EARLY STABILITY AND CEMENT DAMAGE FOR CEMENTED STEMS WITH TRIPLE AND DOUBLE TAPER

Abstract

Highly polished femoral stems with a double taper have had outstanding long-term clinical results. Recently a stem with a third, cross-sectional taper was introduced with the goal of providing additional stability while still utilising the polished taper concept.

The goal of the present study was to determine if there were differences in the mechanical stability and cement damage due to cyclic loading of a triple-tapered (C-stem, J& J-DePuy) and a double-tapered design (TPS, J& J-DePuy). Six pairs of cadaveric femurs were cemented with either C-stem or TPS stems using contemporary techniques. Specimens were cyclically loaded using a stair-climbing apparatus with femoral head and abductor loads for 1 000 to 266 000 loading cycles. Motion between the stem and bone was measured using a 6 dof measurement system. Following testing, specimens were sectioned at four transverse levels and the number and length of cracks in the cement were measured.

All stems were extremely well fixed after loading. The C-stem did not subside during loading except for one outlier that was cemented ‘high’. The TPS stem had a pattern of rapid subsidence over the first 100 cycles (mean 0.032 mm) followed by a more gradual subsidence (0.05 mm at 266 k cycles). ANCOVA showed that the TPS-stems rotated significantly more than the C-stems (p< 0.0001), that the rotation of both stems increased with number of loading cycles (p=0.022) and that the effect of number of loading cycles was greater for the TPS stems (p=0.047). Total crack length was not a function of number of loading cycles, nor was it different for the two stem designs (p=0.33).

The outlier C-stem had micromotion behavior similar to the TPS stem. The reason for this is unclear, but could be due to reduced lateral-proximal cement. Thus it is possible that both the stem cross-sectional and in-plane shape contribute to the stability of the C-stem design.