Review Question - QID 216800

The subscapularis is the primary restraint to posterior instability in the setting of a posterior labral tear, which is depicted in figure C.

Posterior shoulder instability is often the result of a posterior labral tear. Classic manifestation is shoulder pain that is reproduced with shoulder flexion, adduction, and internal rotation with axial load. This is often seen in football linebackers and bench pressers. The activity of the subscapularis can act as a stabilizer to the posterior instability in these cases.

Millett et al. reviewed the diagnosis and treatment of recurrent posterior dislocations. The authors stated nonoperative treatment is often effective, but surgical intervention targeting the salient aspects of instability may be required in refractory cases. They concluded arthroscopic repair of the caspsulolabral pathology to be the preferred treatment for surgical cases.

Hawkins et al. evaluated glenohumeral joint translation in anesthetized patients. They reported patients with anterior instability demonstrated 29% anterior translation, 21% posterior translation, and 49% inferior translation compared to 17%, 26%, and 29% translation for normal shoulders, respectively. The authors concluded clinically assessing the position of the humeral head relative to the glenoid rim is the most practical method for determining humeral head translation.

Kido et al. performed a cadaveric study determining the role of the deltoid on dynamic shoulder stability. They reported muscle contraction of the three heads of the deltoid significantly reduced glenohumeral translation in anterior capsule vented and Bankart lesion models. The authors concluded the deltoid muscle acts as a dynamic stabilizer of the glenohumeral joint with the shoulder in abduction and external rotation.

Lee and An performed a biomechanical study of the dynamic stability of the glenohumeral joint offered by the deltoid muscle. They reported the middle and posterior heads of the deltoid provide the greatest compressive forces and fewer shear forces than the anterior head. They concluded repair of the glenoid labrum functions to enhance the compressive forces generated by the deltoid.

Figure A is a coronal T2 MRI arthrogram of the right shoulder with a SLAP tear. Figure B is a coronal T2 MRI arthrogram of the right shoulder with a humeral avulsion of the glenohumeral ligament. Figure C is an axial T2 MRI of the shoulder with a posterior labral tear. Figure D is an axial T2 MRI of the right shoulder with a spinoglenoid notch cyst. Figure E is an axial T2 MRI of the right shoulder with an anterior labral tear.

Incorrect answers
Answer 1: A SLAP tear would not result in instability that is stabilized by the subscapularis.
Answer 2: A humeral avulsion of the glenohumeral ligament would result in anterior instability that is stabilized by infraspinatus and teres minor muscle activity.
Answer 4: A spinoglenoid notch cyst may be seen in the setting of a labral tear (which is not shown in this image) and does not result in significant shoulder instability in and of itself.
Answer 5: An anterior labral tear results in anterior instability which is stabilized by muscles activity of the infraspinatus and teres minor.