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COMPARATIVE EVALUATION OF DIFFERENT BONE GRAFT MATERIALS: A REVIEW

Success of implant supported prosthesis depends on many factors. The presence of adequate bone at surgical site being one of the important factor. Periodontal conditions, trauma may lead to inadequacy of bone, however with recent developments implant bed can be successfully grafted with variety of materials to increase the bone volume. Bone graft materials are derived from different sources. Choice of the graft material should be based on the technique, amount of graft required, and healing pattern around the bone. This review provides understanding about different graft materials with comparative evaluation of outcome when used alone or in combination with other graft materials.


INTRODUCTION
With recent developments in field of implantology and long term survival rates of implant supported restoration, replacement of missing teeth with implant supported restoration has become a popular choice among clinicians and patients. However, for the implant to be functional for a longer duration good bone support is required as the masticatory load on the implant is transferred through the bone which lays foundation for the implant. Extraction of tooth is accompanied by significant amount of dimensional changes occurring in socket. 1,2 Loss of bone structure either through bone resorption followed by tooth extraction or due to trauma may preclude placement of implant owing to presence of insufficient bone which may alter the esthetic outcome, phonetics or prognosis of implant restoration. Paucity of bone at surgical site can be reduced by grafting it with bone graft materials or replacing the lost bone with block bone graft. Classification of grafts: 3 1. Autograft (autogenous graft): Tissue transferred from one location to another within the same individual. 2. Allograft: A graft between genetically dissimilar members of the same species i.e. human tissue. 3. Xenograft: A graft taken from a donor of another species i.e. bovine, porcine, equine bone graft material etc 4. Alloplast: Inorganic, synthetic or inert foreign material implanted into tissue. Eg; Hydroxyapatite, βtricalcium phosphate, Bioglass.
Bone healing and new bone formation after grafting occur through osteogenesis, osteoinduction and osteoconduction. Osteogenic graft materials supply actual viable osteoblasts themselves, osteoinductive materials stimulate primitive mesenchymal cells to differentiate into osteoblasts whereas, osteoconductive materials merely act as a lattice or framework for cell growth.

Rationale
A sufficient volume of bone of adequate biological quality is required for placement of dental implant. In aesthetic regions like the maxillary anterior region, soft tissue requires a solid bony basis as 'soft tissue follows hard tissue'. 4 Healing in extraction socket results in reduction of about 50% of the initial ridge width over a 12-month period, with two third of reduction in width taking place during the first 3 months of healing. Loss of width in bone is more in the mandibular posterior region.1 Vertical bone loss of 3-4 mm, or bone volume loss of around 50% has been reported in six months post extraction. 5 Horotwiz et al in their study concluded that less resorption of alveolar ridge occurred when socket preservation was attempted with use of bone graft material as compared to no use of graft material in fresh extraction sockets. 6 In case of infection or trauma additional augmentation may be required to provide bony basis for placement of implant which will be functionally and aesthetically acceptable.

Indications 4
Bone substitutes are indicated in the following conditions: 1. Augmentation of vertical and horizontal dimension in the maxilla and mandible. 2. In treatment of peri-implantitis to refill the periimplant bone defect. 3. For preservation of alveolar socket following tooth extraction.
For any bone graft to be successful 4 conditions must exist: 7

Autografts
Autografts are the bone graft materials harvested from intraoral or extraoral site of the same individual. Autogenous bone has osteogenic, osteoinductive and osteoconductive properties, and is considered to be ideal bone graft material. 8 However morbidity, unavailability and unpredictable graft resorption are the associated drawbacks. 5. There is high variability in quality of harvested bone tissue.
The common sites for harvesting of an autograft in the mandible are chin, angle, lineaobliqua, and corpus, in maxilla tuber, spinanasalis, and crista zygomaticoalveolaris. Calvaria, Pelvic rim, Tibia are the extra oral sites. Autogenous bone graft harvested from mandible has certain advantages as they maintain dense quality during healing period and exhibit minimal resorption. 10 Autograft can be a cortical bone, cancellous bone or combination of both. Cancellous bone graft survive better as compare to cortical bone due to diffusion of nutrients and better revascularization. 8

