MANTLE CONFORMITY EFFECTS PRESSURE GENERATION AT THE STEM/CEMENT INTERFACE IN HIP ARTHROPLASTY

Abstract

Hypothesis Stem surface finish & cement mantle conformity influences pressure at the stem/cement interface, under physiological load.

Method We developed a scaled mechanical analogue of a cemented Exeter femoral stem with a temperature and pressure controlled fluid environment. The stem was subjected to physiological torsional & axial loads using a material testing machine with two perpendicularly mounted actuators. Rough (Ra=2.2μm), matt (Ra=1.16μm) & polished (Ra=0.02μm) stems were tested in both conforming & artificially created, asymmetrically worn, cement mantles. Pressure was recorded at five sites along the interface.

Results Pressure was generated in both conforming and worn mantles. Peak pressures recorded in worn mantles were nearly four times greater than in conforming; peak stem tip pressures, worn: 12000Pa, versus conforming: 4680Pa. The axial load was the main determinant of pressure generation in the conforming mantle. Torsional loads generated a rise in interface pressure in both mantle types but the resultant stem toggle seen in the worn mantle had a significant positive effect on pressure. Pressure fluctuations generated in the conforming mantle had the greatest range at the tip. Peak pressures within the worn mantle were more uniform, but marginally greater on the posterior wall. Surface finish influenced pressure; surface roughness had a positive association with pressure within conforming mantles & the reverse effect in worn mantles.

Conclusion Asymmetrical wear leads to increased pressure generation at the stem/cement interface under physiological loads, with the torsional load playing a key part in pressure generation. Well fixed, debonded stems also generate limited pressure fluctuations at their mantle interface. This is principally due to axial load. Mantle shape dictates the influence of surface finish on pressure; surface roughness increases pressure within conforming mantles, but reduces pressure when the mantle is worn. This may be a confounding effect of worn mantle shape, restricting non-polished stem movement.