Neural and Collagen Tissue Properties As They Relate to Glenohumeral Joint Laxity and Stiffness
Witherspoon, Jessica Waine'ette
University of Kansas
Physical Therapy & Rehabilitation Sciences
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Shoulder instability is the most frequently occurring shoulder injury. Instability is caused by repetitive stress or a traumatic event that leads to excessive joint laxity. A major concern with instability is the high recurrence rate post treatment. Although the occurrence and recurrence of instability are primarily associated with capsule and labrum injury, the tissue characteristics of the capsule and labrum are not well understood. Therefore, we sought to explore the tissue properties of the capsule and labrum. We also explored the relationship between the tissue properties and passive joint motion. To carry out this body of work, we dissected five fresh and three frozen human cadaveric shoulder pairs, removing the muscle and subcutaneous tissue while leaving the capsule and labrum intact. The cadaveric specimens included four males and four females of ages 23, 55, 58, 62, 76, 81 (x2), and 98. We then assessed glenohumeral joint laxity and joint stiffness using a materials testing machine. Following this assessment, we further dissected the shoulder removing the capsule and labrum from the anteroinferior (most common region of injury) and posteroinferior (least common region of injury) areas of the glenohumeral joint. The majority of the tissue was used for studying mechanoreceptors. A small portion was stored and used later for determining collagen content. Mechanoreceptor distribution and neural count were determined by first staining the tissue using our improved gold chloride staining method and light microscopy. Collagen content was determined using an acid-pepsin digestion for extracting collagen and the Sircol Collagen Dye assay for quantifying collagen. Joint laxity and joint stiffness assessment revealed a significant relationship (r = -.824, P<.001) between joint laxity and joint stiffness where joint stiffness decreased as joint laxity increased. From the neural staining, we created neuroanatomical maps for the capsule and labrum based on mechanoreceptor distribution. Not only did we discover a mechanoreceptor distribution pattern, but we also observed a strong positive relationship between neural count and joint laxity specifically in the capsule (r = .710, p = .003). Though this relationship was not significant in the labrum, when taking the capsule and labrum neural count into consideration, the overall relationship was significant (r =.646, p =.009). Unlike neural count and joint laxity, we did not observe a significant relationship between collagen content and joint stiffness in the capsule or labrum except when injury was taken into consideration. In the presence of injury, we observed strong negative relationships in the capsule (r = -.803, p =.016), labrum (r = -.755, p = .030), and overall, when including the capsule and labrum (r = -.814, p = .014). Based on our results, we were able to identify potential contributing factors to the unstable shoulder that may help guide future treatment thereby reducing the recurrence rate. We identified mechanoreceptor scarce regions from the neuroanatomical map that may be more apt for surgical repair that allows for mechanoreceptor preservation. We also observed a relationship between neural count and joint laxity that further supports the need to preserve neural structure integrity during treatment. The relationship between collagen content and joint stiffness, in the presence of injury, suggests that changes in collagen content should be considered during rehabilitative treatment when joint stiffness is compromised. Joint laxity, joint stiffness, mechanoreceptor distribution, neural count, and collagen content are all factors that should be studied in greater detail when developing treatments for the unstable shoulder.
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