Though it has been reported that the humeral head migrates antero-superiorly when massive rotator cuff tear occurs, we sometimes find CTA cases which show postero-superior migration of the humeral head. CT osteoabsorptiometry (CTOAM) has been used to study the stress distribution changes of glenoid and coracoacromial arch in cuff tear shoulders. However, there is no study about the relationship between stress distribution and number or location of torn tendons and about CTA. The purpose of this study was to evaluate the stress distribution of the glenoid and undersurface of the acromion in shoulders with various degree of cuff tear including CTA using CTOAM. 100 shoulders of 91 patients, which underwent 3D-CT, were analyzed for this study. We divided the 100 shoulders into 4 groups as follows: 1) supraspinatus tear (SSP group); 41 shoulders, 2) massive tear without subscapularis tendon involvement (M-SSC(-) group); 14 shoulders, 3) massive tear with subscapularis tendon involvement (M-SSC(+) group); 11 shoulders, 4) CTA group; 34 shoulders. As results, 3 patterns of stress distribution (antero-superior, anterior-posterior, postero-superior) were observed for the glenoid and 2 patterns (anterior, central) for the acromion. In the SSP group, 100% of glenoid and 75% of acromion showed high stress in the anterior area. In the M-SSC(-) group, high stress of glenoid was located in the anterior area in 50% and posterior in 50%, while 100% of acromion showed high stress in the central area. In the M-SSC(+) group, 100% of both glenoid and acromion showed high stress in the anterior area. In the CTA group, however, all patterns were mixed. Our results suggested that cuff tear shoulders without arthropathy showed certain patterns of stress distribution according to the torn tendons; whereas various patterns were observed once they progress to CTA probably because of the gross instability. We believe the results of this study will provide useful information to understand the pathogenesis of CTA.
