Influence of femoral rotation on femoral offset determined from radiographs

To study how the size of femoral offset, judged from radiographs, depends on the femur's degree of internal-external rotation during radiography, this report studied 108 hips (55 right, 53 left; 60 male, 48 female). For each hip, the femoral offset was determined from radiographs taken with the femur in 15 degrees of internal rotation (standardised femoral offset), and also from radiographs taken when no special attempt was made to have the femur in this position (control femoral offset - corrected for magnification). Control femoral offset (corrected) was less than the standardised femoral offset in 102 of 108 hips. On average, for the 108 hips as a group, control femoral offset (corrected for magnification) was 7.7 (CI 8.7, 6.6) mm smaller than standardised offset, a significant difference (p < 0.0001, t-test, two-tailed). For individual hips, the difference was as great as 25 mm. Depending on the femoral rotation during radiography, calculating femoral offset from x-rays may underestimate the femoral offsets sufficiently to compromise hip joint replacement in clinical orthopaedic practice