Rotator cuff (RTC) tears impair upper limb mobility and affect 20% of the adult population. Unfortunately, surgical repair of major RTC tears often fails to restore shoulder function and has a high risk of re-tear. RTC tears induce irreversible, degenerative changes to the muscle that may hinder the recovery of shoulder function. Currently, very few studies have comprehensively assessed RTC muscle function, thus, little is known about which markers may be able predict changes in function after RTC tear. In this dissertation, I present three studies designed to systemically determine the impact of a RTC tear on contractile function of the supraspinatus (SS), the muscle most commonly affected in the RTC.

In study #1 I developed a novel method to test in vivo SS contractile function using animal species common to RTC research. In study #2, I found that the SS exhibited a 30% loss in force prior to onset of muscle atrophy after acute RTC tear using the rat model. The initial loss of force was associated with a decrease in the size and continuity of the neuromuscular junction (NMJ). The SS muscle was also more susceptible to injury, which was associated with a reduction in collagen packing density. Therefore, SS size is not the strongest predictor of force output with acute RTC tears. In addition, the increased susceptibility to injury could compound the dysfunction already apparent in the SS muscle after RTC tear.

In study #3, I found that the rabbit model experienced a 40% loss of force after 6 weeks of RTC tear that persisted at 12 weeks. Using a number of different in vivo and ex vivo imaging approaches I found the degree of fatty infiltration (FI) to be the strongest predictor of muscle force production after RTC tear. Surprisingly, the data suggested that muscle atrophy only explained the loss in force in torn muscles when little to no FI was present. Therefore, FI is a prognostic marker for muscle weakness after RTC tear, and can help clinicians predict the force generating capacity of the SS for surgery and rehabilitation decision-making.

Results from both studies found that SS contractile function was significantly impaired after RTC tear, and identified measureable markers beyond muscle atrophy that were associated with the loss in muscle force that may act as potential therapeutic targets to improve functional outcomes after RTC tear.