Posterior instability of the shoulder. A cadaver study

Area of Science:

• Orthopedic Surgery
• Biomechanics
• Anatomy

Background:

• Shoulder joint internal rotation is critical for function.
• Posterior shoulder structures' roles in stabilizing internal rotation are not fully elucidated.

Purpose of the Study:

• To investigate the contribution of posterior shoulder structures to internal rotation stability.

Main Methods:

• Cadaveric study using 15 shoulder specimens.
• Applied 1.5 Nm internal torque to humerus.
• Measured internal rotation at 0-90 degrees abduction with varying posterior structure integrity.

Main Results:

• Teres minor and infraspinatus tendon transection increased internal rotation up to 40 degrees abduction.
• Posterior capsule lesion increased internal rotation from 40 degrees abduction onwards.
• Combined transection of tendons and capsule further increased internal rotation.

Conclusions:

• Teres minor and infraspinatus tendons are key stabilizers for internal rotation in early abduction (0-40 degrees).
• The posterior capsule significantly stabilizes internal rotation in later abduction (40-90 degrees).
• Understanding these roles is vital for managing shoulder instability.