Volumetric evaluation of the rotator cuff musculature in massive rotator cuff tears with pseudoparalysis.

Background: If the balance of the rotator cuff force couple is disrupted, pseudoparalysis may occur, but the exact mechanism remains unknown. This study investigated the effect of rotator cuff force couple disruption on active range of motion in massive rotator cuff tear (mRCT) by rotator cuff muscle volume analysis.

Methods: The study included 53 patients with irreparable mRCT: 22 in the nonpseudoparalysis group and 31 in the pseudoparalysis group. The volumes of the subscapularis (SBS), infraspinatus (ISP), and teres minor (TM) muscles were measured using magnetic resonance imaging (MRI), and the ratios of each muscle volume to the anatomic external rotator (aER) volume were calculated. A control group of 25 individuals with normal rotator cuffs was included.

Results: Anterior-to-posterior cuff muscle volume ratio (SBS/ISP + TM) was imbalanced in both mRCT groups (1.383 nonpseudoparalysis and 1.302 pseudoparalysis). Between the 2 groups, the ISP/aER ratio (0.277 vs. 0.249) and the inferior SBS/aER ratio (0.426 vs. 0.390) were significantly decreased in the pseudoparalysis group (P= .022 and P= .040, respectively). However, neither the TM/aER ratio (0.357 vs. 0.376) nor the superior SBS/aER ratio (0.452 vs. 0.424) showed a significant difference between the two groups (P= .749 and P= .068, respectively). If the inferior SBS was torn, a high frequency of pseudoparalysis was noted (81.0%, P= .010).

Conclusion: The disruption of transverse force couple was noted in both irreparable mRCT groups, although no significant difference was found between the nonpseudoparalysis and pseudoparalysis groups. ISP and inferior SBS muscle volumes showed a significant decrease in pseudoparalysis group and, therefore, were considered to greatly influence the loss of active motion in mRCT. The TM did not exert significant effect on the incidence of pseudoparalysis.