Anatomical, biomechanical and histological evaluation of the glenoid labrum

Abstract

The glenoid labrum is an integral part of the highly mobile glenohumeral joint. Labral injury can be detrimental to the movement and function of the shoulder. Therefore, an in-depth investigation of glenoid labrum microanatomy may improve our understanding of its roles and functions in the glenohumeral joint. This project aimed to explore the (1) composition and attachment, (2) vascularity, and (3) innervation of the glenoid labrum. Labral degenerative changes were also assessed relative to the changes in labral neurovascularity that were observed. Sixteen cadaveric shoulders were obtained under the supervision of a licensed anatomist from the Department of Anatomy, University of Edinburgh and in accordance with the Human Tissue Act (Scotland) of 2006. After dissection, the glenoid, including the labrum and the attachments of its associated structures, was segmented into eight regions and processed for histological analysis. Three histological stains were utilised to evaluate the composition of the labrum and assess its attachments. General labral morphology was first assessed in haematoxylin and eosin-stained sections. This was followed by staining with toluidine blue to assess the total and percentage of the labral area occupied by proteoglycans as an indirect measure of its capacity to accommodate a compressive load. Finally, Verhoeff-Van Gieson stain was used to localise elastin fibres within the labral cross-section. Labral vascularity was assessed immunohistochemically using an anti-alpha-smooth muscle actin antibody and evaluated quantitatively by determining the vascular count, vascular density, and blood vessel distribution within the labrum. Labral innervation was assessed immunohistochemically with an anti-p75 antibody that facilitated the quantitative analysis of nerve count and nerve density. Nerve types were identified following the modified Hagert (2008) classification. Degenerative changes detected in the labrum were assessed following a histopathologic grading system described by Pauli et al. (2011). After microanatomical quantification, differences in proteoglycan content, vascularity, and innervation were assessed across various labral regions, and correlations of labral vascularity and innervation to proteoglycan area and proteoglycan percentage were determined. Finally, labral vascularity and innervation were also evaluated in labral tissues exhibiting various grades of degeneration. The glenoid labrum is connected to the glenoid bone via a fibrocartilaginous attachment that includes a fibrocartilaginous transition zone that links the bone to the fibrous labrum. Elastin fibres exhibited a distinct localisation within the labrum, including the capsular attachment, between large collagen bundles and within the fibrocartilaginous regions. Blood vessels and nerves were identified in all labral regions. Four types of mechanoreceptor nerve endings were identified within the labrum, including Ruffini corpuscles, Pacini corpuscles, Golgi-like nerve endings and unclassified corpuscles. The labral sections ranged from normal to moderately degenerated; no severely degenerated labra were identified. The analysis revealed significant variation across all labral regions and a significant inverse correlation between vascularity and innervation with proteoglycan content. However, there were no significant differences in vascularity and innervation across the degeneration spectrum. In conclusion, the glenoid labrum was vascularised and innervated throughout its circumference, but exhibited significant microanatomical variations that were linked to compressive load accommodation properties. The different types of nerve endings found in the labrum suggest that the labrum contribute to the sensorimotor function of the glenohumeral joint. Whilst the degree of degeneration had no apparent influence on labral vascularity or innervation, a larger sample size will be needed to substantiate these findings. The results of this study will lead to improvements in our understanding of the microanatomy of the glenoid labrum and have important implications for shoulder function and stability. The findings may also provide critical insights into methods that may be used for injury management and new rehabilitation strategies.