Abstract
ObjectiveAfter reverse shoulder arthroplasty (RSA) external and internal rotation will often remain restricted. A postoperative alteration of the biomechanics in the remaining cuff is discussed as a contributing factor to these functional deficits.MethodsIn this study, muscle moment arms as well as origin-to-insertion distance (OID) were calculated using three-dimensional models of the shoulder derived from CT scans of seven cadaveric specimens.ResultsMoment arms for humeral rotation are significantly smaller for the cranial segments of SSC and all segments of TMIN in abduction angles of 30 degrees and above (p ≤ 0.05). Abduction moment arms were significantly decreased for all segments (p ≤ 0.002). OID was significantly smaller for all muscles at the 15 degree position (p ≤ 0.005), apart from the cranial SSC segment.ConclusionsReduced rotational moment arms in conjunction with the decrease of OID may be a possible explanation for the clinically observed impaired external and internal rotation.
Highlights
Promising early functional results can be achieved with reverse shoulder arthroplasty (RSA), especially in patients with severe cuff tear arthropathy[1,2,3]
Subscapularis There was a significant change of abduction moment arm values for all three muscle segments in all tested positions after reverse arthroplasty (p ≤ 0.0012), except for the most cranial segment at 60 degree abduction (p = 0.86)(Figure 3)
Postoperative rotational moment arms of the two more cranial segments were significantly smaller at all positions (p ≤ 0.05), whereas no difference could be seen for the distal segment (p ≥ 0.45)
Summary
Promising early functional results can be achieved with reverse shoulder arthroplasty (RSA), especially in patients with severe cuff tear arthropathy[1,2,3]. It is a salvage procedure for fracture sequelae[4,5,6,7] and revision of failed hemiarthroplasty[8,9], even though outcome is less predictable in these patients. The deltoid’s proportion, contributing to active elevation, is enlarged and the hemispheric design provides stability and constraint These changes result in a significantly improved ability to actively abduct and forward-flex the arm[11], while internal and external rotation often remains impaired or even decreases postoperatively[12]
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