Dr. Cree M. Gaskin is currently a third-year radiology resident at Duke University Medical Center. He graduated as co-valedictorian from the University of Florida with a BS in Basic Medical Science in 1994. He subsequently received his MD from the University of Florida in 1997 as a member of Alpha Omega Alpha. Dr. Gaskin will remain at Duke to begin a fellowship in musculoskeletal/MR imaging in 2002.

The glenohumeral joint is a relatively unstable joint. The inherent biomechanics that allow for its exceptional range of motion are equally responsible for it being the most frequently dislocated joint in the body. Glenohumeral instability is defined simply as the recurrence of dislocation or subluxation of the joint with the performance of activity. ³

Instability is classified by the direction of abnormal movement of the humeral head relative to the glenoid margin. It can be unidirectional (anterior or posterior) or multidirectional (including inferior). Isolated inferior or superior instabilities are rarely encountered. ³ Orthopedists often lump shoulder instability into two broad categories under the acronyms TUBS and AMBRI. TUBS is used to describe patients with traumatic unidirectional anterior-inferior instability with a Bankart lesion (generally requiring surgical intervention). AMBRI refers to patients with atraumatic multidirectional bilateral instability who can be treated successfully with rehabilitation or with surgery in the form of an inferior capsular shift procedure. ⁴ This article is largely concerned with the TUBS population as traumatic anterior glenohumeral joint instability constitutes > 95% of all glenohumeral instabilities encountered in clinical practice. ³

Clinical presentation

Virtually all patients with glenohumeral instability have an unmistakable history of experiencing displacement of the humeral head. This is most often associated with trauma in the form of forced abduction and external rotation, such as in a fall on the outstretched arm. Subsequent to this acute event, the patient may complain of anterior shoulder pain, joint clicking or catching, or frank recurrent dislocation. ^1,3 These symptoms are often exacerbated by overhead activities. ¹

The physical examination for instability includes a multitude of techniques, including the apprehension, relocation, load and shift, inferior sulcus sign, and crank tests. The combination of clinical history and physical examination are considered to be quite accurate in determining the presence of instability lesions requiring surgery. ^1,2 Liu et al ² found physical examination to be 90% sensitive and 85% specific for labral tears confirmed on arthroscopy. Considering this inexpensive and relatively accurate preoperative assessment, what is the role for MR imaging in the evaluation of instability?

The answer to this question is not straightforward. Clearly, arthroscopy can be used to adequately delineate and treat abnormalities in patients with clinically obvious instability. Such patients do not necessarily require diagnostic imaging. However, preoperative MR (with or without arthrography) may be useful to determine the presence and type of capsulolabral lesion in order to aid in the selection of the appropriate surgical procedure. ^1,5,6 Preoperative determination that the lesion will require an open procedure (arthrotomy) prevents edematous saturation of tissues from arthroscopy prior to conversion to the necessary arthrotomy. This obviously also saves operating room time and expense. ¹

In contemporary times of managed care and patient convenience, a great deal of emphasis has been placed upon ambulatory surgery. In some orthopedic practices, this has essentially mandated preoperative imaging to determine whether a procedure will be open or closed. The reason for this is that the ambulatory surgery center may be remote from the main hospital, restricting access to the inpatient services needed in cases requiring arthrotomy. Therefore, in some orthopedic practices, preoperative imaging may actually determine which surgical suite (main hospital versus ambulatory surgical center) and accompanying fee schedule are chosen for an operation. ¹

As explained above, there are varying rationales behind preoperative imaging in patients in whom instability is clinically obvious. MR imaging can be even more valuable when the diagnosis is uncertain. In these cases, MRI can confirm or deny the presence of instability lesions as well as identify other abnormalities that may have mimicked the clinical manifestations of instability. ⁵ Examples of such pathology include a displaced labrum, a loose body, or a subluxating tendon of the long head of the biceps. ¹

