Posterior instability of the shoulder. A cadaver study.
In a cadaver study of 15 shoulder specimens, the internal rotation of the joint was measured applying a constant internal torque of 1.5 Nm to the humerus. The specimens were suspended with the medial border of the scapula in vertical position. A lever fixed to the humerus was fitted with strain gauges for measurement of internal torque and sensors for measurement of internal rotation at different degrees of abduction from 0-90 degrees. Cutting the teres minor and infraspinatus muscle tendons increased internal rotation in the first 40 degrees of abduction. Internal rotation was further increased in this range by cutting also the proximal half of the posterior capsule. Lesion to the posterior capsular structures alone increased internal rotation from 40 degrees of abduction. In conclusion, among the posterior structures of the shoulder joint, the teres minor and the infraspinatus muscle tendons stabilize the joint for internal rotation in the first half of abduction, and the lower half of the capsule in the last part.
D001696:Biomechanical Phenomena; D002102:Cadaver; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology*; D008566:Membranes / Q000502:physiology; D009132:Muscles / Q000502:physiology; D012399:Rotation; D012785:Shoulder Joint / Q000502:physiology*; D013710:Tendons / Q000502:physiology