Abstract
Introduction
In cemented total hip arthroplasty (THA), proper cement mantle thickness in the femoral canal is still controversial subject. It is widely accepted that the cement mantle around a femoral stem should be at least 2 mm in thickness. But articles from France reported good long-term result with thin cement mantle. It is so called “The French paradox”. We have already reported that the greater compressive force at the cement-bone interface was seen in collarless polished tapered (CPT, Zimmer, USA) stem with thick cement mantle than that with thin cement mantle. However, the stem with thick cement mantle subsided more than with thin mantle. It may have a possibility to cause an early mechanical failure of cemented THA. We compared to stem and cement subsidence in various cement mantles using tantalum ball into cement in this study.
Methods
A cemented stem model was used for this study with a CPT stem into composite femur.
Three sizes of CPT stems (No. 1, No. 2 and No. 3) and one size composite femur were prepared for this study. We inserted two stems for each size, for a total of six stems. Composite femurs were reamed with a No. 3 rasp, and various size of stem was fixed with cement in each composite femur to make a various thicknesses of cement mantle. Two to three tantalum marker balls were injected into the cement in each femur before cement was hardened. 1-Hz dynamic load applied to the stems for half a-million cycles. Each 16 hours of loading was followed by 8 hours without loading. We used micro-CT before and after loading to measure the movement of the tantalum balls in three dimensions. And we analyzed occupation ratio of stem in the femoral canal by computed reconstructed three dimensional model of bone cement and stem.
Results
We were able to detect a total of 13 balls in the cement for the 6 stems. The range of cement thickness in the CT slice on balls was 1.52 to 5.32 mm. Stem subsidence showed a significantly positive correlation to the thickness of cement mantle. In the horizontal plane, 8 of these 13 balls moved in an outside direction and five moved in an inside direction. The horizontal/perpendicular ratio for the tantalum balls showed a significant negative correlation to the thickness of the cement mantle. The subsidence rate (ratio of stem subsidence to ball subsistence) showed a significant negative correlation to the stem occupation ratio. The effective radial cement creep was also detected when the cement mantle was less than 2.5 mm thick and more than 60% in the occupation ratio.
Conclusions
Stem subsistence was more commonly associated with cement subsidence in the thicker cement mantles and with a low stem occupation rate in CPT stems.
