Quantifying supraspinatus tendon responses to exposures emulative of human physiological levels in an animal model

Rotator cuff pathology typically originates in the supraspinatus tendon, but uncertainty exists on how combinations of glenohumeral elevation angle and load intensity influence responses of the intact, functional supraspinatus unit. This study exposed the supraspinatus tendon to mechanical loading scenarios emulative of derived muscle force and postural conditions measured in vivo to document its responses. Right shoulders from 48 Sprague-Dawley rats were placed into one of eight testing groups combining glenohumeral elevation angles (0/30/60/75°) and a high or low load intensity for 1500 cycles at 0.25 Hz using a custom mounting apparatus attached to a tensile testing system. Load intensities were derived from in vivo human partitional muscular activation levels collected previously and scaled to the animal model. Mechanical response variables examined included tangent stiffness and hysteresis, in addition to localized surface stretch ratios calculated via virtual tracking points. A significant three-way interaction (p = 0.0009) between elevation angle, load magnitude and cycle number occurred for tangent stiffness, with increasing angles, loads and cycles increasing stiffness by up to 49%. Longitudinal stretch ratios had significant interactions (p = 0.0396) with increasing elevation angles, load intensities and cycle numbers, and differences existed between the articular and bursal sides of the tendon. Complex interactions between angle, load and cycle number suggest higher abduction angles, increased load magnitude and higher loading cycles increase tangent stiffness, stretch ratios and hysteresis within the tendon.