In vitro testing of a novel limb salvage prosthesis for the distal femur

OBJECTIVE: The aim of this paper was to define strain pattern in the host bone following distal femoral resection and implantation of a massive prosthesis. Two methods of coupling the prosthesis to the bone were compared: the Compliant Pre-Stress device, and a standard cemented tumour prosthesis. DESIGN: The composite femur model was selected to minimize variables. Four femurs were tested before and after implantation. Both coaxial and cantilever loading were applied. BACKGROUND: Cemented distal femoral replacement following resection of malignant tumours has a high failure rate at 5 years and is associated with extensive bone resorption thought to be secondary to stress shielding. METHODS: Strain was measured in the medial and lateral sides at four levels with physiologic loads applied, in the intact, Compliant Pre-Stress, and cemented femurs. Repeated measurements were taken. Strains in the implanted femur were calculated as percentage of the intact, and statistically analyzed. RESULTS: The most reproducible results were noted in cantilever bending (variability <5%). The Compliant Pre-Stress device demonstrated a more physiologic strain pattern than the cemented stem. The most significant difference between the two implants was in the area adjacent to the interface. CONCLUSIONS: The Compliant Pre-Stress device shows less stress shielding than a standard cemented implant. The protocol described and the use of composite femurs demonstrated reproducible results. RELEVANCE: Massive prosthesis are commonly used following tumour resection or removal of failed primary joint replacement prostheses. The failure rate for aseptic loosening for cemented implants is 25% at 5 years with significant bone resorption about the implant. Compliant Pre-Stress is an innovative technology that allows coupling of metallic implants to bone with little stress shielding. This paper aims to define the strain patterns about the implant and compare them to a standard cemented device. The reduced stress shielding of the Compliant Pre-Stress fixation system should guarantee reduced bone loss around the implant and help to obtain improved clinical results.