Surface roughness of debonded straight-tapered stems in cemented THA reduces subsidence but not cement damage

Abstract

Although stress analyses have shown that the mechanical endurance of cemented femoral THA reconstructions is served by stems that firmly bond to their cement mantles, retrieval studies suggest that this may be difficult to achieve. Clinical studies with roentgen stereophotogrammetric analyses have shown that stems may gradually debond from their cement mantle. Accepting the fact that stem debonding is unavoidable, stem subsidence and cement stresses can be reduced by increasing stem–cement friction, as indicated by finite element stress analyses. Hence, it can be hypothesized that debonded stems with high surface roughness values would damage the cement mantle to a lesser extent as compared to polished ones. To confirm this hypothesis, tapered stems with polished and rough surface finishes were implanted in cement mantles and cyclically loaded for 1.7 million times. It was investigated how surface roughness affected the damage in the cement mantle, and how it was related to prosthetic subsidence. The polished taper subsided considerably more than the rough one (630 vs. 270 μm at the end of the experiments). In addition, it was found that the polished taper displayed step-wise subsidence, which is probably due to the interaction of stick–slip processes at the interface, associated with creep of the acrylic cement. The rough taper subsided monotonously. Scanning Electron Microscopic (SEM) analysis of the taper–cement structures showed that the rough taper was completely debonded from the cement mantle, creating a gap at the interface, and that many large cement cracks and particles were created. Around the polished taper, only a few cracks were found and the taper–cement interface seemed undamaged. It was concluded that an increased surface roughness does not necessarily lead to a reduction in cement damage. On the contrary, compared to polished ones, debonded rough stems may produce more cement cracks and acrylic cement debris, and provide routes to transport these wear products. Hence, after failure of the stem–cement interface, straight–tapered stems with an increased surface roughness accelerate the failure process due to inferior fail–safe features. Consequently, in vivo subsidence patterns at the stem–cement interface should be considered in combination with the surface finish of the implant. An amount of post-operative subsidence of a rough stem may be much more damaging for the reconstruction than the same amount for a polished stem.