IS REMOVAL AND REINSERTION OF CEMENTED FEMORAL STEMS DURING REVISION HIP ARTHROPLASTY A MECHANICALLY SAFE PROCEDURE?

Abstract

This experimental study aimed to determine the pattern of load transmission to the cement mantle and to the outer surface of six composite femurs implanted with three different designs of polished, collarless, tapered stems (2 specimens for each type), before and after the removal and reinsertion of the same stem. Strain distribution was measured with uni-axial and tri-axial strain-gauges before, after implantation and after reinsertion of the stems. Additionally, axial and rotational stability of the stem relative to the cement mantle and to the composite were determined by means of one extensometer and two linear variable displacement transducers (LVDT’s). All specimens were loaded simulating single leg stance of 3,25 body weight for a 708N subject. Static load were applied to the intact femurs, after implantation, after cyclic loads of 1Hz up to 3x10 5 cycles followed by 7Hz up to 1million cycles, and after reinsertion. Variation of strain and micro-motion during static loading following reinsertion were determined by the average of 10 cycles, with each cycle being represented by the difference between maximum and minimum values.

Linear regression analysis of the strain values obtained by the sensors in the cement mantle after reinsertion plotted with regard to the strains obtained initially by the same sensors before and after cyclic loading showed coefficients R2= 0.95; 0.91,with slopes of 1.12 and 1.03 respectively. The values of static strain of all sensors plotted with regard to values of initial static strain showed a very strong positive correlation (R2 = 0.98; slope = 0.96).

These findings support the concept that reinsertion of same design and size polished, collarless tapered stems may not alter the pattern of load transmission and stability at the interfaces between stem/cement, and cement/bone to the outer surface of bone. The mechanical conditions at the interfaces are restored with no need for additional cement during reinsertion if the cement envelope is preserved. The same mechanical principle that maintains the stability of the stem during subsidence for ”force-closed” fixation, may keep the stability of the interface following reinsertion. For these reasons this procedure may not be applicable to designs with texturing or pre-coating, and cylindrical-collared designs because in such conditions (“shaped-closed “fixation) the mechanics of stem/cement interface may not be restored.