Introduction: Conservative hip replacements are advantageous because resection of bone in the proximal femur is minimised. This study investigated a new design of conservative hip in the goat model where the femoral head was resected and two hydroxyapatite coated ‘pegs’ were introduced into the femoral neck. The hypothesis was that the ‘pegs’ would provide a direct method of transmitting forces within the femoral neck thus resulting in less adverse bone remodelling and reduced loosening. Bone stock is also preserved should subsequent revision be required.

Methods: Eight unilateral implants were inserted into the right femur of adult female goats for 1 year. Retrieved specimens were analysed radiographically and histologically. Image analysis was used to quantify bone attachment and total bone area adjacent to the implant. Tetracycline bone markers quantified bone turnover. Operated hips were compared with non-operated hips. The students t-test was used for comparative statistical analysis where p< 0.05 were classified as significant.

Results: Radiographic analysis demonstrated bone loss beneath the cup with increased bone density at the distal end of the pins (fig.1). Light microscopy revealed areas of new and mature bone adjacent to the implant. Osseointegration to the HA coating was observed. Bone markers established significantly decreased bone formation rates (p< 0.05) in bone adjacent to the implant in the operated versus control hips.

Image analysis results demonstrated an average bone attachment of 30.94% to the implant surface (fig 2). Greatest bone attachment occurred at the end of the pins (78.99%) contributing 22% of overall attachment to the implant. Least attachment occurred beneath the prosthetic cup (13.82%) and in the medial aspect adjacent to the central pin. Greater total bone area was measured in control hips and no significant correlation between bone attachment to the ‘pegs’ and bone area beneath the prosthetic cup was identified.

Discussion: From this study we have concluded that despite the resorption of bone beneath the prosthetic cup, the conservatve hip design investigated remained well fixed in the femur during the 1 year in vivo period. It appears that an implant design that resurfaces the femoral head with two pins used to transmit forces into the femoral neck is a useful approach in conservative hip design.