Varus Malalignment in Hip Replacement Impacts Femoral Offset and Canal-Fill Ratio

A master thesis from Linz examined the effect of an intentional 8° varus malalignment of a femoral stem in primary hip endoprosthetic surgery. Using radiographs from a biomechanical study conducted at the AO Research Institute in Davos, ten paired donor femora were implanted with either a neutral or a varus‑aligned stem. Measurements of the centre of rotation (COR), femoral offset (FO) and canal‑fill ratios (CFR I‑III) were taken with Deep Unity and MediCAD and analysed with Mann‑Whitney‑U tests.

The study found no difference in COR between the groups. The varus group showed a statistically significant increase in femoral offset (p = 0.0079) and a marked reduction in canal‑fill ratio II, while CFR I and III were unchanged. These results suggest that even small angular deviations of the stem can alter key morphological parameters, potentially affecting primary stability, long‑term osseointegration and joint mechanics. The findings underline the importance of precise stem alignment in hip arthroplasty.