Posterior Glenohumeral Instability

Clinical History: A 72 year-old male presents with severe left shoulder pain and limited motion following a fall 10 days earlier.  What are the findings? What is your diagnosis?

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Figure 1:

(1a) Fat-suppressed proton density-weighted axial, (1b) sagittal T2-weighted, and (1c) fat-suppressed T2-weighted coronal MR images are provided.

Findings

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Figure 2:

(2a) The fat-suppressed proton density-weighted axial image reveals alignment of the humeral head posteriorly relative to the glenoid, with an impaction fracture of the humeral head articular surface (red arrow). The posterior labrum is avulsed, and stripped scapular periosteum remains attached to the posterior labrum (arrowhead). An impaction fracture is also present at the posterior glenoid rim (blue arrow). (2b) The T2-weighted sagittal image confirms posterior displacement of the humeral head (arrow) relative to the glenoid (asterisk). Low signal intensity blood clot (arrowhead) is present within the subscapularis recess. (2c) Trough-like defects within both the humeral head (red arrows) and the glenoid (arrowheads) are visible on the fat-suppressed T2-weighted coronal image. Tearing of the inferior glenohumeral ligament at the humeral attachment (blue arrow) is also evident.

Diagnosis

Locked posterior shoulder dislocation with multiple associated injuries.

Introduction

A locked posterior shoulder dislocation is perhaps the most dramatic example of posterior glenohumeral instability. The shoulder, because of its wide range of motion, is anatomically predisposed to instability, but the vast majority of shoulder instability is anterior, with posterior instability estimated to affect 2-10% of unstable shoulders.¹ Although anterior shoulder dislocations have been recognized since the dawn of medicine, the first medical description of posterior shoulder dislocation did not occur until 1822.² In modern times, posterior shoulder instability is still a commonly missed diagnosis, in part due to a decreased index of suspicion for the entity among many physicians. It is, however, becoming more frequently recognized, particularly in athletes such as football players and weightlifters, in which posterior glenohumeral instability has achieved increased awareness.³ As McLaughlin stated in 1963⁴, “the clinical diagnosis is clear-cut and unmistakable, but only when the posterior subluxation is suspected”. Modern imaging techniques, in particular MRI, have greatly increased our ability to accurately diagnose posterior glenohumeral instability, and accurate recognition and characterization of the relevant abnormalities are critical for proper diagnosis and patient management.⁵

Relevant Anatomy

Multiple shoulder structures are important in resisting shoulder instability. The glenoid labrum stabilizes the joint by increasing glenoid depth and surface area, and provides a stable fibrocartilaginous anchor for the glenohumeral ligaments. The shoulder capsule, including the glenohumeral ligaments, is one of the most important structures for restricting posterior translation of the humeral head.⁶ The subscapularis, and to a lesser extent the infraspinatus and teres minor muscles, provide dynamic restriction of posterior humeral head translation.⁷ The rotator interval is also thought to play a role, though its significance is somewhat controversial.⁸

Glenoid dysplasia, also referred to as glenoid hypoplasia and posterior glenoid rim deficiency, is now increasingly recognized as an anatomic variant that predisposes patients to posterior glenohumeral instability. Once thought to be a relatively rare entity, a study by Harper et al. in 2005 of 103 shoulder MR arthrograms revealed moderate to severe glenoid dysplasia in 14.3% of patients, and including mild cases increased the incidence to 39.8%.⁹ The study also provided a simplified classification system for glenoid dysplasia (Fig. 3).

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Figure 3:

A normal glenoid labrum has a laterally pointing edge and normal posterior labral morphology. Mild glenoid hypoplasia results in a rounded contour of the posterior glenoid with normal or only mildly thickened posterior labral tissue. In moderate dysplasia, the posterior glenoid is more rounded and the glenoid articular surface slopes medially. The posterior labrum is enlarged to replace the deficient glenoid rim. In cases of severe dysplasia, advanced rounding and posterior sloping of the posterior glenoid is seen, and pronounced thickening of the labrum and other adjacent posterior soft tissues is apparent.

In patients with posterior instability, the presence of glenoid hypoplasia is predictably higher, with one report finding deficiency of the posteroinferior glenoid in 93% of patients with atraumatic posterior instability.¹⁰ When diagnosing posterior glenoid hypoplasia on MRI, care should be taken not to overcall the entity, as volume averaging can result in a false appearance of dysplasia on the most inferior axial slice. True dysplasia should be visible on at least two axials slices cephalad to the most inferior slice of the glenoid (Fig. 4).

