Abstract
BackgroundThe transverse force couple (TFC) of the rotator cuff (subscapularis vs. infraspinatus and teres minor muscle) is an important dynamic stabilizer of the shoulder joint in the anterior-posterior direction. In patients with posterior static subluxation of the humeral head (PSSH), decentration of the humeral head posteriorly occurs, which is associated with premature arthritis. We hypothesize that not only pathologic glenoid retroversion but also chronic muscle volume imbalance in the transverse force couple leads to PSSH.MethodsA retrospective analysis of the TFC muscle volumes of 9 patients with symptomatic, atraumatic PSSH, within 8 were treated with glenoid correction osteotomy, was conducted. The imaging data (CT) of 9 patients/10 shoulders of the full scapula and shoulder were analyzed, and the muscle volumes of the subscapularis (SSC), infraspinatus (ISP) and teres minor muscles (TMM) were measured by manually marking the muscle contours on transverse slices and calculating the volume from software. Furthermore, the glenoid retroversion and glenohumeral distance were measured.ResultsThe mean glenoid retroversion was − 16° (− 7° to − 31°). The observed mean glenohumeral distance was 4.0 mm (0 to 6.8 mm). Our study population showed a significant muscle volume imbalance between the subscapularis muscle and the infraspinatus and teres minor muscles (192 vs. 170 ml; p = 0.005). There was no significant correlation between the subscapularis muscle volume and the glenohumeral distance (r = 0.068), (p = 0.872).ConclusionThe muscle volume of the SSC in patients with PSSH was significantly higher than the muscle volume of the posterior force couple (ISP and TMM). This novel finding, albeit in a small series of patients, may support the theory that transverse force couple imbalance is associated with PSSH.Level of evidenceLevel 4 – Case series with no comparison group.
Highlights
The transverse force couple (TFC) of the rotator cuff is an important dynamic stabilizer of the shoulder joint in the anterior-posterior direction
The glenoid retroversion ranged from − 7° to − 31° and the mean glenohumeral distance was 4.0 mm (Table 2)
It seems that the transverse force couple is balanced, meaning that the muscle volume of the subscapularis muscle and infraspinatus plus teres minor muscle are not significantly different in healthy, nonpathologic shoulders [13]
Summary
The transverse force couple (TFC) of the rotator cuff (subscapularis vs. infraspinatus and teres minor muscle) is an important dynamic stabilizer of the shoulder joint in the anterior-posterior direction. We hypothesize that pathologic glenoid retroversion and chronic muscle volume imbalance in the transverse force couple leads to PSSH. PSSH leads to eccentric loading on the glenoid and arthritis [1]. The classification has been extended to include B0 [3], which corresponds to a prearthritic state of PSSH without signs of arthritis on the glenoid side. It appears logical that classes B0 to B3 correspond to chronologic progression over time, with B0 indicating a prearthritic condition that progresses to B1 and to B3. There is no clear scientific evidence supporting this hypothesis
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