MRI assessment of shoulder suggests the presence of a strong fibrous structure in the depth of the supra spinatus muscle. An anatomical study has been performed in 20 fresh human shoulders. There was a very strong fibrous frame within the anterior part of the supra spinatus m. and within the upper part of the sub scapularis. The upper part of the infra spinatus m. contained a significant fibrous reinforcement too so as in the teres minor m. The authors describe fibrous structures around the rotator interval, which constitute an antero-superior Fibrous Lock of the scapulo humeral joint and may help the repair of the full thickness tears of the rotator cuff.