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Figure 4:

Consecutive fat-suppressed proton density-weighted axial images at the mid glenoid in a football player with persistent shoulder pain reveals mild glenoid dysplasia, with a rounded contour of the posterior glenoid rim (arrows). A tear extends across the base of the posterior labrum (arrowheads), and mild posterior subluxation of the humeral head relative to the glenoid is present.

In more advanced cases of glenoid dysplasia, hypertrophic changes of the labrum and hyaline cartilage are pronounced. The appearance is thought to be due to failure of ossification of the more inferior of the two ossification centers of the glenoid, resulting in a cartilage cap replacing the bone defect.¹¹ The presence of the hypertrophied tissue and associated labral tears is well demonstrated on MRI (Fig. 5).

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Figure 5:

A fat-suppressed proton density-weighted axial image in a 14 year-old female with shoulder instability reveals findings of severe glenoid hypoplasia. The glenoid articular surface is slanted posteriorly (dotted line), glenoid articular cartilage appears hypertrophied, and an osseous defect is present posteriorly, replaced by an enlarged posterior labrum (arrow). Posterior subluxation of the humeral head is also apparent.

Spectrum of MR findings in posterior glenohumeral instability

Numerous labral abnormalities may be encountered in patients with posterior glenohumeral instability. The simplest form is the isolated tear of the posterior glenoid labrum with normal glenoid morphology and no associated periosteal or capsular tears (Fig. 6). Such lesions are generally found in patients with atraumatic posterior instability. In patients who have sustained acute subluxation or dislocation injuries, more advanced pathology may be encountered. These tears include numerous variations designated by acronyms similar to those used for the more commonly seen anterior labral tears. Examples include the reverse Bankart lesion, the POLPSA lesion, and the posterior GLAD lesion (sometimes referred to as a PLAD lesion) (Figs. 7-9).

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Figure 6:

Fluid undermines a tear of the posterior glenoid labrum (arrow) in a 42 year-old male with persistent posterior shoulder pain. Normal glenoid morphology is present.

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Figure 7:

In a 20 year-old football player following acute injury, a reverse Bankart lesion is present. A displaced tear of the posteroinferior labrum is present, with a torn piece of periosteum (arrow) remaining attached to the posterior labrum.

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Figure 8:

Posterior labral periosteal sleeve avulsion injury (POLPSA) in a 19 year-old football player following acute injury. A displaced tear of the posterior labrum (arrow) is present. Posterior periosteum (arrowheads) is extensively stripped but remains attached to the posterior labrum. Posterior subluxation of the humeral head is readily apparent.

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Figure 9:

In a 34 year-old male following an acute subluxation event, a tear is present along the base of the posterior labrum with edema and irregularity noted at adjacent posterior periosteum (arrow). A small chondral defect is present (arrowhead) adjacent to the free edge of the posterior labrum. The findings are compatible with a posterior GLAD lesion (glenolabral articular disruption). The chondral lesion is thought to arise secondary to impaction injury from the humeral head.

Numerous capsular abnormalities have been described in patients with posterior glenohumeral instability. The Bennett lesion (Fig. 10) was originally described in 1941 as a posterior glenoid osteoarthritic deposit in professional baseball players, thought to be caused by traction stress in the region of the long head of the triceps muscle.¹² More contemporary data suggest that the lesion is due to a traction injury of the posterior shoulder capsule, particularly the posterior band of the inferior glenohumeral ligament.¹³ Posterior labral tears and a history of previous shoulder posterior subluxation are found with high frequency in patients with the Bennett lesion.

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Figure 10:

(10a) Ossification is seen along the posterior glenoid (arrows) in a professional baseball pitcher with a history of posterior instability. Posterior labral tearing was apparent on contiguous images (not shown). (10b) A corresponding T2-weighted sagittal view in the same patient confirms the large ossification along the posteroinferior glenoid rim (arrows), compatible with a Bennett lesion.

The posterior shoulder capsule plays a significant role in preventing posterior shoulder dislocation, particularly at the extremes of internal humeral rotation, the position in which most posterior dislocations occur. As a result, in cases of posterior shoulder instability, particularly dislocation, capsular tears are frequently identified on MR imaging.¹⁴ The posterior capsule injuries most commonly involve the humeral attachment inferiorly¹⁵, in the region known as the posterior band of the inferior glenohumeral ligament. Such injuries may be referred to as reverse HAGL (humeral avulsion of the glenohumeral ligament) or RHAGL lesions (Fig. 11). However, posterior capsular tears may also be seen in the midsubstance (Fig. 12) or at the humeral attachment (Fig. 13) of the posterior capsule.

