Introduction: Replacement of extensive local bone loss especially in revision joint arthroplasties is a significant clinical challenge. Autogenous and allogenic cancellous bone grafts have been the gold standard in reconstructive orthopaedic surgery, but it is well known that there is morbidity associated with harvesting of autogenous bone graft and limitations in the quantity of bone available. Disadvantages of allograft include the risk of bacterial or viral contamination and non union as well as the potential risk of disease transmission. Alternative options are attractive and continue to be sought. Hydroxyapatite and collagen composites have the potential in mimicking and replacing skeletal bones. Aim: This study attempted to determine the effect of hydroxyapatite/collagen composites in the fixation of bone implants. The composites used in this study is produced by Institute of Science and Technology for Ceramic (ISTEC), Faenza Italy. Material and method: Titanium alloy implants were inserted into the femoral condyles of mature 8 sheep. The implants were inserted extra-articularly into the medial and lateral femoral condyle. Thus four implants were inserted in each sheep. The implant device has a length of 10 mm and a diameter of 6 mm, a footplate and a top washer of 10 mm diameter. The circumferential gap around the cylinder is of 2 mm. The circumferential gaps were filled with the one of the following bone substitute: ISTEC, ISTEC mixed with bone marrow aspirate (BMA), autograft or allograft (control group). The bone marrow was harvested by aspiration from the proximal part of the tibia. Autologous bone graft harvested from the four drill holes in the femoral condyles. A standardised surgical procedure was used. After surgery the sheep were observed at the outdoor facilities of the Biomedical Laboratories, University of Southern Denmark, Odense. The observation period was 5 weeks. The sheeps were euthanized and both of femurs were harvested and sectioned in two parts by using a water-cooled diamond band saw. Specimens for mechanical testing were always taken from the most superficial part of the implant. Specimens are preserving now at - 20°C and wait for the push-out test which is destructive and will be performed on an 858 Bionex MTS hydraulic material testing machine (MTS system cooperation, Minneapolis, Minnesota, USA). The specimens for histological analysis were taken from the more profound part of the implant. They were dehydrated in graded series of ethanol (from 70% to 99%) containing 4% basic fuchin followed by embedding in methylmetacrylate. Sections for histomorphometric analysis using stereological methods in order to get unbiased estimates will be done following.