Simulated tears of antero-posterior rotator cuff force-couple and reduced glenoid concavity decrease anterior glenohumeral stability: a robot-assisted biomechanical analysis of a load and shift sequence

Abstract Backround The glenoid concavity and the compression applied by the rotator cuff (RC) are essential for glenohumeral stability (GHS). This study aimed to determine how different simulated rotator cuff tears (RCT) and the glenoid depth influence GHS. Methods A Load and Shift sequence was performed with eight fresh-frozen cadaveric shoulders in a robotic-assisted setup. Differently configured static loading of the reinforced RC and deltoid muscle (DLT) simulated intact RC and anterior, superior, anterosuperior, posterosuperior, mass, and complete RC plus DLT tears. Anterior dislocation forces and their changes were determined as indicators of stability. The glenoid depth and Bony shoulder stability ratio (BSSR) were defined as indicators of concavity. To assess GHS, the maximal force ( F avg ), the maximal force increase ( dF avg ), and their mean deviations ( ΔF max , ΔdF max ) to the intact configuration during the sequence were evaluated. Results Simulated tears of the subscapularis tendon (SCP) ( ΔF max = 7.90 N, ΔdF max = 1.54 N/mm) and simulated tears of the infraspinatus (ISP) + teres minor (TM) tendons ( ΔF max = 7.19 N, ΔdF max = 1.48 N/mm) resulted in greater differences to the intact shoulder than simulated tears of the supraspinatus tendon ( ΔF max = 3.82 N, ΔdF max = 1.16 N/mm). High correlations were observed between maximal force concerning glenoid depth ( r = 0.81) and BSSR ( r = 0.79). Conclusion Simulated tears of the SCP or ISP + TM significantly affect the anterior GHS in this model. These findings highlight the importance of careful evaluation regarding the indication for surgical reconstruction in such configurations. Decreased glenoid concavity reduces the anterior GHS and should be considered in treatment algorithms for shoulder instability. Study design Controlled Laboratory Study. Level of evidence IV.