Introduction A consensus exists regarding the optimal range of femoral cement mantle thickness in hip replacement. However, within this range surgical preferences differ, surgeons in Europe generally preferring thinner cement mantles whilst those in the US prefer a thicker mantle. For a given implant size, the rasps provided in the US for use with the Stanmore Hip are larger than those used in Europe, producing a thicker cement mantle. The integrity of the femoral cement is considered to be crucial to the long-term survival of cemented hip replacements. Previous studies have used cement cracking under fatigue loading as a comparative measure of implant survival. Damage accumulation levels between different implants are associated with clinical failure rates. The aim of this study was to compare the cracking behaviour of cement mantles of different thicknesses around Stanmore Hip replacements. We hypothesised that a thicker cement mantle would lead to reduced cement cracking.

Methods Ten synthetic femurs (Sawbones) were prepared following standard surgical practice for the Stan-more Hip. Five of these were rasped using the larger US rasp, and five using the European version. Stanmore Hip femoral components were then cemented into the femurs with Palacos-R cement and using a custom insertion rig to ensure good alignment and centralisation, confirmed by radiographs. The femurs were then cyclically loaded with an aggressive 4 kN stair-climbing load for 4 million cycles at 3 Hz. The femurs were sectioned at 5 mm intervals and dye penetrant used to highlight cement cracks. Image analysis software was used to measure cement thickness and crack lengths under light microscopy.

Results The minimum cement mantle thickness per section was found to average 0.8 mm and 2.0 mm for the thin and thick mantle groups respectively, measured around the proximal half of the implant. This was significantly different (p< 0.05). Cracks in the cement mantle were irregularly distributed along the length of the prostheses. We found no significant difference in either the total number or total length of cracks found in each group. These were investigated over the whole mantle and by Gruen Zone.

Discussion The geometric and mechanical properties of human femurs vary considerably, which might be expected to increase dramatically the scatter in any clinical trend relating cement thickness to cracking. Our study, using identical synthetic femurs and well-centralised prostheses to minimise experimental variability, found no difference in cracking. Given this experimental consistency, it is thought that there would be no clinically significant difference in cracking rates between different cement thicknesses within the normal range for the Stanmore Hip replacement. The Stanmore Hip is designed to minimise cement stress. A collar prevents subsidence-related hoop stresses, and smooth corners minimise stress concentration in the cement. It is likely that, for a sub-optimal implant design with higher stress risers, cement thickness might have a more noticeable effect on crack propagation.