Muscle and joint function in the rotator cuff deficient shoulder

Area of Science:

• Biomechanics
• Orthopedic Surgery
• Human Anatomy

Background:

• Full-thickness rotator cuff tears compromise glenohumeral joint stability by impairing humeral head coaptation.
• Tears disrupt normal muscle force dynamics, potentially leading to joint subluxation and altered joint mechanics.

Purpose of the Study:

• To quantify changes in muscle forces and glenohumeral joint translations during arm elevation with various rotator cuff tear configurations.
• To investigate the biomechanical consequences of isolated and combined rotator cuff tears on shoulder joint stability.

Main Methods:

• Utilized a computer-controlled testing apparatus to simulate muscle forces and shoulder abduction in eight fresh-frozen upper limbs.
• Measured glenohumeral joint translations using an optoelectronic system under intact and simulated tear conditions (supraspinatus, subscapularis, infraspinatus, teres minor).

Main Results:

• Rotator cuff tears significantly increased middle deltoid force during abduction (30°, 60°, 90°) compared to the intact shoulder (p < 0.05).
• Combined supraspinatus and infraspinatus tears led to significantly greater superior humeral head translations at 30° and 60° of abduction (p < 0.05).
• The infraspinatus-teres minor complex demonstrated a significant role as a humeral head depressor.

Conclusions:

• Increased deltoid force in rotator cuff tears exacerbates superior humeral head migration.
• The infraspinatus-teres minor complex is vital for maintaining glenohumeral joint stability.
• Findings support improved diagnostic tools, surgical planning, and rehabilitation strategies for rotator cuff injuries.