Anatomic, Histologic and Histomorphometric Analysis of the Acetabular Labrum and its Enthesis

Abstract

The acetabular labrum is a fibrocartilaginous tissue attached to the acetabular rim of the hip joint. It plays a vital role in maintaining both static and dynamic hip joint stability, with multiple associations to pain sensation, proprioception, and various hip joint disorders. Various traumatic and pathological conditions can compromise the structural integrity of the labral body and its attachment, leading to disruptions in hip joint stability. The localisation of the labrum within a large load-bearing joint has led to disparities in its structural and functional characteristics at the cross-sectional level. These disparities are localised within distinct labral zones (i.e., inner and outer zones), which are commonly attributed to the functional adaptation of each zone to mechanical loads. Despite the labrum's critical role in hip joint function, its intricate micro-anatomical differences, both within its main body and at its attachment to bone, have remained largely unexplored in the current research.

The primary objective of this project was to provide a detailed investigation of several micro-anatomical aspects of the labrum, encompassing labral innervation, vascularity, attachment features, degrees of degenerative changes, and levels of tears. The aim was placed on understanding the effects of labral morpho-functional zones on the distribution dynamics of these micro-anatomical characteristics. Secondly, this project also assesses the relationship between labral vascularity and innervation and their potential implications on labral healing and degeneration.

Human tissue was obtained from 9 embalmed cadavers, comprising a total of 16 hemipelves (10 males and 6 females) with an average age of 80 years. Each hip was divided into 8 distinct regions, resulting in a total of 128 regional segments. The histological investigations produced a total of 742 tissue sections, which were utilised in a variety of histological and immunohistochemical staining techniques.

Initially, a systematic zone-specific approach for labral assessment was developed to achieve a thorough understanding of the distribution dynamics of labral histologic and histopathologic characteristics across different zonal territories. The histopathologic evaluation of the labrum revealed a significant increase in the severity of multiple degenerative features, which were predominantly concentrated in the inner labral zone near the articular surface. These degenerative changes encompassed alterations in matrix proteoglycan content, cellularity, collagen organisation, and surface, including the lamellar layer. Immunohistochemical analysis and quantification of labral neuro-vascularity revealed a significantly higher concentration in the outer labral zone, near the joint capsule, with notably reduced neuro-vascularity in the inner zone. The analysis of sensory nerve endings revealed distinct distribution patterns for proprioceptive and nociceptive innervations within the labrum. Various types of sensory corpuscles (including Pacini, Ruffini, Golgi, and unclassified corpuscles), along with free nerve endings (both perivascular and non-perivascular), were significantly more concentrated within the outer labral zone.

The histological examination of the labral enthesis structure and morphology unveiled a fibrocartilaginous type of attachment. Notably, the attachment of the labrum to the bone exhibited marked differences in structural morphology between the inner and outer zones. The histopathologic and histomorphometric analysis of the enthesis’s layers, including calcified fibrocartilage region (CFC), tidemark (TM) and cement line (CL), revealed a significantly more developed and compact attachment in the inner zone. In contrast, the outer enthesis displayed a notably weaker anchorage, characterised by less defined entheseal features, a higher frequency of entheseal and cortical bone micro-damage, and a greater incidence of inflammatory and degenerative changes.

The labral morpho-functional zones play a significant role in shaping multiple micro-anatomic features of the labrum, affecting the distribution of degeneration processes and labral healing capability. The delineation of these distinct zonal frameworks offers insights into the labrum's functional adaptation to its mechanical environment and a zone-specific vulnerability to injury and degeneration.