The Long Head of the Biceps Has a Stabilizing Effect on the Glenohumeral Joint in Simulated Infraspinatus or Subscapularis but Not Supraspinatus Rotator Cuff Deficiency—A Biomechanical Study

Abstract

To investigate the stabilizing role of the long head of the biceps (LHB) for different simulated rotator cuff (RC) tears. Human cadaveric specimens (n= 8) were fixed in a robotic-based experimental setup with a static loading of the RC, deltoid, and the LHB. RC tears were simulated by unloading of the corresponding muscles. A throwing motion and an anterior load-and-shift test were simulated under different RC conditions by unloading the supraspinatus (SS), subscapularis (SSc), infraspinatus (IS), and combinations (SS+ SSc, SS+ IS, SS+ SSc+ IS). The LHB was tested in 3conditions: unloaded, loaded, and tenotomy. Translation of the humeral head and anterior forces depending on loading of the RC and the LHB was captured. Loading of LHB produced no significant changes in anterior force or glenohumeral translation for the intact RC or a simulated SS tear. However, if SSc or IS were unloaded, LHB loading resulted in a significant increase of anterior force ranging from 3.9 N (P= .013, SSc unloaded) to 5.2 N (P= .001, simulated massive tear) and glenohumeral translation ranging from 2.4 mm (P= .0078, SSc unloaded) to 7.4 mm (P=.0078, simulated massive tear) compared to the unloaded LHB. Tenotomy of the LHB led to a significant increase in glenohumeral translation compared to the unloaded LHB in case of combined SS+ SSc (2.6 mm, P= .0391) and simulated massive tears of all SS+ SSc+ IS (4.6 mm, P= .0078). Highest translation was observed in simulated massive tears between loaded LHB and tenotomy (8.1 mm, P= .0078). Once SSc or IS is simulated to be torn, the LHB has a stabilizing effect for the glenohumeral joint and counteracts humeral translation. With a fully loaded RC, LHB loading has no influence. With an intact RC, the condition of the LHB showed no biomechanical effect on the joint stability. Therefore, from a biomechanical point of view, the LHB could be removed from the joint when the RC is intact or reconstructable. However, since there was a positive effect even of the unloaded LHB in this study when SSc or IS is deficient, techniques with preservation of the supraglenoid LHB origin may be of benefit in such cases.