Abstract
Background: Osteoarthritis affects 0.9 - 45% of individuals worldwide. One method to manage this within the hip joint surgically is total hip arthroplasty (THA), however, one common resulting complication is post-operative dislocation. Capsular repair, which aims to minimise damage to the hip joint capsular complex (HJCC) has been developed to reduce the risk of dislocation. Numerous hypotheses for its success have been proposed, one of which being the role and location of neural components of the HJCC.
Aims: The main aim of this thesis was to provide a detailed description of, and therefore enhance understanding of, the HJCC in relation to proprioception, neuromechanics, and stability of the hip joint. The sub-aims were: (1) to review the current literature on HJCC innervation, (2) to investigate the micro-scopic innervation of the anterior, medial, and lateral aspects of the central HJCC in a cohort with no known pathology, (3) to investigate the innervation of other tissues adjacent to the central portion of the HJCC. (4) A further sub-aim was to map the course of nerves within and into the HJCC.
Methods: To address the first objective, two systematic literature reviews were performed - the first on the micro-scopic innervation and the second on the macro-scopic innervation of the HJCC. The second objective was addressed with an immunohistochemical and stereological study; this utilised 33 HJCC tissues from the anterior, medial, and lateral aspects of the central portion of the HJCC (22 left, 11 right; 29 cadavers; 20 male, 9 female; 2-83 years old). These tissues were serially sectioned and stained with the histological stains haematoxylin and eosin, and several immunohistochemical markers (anti-S100, anti-neurofilament, anti-von Willebrand factor and anti-protein gene product 9.5). The resulting sections were assessed manually to identify and count encapsulated mechanoreceptors (Ruffini, Pacinian corpuscles and Golgi tendon organs) according to a modified version of the Freeman and Wyke classification. The third objective was addressed by manually observing the tissue overlying the HJCC in the same cohort, also within the anterior and posterior central HJCC and regions overlying the bone of one pilot specimen (a 28-year-old male). The fourth objective was addressed with immunohistochemical, ultra-thin E12 plastination and confocal technologies to observe how nerves course from a micro-, meso- and macro-scopic perspective in a total cohort of 41 cadavers (initial cohort of 33 HJCC tissues and further HJCC tissues from 3 male, 5 female; 5 right, 3 left; 64 - 88 years old).
Results: Few encapsulated mechanoreceptors were found in the anterior, medial, or lateral aspects of the central HJCC across the 33 HJCC tissues studied. Furthermore, few mechanoreceptors were found in the tissue overlying the HJCC, posterior central HJCC and HJCC overlying the bone, in the specimen studied. A greater density of mechanoreceptors was found in the proximal lateral aspects of the HJCC. Nerves also appeared to course most commonly into the proximal HJCC, yet this was variable. Nerves were shown to consistently course within the HJCC in the anterior, medial, and lateral central HJCC. No differences in mechanoreceptor density were found in side, sex, or age demographics.
Conclusion: The HJCC appears to have a subordinate or no role in proprioception of the hip joint and consequently potentially in mechanical stabilisation fed by neuromuscular feedback loops originating in the HJCC. The neural components of the HJCC likely contribute in tandem with the active and passive components of the joint to form the hip joint core complex of stability.
Clinical Relevance: Dislocation following THA is further reduced with capsular repair, and this occurs in few (0.65%) individuals. Greater understanding of the anatomy of the HJCC is required, which may aid in developing and utilising procedures which reduce this risk further. The proximal-lateral and distal-medial aspects of the HJCC appear to be crucial areas to avoid during THA from a neuromechanical standpoint, but this should be considered alongside other variables which contribute to hip dislocation.