Introduction: Leg length and offset restoration are known to improve function after total hip arthroplasty, and to minimize the risk of dislocation and limp. Anatomic data of the hip are needed to determine specifications for prosthesis design that restore patient hip anatomy more closely. Furthermore, femoral off-set values calculated on X-Rays may be inaccurate in case of external rotational contracture or high femoral ante-version. The goal of this study was to determine three-dimensional morphological data of the hip in case of primary osteoarthritis, especially for femoral off-set.

Material and Method: 223 hips with primary osteoarthritis have been analysed using a CT-scan and a specific software (HIP-PLAN^®) that allows image post-processing for re-orienting the pelvis or the femur to a standardized orientation. Femoral and acetabular anteversions were measured. The planar (2D) and three-dimensional (3D) values of femoral offset were determined. 3D values were measured as the distance between the femoral head centre and the diaphyseal femur axis; 2D values were calculated as the projection of this distance on the frontal plan.

Results: Measurements precision was good with correlation scores ranging between 0.91 and 0.99. Mean acetabular anteversion angle was 26° +/−6.6° when measured in the Anterior Pelvic Plane and 21.9° +/−6.6° in the frontal plane according to the method of Murray. Mean femoral anteversion was 21.9° +/−9.4 according to the method of Murphy. The Sum of acetabular and femoral anteversion was found to be out of the safe zone regarding dislocation risk in 47% of patients. Mean 3D femoral off-set was found to be 42.2 mm+/− 5, significantly increased by 3.5 mm +/− 2.5 when compared to the 2D femoral off-set values. Femoral off-set was above 45mm in 31% of cases and higher than 50 mm in 12% of cases. The tip of the great trochanter was located higher than the femoral head centre, at a mean distance of about 9 mm.

Discussion: When measured on X-rays, femoral off-set may be significantly under-estimated. This error is probably due to the external rotational contracture of the hip induced by osteoarthritis. If the implants are positioned using the anatomical preoperative anteversion angles, 47% of patients would not be in the safe zone regarding posterior dislocation risk.

Conclusions: Planar measurement using X-Rays underestimates significantly the femoral off-set. Neck and head modularity may be useful to achieve simultaneous restoration of femoral off-set and leg length in 12 to 31% of cases.

Pre-operative computerised three-dimensional planning was carried out in 223 patients undergoing total hip replacement with a cementless acetabular component and a cementless modular-neck femoral stem. Components were chosen which best restored leg length and femoral offset. The post-operative restoration of the anatomy was assessed by CT and compared with the pre-operative plan.

The component implanted was the same as that planned in 86% of the hips for the acetabular implant, 94% for the stem, and 93% for the neck-shaft angle. The rotational centre of the hip was restored with a mean accuracy of 0.73 mm (sd 3.5) craniocaudally and 1.2 mm (sd 2) laterally. Limb length was restored with a mean accuracy of 0.3 mm (sd 3.3) and femoral offset with a mean accuracy of 0.8 mm (sd 3.1).

This method appears to offer high accuracy in hip reconstruction as the difficulties likely to be encountered when restoring the anatomy can be anticipated and solved pre-operatively by optimising the selection of implants. Modularity of the femoral neck helped to restore the femoral offset and limb length.