Rodosky and colleagues tested whether the long head of the biceps and its anchor on the superior labrum help stabilize the shoulder in the overhead position.
Dynamic cadaveric study of 7 shoulders from 5 male donors aged 45 to 60 (mean 55.6).
Methods
Model
7 cadaveric shoulders from 5 male cadavers
Mean donor age 55.6
Simulated rotator cuff and long head of biceps forces
Constructs
Intact superior labrum
Superior labrum and biceps origin stripped from 10 to 2 o'clock, retested in the same shoulders
Loading
External torque rotating the humerus from about 70° to 90° of external rotation at about 90° of abduction
Biceps force at 0%, 25%, 50%, 75% and 100% of predicted maximum
Outcome measures
Peak external rotation torque
Torsional rigidity
Strain in the anterior superior band of the inferior glenohumeral ligament
Key Findings
Raising biceps force to 100% of predicted increased the peak torque needed to externally rotate the shoulder by 22%, from 8.04 to 9.85 N‑m (p < 0.01).
Torsional rigidity rose 32% with full biceps force, from 18.59 to 24.50 N‑m/radian (p = 0.0106).
Inferior glenohumeral ligament strain stayed essentially constant as larger external rotation torques were applied with rising biceps force, suggesting the biceps shares the load.
With a superior labral lesion, inferior glenohumeral ligament strain was 120% larger than in intact shoulders at zero biceps force and 102% larger at full force (p < 0.01 at each level).
Intact shoulders needed 12% more peak torque than lesioned shoulders at zero biceps force, a gap that narrowed to 7% at full force (p < 0.01 at each level).
Torsional rigidity was 19% higher in intact shoulders at zero biceps force and 11% higher at maximal biceps force (p < 0.01).
Board pearl
In abduction and external rotation, a superior labral lesion more than doubled strain in the inferior glenohumeral ligament at every level of biceps force.
Authors' conclusion
Our dynamic study suggests that the long head of the biceps muscle contributes to anterior shoulder stability by increasing the torsional rigidity and thereby resisting the excessive external rotatory forces occurring in the vulnerable abducted and externally rotated position. The biceps muscle may also play a protective role by diminishing the stress placed on the inferior glenohumeral ligament. The superior labral lesion is detrimental to anterior shoulder stability as it decreases the shoulder's torsional rigidity and places a greater magnitude of strain on the inferior glenohumeral ligament.
Rodosky and colleagues tested whether the long head of the biceps and its anchor on the superior labrum help stabilize the shoulder in the overhead position.
Dynamic cadaveric study of 7 shoulders from 5 male donors aged 45 to 60 (mean 55.6).
Methods
Model
7 cadaveric shoulders from 5 male cadavers
Mean donor age 55.6
Simulated rotator cuff and long head of biceps forces
Constructs
Intact superior labrum
Superior labrum and biceps origin stripped from 10 to 2 o'clock, retested in the same shoulders
Loading
External torque rotating the humerus from about 70° to 90° of external rotation at about 90° of abduction
Biceps force at 0%, 25%, 50%, 75% and 100% of predicted maximum
Outcome measures
Peak external rotation torque
Torsional rigidity
Strain in the anterior superior band of the inferior glenohumeral ligament
Key Findings
Raising biceps force to 100% of predicted increased the peak torque needed to externally rotate the shoulder by 22%, from 8.04 to 9.85 N‑m (p < 0.01).
Torsional rigidity rose 32% with full biceps force, from 18.59 to 24.50 N‑m/radian (p = 0.0106).
Inferior glenohumeral ligament strain stayed essentially constant as larger external rotation torques were applied with rising biceps force, suggesting the biceps shares the load.
With a superior labral lesion, inferior glenohumeral ligament strain was 120% larger than in intact shoulders at zero biceps force and 102% larger at full force (p < 0.01 at each level).
Intact shoulders needed 12% more peak torque than lesioned shoulders at zero biceps force, a gap that narrowed to 7% at full force (p < 0.01 at each level).
Torsional rigidity was 19% higher in intact shoulders at zero biceps force and 11% higher at maximal biceps force (p < 0.01).
Board pearl
In abduction and external rotation, a superior labral lesion more than doubled strain in the inferior glenohumeral ligament at every level of biceps force.
Authors' conclusion
Our dynamic study suggests that the long head of the biceps muscle contributes to anterior shoulder stability by increasing the torsional rigidity and thereby resisting the excessive external rotatory forces occurring in the vulnerable abducted and externally rotated position. The biceps muscle may also play a protective role by diminishing the stress placed on the inferior glenohumeral ligament. The superior labral lesion is detrimental to anterior shoulder stability as it decreases the shoulder's torsional rigidity and places a greater magnitude of strain on the inferior glenohumeral ligament.