muschler GF: Bone graft materials: An overview of the basic science, Clin Orthop 2000 Feb(371):10-27 * Passi P, Girardello G, PiatelliA, Scarano A: Synthetic bone graft in peri implant bone dehiscences: histological results in humans, Gen Dent 1999 May-Jun;47(3):290-5 * Haris AG, Szabo G, Ashman A, Divinyi T, Suba Z, Martonffy K: Five year 224 patient prospective histological study of clinical applications using a synthetic bone alloplast, Implant Dent 1998;7(4):287-99 * Oonish H, KushitaniS, Yasukawa E, Iwaki H, Hench LL, Wilson J, Tsuji E, Sugihara T: Particulate bioglass compared with hydroxyapatite as a bone graft substitute, Clin Orthop 1997 Jan;(334):316-25. * Dental Implants The Art and Science; Charles A Babbush, 2nd Edition.2010; 79-80. Go Back