Cup-To-Neck Contact and Range of Motion after Total Hip Arthroplasty with Large Head Diameters: An Original Three-Dimensional Combined Gait and Videofluoroscopy Analysis

After a total hip arthroplasty, a limited range of motion and lower-limb disability continue to be observed, with these being mainly associated with the implant design and the head-to-neck ratio. Larger diameters of the head bearings were assumed to provide better stability, a larger range of motion, and smaller risks of dislocation and stem-to-liner impingement. However, these claims have never been demonstrated in real patients. The specific aim of this study was to assess, via multi-instrumental analysis, whether the range of motion of a replaced hip is limited by the stem-to-liner contact in patients with large femoral head diameters. Twenty-three patients with a total hip arthroplasty were evaluated at their one-year follow-ups using clinical and instrumental examinations. A combined three-dimensional gait analysis of the full body and videofluoroscopy analysis of the replaced hip were performed during the execution of standard, i.e., daily living, and more demanding motor tasks. The latter were meant to reach the extreme range of motion at the replaced hip site, thus revealing possible stem-to-liner contact. An original technique based on imaging and computer-aided design (CAD) models of the prosthesis components was developed to calculate the stem-to-liner distance. Excellent clinical scores were observed in the study. The gait analysis showed that the range of motion of the replaced hip in the sagittal plane, averaged over all patients, ranged from 28 degrees to 78 degrees in standard activities. In more demanding tasks, single peaks were as high as 110 degrees, 39 degrees, and 60 degrees in the sagittal, frontal, and transverse anatomical planes, respectively. In all motor tasks, the stem-to-liner distances ranged from 8.7 to 13.0 mm on average, with one outlier minimum distance being 2.2 mm. This study shows that, even in demanding motor tasks and with an extreme range of motion, the hip joint replaced with large femoral head diameters did not experience impingement between the prosthesis components.