The role of tension on supraspinatus tendon to bone healing in a newly developed animal model of chronic rotator cuff tears

More than 3 million people in the United States seek medical care each year for treatment of shoulder injuries. One of the most common injuries is tears to the rotator cuff tendons of the shoulder. Since the primary function of these tendons is to help move and stabilize the shoulder joint, tears are often associated with pain, instability, and decreased shoulder function. When conservative treatments are not successful in managing these symptoms, surgery is often performed to repair the torn tendons back to bone. While this is typically beneficial, re-tearing of the tendon(s) following repair of chronic injuries frequently occurs partly because of inadequate tendon to bone healing. Healing is a complex process and mechanical forces play a large role in the maintenance and remodeling of tendon. Therefore, one potential cause for poor healing is the large tensions that are sometimes required to repair the tendon back to bone. Animal models are ideal for investigating this problem since a consistent injury and repair scenario can be employed and healing can be rigorously measured. Therefore, the objective of this study was to develop and utilize an animal model of chronic rotator cuff tears to investigate the role of increased tension on tendon to bone healing. We hypothesized that the creation of a rotator cuff tear by detaching the supraspinatus tendon from the humerus would result in detrimental changes to the supraspinatus tendon and an increase in the tension required to repair the tendon back to bone. Furthermore, we hypothesized that an increase in repair tension would be related to detrimental changes to the healing tendon insertion site. We found that the detached supraspinatus musculotendinous unit of the rat developed characteristics that were similar to the chronic musculotendinous unit of humans. The tendon became stiff and the tension required to repair the tendon back to bone rapidly increased with time following detachment. Additionally, we found that increased repair tension was related to inferior failure properties of the healing tendon to bone insertion site. Therefore, repair tension should be minimized in the clinical setting to decrease the likelihood of tendon to bone failure.