MR imaging may also uncover unsuspected instability lesions in patients presenting solely with complaints of shoulder pain. ¹ For example, instability may be the underlying cause of impingement syndrome in young athletes, although its manifestations may be obscured by symptoms related to the rotator cuff pathology. Accurate diagnosis and treatment of instability in these patients is required to successfully alleviate the impingement syndrome. ^7,8 Whatever the case may be, MR imaging can lead to modification in arthroscopic technique, the choice of an open surgical procedure as a more appropriate therapeutic approach, or it may deem surgery unnecessary altogether. ⁵

One recent study in the orthopedic literature objectively evaluated the impact of shoulder MRI on clinical decision making. This study achieved statistical significance in demonstrating that shoulder MRI resulted in major alterations in clinical management by orthopedic surgeons. ⁹ MRI altered both the primary diagnoses and the types of therapy (operative versus nonoperative) chosen for shoulder disorders of multiple etiologies, including anterior instability. ⁹ A similar study within the radiologic literature also achieved statistical significance in demonstrating that shoulder MRI both changed patient management (62% of cases) and improved clinicians' diagnostic confidence. ¹⁰

Anatomy of anterior shoulder instability

The glenoid fossa of the scapula and the head of the humerus form a ball-and-socket joint. The osseous glenoid is covered by articular cartilage that thins centrally to form a shallow fossa. The glenoid fossa is opposed to only one-third to one-fourth of the articular surface of the humeral head at any given time. This allows for great degrees of motion at the unfortunate expense of stability. To compensate, the joint is reinforced by the rotator cuff and the capsulolabral complex. ^6,11

The joint capsule has both an outer fibrous layer and an inner synovial layer. The capsule exhibits inherent laxity to allow for wide ranges of motion, while becoming taut at extremes of motion to provide support for the joint. ¹¹ The anterior capsular mechanism provides stability during abduction and external rotation. It is composed of the labrum, subscapularis muscle and tendon, coracohumeral and glenohumeral ligaments, scapular periosteum, and capsule. ^6,11 The posterior capsular mechanism protects against posterior dislocation. It is composed of the labrum, capsule, inferior glenohumeral ligament (posterior band), rotator cuff muscles, and scapular periosteum. ¹¹

The glenoid labrum (figure 1) is a capsular fold that inserts onto the periphery of the bony glenoid, serving to deepen the articular surface. It is composed of fibrous tissue with a fibrocartilaginous transition zone at its insertion. In addition to serving as an insertion site for much of the capsule, the labrum and the adjacent superior glenoid tubercle serve as the origin for the tendon of the long head of the biceps. ¹¹

There are three capsular thickenings that provide extra stability against anterior dislocation during abduction and external rotation. These structures are known as the superior, middle, and inferior glenohumeral ligaments. The superior and middle glenohumeral ligaments originate from the anterior-superior labrum (and sometimes the glenoid margin) immediately anterior to the labral-bicipital anchor. ^11,12 The superior glenohumeral ligament passes anteriorly until it merges with the coracohumeral ligament in the rotator interval. ¹² The rotator interval is the space between the superior border of the subscapularis and the adjacent supraspinatus. ¹³ The middle glenohumeral ligament courses inferiorly along the glenoid rim prior to merging with the subscapularis tendon. It ultimately inserts upon the lesser tuberosity. ¹²

The inferior glenohumeral ligament attaches to the inferior two-thirds of the entire labral circumference. It is also known as the inferior glenohumeral ligament complex as it has three components. The axillary pouch is the hammock-like portion intervening between the ligament's thick anterior and posterior bands. The inferior glenohumeral ligament is lax in the adducted position. During abduction, the anterior and posterior bands move superiorly with respect to the humeral head, ultimately becoming taut and serving as the primary restraint against anterior and posterior dislocations at 90 degrees of abduction. ¹⁴

The coracohumeral ligament arises from the lateral margin of the coracoid process and courses through the rotator interval. It has two separate insertions onto the greater and lesser tuberosities of the humerus, forming a tunnel for the biceps tendon. ¹¹ In addition to stabilizing the biceps tendon, ¹⁵ the ligament also provides support during external rotation ¹⁶ and while the arm is in a dependent position. ¹⁷

