Introduction The effect of hip rotation on the measurement of femoral offset is determined firstly using artificial bones in an anatomical study and then in a patient population. Its effect on the choice of femoral component in total hip arthroplasty is discussed

Methods X-rays were taken of a series of saw bone models rotated through a range of angles. The resultant offset was then measured

Standardised and Control (unstandardised) x-rays of the pelvis were taken of patients presenting to orthopaedic outpatients. Femoral offset was measured from each x-ray

Results In the anatomical study angles of rotation differed significantly with respect to measurement of offset (p< 0.0001 Friedman 2-way analysis of variance by ranks). The greatest measurement of offset was at 15 degrees internal rotation. Offset decreased with external rotation.

The clinical study had power of 80%. Femoral offset was increased in all the standardised x-rays compared with their controls (n=64, mean=8.68, SD=5.56, 95% CI (7.34,10.01) A one-sample t-test was performed to see if the standardised and control films were greater than 5mm different (t=12.94 (63df), p< 0.01).

Conclusions The clinical study confirmed the findings of the anatomical study. A standardised AP x-ray of the pelvis improves the measurement of femoral offset.

For surgeons using the Exeter hip system failure to account for offset could lead to the selection of a stem two sizes too small with regards to offset. Lesser degrees of rotation, not readily identified by looking at the x-ray, could still lead to the selection of an incorrectly sized stem.

Offset has been shown to increase the range of movement, abductor strength and stability of the hip joint whilst decreasing the rate of wear. It therefore benefits patients to account for offset, ensuring a correctly sized hip replacement.