| Introduction. Tears of the rotator cuff of the shoulder account for a significant portion of upper extremity soft tissue injuries. A large number of these tears occur at the tendon to bone interface. However, little knowledge currently exists about the natural healing process of this region, and hence little can be done to improve the tissue's ability to heal. The goal of this thesis was to study the healing of the supraspinatus tendon to its bony insertion under a variety of clinically relevant loading conditions.;Methods. Using the rat as an animal model, supraspinatus tendons were surgically detached and repaired. Post-operatively, rat shoulders were immobilized, allowed normal cage activity, or exercised. Biomechanical, structural, and compositional assays were performed after sacrifice at either two, eight, or sixteen weeks.;Results. Rat shoulders that were immobilized demonstrated superior properties compared to those that were exercised, a result contrary to expectations. When comparing the immobilization group to the exercise group, it was found that cross-sectional area was larger in the exercise group, structural properties were equivalent between the two groups, apparent viscoelastic properties were superior in the immobilization group, and collagen organization was superior in the immobilization group. Evaluation of the expression of extracellular matrix genes at the insertion site provided insight into these results. The ratio of type III to type I collagen, an indication of the level of scar in healing tissue, was highest in the exercise group. The level of expression of genes normally found in fibrocartilage was highest in the immobilization group. No activity level groups approached the structural, material, organizational, or compositional properties of normal uninjured tissue, even after sixteen weeks of healing.;Conclusion. More material of lesser quality was produced due to increased activity level. This material was inferior in material properties, organization, and composition. With knowledge of the healing response under various loading conditions, treatment modalities for rotator cuff tears can be developed. |
