Abstract
Introduction
Large diameter femoral heads provide increased range-of-motion and reduced dislocation rates compared to smaller diameter femoral heads. However, several recent studies have reported that contemporary large head prostheses can directly impinge against the local soft tissues leading to anterior hip pain. To address this we developed a novel Anatomically Contoured large diameter femoral Head (ACH) that maintains the profile of a large diameter femoral head over a hemispherical portion and then contours inward the distal profile of the head for soft-tissue relief. We hypothesized that the distal contouring of the ACH articular surface would not affect contact area. The impact of component placement, femoral head to acetabular liner radial clearance, and joint loading during different activities was investigated.
Methods
A finite element model was used to assess the femoroacetabular contact area of a 36 mm diameter conventional head and a 36 mm ACH (Fig. 1). It included a rigid acetabular shell, plastically deformable UHMWPE acetabular liner, rigid femoral head and rigid femoral stem. The femoral stem was placed at 0°, 10° and 20° of anteversion. The acetabular shell and liner were placed in 20°, 40° and 60° of abduction and 0°, 20° and 40° of anteversion. The femoral head to acetabular liner radial clearances modeled were 0.06 mm, 0.13 mm and 0.5 mm. Three loading cases corresponding to peak in vivo loads during walking, chair sit and deep-knee bend were analyzed (Fig. 2). This allowed a range of component positions and maximum joint loads to be studied.
Results
Under all tested conditions there was no difference between the two implants (Fig. 3). The contact area for both prosthesis depended on the radial clearance between the head and liner. The conventional head contact area (standard deviation) in mm2 for 0.5 mm, 0.13 mm and 0.06 mm of radial clearance was 230.5 (70.2), 419.8 (48.7) and 575.4 (60.1) respectively. Similarly, for the ACH these were 230.5 (70.4), 420.1 (48.7) and 575.9 (59.4). The average data for a head and radial clearance combination included all component placements and load conditions completed. A student T-Test (p = 0.05) confirmed that the ACH had the same contact area as the conventional head for all radial clearances.
Conclusion
This study showed that, as intended, an anatomically contoured large diameter femoral head designed to provide soft-tissue relief maintained the load bearing articular contact area of a conventional implant. The novel ACH prosthesis could mitigate the risk of soft-tissue impingement with contemporary large head implants while retaining their benefits of additional stability and range-of-motion.
