D.H. Williams; U. Masood; and M.N. Norton

Royal Cornwall Hospital, Truro, Cornwall TR1 3LJ, UK.

Decreased head-neck ratio diameter and component malposition in
total hip arthroplasty are factors known to result in impingement,
increased rates of dislocation, wear and failure. In addition to
these complications, impingement of the femoral neck on the
acetabular component of a hip resurfacing may result in femoral neck
fracture and loosening of the acetabular component. Little is known
regarding the optimum femoral and acetabular hip resurfacing
component position to avoid impingement.

In the first part of this study we analyzed the radiographic
component position of 131 consecutive hip resurfacings. In the second
part the effect of three component variables on the range of motion
to impingement were analyzed using a dry bone model:

1. Inclination of the acetabular cup

2. Version of the acetabular cup

3. Femoral head-neck diameter ratio

The mean femoral-stem shaft angle in the first part of the study
was 138° (range 121° to 158°). The mean acetabular inclination angle
was 45° (range 30° to 63°). This wide range in position mirrors that
described in the literature. The dry bone study revealed an optimum
acetabular cup inclination tending towards 50° and an anteversion of
25°. A large diameter femoral head relative to the femoral neck
resulted in a greater range of motion to impingement. A fine balance
however exists, to remove a minimum amount of pelvic bone to
accommodate a larger acetabular component with an ‘oversized’ femoral
component.

The acetabular resurfacing cup positions described allow the
greatest range of physiological hip movement. New technology and
improvements to existing equipment and techniques will hopefully lead
to more accurate placement of hip resurfacing components minimising
the risk of impingement and its complications in this high demand
group of patients

Correspondence should be addressed to SWOC, c/o Mr
David Bracey, Royal Cornwall Hospital, Truro, Cornwall, TR1 3LJ.