Backround: The concavity of the glenoid and the concavity compression exerted by the rotator cuff (RC) are important biomechanical factors for glenohumeral stability.
Aim
The influence of different degrees of RC tears (RCT) on glenohumeral joint (GHJ) stability in relation to glenoid depth was to be investigated by this biomechanical cadaveric laboratory study.
Methods
Eight shoulders from humane donors (Mean age 82 ± 8 years) were placed in an item construction for the experiment. The performance of the load & shift sequence and the measurement of the resulting forces were realized with an industrial robot (KR 60-3, KUKA, Augsburg, Germany), a robot-specific software for joint tests (simVITRO, Cleveland Clinic BioRobotics Lab, Ohio, USA) and a force-torque sensor (FTS) (Mini45, ATI Industrial Automation, Apex, USA). A coordinate system aligned with the scapula, in which the robot could move, was created using computed tomographic and anatomical data. The different RCTs were simulated by different configured static loadings of the previously prepared and with FiberWire (US 5, Arthrex, Munich, Germany) reinforced muscles of the RC and the deltoid muscle (DLT) by weights. The weight was defined by the cross-sectional area of the specific muscle. The tests were performed in GH abduction of 60°, and thus low ligamentous securing of the GHJ. The humeral head was centered in the glenoid with a compression force of 10 N generated by the robot. Subsequently, the humeral head was pushed anteriorly by 90% of half the glenoid width starting from its initial position at a speed of 1 mm/s generated by the robot, after which the humeral head passed through the initial position again and was moved posteriorly by 50% of half the glenoid width. To assess stability, the maximum force (Favg), maximum force change (dFavg) of the anterior dislocation force at differently configured simulated RCT, and their mean deviation (DFmax, DdFmax) from the intact shoulder during the sequence were evaluated. Glenoid depth was determined as an indication of concavity.
Results
For a loaded RT as well as a loaded DLT, a Favg of 72.37 ± 16.29 N was measured. For a shoulder joint without any loading of the RT and DLT, a Favg of 40.94 ± 15.67 N was determined. Simulated ruptures of the subscapularis muscle (SSC, DFmax = 7.90N or DdFmax = 1.54 N/mm) and the infraspinatus and teres minor muscles (ISP teres minor (ISP+TM, DFmax = 7.19 N or DdFmax = 1.48 N/mm) showed a greater difference from the intact shoulder than simulated ruptures of the supraspinatus muscle (DFmax = 3.82N or DdFmax = 1.16 N/mm). The dependence of the maximum force and the increase in force on the glenoid depth was shown in a linear model. A high correlation with an r = 0.81 was calculated for the force maximum, while the force increase showed a significantly poorer correlation with an r = 0.58.
Conclusion
Ruptures of the SSC as well as the ISP+TM significantly affect GH stability; therefore the study emphasizes the need for surgical reconstruction. The glenoid concavity has a decisive influence on GH stability and should therefore be included in future treatment algorithms for shoulder instability. The surgeon should consider not only the location, shape, and age of soft tissue lesions, such as RCTs, labral, ligamentous lesions, bone loss, and shape-but also, in particular, the concavity of the glenoid.