TELEMETRY OF FORCES FROM PROXIMAL FEMORAL REPLACEMENTS AND RELEVANCE TO FIXATION

Two proximal femoral replacements were instrumented to enable axial fo rces 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 mea sure 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 re corded over the first two years following surgery. For the first subje ct, there was an increase in mean shaft force excursions (peak force m inus resting force) during level walking from 0.53 x BW after 1 week t o 2.77 x BW after 23 months. The corresponding mean tip force excursio ns were 0.13 x BW and 1.74 x BW, respectively. The ratio of mean tip f orce excursions to shaft force excursions steadily increased over the same period from 25 to 63%. Similar increases over time in the tip/sha ft 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. Th e progressive transfer of axial load from the proximal to the distal p art of the IM stem recorded telemetrically, together with radiographic observations, suggested that bone remodelling had taken place togethe r with a less stable interface around the proximal part of the stem. T his process evidently began soon after implantation. (C) 1997 Elsevier Science Ltd.