LIGAMENTOUS RESTRAINTS AND MUSCLE EFFECTS LIMITING EXTERNAL ROTATION OF THE GLENOHUMERAL JOINT IN THE NEUTRAL AND ABDUCTED POSITIONS

Participants: J.E. Kuhn, L.J. Huston, L.J. Soslowsky, R.B. Blasier

Keywords: shoulder, ligaments, muscle, biomechanical model

Introduction

Pathology in the thrower’s shoulder is believed to involve damage to the anterior band of the inferior glenohumeral ligament with subsequent instability and secondary impingement. Clinical examination of throwing athletes demonstrates significantly more external rotation in the abducted dominant arm compared to the non-dominant extremity. These findings suggest that the pathology in the thrower’s shoulder may result from damage to restraints of external rotation, and not to restraints of anterior translation. Thus, the purpose of this study was to determine the ligamentous restraints and rotator cuff and biceps muscle effects on limiting external rotation of the glenohumeral joint in the neutral and abducted positions.

Materials and Methods

Fifteen unpaired shoulders (age 56+19 years; 8 male and 7 female) were dissected, leaving the glenohumeral and coracohumeral ligament, rotator cuff and biceps tendons, scapula, and humerus intact. The scapula was mounted in a fixture which allowed adjustments in glenohumeral abduction and the application of loads to the subscapularis, supraspinatus, and combined teres minor and infraspinatus, using weights and pulleys. The biceps was loaded with a pneumatic cylinder. The humerus was mounted to a sliding universal joint in series with a servohydraulic materials torsion testing machine. Specimens were preconditioned for each test. Standard loads of 22N were applied to each rotator cuff and biceps tendon. A reference position was established as that rotational position obtained with 113 Nm of torque (a value which pilot studies showed reproducibly locates the toe-region of the torque-rotation curve). Testing for each specimen was carried out at 1°/second over the range of -70° to +20° of external rotation from the reference position.

The experimental result is the torque-rotation curve and alterations to the curve caused by changing test conditions. Individual muscle effects were tested by varying the force in each test tendon through a range of 0, 50%, 100%, and 200% of the standard load. In each specimen, a single randomly chosen ligament was cut: coracohumeral (CHL), combined superior and middle glenohumeral (S+M), entire inferior glenohumeral (IGHL), anterior band of IGHL (IGHL-A), or posterior capsule (PC). Every specimen was tested in both neutral and abduction, with a range of various forces in each cuff or biceps tendon, both before and after ligament cutting. Analysis of the muscle effects was carried out over the first 70° of motion. Peak torque at +20° was used to evaluate the effects of sectioning the ligament. Statistical analysis was performed using ANOVA with Tukey post hoc correction for multiple comparisons.

Results

In the neutral position, the IHGL and CHL had significantly greater effects at limiting external rotation than the PC. Force in the subscapularis acted as a restraint to external rotation in the early ranges of motion. The external rotators decreased the torque required to passively externally rotate the shoulder. The supraspinatus acted as a weak external rotator. The biceps had a bimodal curve, acting initially as a restraint to external rotation then acting as an external rotator in the higher ranges of external rotation.

In the abducted shoulder, the IGHL provided the greatest restraint to external rotation and had a significantly greater effect than the CHL, S+M, and PC. The subscapularis acted as a restraint to external rotation. The external rotators reduced the torque required to passively externally rotate the shoulder. The supraspinatus and biceps had little effect on the force required to rotate the abducted shoulder.

Conclusions

The IHGL is an important restraint to external glenohumeral rotation, particularly in the abducted shoulder. Injury to the IGHL may allow excess external rotation, and excess external rotation may injure the IGHL. The subscapularis and external rotators behave predictably, with the subscapularis resisting and the external rotators promoting external rotation. The biceps and supraspinatus have effects on external rotation in the neutral position, but have little effect on the forces required to externally rotate the abducted shoulder.