Stabilising function of the biceps in stable and unstable shoulders

Area of Science:

• Orthopedic Surgery
• Biomechanics
• Sports Medicine

Background:

• The glenohumeral joint's stability is crucial for shoulder function.
• The role of the biceps brachii muscle heads in anterior shoulder stability is not fully elucidated.

Purpose of the Study:

• To investigate the quantitative contributions of the long head of the biceps (LHB) and short head of the biceps (SHB) to anterior glenohumeral stability.
• To assess how capsular integrity and external rotation affect the stabilizing function of the biceps heads.

Main Methods:

• Utilized 13 cadaver shoulders, replacing biceps heads with spring devices.
• Performed anterior translational force tests (1.5 kg) at 90 degrees abduction.
• Monitored humeral head position using electromagnetic tracking under varying loads (0-3 kg), external rotation (60-120 degrees), and capsular conditions (intact, vented, Bankart lesion).

Main Results:

• Both LHB and SHB loading significantly reduced anterior displacement at 60 and 90 degrees external rotation across all capsular states.
• At 120 degrees external rotation, stabilization was significant only in the presence of a Bankart lesion.
• The stabilizing effect of both biceps heads increased as shoulder stability decreased.

Conclusions:

• The LHB and SHB act as significant anterior stabilizers of the glenohumeral joint, particularly during abduction and external rotation.
• Their stabilizing role becomes more pronounced with diminished shoulder stability, such as in cases of capsular damage.
• Strengthening the biceps should be considered in rehabilitation for chronic anterior shoulder instability.