The effects of overuse combined with intrinsic or extrinsic alterations inan animal model of rotator cuff tendinosis

An in vivo animal model was used to evaluate overuse and overuse plus intri nsic tendon injury or extrinsic tendon compression in the development of ro tator cuff injury. Forty-four male Sprague-Dawley rats were divided into gr oups of 22. Each left shoulder received an intrinsic or extrinsic injury pl us overuse (treadmill running), and each right shoulder received only overu se. Eleven rats from each group were sacrificed at 4 and 8 weeks. Supraspin atus tendons were evaluated histologically or geometrically and biomechanic ally. Ten rats constituted a cage-activity control group. Both supraspinatu s tendons of the experimental groups had increases in cellularity and colla gen disorganization and changes in cell shape compared with control tendons . Tendons with injury plus overuse exhibited a worse histologic grade than those with overuse alone. The cross-sectional area of both supraspinatus te ndons of the experimental rats was significantly more than in control tendo ns. The area of the injury plus overuse tendons was increased on average co mpared with overuse-alone tendons. Biomechanically, the tissue moduli of ov eruse/intrinsic injury tendons at 4 weeks and those of the overuse/extrinsi c injury tendons at 8 weeks were significantly lower than in control tendon s. Tissue moduli of the overuse/injury tendons were significantly lower tha n in the overuse-alone tendons at 8 weeks. This study demonstrated that dam age to the supraspinatus tendon can be caused by overuse and intrinsic inju ry, overuse and extrinsic compression, and overuse alone.