It is helpful to consider this anatomy while looking upon the glenoid labrum from a lateral viewpoint (figure 2). Consider the labrum as a clock face with 3 o'clock anteriorly, 6 o'clock inferiorly, and 9 o'clock posteriorly. The important anatomy and pathology in anterior instability can be grouped into structures and lesions between the 11 and 3 o'clock positions and those between the 3 and 6 o'clock positions. The 11 to 3 o'clock interval includes more structures (superior labrum, biceps-labral complex, biceps tendon, coracohumeral ligament, and superior and middle glenohumeral ligaments), but has less pathology (predominantly superior labral anterior-posterior [SLAP] lesions). Unusual findings involving this region on MRI are often related to anatomic variants rather than pathology. Conversely, the 3 to 6 o'clock position has fewer structures (labrum, subscapularis tendon, and inferior glenohumeral ligament), but more pathology (Bankart, GLAD, ALPSA, HAGL, and Perthes lesions). ⁶ The MRI findings of these lesions and anatomic variants will be discussed following an introduction to the technical aspects of shoulder MR imaging.

Technical overview

There is little agreement regarding a specific MR imaging technique to evaluate glenohumeral instability. Techniques vary, although with frequent overlap, secondary to individual preferences, the type of MR imaging device available, and the specific clinical question that has been posed. Various techniques and their rationales will be discussed.

Patient positioning and surface coils --Patients are generally imaged supine, going head-first into the scanner. Their comfort level is optimized with padding and sometimes pharmacologic sedation in order to reduce motion. The arm is generally at the patient's side in a neutral to slightly externally rotated position (thumb pointing up or slightly lateral). Elbow support or arm board fixation can help to reduce motion. ¹⁸

Thin slice (3 mm), small field of view (14-18 cm), high-resolution images are desired. Surface coils are required to ensure adequate signal-to-noise ratios to achieve these results. A single loop coil is commonly used in an anterior coronal oblique position. Alternatively, two flat circular coils can be placed both anterior and posterior to the shoulder to be used as a Helmholtz pair. Recently, further improvements in signal-to-noise ratios have been achieved by more contemporary quadrature and phased-array coils. ¹⁸

Throughout the exam, the coil is generally held in place with the use of restraining straps. Very bright signal, known as coil burnout, may be problematic in tissues immediately adjacent to the coil. This artifact may be alleviated by placing a thin pad between the coil and the skin. ¹⁸

Imaging planes --A set of scout coronal images is obtained with a large field-of-view and a short repetition time (TR) spin-echo or gradient-echo sequence. Utilizing an imaging sequence based on physician preference, a series of axial images is then obtained from the acromioclavicular joint through the inferior aspect of the glenoid. This set of axial images subsequently serves as the localizer for the oblique coronal and oblique sagittal images. The oblique coronal images are prescribed parallel to the supraspinatus tendon, whereas the oblique sagittals are prescribed perpendicular to this tendon. Coronal coverage should extend from the subscapularis muscle anteriorly through the infraspinatus-teres minor complex posteriorly. Sagittal images should be obtained from the coracoid process medially to the lateral cortex of the humerus. ¹⁸

Pulse sequences --The precise imaging sequences employed vary from institution to institution. Typical protocols include conventional and fast spin-echo, as well as two- and three-dimensional gradient echo sequences. Some protocols also utilize fat-suppression with fast spin-echo T2-weighted sequences.

