MRI Web Clinic — June 2005

Michael E. Stadnick, M.D.

Clinical History: A 37 year-old male presents with shoulder discomfort, particularly in adduction and mild internal rotation. A fat suppressed proton density-weighted axial image (1a) is provided. What are the findings? What is your diagnosis?

Figure 1:

(1a) A fat suppressed proton density-weighted axial image


Figure 2:

(2a) The posterior labrum (arrow) is torn from the posterior glenoid and displaced posteriorly. The periosteum (arrowhead) is stripped from the posterior glenoid remaining attached to the displaced labral tissue. The humeral head (asterisk) is posteriorly subluxed.


Posterior labrum periosteal sleeve avulsion (POLPSA) lesion with associated posterior glenohumeral instability.


Posterior instability of the shoulder results from excessive posterior glenohumeral translation. Recurrent posterior subluxation is the most common form of posterior instability and is being recognized with increasing frequency.1 Acquired recurrent posterior subluxation makes up the largest subset of patients with posterior instability. Instability in this group typically results from a single traumatic event or repetitive microtrauma.

The most common traumatic event resulting in posterior instability is a posterior shoulder dislocation. Direct trauma to the anterior shoulder, a posteriorly directed force on an adducted arm (fall on outstretched hand), and indirect muscle forces (seizure and electrical shock) are typical etiologies. In atraumatic posterior instability there is no history of major trauma, however, there is almost always an element of repetitive microtrauma causing labral pathology and posterior capsular stretching. Underlying architectural abnormalities of the glenoid are also important to recognize in the development of atraumatic posterior instability. Constant balancing of static and dynamic stabilizers is required to maintain glenohumeral stability. The major restraints to posterior instability include the posterior capsule and glenohumeral ligaments, the rotator interval, the labrum, the glenoid, and the musculature of the rotator cuff and shoulder. The importance of these structures is reviewed in the following:

1. The posterior capsule serves as the primary static stabilizer to unidirectional posterior translation. A number of biomechanical studies have demonstrated the importance of both posterior and anterior capsuloligamentous structures in maintaining static posterior stability. Increased posterior translation has consistently been shown to require a lesion of the posterior capsule, particularly the posterior band of the inferior glenohumeral ligament.2

2.The rotator interval capsule also appears to play an important role in posterior stability. Sectioning of the rotator interval capsule has been shown to increase posterior and inferior translation of the humeral head.3

3. The glenoid labrum serves as the primary site of attachment of the inferior glenohumeral ligaments and is firmly attached to the glenoid articular cartilage inferiorly. The labrum serves to deepen the glenoid fossa and helps maintain glenohumeral alignment.

4. Variations in osseous anatomy at the glenoid can significantly affect shoulder stability. “Glenoid hypoplasia” or “posterior glenoid rim deficiency” refer to a spectrum of bony abnormalities involving the posteroinferior glenoid (Figure 3a). Glenoid hypoplasia is associated with an increased incidence of posterior labral tears and has been identified as a potential cause of posterior instability and accelerated degenerative joint disease.4 Glenoid retroversion describes an excessively posteriorly directed glenoid articular surface, which can contribute to posterior instability (Figure 4a).

5. Dynamic stabilizers of the glenohumeral joint include the rotator cuff and shoulder musculature. The subscapularis muscle has been identified as the most important muscle in resisting posterior subluxation of the humerus.5 Asynchrony of scapulothoracic and glenohumeral muscle contraction may compromise the stability of the glenohumeral joint. Alterations in function of the serratus anterior muscle may disrupt the scapulothoracic rhythm leading to loss of power and stability of the glenoid and variable amounts of scapular winging.6

Figure 3:

(3a) An axial fat-suppressed proton density weighted image demonstrates a rounded posterior margin (arrows) and a prominently hypertrophied posterior labrum (arrowhead) compatible with posterior glenoid hypoplasia.

Figure 4:

(4a) An axial fat-suppressed proton density weighted image demonstrates a severely retroverted glenoid (arrowheads) and posterior glenoid hypoplasia with a hypertrophied posterior labrum (arrow). A tear undercuts the posterior labrum (small arrow). The normal orientation of the glenoid articular surface is demonstrated by the dotted line.

Patients with posterior instability typically complain of pain or a sensation of the shoulder “coming out” when the arm is placed in a provocative position. This position varies but usually includes some degree of flexion, adduction, and internal rotation. Pain is usually limited to the time of subluxation. Persistent pain is not typical and may point to additional pathology of the rotator cuff or biceps tendon6. Patients most at risk for posterior instability include athletes such as weight lifters, throwers, tennis players, and swimmers. Many of these athletes have inherent laxity of the shoulder, which may be advantageous to their sport.

