This trial aims to determine the benefit of using patient's own bone marrow derived stem cells in repairing torn shoulder rotator cuffs. The trial is being conducted by the Hospital San Carlos in Madrid and expects to enrol 10 subjects by December 2013. Participants, all of who have a severe tear of the supraspinatus tendon, will be randomly divided into two groups. The control group will receive a conventional surgical repair using a collagen type I membrane (orthADAPT product from Penta Biomedical). The experimental group will undergo the same repair except that the collagen membrane will be impregnated with the patient's own bone marrow stem cells. Participants will be monitored for complications and for changes in clinical condition. The clinical and radiological practitioners involved in these assessments will not know what treatment each patients receives.