Microstructural analysis on the innervation of the anterior, medial, and lateral human hip capsule: Preliminary evidence on its neuromechanical contribution.

OBJECTIVE: Capsular repair aims to minimize damage to the hip joint capsular complex (HJCC) and subsequent dislocation risk following total hip arthroplasty (THA). Numerous explanations for its success have been advocated, including neuromuscular feedback loops originating from within the intact HJCC. This research investigates the hypothesis that the HJCC contributes to hip joint stability by analyzing HJCC innervation. METHOD: Twenty-nine samples from the anterior, medial, and lateral aspects of the midportion HJCC of 29 individuals were investigated stereologically and immunohistochemically to identify encapsulated mechanoreceptors according to a modified Freeman and Wyke classification, totaling 11,745 sections. Consecutive slices were observed to determine the nerve course within the HJCC. RESULTS: Few encapsulated mechanoreceptors were found in the HJCC subregions and overlying tissues across the cohort studied. Of regions studied, no significant regional differences in the density of mechanoreceptors were found. No significant difference in mechanoreceptor density was found between sides (left, 10.2×10-4/mm3, 4.0×10-4 - 19.0×10-4/mm3; right 12.9×10-4/mm3, 5.0×10-4 - 22.0×10-4/mm3; mean, 95% confidence intervals) sexes (female 10.4×10-4/mm3, 4.0×10-4 - 18.0×10-4/mm3; male 11.6×10-4/mm3, 5.0×10-4 - 20.0×10-4/mm3; mean, 95% confidence intervals), nor in correlation with age demographics. Myelinated nerves coursed consistently within the HJCC in various orientations. CONCLUSION: Sparse mechanoreceptor density suggests that the HJCC contributes to a limited extent to hip joint stabilization. HJCC nerve terminals may potentially contribute to neuromuscular feedback loops with associated muscles to mediate joint stability in tandem with the active and passive components of the joint.