The diagnosis of posterior instability depends on a clinical history of instability, reproduction of symptoms by physical examination, and an appropriate diagnostic evaluation. Careful physical examination is critical in diagnosing and characterizing the patient’s shoulder instability pattern. The majority of patients can demonstrate their subluxation. Comparison with the contralateral shoulder is critical in identifying significant shoulder subluxation versus normal laxity. Plain film and CT may be utilized to evaluate bony contour abnormalities such as the reverse Bankart lesion or retroversion of the glenoid. However, for successful treatment there must be a clear understanding of underlying soft tissue abnormalities. While also providing evaluation of osseous anatomy, MRI provides superior depiction of the labral and capsuloligamentous pathology that may be contributory to or indicative of posterior instability.

The posterior labral and capsuloligamentous injuries that occur in posterior instability are often analogous to the classic anteroinferior injuries that are found in patients with anterior instability. Posterior instability lesions include the reverse Bankart (Figure 5a), the posterior labrum periosteal sleeve avulsion injury (POLPSA) (Figure 6a), and the posterior band inferior glenohumeral ligament avulsion from the humerus (PHAGL) (Figure 7a).7,8

The MRI findings in these patients often provide useful indicators of clinical significance. For example, a direct correlation has been found between the length of posterior labral tears and the degree of posterior humeral translation. Patients with periosteal sleeve avulsions, such as the POLPSA, are more likely to be symptomatic.9

Figure 5:

(5a) An axial fat-suppressed proton density weighted image in a patient after posterior glenohumeral dislocation demonstrates a posterior labral tear (reverse Bankart) (arrow) and bone bruise (arrowheads) at the site of a reverse Hill-Sachs fracture (short arrow).

Figure 6:

(6a) An axial fat-suppressed proton density weighted image demonstrates a posterior labrum periosteal sleeve avulsion (POLPSA). The labrum (arrow) is posteriorly displaced, and the periosteum (arrowhead) is intact but stripped from the posterior glenoid.

Figure 7:

(7a) A coronal T2-weighted fat-suppressed image through the posterior glenohumeral joint in a patient following posterior glenohumeral dislocation demonstrates hemorrhage and edema at the interrupted humeral insertion of the inferior glenohumeral ligament compatible with a posterior band inferior glenohumeral ligament avulsion (PHAGL). Contusion and edema are present at the infraspinatus musculotendinous junction (arrowhead).

The treatment options for posterior instability should be guided by the underlying pathology. A non-operative exercise program is almost always the initial step in treatment. In the past, broad application of surgical repair without an understanding of the underlying anatomic abnormality met with poor results. Currently, the improved appreciation of the various components providing stability to the glenohumeral joint and the more accurate diagnosis of soft tissue injuries through MRI have allowed a more tailored approach. This method appears to favorably improve treatment outcomes.10


Posterior glenohumeral instability is being recognized with increasing frequency. Lesions of the labrum, rotator cuff musculature, and glenoid may contribute to recurrent posterior glenohumeral subluxation. Appropriate treatment requires a thorough clinical and diagnostic evaluation focused on identifying the underlying pathology. The information provided by MRI facilitates treatment specific to the anatomic abnormality, thus positively influencing treatment outcomes.


1 Hawkins RJ, Koppert G, Johnston G. Recurrent posterior instability (subluxation) of the shoulder. Journal of Bone and Joint Surgery 66A:169-74, 1984.

2 Ovesen J, Sojbjerg JO. Posterior shoulder dislocation: Muscle and capsular lesions in cadaver experiments. Acta Orthop Scand 57:535-36, 1986.

3 Harryman DT, Sidles JA, Harris SL, Matsen FA. The role of the rotator interval capsule in passive motion and stability of the shoulder. Journal of Bone and Joint Surgery 74A:53-66, 1992.

4 Harper KW, Helms CA, Haystead CM, Higgins LD. Glenoid Dysplasia: Incidence and Association with Posterior Labral Tears as Evaluated on MRI. AJR 2005;184:984-88.

5 Blasier RB, Soslowsky LJ, Malicky DM, et al. Posterior glenohumeral subluxation: Active and passive stabilization in a biomechanical model. Journal of Bone and Joint Surgery 79A:433-40, 1997.

6 Fery A: Results of treatment of anterior serratus paralysis. In: Post M, Morrey BF, Hawkins RJ, editors. Surgery of the Shoulder. St. Louis, MO: Mosby Year Book; p325-9, 1990.

7 Yu JS, Ashman CJ, Jones G. The POLPSA lesion: MR imaging findings with arthroscopic correlation in patients with posterior instability. Skeletal Radiol. 2002 Jul;31(7):396-9.

8 Chung CB, Sorenson S, Dwek JR, Resnick D. Humeral Avulsion of the Posterior Band of the Inferior Glenohumeral Ligament: MR Arthrography and Clinical Correlation in 17 Patients. AJR 2004;183:355-59.

9 Tung GA, Hou DD. MR Arthrography of the Posterior Labrocapsular Complex: Relationship with Glenohumeral Joint Alignment and Clinical Posterior Instability. AJR 2003;180:369-75.

10 Lamar DS, Williams GR, Iannotti JP, Ramsey ML. Posterior Instability of the Glenohumeral Joint: Diagnosis and Management. The University of Pennsylvania Orthopaedic Journal 14:5-14,2001.

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