Introduction: A reduced range of movement and associated pain due to impingement following hip resurfacing is a side effect leading to significant morbidity in the affected patient. We have found it may possible to identify those patients most at risk of impingement from plain radiograph analysis.

Methods: Using Corin Cormet^TM resurfacing templates angles of impingement of different size femoral heads were measured and compared by using a theoretical head neck ratio of one. Following this a single femoral head size of 56mm was taken and the impingement angle was measured with altering neck sizes.

Femoral head: femoral neck ratios were then measured on a series of 43 plain AP pelvic radiographs using calibrated digital calipers. The range of values was compared to a normal distribution curve. Inter and intra observer variation was calculated.

Results: Varying the template femoral head size with the corresponding acetabular component will give the same impingement angle. With a single femoral head size and altering femoral neck sizes the angle of impingement in the AP view decreases with an increasing head neck ratio.

Analysis of 43 pelvic radiographs revealed a range of head neck ratios from 0.64 to 0.80 with a mean of 0.71 and standard deviation of 0.038. This data compares with a normal distribution.

Conclusion: The risk of impingement in hip resurfacing is not related to femoral head size alone, but also to the head neck ratio. Furthermore it is evident that range of movement is a function of the head neck ratio and not the femoral head size alone. Those ratios which fall within the upper standard deviation may require further consideration as to the suitability of a planned resurfacing procedure. This study also highlights the need for surgeons to become experienced in the technique of osteochondroplasty to minimise the head neck ratio.