To establish and report a dynamic robotic shoulder model, and assess the influence of rotator cuff tear patterns and SCR following a posterosuperior cuff tear on glenohumeral biomechanics. It was hypothesized that a posterosuperior rotator cuff tear would diminish glenohumeral stability when compared to the intact and supraspinatus tendon- deficient state, and that SCR would reverse this instability in the supra- and infraspinatus- deficient shoulder.
Twelve fresh-frozen cadaveric shoulders were tested using a 6-degrees-of-freedom robotic arm (KR 60-3; KUKA Robotics). Kinematic testing was performed in 4 conditions: (1) intact, (2) simulated irreparable supraspinatus tendon tear, (3) simulated irreparable supra- and infraspinatus tendon tear, and (4) SCR using a 3 mm thick dermal allograft (DA). Kinematic testing consisted of dynamic flexion and static 40-N superior force tests at neutral abduction, 30°, 60° and 90° of abduction. In each test, superior glenoid translation was measured, and for static testing, linear mixed-effects models were used to compare across repeated measures shoulder conditions at each abduction angle.
In dynamic flexion testing, there was an average increase in superior translation of 2.1±1.7 mm for the supraspinatus cut state, 2.8±1.8 mm for the supra- and infraspinatus cut state, and 1.3±1.7 mm for SCR, when compared to the native state. In static testing, in all degrees of abduction the supraspinatus cut and the supra- and infraspinatus cut states showed a significant increase in superior translation compared to the native state (all p< 0.001). Supra- and infraspinatus cut increased superior translation significantly when compared to supraspinatus cut only in neutral and 60° of abduction (p=0.030 and p=0.022). SCR was able to significantly decrease superior translation in all degrees of abduction when compared to the supra- and infraspinatus cut (all p< 0.02), and in neutral position and 60° of abduction when compared to supraspinatus cut only (all p< 0.05). When compared to the native state, SCR was not able to restore superior stability with a significant increase of translation for neutral position, 30°, and 60° of abduction (p< 0.001).
Superior capsule reconstruction using a dermal allograft partially restored superior stability of the glenohumeral joint in the presence of a simulated massive posterosuperior rotator cuff tear in a dynamic and static robotic shoulder model.