Biomechanical effects of different rotator cuff tear conditions and mediolateral offset in lateralized reverse total shoulder arthroplasty (Coralis Reverse Shoulder System).

BACKGROUND

The indications for reverse total shoulder arthroplasty (rTSA) have expanded to include various shoulder pathologies regardless of rotator cuff condition. However, the biomechanical effects of the procedure based on the extent of the rotator cuff tear remain uncertain in medialized rTSA (m-rTSA) and lateralized rTSA (l-rTSA). Furthermore, the necessity of subscapularis repair for l-rTSA has been debated. Therefore, this study aimed to quantify the biomechanical characteristics of m-rTSA and l-rTSA based on the extent of the rotator cuff tear and subscapularis repair.

METHODS

Four matched pairs of human cadaveric shoulders were subjected to m-rTSA and l-rTSA. Both m-rTSA and l-rTSA were modeled by adjusting the lateralization offset in a single type of lateralized rTSA (Coralis Reverse Shoulder System; Corentec, Seoul, Republic of Korea). Deltoid and rotator cuff conditions were simulated using specific loads on various muscle segments. Seven rotator cuff conditions were replicated according to the extent of the rotator cuff tear: intact, complete supraspinatus tear, complete supraspinatus tear with a superior half infraspinatus tear, and complete supraspinatus and infraspinatus tear, each with or without subscapularis repair. Deltoid length, for indirect assessment of the risk of acromial or scapular spine stress fracture (ASF), and impingement-free range of motion were measured in m-rTSA and l-rTSA. Joint stability was assessed using the anterior dislocation force; abduction capability was evaluated using the load required for initial and maximal abductions. Joint stability and abduction capability were measured for all combinations of the rTSA type and rotator cuff conditions.

RESULTS

L-rTSA, compared with m-rTSA, demonstrated increased deltoid lengths (all P < .05) and decreased impingement-free abduction angles (m-rTSA vs. l-rTSA, 80.2° ± 8.0° vs. 72.6° ± 8.4°; P = .039). Notably, joint stability and abduction capability showed an inverse relationship with the extent of the rotator cuff tear (all P < .05) and were improved by subscapularis repair to the same extent as the rotator cuff tear (all P < .05).

CONCLUSION

The extent of implant lateralization affected deltoid length and impingement-free abduction angle. Joint stability and abduction capability were compromised by more extensive rotator cuff tears, and subscapularis repair might be essential to enhancing biomechanical effectiveness, even in l-rTSA. These biomechanical findings provide insights for optimizing implant lateralization and refining surgical technique selection in rTSA. However, further clinical validation, including comparisons of different rTSA designs, may be necessary to extrapolate the findings on the Coralis implant to other rTSA systems.