Rotator cuff muscle imbalance associates with shoulder instability direction

Area of Science:

• Orthopedics
• Sports Medicine
• Anatomy

Background:

• Rotator cuff muscle dysfunction is implicated in shoulder instability.
• The precise relationship between muscular imbalance and specific shoulder instability types remains unclear.

Purpose of the Study:

• To assess rotator cuff and deltoid muscle cross-sectional areas in patients with anterior, posterior, and multidirectional instability (MDI).
• To investigate associations between muscular imbalance and the direction of shoulder instability.

Main Methods:

• Magnetic resonance images of 189 patients with shoulder instability were analyzed.
• Rotator cuff (supraspinatus, subscapularis, infraspinatus + teres minor) and deltoid muscle areas were quantified.
• Ratios of subscapularis to infraspinatus + teres minor and anterior to posterior deltoid areas were calculated to assess imbalance.

Main Results:

• Anterior instability group showed smaller posterior rotator cuff muscles (infraspinatus + teres minor) compared to the subscapularis.
• Posterior instability and MDI groups exhibited smaller anterior rotator cuff muscles (subscapularis) compared to the posterior rotator cuff.
• All groups demonstrated smaller anterior deltoid muscle areas than posterior deltoid areas.

Conclusions:

• Shoulder instability direction correlates with rotator cuff muscle size.
• Anterior instability is associated with relative posterior rotator cuff weakness.
• Posterior and MDI are linked to relative anterior rotator cuff weakness.