The upper band of the subscapularis tendon in the rat has altered mechanical and histologic properties

Abstract

The subscapularis is an important mover and stabilizer of the glenohumeral joint. Since the advent of shoulder arthroscopy, partial tears are found in 43% of rotator cuff patients. While partial tears to the upper band occur more commonly, little is known about the structure and mechanical behavior of the individual bands. Therefore, the objective of this study was to measure tensile mechanical properties, corresponding collagen fiber alignment, and histology in the upper and lower bands of the rat subscapularis tendon. Thirty adult Sprague-Dawley rats were euthanized and subscapularis tendons dissected out for mechanical organization (n = 24) and histologic assessment (n = 6). Collagen organization was measured with a custom device during mechanical testing. Linear-region modulus at the insertion site was significantly lower in the upper band compared to the lower band, while no differences were found at the midsubstance location. The upper band was found to be significantly less aligned and demonstrated a more rounded cell shape than the lower band at the insertion site. This study demonstrated that the 2 bands of the subscapularis tendon have differential mechanical, organizational, and histological properties, which suggests a functional deficit exists to the upper band of the subscapularis and may be contributing to the prevalence of partial subscapularis tears. Clinicians should be aware that the upper band of the subscapularis tendon may be at higher risk of developing tears, based on decreased mechanical properties and a more disorganized collagen fiber distribution.