We studied the effect of the surface finish of the stem on the transfer of load in the proximal femur in a sheep model of cemented hip arthroplasty. Strain-gauge analysis and corresponding finite-element (FE) analysis were performed to assess the effect of friction and creep at the cement-stem interface. No difference was seen between the matt and polished stems. FE analysis showed that the effects of cement creep and friction at the stem-cement interface on femoral strain were small compared with the effect of inserting a cemented stem

Interfacial defects between the cement mantle and a hip implant may arise from constrained shrinkage of the cement or from air introduced during insertion of the stem. Shrinkage-induced interfacial porosity consists of small pores randomly located around the stem, whereas introduced interfacial gaps are large, individual and less uniformly distributed areas of stem-cement separation. Using a validated CT-based technique, we investigated the extent, morphology and distribution of interfacial gaps for two types of stem, the Charnley-Kerboul and the Lubinus SPII, and for two techniques of implantation, line-to-line and undersized.

The interfacial gaps were variable and involved a mean of 6.43% (sd 8.99) of the surface of the stem. Neither the type of implant nor the technique of implantation had a significant effect on the regions of the gaps, which occurred more often over the flat areas of the implant than along the corners of the stems, and were more common proximally than distally for Charnley-Kerboul stems cemented line-to-line. Interfacial defects could have a major effect on the stability and survival of the implant.