Abstract
Introduction
Range of motion (ROM) simulation of the hip is useful to understand the maximum impingement free ROM in total hip arthroplasty (THA). In spite of a complex multi-directional movement of the hip in daily life, most of the previous reports have evaluated the ROM only in specific directions such as flexion-extension, abduction-adduction, and internal - external rotation at 0° or 90° of hip flexion. Therefore, we developed ROM simulation software (THA analyzer) to measure impingement free ROM in any positions of the hip. Recent designs of the hip implants give a wider ROM by increasing the head diameter and then, bone to bone impingement can be a ROM limit factor particularly in a combination of deep flexion, adduction and internal rotation of the hip. Therefore, the purpose of this study were to observe an individual variation in the pattern of the bone impingement ROM in normal hip bone models using this software, to classify the bone impingement ROM mapping types and to clarify the factors affecting the bone impingement type.
Methods
The subjects were 15 normal hips of 15 patients. Three dimensional surface models of the pelvis and femur were reconstructed from Computer tomography (CT) images. We performed virtual hip implantation with the same center of rotation, femoral offset, and leg length as the original hips. Subsequently, we created the ROM mapping until bone impingement using THA analyzer. We measured the following factors influenced on the bone impingement map patterns; the neck shaft angle, the femoral offset, femoral anteversion, pelvic tilt, acetabular anteversion, sharp angle, and CE angle. These factors were compared between the two groups. Statistical analysis was performed with Mann-Whitney U test, and statistical significance was set at P<0.05.
Results
According to the borderline of ROM at the flexion-internal rotation corner on the bone impingement map, the hips were classified into two groups; group-A showed more than 45° of the borderline slope at the flexion-internal rotation corner and the remaining hips were group-B. (Fig.1). There were 7 hips in group-A and 8 hips in group-B. Femoral offset was 36.8±2.2 mm in group-A and 30±2.7 mm in group-B. Femoral anteversion was 32±6.4° in-group A and 43 ±4.8° in group-B. There were statistically significant differences in the femoral offset and femoral anteversion between the groups. There were no significant differences in the other factors.
Discussion
The results of this study showed various ROM map patterns even in normal hips and we classified them into two groups. An increased femoral offset or a decreased femoral anteversion revealed an early impinge in internal rotation. ROM until bone impingement is affected by the individual bone morphology. However, it is not easy to evaluate bony ROM in complex hip positions. THA analyzer shows the impingement position visually on the map and it is easy to understand the hip positions with reduced ROMs.
Conclusion
There are two patterns on the bony ROM map in normal hips, and an early impinge in internal rotation occurred by increasing the femoral offset or decreasing the femoral anteversion.
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