Telemetry of forces from proximal femoral replacements and relevance to fixation

Two proximal femoral replacements were instrumented to enable axial forces to be determined at two sites within the prosthesis: in the main shaft and near the tip of the intramedullary stem. The goal was to measure the changes in force distribution over time, as indicated by the ratio of the two forces. Inductive coupling between a coil worn around the leg and a small implanted coil was used, both to supply power to electronic circuits sealed into a welded cavity in the prosthesis and to telemeter data from the prosthesis. Data from both subjects were recorded over the first two years following surgery. For the first subject, there was an increase in mean shaft force excursions (peak force minus resting force) during level walking from 0.53 x BW after 1 week 2.77 x BW after 23 months. The corresponding mean tip force excursions were 0.13 x BW and 1.74 x BW, respectively. The ratio of mean tip force excursions to shaft force excursions steadily increased over the same period from 25 to 63%. Similar increases over time in the tip/shaft ratio were found during treadmill walking, stair climbing and stair descending. Data from the second subject were obtained for the shaft forces only, and were consistent with those from the first subject. The progressive transfer of axial load from the proximal to the distal part of the IM stem recorded telemetrically, together with radiographic observations, suggested that bone remodelling had taken place together with a less stable interface around the proximal part of the stem. This process evidently began soon after implantation.