Introduction: Femoral offset plays an important part in the biomechanics of the hip with inaccurate balancing at the time of arthroplasty leading to abductor weakness, leg length discrepancies and altered wear characteristics.

Aim: To look at the degree of external rotation of the leg at the time of pre-operative x-rays and to assess the effect of this on templating for femoral offset.

Methods: A radiological review of a cohort of patients attending for hip arthroplasty with unilateral osteoarthritis.

Results: Externally rotated posturing of the affected leg is common amongst patients with osteoarthritis attending for hip arthroplasty. The effect of this change in the positioning of the femoral neck is to create an apparent reduction in the offset of the femoral shaft. This will lead to underestimation of the correct offset for the chosen prosthesis. In over 50% of cases templating would have been incorrect.

Conclusions: When templating prior to hip arthroplasty surgery an assessment of the profile of the lesser trochanter will give an impression of the leg position. If this is externally rotated it should be presumed that the offset will be larger than the measured value. If the other hip is in a better rotational position this should be used for assessment.