Length of clinically proven cemented hip stems: state of the art or subject to improvement?

PURPOSE: The purpose of this study was to identify the shortest possible length of an established cemented hip stem in order to reduce stress shielding and optimise its bending behaviour. METHODS: Twenty-five prototypes from the same batch (five for each stem length) were included. Lengths resulted from the original length (100 %) and four distal shortenings to 94, 88, 83 and 78 % of original length. For standardised implantations, synthetic femurs were prepared. Relative movements under axial torque and stem bending under varus-valgus torque applications were investigated consecutively. Analyses of variance (ANOVA) were applied to detect differences between lengths. RESULTS: The maximum torque transfer occurred at the level of the lesser trochanter (p < 0.01). In particular, for the 78 % version, relative motion of the stem tip increased significantly compared with all other lengths (p = 0.02-0.04). Comparable findings were made for mean overall movements. In regards to varus-valgus torque, both the distal tip and the proximal shoulder always bent in the same direction following the femoral deflection. For the 83 % version bending of stem's shoulder was increased compared with the other length (p = 0.01 - 0.02), whereas tip bending was always comparable (p = 0.45-0.91). CONCLUSIONS: Our data show that the intention to preclinically optimise the mechanical behaviour of cemented stems by modifying their length seems to be a reasonable option. Advantages are decreased inner bone stiffening to reduce stress shielding and a lower cement volume, which is involved in cellular interactions.