The reproduction of ideal offset is an aim of hip replacement. Determining this measurement from traditional radiology techniques is inaccurate because femoral neck anteversion will foreshorten the femoral neck offset in a standard two dimensional x-ray making the measurement “apparent”. A novel method of determining offset is presented. A computer software program has been developed for pre-operative planning of joint replacements, (Orthopaedic Work Station) The program relies on using a CT scout film for magnification correction and to determine measurement parameters including leg length difference. It was recognised that by collecting extra cross-sectional references that three-dimensional measurement of offset would be possible. The CT scanner has software that allows determination of:
The location of the centre of the femoral head The centroid of the femoral shaft at a point just below the lesser trochanter The centroid of the femoral shaft at a point 150mm below the lesser trochanter For this study the line joining the two centroids is considered the longitudinal axis of the femur. The CT scanner has software that also allows for the centroids to be moved along the longitudinal axis into the plane represented by a perpendicular line from the longitudinal axis to the centre of the femoral head. It is a simple matter to measure the distance between the centroid and the centre of the femoral head to obtain a true offset. A phantom femur was measured using the radiology method described and then measured directly. Exact correlation was established. A study of inter-observer measurement has shown statistically consistent agreement using six observers in twenty cadaver femurs. The method is accurate and uses existing data collected as part of the pre-operative planning process. CT scanning prior to hip replacement, gives less radiation exposure and is more efficient with respect to radiology services than conventional radiology. An intraoperative study may require ethics approval.