Allografts
Allografts are the graft materials which are harvested from members of same species which are genetically non-identical. Their incorporation in existing bone is slower as they lack living cells. Allografts have the advantage of no donor site morbidity but are associated with certain disadvantages like disease transmission, slow and less graft complete incorporation, limited graft availability and cost. 11 Allografts are used either fresh or processed before being used. Allografts show some incidence of immunogenic reaction or cross infection. 12 They have been used independently or in combination with autogenous bone or xenografts.
Allografts are generally used generally used in two formsfreeze dried bone allograft (FDBA) and demineralized freeze dried bone allograft (DFDBA).
Mineralized freeze-dried bone allografts FDBA is not demineralized and considered to be osteoconductive. 13 Demineralized freeze-dried bone allografts They are frequently used for maxillofacial and periodontal grafting. They are osteoinductive owing to presence of bone morphogenetic proteins (BMPs). 14

Xenografts
Xenograft is the graft obtained from different species mainly bovine bone and porcine bone. Proteins from the xenografts are extracted to avoid any immunologic reaction, however this procedure makes them osteoconductive. Recently enzymetreated equine bone graft material have also been used in which bone collagen is preserved in its native state. 15,16

DISCUSSION
Bone grafts serve as source of osteogenic cells and provide mechanical support too. 13 The potential for regeneration of functional attachment apparatus has been demonstrated in autogenous and allografts.
Autogenous bone has been considered for the augmentation of bone in case of atrophic maxilla and mandible. Autogenous iliac graft when used for ridge augmentation of atrophic maxilla showed good survival result with clinical and radiographic evidence showing low rate of resorption after grafting and implant placement. 17 Similar results were found when lateral or vertical augmentation was done in mandible and maxilla before implant placement with onlayautografts harvested from mandibular ramus or symphysis region. Healing period was followed by resorption at graft site which was more in mandible than maxilla. 18  Hallman et al used autogenous bone, bovine bone and the combination in ratio of 20:80 autogenous bone and bovine bone for maxillary sinus floor augmentation and found similar results. They concluded autogenous bone can be replaced with bovine bone or different combination of autogenous and bovine bone can be used. 35 Another study also revealed satisfactory result with combination of autogenous and bovine bone and concluded bovine bone can alone be used for bone augmentation. 36 Deproteinized bovine bone was used successfully used for GBR procedures in dehiscence defect with respect to vertical and horizontal growth of bone. 37 Lekovic V et al found combinations of platelet rich plasma (PRP) bovine porous bone mineral (BPBM) and guided tissue regeneration GTR. are effective in the treatment of intrabony defects present in patients with advanced chronic periodontitis but, GTR adds no clinical benefit to platelet rich plasma and bovine porous bone mineral. 38 Most alloplastic materials consist of hydroxyapatite, β-tricalcium phosphate, biphasic calcium phosphate, or some type of nonsintered calcium phosphate (reduced calcium content). 8 Synthetic bone graft materials has shown potential for new bone formation. 39 Hydroxyapatite is a biocompatible slowly resorbing osteoconductive material. 40,41 Osteoblast differentiation and new bone formation at the surface of porous HA ceramic granules has also been demonstrated. 42 Pure dense hydroxyapatite is also well accepted by hard and soft tissue in intrabony defects. 43 Pure β-tricalcium phosphate was used for socket preservation after tooth extraction followed by implant placement. After six months surgical site was found to contain dense bone supporting stable implant. 44 Shalash MA et al conducted ridge augmentation following GBR principle using β-tricalcium phosphate and its combination with demineralized bone matrix and found that combination of both is more effective in cases of minimal alveolar ridge defects. 45 Combination of βtricalcium phosphate and hydroxyapatite in ratio of 60:40 showed better result in bone augmentation compared to allograft (Bio-oss). 46 Combination of bioactive glass and autografts have also shown satisfactory result. 47

CONCLUSION
Regenerative or bone filling capacity of different graft materials are comparable. However to increase the potential for regeneration combination of different materials can also be done. Selection of graft material is based on operator preference, type and size of the defect, resorbability of graft material, cost and finally patient acceptance.