In this paper the authors sought to determine the rotational moment arms, the origin-to-insertion distances of the teres minor and subscapularis, and the flexion and abduction moment arms. In seven cadaveric shoulder specimens. Three-dimensional shoulder surface models were created from CT scans before and after implantation. The implant investigated is shown below.
Inspection of this illustration in relation to the scaplar anatomy makes us wonder how contact between the medial humeral component and the lateral scapula (notching) is avoided with this implant.
While the authors did not examine the effects of non-lateralized glenospheres in this study, a prior study, Reverse shoulder arthroplasty leads to significant biomechanical changes in the remaining rotator cuff., found that moment arms for humeral rotation were significantly smaller for the cranial segments of the subscapularis and the teres minor in abduction angles of 30 degrees and above. Origin to insertion distances were significantly smaller for all muscles. These findings may be a possible explanation for the clinically observed impaired external and internal rotation when the center of rotation
In their study, the authors achieved lateralization using an 8 mm thick block implanted between the baseplate of the glenosphere and the glenoid. Not only does this lateralization reduce the risk of notching but the authors found that after lateralized reversed total shoulder arthroplasty, the subscapularis and teres minor maintained their length and rotational moment arms, their flexion forces were increased, and abduction capability decreased. They suggested that these finding might explain clinically improved rotation in lateralized RSA in comparison to nonlateralized reverse total shoulders.
It is of interest that different designs of reverse total shoulder result in different locations of the center of rotation, these differences result in differences in stability and as this recent article shows, differences in the moment arms for the residual external rotators. Thus it is important to understand these effects and to know the reverse system you are using. Some designs of reverse total shoulder have the lateralization built into the design of the glenoid component, so that a bone graft (with possible risk of resorption or collapse) is not needed.
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