TRADITION AND MODERNITY IN VETERINARY MEDICINE, 2017, vol. 2, No 1(2): 17–20 BONE REGENERATION IN CRITICAL-SIZE CALVARIAL DEFECT IN RATS USING INNOVATIVE NANO COMPOSITE MATERIAL OF CHITOSAN/NANO- HYDROXYAPATITE COMPOSITE Petar Dimitrov 1 , Elena Dyulgerova 2 , Radost Ilieva 3 , Radosveta Vasileva 2 , Veselin Nanev 1 , Ivelin Vladov 1 , Margarita Gabrashanska 1 , Marin Alexandrov 1 , Neli Tsocheva-Gaytandzhieva 1 1 Institute of Experimental Morphology, Pathology and Anthropology with Museum, Bulgarian Academy of Sciences, Sofia, Bulgaria 2 Medical University, Faculty of Dental Medicine, Sofia, Bulgaria 3 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria E-mails: drpdimitrov@abv.bg; tsocheva_n@abv.bg ABSTRACT Healing of standardized critical-size calvarial defects in rats was performed with nano composite material of nano-hydroxyapatite/3 % solution of chitosan in citric acid, and electrospinning fibres of poly lactic acid. Histologically at the 84 day, the implanted rats exhibited full closure of the defect by new developed connective and osteoid tissues penetrating in reticular pattern within the implant components. These results indicate that by using a newly created composite paste critical cranial defects can be successfully retrieved and probably is also suitable for other cases in bone augmentation surgeries. Key words: calvarial defect in rats, chitosan/nano-hydroxyapatite, bone regeneration. Introduction Reconstruction of bone defects caused by severe trauma, removal of tumors or congenital mal- formations still remains a challenge for surgical practice. Therefore, many scientists use different approaches to establish useful bone substitutes for tissue engineering of critical bone defects provid- ing regeneration of bone with mechanical properties similar to the original one. Recently, tissue engineering has become a promising approach in managing bone loss using tissue scaffolds. Tissue scaffolds are the most important issue in tissue engineering and could be divided into two main categories including biological (physical or organic) and synthetic (artificial) materials. Natural pol- ymers such as collagen type I or demineralized bone matrix belong to the first type, while porous metals, bioactive glasses, polylactic acid (PLA) and polyglycolic acid (PGA), and calcium phos- phate ceramics such as hydroxyapatite (HA) and tricalcium phosphates (TCP) are examples of syn- thetic materials [1, 5]. In recent years, particularly useful and reliable as bone substitutes were es- tablished some innovative nano-composite materials [6, 9]. Therefore, the aim of the present study was to perform a histological evaluation on the healing potential of a new developed artificial bone graft based on a chitosan/nano-hydroxyapatite composite complex in a standardized critical calvarial bone defect model in rat. Experimental model. Eight male rats (Wistar) with critical calvarial defects were divided into two groups with four animals each. Calvarial defects of the rats from group 1 remained untreated, while those in group 2