Abstract
Background
The hip joint capsular ligaments passively restrain extreme range of motion (ROM), protecting the native hip against impingement, subluxation, edge loading and dislocation. This passive protection against instability would be beneficial following total hip arthroplasty (THA), however the reduced femoral head diameter postoperatively may prevent a wrapping mechanism that is essential to capsular ligament function in the native hip. Therefore we hypothesized that, post-THA, the reduced femoral head size would prevent the capsular ligaments protective biomechanical function.
Methods
In vitro, THA was performed through the acetabular medial wall preserving the entire capsule, avoiding targeting a particular surgical approach. Eight fresh-frozen cadaveric hips were examined and capsular function was measured by internally/externally rotating the hip in five positions ranging from full extension with abduction, to full flexion with adduction. Three head sizes (28, 32, 36 mm) with three neck lengths (restored native 0, +5, +10 mm) were compared.
Results
Internal and external rotation ROM increased following THA, indicating late engagement of the capsule and reduced biomechanical function (p<0.05). Internal rotation was affected more than external. Increasing neck length restored ROM more towards the native condition, but too much lengthening over-constrained external rotation. Increasing head size only had a small effect, restoring ROM towards the native condition.
Conclusions
Following THA, the capsular ligaments were unable to wrap around the reduced diameter femoral head to restrain excessive hip movement. The posterior capsule was the most affected, indicating native posterior capsule preservation is not advantageous at least in the short-term. Decreased neck length could cause capsular dysfunction, whilst increased could over-tighten the anterior capsule.
Relevance
Increased understanding of soft tissue balancing following THA could help prevent instability, a frequent and long-standing THA complication. This study illustrates how the capsule will function according to its preservation or repair following THA.
