Does stem preheating have a beneficial effect on PMMA bulk porosity in cemented THA?

In cemented total hip arthroplasty (THA), porosity plays a major role in the fatigue failure of bone cement. Stem preheating procedure is known to reduce the stem/cement interfacial porosity. In the literature, no information is available about the effect of such procedure on cement bulk porosity. This study helps to find out if stem preheating can have a beneficial effect on bulk porosity, thus enhancing long-term bone cement integrity. A simplified experimental model of a stem/cement/bone construct of a cemented THA is designed to reproduce the mechanical boundary conditions of polymerizing cement. Effect of stem preheating and polymethylmethacrylate prechilling and mixing method (hand mixed and vacuum mixed) on cement porosity are investigated. Bulk porosity is analysed within three zones across the cement mantle in terms of pore number, pore area, and mean pore size. The results demonstrate that bulk cement porosity is strongly influenced by stem preheating, cement precooling as well as cement composition and mixing method. Stem preheating procedure displaces the porosity away from stem/cement interface toward bone; consequently reducing the pore area within the zone near the stem and increasing it in the middle and bone/cement zone. The most pronounced beneficial effect of stem preheating before implantation is visible for vacuum mixed procedure as the cement contains few pores of very small size (<100 μm). However, if stem is preheated, cement precooling should be avoided as it could counteract the beneficial effect of reduced porosity inside cement mantle.