Abstract
The hip's capsular ligaments (CL) passively restrain extreme range of motion (ROM) by wrapping around the native femoral head/neck, and protect against impingement and instability. We compared how CL function was affected by device (hip resurfacing arthroplasty, HRA; dual mobility total hip arthroplasty, DM-THA; and conventional THA, C-THA), and surgical approach (anterior and posterior), with and without CL surgical-repair. We hypothesized that CL function would only be preserved when native head-size (HRA/DM-THA) was restored.
CL function was quantified on sixteen cadaveric hips, by measuring ROM by internally (IR) and externally rotating (ER) the hip in six functional positions, ranging from full extension with abduction to full flexion with adduction (squatting). Native ROM was compared to ROM after posterior capsulotomy (right hips) or anterior capsulotomy (left hips), and HRA, and C-THA and DM-THA, before and after CL repair.
Independent of approach, ROM increased most following C-THA (max 62°), then DM-THA (max 40°), then HRA (max 19°), indicating later CL engagement and reduced biomechanical function with smaller head-size. Dislocations also occurred in squatting after C-THA and DM-THA. CL-repair following HRA restored ROM to the native hip (max 8°). CL-repair following DM-THA reduced ROM hypermobility in flexed positions only and prevented dislocation (max 36°). CL-repair following C-THA did not reduce ROM or prevent dislocation.
For HRA and repair, native anatomy was preserved and ligament function was restored. For DM-THA with repair, ligament function depended on the movement of the mobile-bearing, with increased ROM in positions when ligaments could not wrap around head/neck. For C-THA, the reduced head-size resulted in inferior capsular mechanics in all positions as the ligaments remained slack, irrespective of repair.
Choosing devices with anatomic head-sizes (HRA/DM-THA) with capsular repair may have greater effect than surgical approach to protect against instability in the early postoperative period.