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Figure 11:

Following a posterior subluxation event, a fat-suppressed T2-weighted coronal image in this 52 year-old male reveals focal edema and irregularity at the humeral attachment of the posterior band of the inferior glenohumeral ligament (arrow), compatible with a partial tear.

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Figure 12:

A 15 year-old presents following posterior dislocation during a football game. A mid-substance tear of the posterior capsule is present with the medial component appearing lax and retracted (arrow).

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Figure 13:

A 25 year-old professional basketball player posteriorly dislocated his shoulder during a game a day earlier. The posterior capsule is torn at the humeral attachment (arrow). A posterior labral tear (reverse Bankart) is also present (arrowhead), and a bone bruise is seen within the anterior humeral head (asterisk).

MR interpreters should be aware that at times capsular tears are quite subtle. A useful indirect sign to be aware of, whether using MR arthrography or routine MR, is to recognize that normally the shoulder capsule should only be outlined by fluid along its inner margin. Fluid should not lie along both sides of the shoulder capsule. If this appearance is present, a capsular tear should be strongly suspected (Fig. 14). Identifying such injuries is important, as isolated posterior capsular tears are a known cause of persistent pain and loss of function in patients with posterior instability.¹⁶

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Figure 14:

(14a) Normal capsular appearance on an axial fat-suppressed T1-weighted MR arthrographic image. Fluid distends the joint and only lies along the inner margin of the joint capsule (arrowheads). (14b) In a 39 year-old weightlifter with persistent posterior shoulder pain and instability, the axial image reveals the posterior capsule outlined by arthrographic fluid along both sides of the capsule, strongly suggestive of a capsular tear. An area of capsular irregularity (arrow) is apparent as well. (14c) An arthroscopic examination confirms the tear in the posterior capsule (arrow), which was subsequently repaired. Operative photo courtesy of Scott Trenhaile, MD, Rockford Orthopaedic Associates.

Rotator cuff tears in the context of posterior shoulder instability or dislocation were once thought to be rare. However, a study by Saupe et al. in Radiology in 2008 examined 36 patients following acute traumatic shoulder dislocation and revealed full-thickness tears in 19% of patients and partial or full-thickness tears in 42%.¹⁷ As would be expected, subscapularis tears were most common, but tears were also identified in the supraspinatus and the infraspinatus. Posterior shoulder subluxation or dislocation is also one of the rare entities that may result in tears of the teres minor muscle.¹⁸ MR allows rapid evaluation of the status of the cuff following posterior dislocation, and prompt diagnosis of such lesions avoids delays in treatments that may lead to irreversible fatty atrophy of cuff musculature (Figs. 15,16)

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Figure 15:

An axial image in a 53 year-old male following an acute traumatic posterior dislocation reveals tears of the posterior labrum (arrow) and posterior capsule (arrowhead). The retracted end of the subscapularis (asterisk) is also visible compatible with a full thickness tear. The biceps tendon is medially dislocated (short arrow).

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Figure 16:

(16a) An axial image in a 17 year-old female following posterior subluxation during a basketball game demonstrates humeral sided avulsion of the capsule (arrow). (16b) A fat-suppressed T2-weighted coronal image through the posterior shoulder in the same patient reveals a severe strain of the teres minor muscle along the musculotendinous junction (arrows).

Treatment

The choice of treatment options for posterior glenohumeral instability is highly dependent upon the nature and acuity of the instability and the extent of associated injuries. Non-surgical treatment tends to be most successful in patients with a history of atraumatic subluxations, whereas patients who experience an acute, traumatic posterior dislocation are much less likely to report successful outcomes from conservative therapy.¹⁹ Non-operative therapy focuses on strengthening the dynamic shoulder stabilizers and activity modification.

The approach to surgery is dependent upon the type of injuries sustained by the patient, and the developmental or acquired alterations in anatomy that may be present. Injuries isolated to labrum and capsule can often be successfully repaired with arthroscopic techniques including capsulolabral repair, capsular shift, and capsular shrinkage. In patients with glenoid deficiency or large impaction defects, osteotomies and osseous augmentation procedures may be required.

Conclusion

Posterior instability of the shoulder can vary from minor symptoms and findings to dramatic events resulting in extensive, complex injuries to the shoulder. MRI is not uncommonly the key to the diagnosis as patients may present with vague clinical findings that are not prospectively diagnosed, in part because of the relatively less common incidence and awareness of this entity. In patients with traumatic posterior subluxation or dislocation, injuries to labrum, capsule, bone and rotator cuff may be found, and accurate diagnosis with MRI allows the most appropriate treatment pathway to be chosen.

References