Does the Walch type B shoulder have a transverse force couple imbalance? - a volumetric analysis of segmented rotator cuff Muscles in osteoarthritic shoulders

Antonio Arenas-Miquelez, Victor K. Liu, Joseph Cavanagh, Petra L. Graham, Louis M. Ferreira, Desmond J. Bokor, George S. Athwal, Sumit Raniga

Research output: Contribution to journalArticlepeer-review


Background: The etiology of the Walch type B shoulder remains unclear. We hypothesize that a scapulohumeral muscle imbalance, secondary to a disturbed transverse force couple (TFC) between the anterior and posterior rotator cuff muscles, may have a role in the pathogenesis of the type B morphology. The purpose of this study was to determine whether there is a transverse force couple imbalance in the Walch type B shoulder using an imaging-based 3D volumetric and fatty infiltration assessment of segmented rotator cuff muscles.
Methods: CT images of 33 Walch type A and 60 Walch type B shoulders with the complete scapula and humerus including the distal humeral epicondyles were evaluated. The 3D volumes of the entire subscapularis, supraspinatus, infraspinatus and teres minor were manually segmented and analyzed. Additionally, anthropometric parameters including glenoid version, glenoid inclination, posterior humeral head subluxation and humeral torsion were measured. The 3D muscle analysis was then compared to the anthropometric parameters using Wilcoxon rank-sum and Kruskal-Wallis tests.
Results: There were no significant differences (p>0.200) in muscle volume ratios between the infraspinatus-teres minor (Infra-Tm) and the subscapularis in Walch type A (0.93) and type B (0.96) shoulders. The fat infiltration percentage ratio, however, was significantly greater in type B shoulders (0.94 vs 0.75; p<0.001). The Infra-Tm to subscapularis fat infiltration percentage ratio was significantly larger in patients with >75% humeral head subluxation than 60-75% head subluxation (0.97 vs 0.74, p<0.001) and those with >25° retroversion than ≤15° retroversion (1.10 vs 0.75, p=0.004). The supraspinatus fatty infiltration percentage was significantly lower in Walch type B shoulders than type A shoulders (p=0.004). The Walch type A Shoulders had a mean humeral retrotorsion of 22±10°, while the Walch B types had a humeral retrotorsion of only 14±9° relative to the epicondylar axis (p<0.001).
Conclusion: The transverse force couple is in balance in the Walch type B shoulder in terms of 3D volumetric rotator cuff muscle analysis, however, the posterior rotator cuff did demonstrate increased fatty infiltration. Posterior humeral head subluxation and glenoid retroversion, which are pathognomonic of the Walch type B shoulder may lead to a disturbance in the length-tension relationship of the posterior rotator cuff causing fatty infiltration.
Original languageEnglish
JournalJournal of Shoulder and Elbow Surgery
Publication statusE-pub ahead of print - 3 Mar 2021


  • Walch B shoulder
  • Shoulder Osteoarthritis
  • transverse force couple
  • muscle volume
  • fatty infiltration
  • CT images

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