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?
Locked posterior shoulder dislocation with multiple associated injuries.
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
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).
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).
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).
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).
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.
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.
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
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)
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.
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.
- Tannenbaum E and Sekiya JK. Evaluation and management of posterior shoulder instability. Sports Health 2011 May, 3(3):253-263 ↩
- Cooper A. A Treatise on Dislocations and Fractures of the Joints. Philadelphia, Pa: Lea & Blanchard; 1822 ↩
- Pollock RG, Bigliani LU. Recurrent posterior shoulder instability: diagnosis and treatment. Clin Orthop Relat Res 1993 : 85-96 ↩
- McLaughlin, HL. Locked posterior subluxation of the shoulder: diagnosis and treatment. Surg Clin North Am. 1963 Dec. 43:1621-2. ↩
- Shah N and Tung GA. Imaging signs of posterior glenohumeral instability. American Journal of Roentgenology. 2009;192: 730-735. ↩
- Pagnani MJ, Warren RF Stabilizers of the glenohumeral joint. J Shoulder Elbow Surg. 1994 May; 3(3):173-90. ↩
- Lee SB, Kim KJ, O’Driscoll SW, Morrey BF, An KN Dynamic glenohumeral stability provided by the rotator cuff muscles in the mid-range and end-range of motion. A study in cadavers. J Bone Joint Surg Am. 2000 Jun; 82(6):849-57. ↩
- Provencher MT, Dewing CB, Bell SJ, McCormick F, Solomon DJ, Rooney TB, Stanley M. An analysis of the rotator interval in patients with anterior, posterior, and multidirectional shoulder instability. Arthroscopy. 2008 Aug; 24(8):921-9. ↩
- Keith W. Harper1, Clyde A. Helms1, Clare M. Haystead1 and Lawrence D. Higgins Glenoid Dysplasia: Incidence and Association with Posterior Labral Tears as Evaluated on MRI. American Journal of Roentgenology. 2005;184: 984-988. ↩
- Weishaupt D, Zanetti M, Nyffeler RW, Gerber C, Hodler J. Posterior glenoid rim deficiency in recurrent (atraumatic) posterior shoulder instability. Skeletal Radiol 2000; 29:204-210. ↩
- Wirth MA, Lyons FR, Rockwood CA Jr. Hypoplasia of the glenoid: a review of sixteen patients. J Bone Joint Surg Am 1993; 75:1175-1184. ↩
- Bennett GE: Shoulder and elbow lesions of the professional baseball pitcher. J Am Med Assoc 117: 510-514, 1941. ↩
- Ferrari JD, Ferrari DA, Coumas J, Pappas AM. Posterior ossification of the shoulder: the Bennett lesion. Etiology, diagnosis, and treatment. American Journal of Sports Medicine 1994, 22:2:171-176. ↩
- Hottya GA, Tirman PF et al. Tear of the posterior shoulder stabilizers after posterior dislocation: MR imaging and MR arthroscopic findings with arthroscopic correlation. AJR 1998; 171:763-768. ↩
- Chung CB, Sorenson S, Dwek JR and Resnick D. Humeral Avulsion of the Posterior Band of the Inferior Glenohumeral Ligament: MR Arthrography and Clinical Correlation in 17 Patients. AJR 2004; 183(2). ↩
- Shah AA, Butler RB, Fowler R, Higgins LD. Posterior capsular rupture causing posterior shoulder instability: a case report. Arthroscopy. 2011 Sep;27(9):1304-7. ↩
- Saupe N, White LM, Bleakney R, et al. Acute traumatic posterior shoulder dislocation: MR findings. Radiology 2008; 248:185–193 ↩
- Hottya GA, Tirman PF, Bost FW, Montgomery WH, Wolf EM, Genant HK. Tear of the posterior shoulder stabilizers after posterior dislocation: MR imaging and MR arthrographic findings with arthroscopic correlation. AJR Am J Roentgenol. 1998 Sep;171(3):763-8. ↩
- Burkhead WZ, Rockwood CA Treatment of instability of the shoulder with an exercise program. J Bone Joint Surg Am. 1992 Jul;74(6):890-6. ↩