nad a22 4500 551414782 20181214064301.0 cr unu---uuuuu 181214e201812 xx s 000 0 eng 10.5167/uzh-159637 doi 10.1302/0301-620X.100B12.BJJ-2018-0493.R1 doi (ZORA)oai:www.zora.uzh.ch:159637 610 ddc 30499319 pmid The lever arm ratio of the rotator cuff to deltoid muscle explains and predicts pseudoparalysis of the shoulder [Elektronische Daten] [S Bouaicha, L Ernstbrunner, L Jud, D C Meyer, J G Snedeker, E Bachmann] closedAccess eu-repo AIMS In patients with a rotator cuff tear, tear pattern and tendon involvement are known risk factors for the development of pseudoparalysis of the shoulder. It remains unclear, however, why similar tears often have very different functional consequences. The present study hypothesizes that individual shoulder anatomy, specifically the moment arms (MAs) of the rotator cuff (RC) and the deltoid muscle, as well as their relative recruitment during shoulder abduction, plays a central role in pseudoparalysis. MATERIALS AND METHODS Biomechanical and clinical analyses of the pseudoparalytic shoulder were conducted based on the ratio of the RC/deltoid MAs, which were used to define a novel anatomical descriptor called the Shoulder Abduction Moment (SAM) index. The SAM index is the ratio of the radii of two concentric spheres based on the centre of rotation of the joint. One sphere captures the humeral head (numerator) and the other the deltoid origin of the acromion (denominator). A computational rigid body simulation was used to establish the functional link between the SAM index and a potential predisposition for pseudoparalysis. A retrospective radiological validation study based on these measures was also undertaken using two cohorts with and without pseudoparalysis and massive RC tears. RESULTS Decreased RC activity and improved glenohumeral stability was predicted by simulations of SAM indices with larger diameters of the humeral head, being consequently beneficial for joint stability. Clinical investigation of the SAM index showed significant risk of pseudoparalysis in patients with massive tears and a SAM < 0.77 (odds ratio (OR) 11). CONCLUSION The SAM index, which represents individual biomechanical characteristics of shoulder morphology, plays a determinant role in the presence or absence of pseudoparalysis in shoulders with massive RC tears. Balgrist University Hospital, Swiss Spinal Cord Injury Center zora Bouaicha S. joint author Ernstbrunner L. joint author Jud L. joint author Meyer D C. joint author Snedeker J G. joint author Bachmann E. joint author The Bone & Joint Journal 100-B(12):1600-1608 https://www.zora.uzh.ch/id/eprint/159637/1/2018_Bouaicha_The_Lever_Arm_Raatio_Of_The_Rotator_Cuff_BJJ.pdf text/html WWW-Backlink auf das Repository 1 Journal Article PeerReviewed zora ZORA 856 40 https://www.zora.uzh.ch/id/eprint/159637/1/2018_Bouaicha_The_Lever_Arm_Raatio_Of_The_Rotator_Cuff_BJJ.pdf text/html WWW-Backlink auf das Repository ZORA 700 1- Bouaicha S. joint author ZORA 700 1- Ernstbrunner L. joint author ZORA 700 1- Jud L. joint author ZORA 700 1- Meyer D C. joint author ZORA 700 1- Snedeker J G. joint author ZORA 700 1- Bachmann E. joint author ZORA 773 0- The Bone & Joint Journal 100-B(12):1600-1608 BK010053 XK010053 XK010000 ZORA ZORA ZORA Journal Article closedAccess