Complex interactions between dynamic and static stabilizers of the shoulder girdle are critical to produce a biomechanically complex system, allowing for sufficient range of motion in multiple planes. As such, detailed knowledge on the biomechanical consequences of non-retracted, isolated, and massive and retracted posterosuperior rotator cuff tears (RCT) in the glenohumeral joint is needed.
The purpose of the study was to assess the biomechanical consequences of isolated, massive, non-retracted RCT (according to Patte I) and irreparable, retracted posterosuperior RCT (Patte III) on the glenohumeral joint using a validated, dynamic shoulder testing system.
Study Design Controlled laboratory study
Eight fresh-frozen cadaveric shoulders were tested using a dynamic shoulder simulator. Each shoulder was tested in the following conditions: (1) Intact state; (2) Isolated non-retracted supraspinatus tendon (SSP) defect; (3) Isolated non-retracted subscapularis tendon (SSC) defect; (4) Isolated non-retracted infraspinatus tendon (ISP) defect; (5) Massive non-retracted RCT involving all three tendons; (6) irreparable, retracted posterosuperior RCT. Four parameters were measured in each cadaveric shoulder for each testing condition: (1) maximum glenohumeral abduction angle (degrees), (2) glenohumeral superior translation (%), (3) subacromial peak contact pressure (MPa), and (4) cumulative deltoid force (N).
The SSP, SSC and ISP simulated defects showed a significant increase in total deltoid force, respectively (P = 0.012; P = 0.007; P = 0.001). Compared to the intact state, the massive RCT showed a significant decrease in glenohumeral abduction angle (P < 0.001) and a significant increase in total deltoid force (P < 0.001). The irreparable, retracted posterosuperior RCT showed a significant decrease in glenohumeral abduction angle, significant increase of total deltoid force, subacromial peak contact pressure, and glenohumeral superior translation (P > 0.001, respectively) compared to the intact state.
In a dynamic biomechanical shoulder model, isolated non-retracted RCT, located lateral to the rotator cable, can be sufficiently compensated by the remaining intact cuff. However, in irreparable, massively retracted posterosuperior RCT located medial to the rotator cable, devasting effects on the glenohumeral joint can be expected. Along with increased maximum deltoid forces, increased subacromial peak contact pressure, decreased shoulder function can be expected. As such, surgery should be highly recommended for patients with retracted tears involving the cuff located medial to the rotator cable to improve shoulder function and reduce painful direct bone-to-bone contact.
Key Terms Rotator Cuff Tears; Posterosuperior Rotator Cuff Tears; Biomechanics; Dynamic Shoulder Simulator; Superior Capsule, Rotator Cable