BACKGROUND: This study aimed to investigate the biomechanical effect of subacromial spacer (physically depressing the humeral head) and magnets (concavity compression restoration) in massive rotator cuff tear (MRCT). We assessed the influence on superior stability by employing the subacromial spacer (the spacer effect) and magnets (concavity compression effect) separately or in combination in MRCT. METHODS: A customized shoulder testing system tested seven fresh-frozen cadaveric shoulders. The prosthesis implantation was performed prior to all experimental conditions. Each specimen underwent the following conditions: (1) intact rotator cuff without magnets, (2) MRCT without magnets
(3) MRCT with magnets, (4) MRCT with a spacer, (5) MRCT with magnets and a spacer. For each condition, superior migration and subacromial contact pressure were measured at 0°, 30°, and 60° abduction angles. RESULTS: Condition 2 resulted in a significant increase in both superior migration and subacromial contact pressure compared to condition 1. Condition 3 exhibited no significant differences in both parameters compared to condition 2 (p >
0.05). In condition 4, both parameters significantly decreased (p <
0.05), and in condition 5, levels were restored to those of the intact condition with no significant difference. Neither parameter between conditions 4 and 5 differed significantly (p >
0.05). CONCLUSION: The combination of the static (spacer) and dynamic (concavity compression) effect demonstrated restoration of superior migration and subacromial contact pressure after MRCT to levels comparable to the intact condition. However, the subacromial spacer alone also showed positive results, and the influence of concavity compression was minimal. To achieve superior stability in MRCT, the static role (spacer) as superior restraint is more critical for rotator cuff function than the dynamic role (concavity compression).