Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCB160

Poster Communications

Functional and biological changes in supraspinatus muscle after rotator cuff tear

A. P. Valencia2,1, S. Iyer2, M. Gilotra2, R. lovering2

1. Kinesiology, University of Maryland, Baltimore, Maryland, United States. 2. Orthopaedics, University of Maryland School of Medicine, Baltimore, Maryland, United States.

Rotator cuff (RTC) tears, particularly in the supraspinatus muscle (SS), are a common clinical problem that affects the quality of the muscle (muscle atrophy and lipid infiltration). Muscle quality is the strongest predictor of functional and surgical outcomes, but surgical repair often fails to reverse it, leading to high re-tear rates and poor shoulder function. Tenotomy of RTC muscles is an established animal model for RTC tear, but currently no published studies have assessed muscle function or susceptibility to injury after a tear. PURPOSE: 1) To develop a method to assess RTC contractility and assess functional changes in SS after RTC tear. 2) To describe an in situ method to induce injury in the supraspinatus muscle (SS) through eccentric contractions and assess susceptibility to injury in torn versus healthy RTC muscle. METHODS: RTC tears were performed by surgical release of the SS and infraspinatus tendons in anesthetized Sprague Dawley rats (2% isofluorane via nosecone using precision vaporizer). Rats were monitored and allowed normal cage activity. Contractile testing was performed in anesthetized animal in situ 2 days (2D) and 15 days (15D) after tears, with contralateral side serving as control, followed by euthanasia through cardiac puncture. Muscle was harvested to measure wet weight, assess neuromuscular junction (NMJ) morphology, and fiber cross-sectional area (CSA). In a second set of experiments we induced injury to the SS in situ. The tendon was released and tied to a lever arm attached to a stepper motor in line with a load cell. The muscle was set at optimal length (Lo) followed by 30 eccentric contractions with the muscle lengthened 15% of Lo. Muscles were harvested, weighed, snap frozen in liquid nitrogen, and sectioned for labeling of structural proteins. RESULTS: Maximal isometric force was lower in 2D (2.2 ± 0.4 N vs. 2.9 ± 0.4 N), but not 15D. A decrease in CSA and altered NMJ morphology were present for 2D only. After contraction-induced injury, the loss of force in 2D was not different to control (43.5% ± 2.3), but it was significantly greater in 15D (51.8% ± 2.5). Moreover, desmin content was reduced in injured 15D only compared to injured control SS. CONCLUSIONS: This is the first study to assess susceptibility to injury after RTC tear. Shortly after a RTC there is a loss in contractile function with morphological changes to the muscle and NMJ that recover over time. However, SS muscle actually becomes more susceptible to injury over time, with corresponding loss of the intermediate filament protein, desmin, which could contribute to the poor quality of muscle seen clinically after RTC tears. All protocols were approved by the University of Maryland Institutional Animal Care and Use Committee.

Where applicable, experiments conform with Society ethical requirements