Background: This study was motivated by the need to better understand the tribology of the rotator cuff, as it could provide insights into degenerative processes and suggest therapeutic approaches to cuff disorders. The objective was to evaluate the distribution of a known lubricating protein in the infraspinatus tendon of the rotator cuff in adult goats. The hypothesis was that lubricin, also known as superficial zone protein or proteoglycan 4, serves as an interfascicular lubricant.
Methods: Eight infraspinatus tendons were resected from eight Spanish goats, and five patellar tendons and articular cartilage samples were also resected from five of the goats. Samples were processed for immunolocalization of lubricin with use of a purified monoclonal antibody and for histological analysis with Masson trichrome staining for collagen. The locations of lubricin within the tendon were documented, and measurements were made of the distance to which lubricin was detected in the tendon, relative to the humeral insertion site, and the depth of lubricin staining into the fibrocartilage of the fascicle bordering the humeral joint space. Images from polarized light microscopy were used to measure the fascicle diameter and the crimp length.
Results: Lubricin was prominent in layers separating the fascicles in the infraspinatus tendon, with occasional intrafascicular staining. The fibrocartilaginous portion of the fascicle bordering the humeral joint space displayed diffuse lubricin staining. The Masson trichrome staining of the collagen in this lubricin-containing fibrocartilage indicated that it was not being tensioned at the time of fixation, whereas the collagen contained in the body of the tendon was under tension. The crimp of the fascicles near the humeral joint side of the tendon displayed a shorter peak-to-peak length than the crimp of the fascicles in the superior region of the tendon. Only the surface of two of the five patellar tendon samples stained for lubricin; there was no staining within these ligaments.
Conclusions: The sheaths of the fascicles of the infraspinatus tendon near the bone insertion site contain lubricin, indicating that this lubricating protein may be facilitating interfascicular movement. The fact that the crimp pattern of fascicles changes with location in the tendon provides support for the supposition that fascicles move relative to one another as the tendon is loaded, underscoring the importance of a lubricating protein in the layer separating the fascicles.
Clinical Relevance: The results provide a basis for the hypothesis that the loss of lubricin may favor the wear of tendons during interfascicular movement. Moreover, should lubricin in the infraspinatus be found to serve as a lubricant, the present work provides a rationale for the investigation of an injectable form of lubricin as a therapeutic agent to treat certain rotator cuff problems.