Principles for the evaluation and management of shoulder instability

Area of Science:

• Orthopedics
• Biomechanics
• Sports Medicine

Background:

• Normal shoulder function relies on the humeral head being centered within the glenoid and coracoacromial arch.
• Shoulder instability compromises normal upper extremity function.
• Joint laxity, a normal property allowing full range of motion, is distinct from shoulder instability.

Purpose of the Study:

• To define shoulder instability and differentiate it from joint laxity.
• To explain the concavity-compression mechanism responsible for maintaining shoulder centering.
• To identify factors that can compromise shoulder centering and lead to instability.

Main Methods:

• Review of anatomical and biomechanical principles of shoulder joint stability.
• Analysis of the concavity-compression mechanism.
• Identification of etiological factors contributing to shoulder instability.

Main Results:

• Shoulder instability is characterized by the inability to maintain the humeral head's centered position within the glenoid.
• The concavity-compression mechanism, involving glenoid and coracoacromial arch integrity, muscular forces, and ligaments, is crucial for centering.
• Compromise of these elements due to developmental, degenerative, traumatic, or iatrogenic factors can cause shoulder instability.

Conclusions:

• Shoulder instability arises from a failure of the mechanisms that center the humeral head.
• Preservation of the glenoid, coracoacromial arch, muscles, and ligaments is essential for shoulder stability.
• Understanding these factors is key to addressing shoulder instability.