The Effects of Mesenchymal Stem Cells on Rotator Cuff Muscle in a Chronic Injury Model in Sheep

INTRODUCTION Most clinically significant injuries involving the rotator cuff muscle are diagnosed and treated in a chronic setting. Those rotator cuff injuries requiring surgical repair of the tendon have a surprisingly high failure rate when analyzed with imaging; failure rates for massive tears have been reported to be as high as 80%. These failures are due in part to atrophy of the chronically injured rotator cuff muscle. Muscle atrophy is characterized by fatty infiltration, loss of muscle volume and a decrease in the elasticity of the muscle-tendon unit. We have previously described a chronic rotator cuff injury and repair model in sheep. We found that chronic (six weeks) detachment of the tendon from the bone resulted in a 50% decrease in the force of muscle contraction and a twelve-fold increase in fat concentration within both the muscle cells and the interstitial spaces. Repair of the tendon partially reversed these findings. We hypothesize that the injection of stem cells directly into the infraspinatus muscle at the time of surgical repair will improve muscle function and decrease fat content within the muscle compared to controls. We use both muscle derived and marrow derived adult sheep stem cells in our experiments.