The position of the centre of the femoral head relative to the midline of the pelvis: a consistent landmark in total knee replacement surgery.

BACKGROUND: Femoro-tibial malalignment in excess of 3° is a recognised contributor of early mechanical failure in total knee replacement (TKR). Knowledge of the location of the centre of the femoral head is a pre-requisite to identification of the mechanical axis of the femur and can facilitate optimal component orientation. We investigated variation in the location of the centre of the femoral head relative to the midline of the pelvis.
METHODS: We analysed the pelvic radiographs of 150 patients with unilateral total hip replacements. The perpendicular distance from the centre of the femoral head of the non-operated hip to the centre of pubic symphysis was measured.
RESULTS: The mean distance from the centre of the femoral head to the pubic symphysis was 89.2mm (standard deviation, 5.7 mm). Patient height strongly correlated with this distance (r=0.53, p<0.01), as did the diameter of the femoral head (r=0.59, p<0.01). The latter was significantly larger in men than in women (50.9 mm vs. 44.5mm, p<0.01).
CONCLUSION: The results demonstrate that the position of the centre of the femoral head has very little variability, irrespective of patient age or body-mass index. If the gender-specific mean femoral head to midline distance is used to estimate the location of the femoral head centre, a line from this point to the centre of the femoral condyles will deviate from the true mechanical axis by no more than 1.5°, in 98% of cases.