Muscle volume imbalance may be associated with static posterior humeral head subluxation

Abstract

Background 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. 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. Methods A 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. Results The 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). Conclusion The 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 evidence Level 4 – Case series with no comparison group.

Identifying key factors associated with subscapularis tendon tears and developing a risk prediction model to assist diagnosis

“…Rotator cuff injury is a common cause of shoulder pain and dysfunction. As the largest and most powerful tendon in the rotator cuff, the subscapularis (SSC) tendon is an important component in maintaining transverse force couple balance and plays an important role in the stability of the glenohumeral joint and internal rotation [ 1 ]. Approximately 12–50% of patients were reported to have SSC tendon tears undergoing arthroscopy [ 2 – 4 ].…”

Correction of Static Posterior Shoulder Subluxation by Restoring Normal Scapular Anatomy Using Acromion and Glenoid Osteotomies

“…The etiology underlying SPSL with eccentric OA has been debated for years 1,5,8,9,13 . Recent studies have documented that acromial anatomy of shoulders with dynamic and SPSL and/or early eccentric OA differs substantially and typically from healthy shoulders 10,21,22,24 .…”

“…The reasons for SPSL of the humeral head and thereby the etiology of eccentric OA are unknown. A multifactorial etiology is postulated 1,5,[8][9][10][11] : The focus has been on glenoid shape alterations 4,8,12,13 , but such changes are neither consistent, nor do they satisfactorily explain the condition. Currently, correction of SPSL is attempted by arthroscopic soft-tissue procedures 14 , restoration of normal glenoid version using scapular neck osteotomies [15][16][17][18] , or posterior bone block procedures 15,19,20 .…”

CORR Insights®: Imbalance in Axial-plane Rotator Cuff Fatty Infiltration in Posteriorly Worn Glenoids in Primary Glenohumeral Osteoarthritis: An MRI-based Study

“…One idea is that alterations in the transverse force couple of the shoulder (the balance between the anterior subscapularis and the posterior infraspinatus/teres minor muscles) may contribute to eccentric arthritis [10]. The forces of these two groups are equally balanced in stable shoulders [11].…”