Indian Journal of Experimental Biology Vol. 46, December 2008, pp. 836-841 Incorporation and biodegradation of hydroxyapatite–tricalcium phosphate implanted in large metaphyseal defects–An animal study P Sunil*, S C Goel & A Rastogi Department of Orthopaedics and N C Aryya Department of Pathology, Institute of Medical Sciences, Banaras Hindu University, Varanasi 221 005, India Received 25 April 2008; revised 10 October 2008 Incorporation and biodegradation of hydroxyapatite(HA)−tricalcium phosphate(TCP) granules implanted in 5 × 5 × 5 mm distal femoral metaphyseal defects created in 18 adult rabbits were studied. In two rabbits, the defects were left to heal spontaneously without any implant. Roentgenographic and histological study by light microscopy was done on silver nitrate stained undecalcified sections as well as haematoxylin–eosin stained decalcified sections. The synthetic HA-TCP was biocompatible and produced no adverse reactions. The implant was osteoconductive and allowed good new bone formation to occur, mainly from periphery to center, but mature trabeculae could be delineated only at 4-6 months. The HA-TCP biomaterial had very low biodegradability with marked amount of intact implant still present at final follow up. Bonding between implant and bone, though a close biological bond, was not uniformly strong. Rate of bone ingrowth was very slow and large areas of implant at center did not show new bone formation at 12 months. Keywords: Bioactive ceramics, Bone substitutes, Hydroxyapatite, Metaphyseal defects, Tricalcium phosphate Calcium hydroxyapatite and tricalcium phosphate are bioactive ceramics and belong to the calcium phosphate family 1-4 . They have been widely studied as bone graft substitutes 2,3,5,6 . Biological, biomechanical and histological studies have been done regarding their implantation into bone and revealed their excellent osteoconductive properties and biocompatibility 7-16 . Among the many areas of use of hydroxyapatite (HA) and tricalcium phosphate (TCP) as bone graft substitute, one of the important is their use to fill cavities after excision of benign bone tumors 17-19 . But, bone ingrowth in osteoconducive materials is limited largely to periphery of implants which does not make it suitable for repair of large defects. Incorporation at the centre of implant and strength of larger bone defects filled with implant have not been properly assessed. Most workers have studied the ceramic implant by inserting it in small defects which may heal even without any supplementation.The present study has been undertaken to evaluate the incorporation and biodegradation of HA-TCP composite in large metaphyseal defects of bone. This was done through radiological and histological analysis of experimentally created bone defects in adult white rabbits. Materials and Methods The study was carried out on a total of 20 adult Newzealand white rabbits, Oryctolagus cuniculus (Linnaeus, 1758) of which 18 (16 test and two controls) were available for follow up. All animals were maintained as per the norms laid down by the Committee for Control and Supervision of Experiments on Animals, at the Central Animal House, Institute of Medical Sciences, Banaras Hindu University (Regd. No. 542/02/ab/CPCSEA). The exeperimental protocol was approved by the Institutional Animal Ethical Committee. All rabbits were aged more than 16 weeks and weighed more than 2 kg. Synthetic calcium phosphate biomaterial in _____________________ *Present address Consultant Orthopaedic Surgeon Flat No.201; Plot no. G-47, Pride Dream House Apartments Panchvati Township (Near OU Colony & Lanco Hills) Manikonda ; Hyderabad 500 008, India Telephone: +91-9949128282 Fax : +91-40-23608050 E-mail : sunilortho@yahoo.com