Do unloading periods affect migration characteristics of cemented femoral components? An in vitro evaluation with the Exeter stem.

Prosthetic migration has been identified as a marker for future revision of cemented total hip reconstructions. This could be tested at a pre-clinical stage with dynamic loading experiments. The purpose of this study was to assess the effects of resting periods, which are a considerable part of the daily activity cycle, on the migration characteristics of femoral cemented stems. Ten polished Exeter stems were implanted in composite femurs and loaded either with a continuous load or a discontinuous load. Continuous loading involved 345,600 loading cycles at 1 Hz, whereas the discontinuous loading involved loading at 1 Hz for 2.5 hours and a resting period of 21.5 hours for a period of four days. Hence, a total of 36,000 loading cycles were applied to these reconstructions. The subsidence patterns of the prostheses were considerably affected by the resting periods. The prostheses exhibited a step-wise migration pattern with migration steps of about 50 microns after every resting period, whereas the continuously loaded prostheses showed a more gradual migration pattern. The final subsidence of the specimens when loaded with resting periods was significantly less than the subsidence measured without resting periods. However, these specimens were loaded with fewer loading cycles. If this was taken into account, the discontinuously loaded specimens had a four fold higher average migration per loading cycle than the specimens that were loaded continuously. In conclusion, the resting periods had a considerable effect on the migration patterns, which should be realized when these stems are analyzed with pre-clinical tests.