The Influence of the Relative Deltoid Moment Arm on Active Motion in Patients with Massive Cuff Tears.

Background: The underlying mechanisms for why certain patients with massive cuff tears develop pseudoparesis are unclear. A recent biomechanical study described the shoulder abduction moment (SAM) index, which considers the deltoid and rotator cuff moment arms based on a patient's specific anatomy as measured on a plain XR Grashey view. The purpose of this study was to clinically evaluate the correlation of pseudoparesis with the SAM index in patients with massive cuff tears.

Methods: This was a single surgeon retrospective study of 105 patients presenting with MRI-diagnosed massive cuff tears. A massive cuff tear was defined as a full thickness tear of the rotator cuff involving two or more tendons. Pseudoparesis was defined as inability to achieve active elevation of 90° with retained passive motion beyond 90° of elevation. Thirty-six patients (34%) presented with pseudoparesis. Six radiographic measurements including the SAM index and demographics were evaluated as risk factors for pseudoparesis.

Results: Univariate analysis showed no significant differences in demographics between those with and without pseudoparesis. Patients with pseudoparesis more commonly opted for surgery instead of physical therapy as their initial treatment (44% vs 23%; p = 0.04). For those with pseudoparesis, the median SAM index was significantly lower (0.72 [interquartile range 0.70, 0.76] vs 0.75 [0.74,0.78]; p = 0.001). The receiver operating characteristic curves comparing radiographic measurements' ability to predict pseudoparesis demonstrated that the measurement with the highest area under the curve was the SAM index (0.69; 95% CI 0.59 - 0.80).

Conclusions: The SAM index was found to correlate with the clinical presence of pseudoparesis in patients with massive cuff tears. This index is an indication of the relative moment arms of the deltoid and rotator cuff and may provide insight into the variable presentation of massive cuff tear patients.