Conventional spin-echo images are widely used for shoulder MR imaging as they are robust, widely available, and familiar to most radiologists. T1-weighted images provide excellent anatomic definition and are necessary to evaluate the marrow space and the peribursal fat planes. T2-weighted sequences are necessary for the detection of pathology. ¹⁸

Fast spin-echo T2-weighted se-quences are widely used as they offer considerable time savings over conventional spin-echo. This time savings may be utilized simply to shorten acquisition time or it may be reinvested to increase spatial resolution (increased matrix) or signal-to-noise ratio (increased number of signal averages). Fast spin-echo images display similar contrast to those of conventional spin-echo, except for having brighter fat signal. Fat suppression is required in order to avoid obscuring small pathologic lesions adjacent to fat. Satisfactory fat suppression may be achieved with frequency selective techniques or with short tau inversion recovery (STIR). Fast spin-echo sequences also result in some loss of edge definition relative to conventional spin-echo. ¹⁸

Gradient echo images are widely used because of short acquisition time and excellent visualization of the labrum. ^3,18 These sequences allow improved delineation of the low-signal labrum because of relatively increased signal intensity of the adjacent articular cartilage and synovium secondary to their water content. ³

MR arthrography and provocative positioning of the shoulder --To improve lesion detection when instability is questioned, many institutions will perform MR arthrography. Additional modifications in technique may also include imaging the shoulder in provocative positions, such as the ABER (abduction external rotation) and apprehension test (requires the lateral space of an open magnet) positions. ^19-21 The success of these techniques will be discussed later.

Briefly, MR arthrography can be performed by direct injection of saline or gadolinium into the joint space, or by indirect (intravenous) injection of gadolinium. A major advantage of direct injection of gadolinium over saline is that it allows determination of the origin of extra-articular fluid collections (i.e., inflammation versus abnormal communication with the joint) encountered upon imaging. ¹² Direct injection is preferred over indirect injection because it provides the capsular distension necessary to optimally visualize important structures, particularly the capsular ligaments. ^14,18

Anterior instability lesions and the efficacy of MRI

Bankart lesion --The Bankart lesion is the most common injury sustained from anterior shoulder dislocation. Classically, this lesion refers to an avulsion of the labral-ligamentous complex from its attachment to the anterior-inferior glenoid in association with disrupted scapular periosteum. The lesion may occur with or without a fracture of the bony glenoid. ²²

The hallmark finding of a Bankart lesion on MRI is a band of abnormally increased signal intensity separating the anterior-inferior capsulolabral complex from the osseous glenoid margin (figures 3 and 4). ³ In the acute setting, the lesion may be more conspicuous with the additional findings of a joint effusion, bony fracture, and edema within the bone marrow and surrounding soft tissues. The labral tear is often large enough that it extends upward to involve the middle and sometimes the superior anterior labrum. If the lesion does not heal correctly following dislocation, the shoulder will remain unstable. Partial healing, including fibrosis and resynovialization, may make detection of the chronic Bankart lesion difficult with conventional MRI. ²²

Multiple studies have evaluated the abilities of conventional MRI and MR arthrography to detect anterior labral tears in patients with instability. Liu et al ² evaluated conventional MRI, reporting a sensitivity of 59% and a specificity of 85%. Palmer and Caslowitz ²³ evaluated MR arthrography, reporting 92% for both sensitivity and specificity. In a smaller series offering direct comparison of the two techniques, MR arthrography compared favorably with conventional MRI. ²⁴ In this study, conventional MRI failed to detect six of nine labral tears found at arthroscopy, while all nine lesions were successfully identified with MR arthrography. ²⁴

The additional use of the ABER position with MR arthrography has been shown to improve labral tear detection when compared to MR arthrography performed in the standard neutral position alone. Cvitanic et al ¹⁹ demonstrated improvement in sensitivity (from 48% to 89%) and specificity (from 91% to 95%) with the addition of ABER positioning to their standard MR arthrographic protocal.

Bankart variant lesions --In the Bankart lesion, the anterior scapular periosteum ruptures as the anterior-inferior labroligamentous complex is avulsed from the glenoid. This results in anterior displacement of the detached labroligamentous structure. In the Bankart variant lesions, the stripped scapular periosteum remains attached to the torn labroligamentous complex (figure 4). In one such lesion termed ALPSA (anterior labroligamentous periosteal sleeve avulsion) (figures 4 and 5), the stripped periosteum rolls up like a long shirt sleeve causing the detached labroligamentous structure to displace medially and rotate inferiorly toward the scapular neck. ²⁵ In the chronic state, the detached structure becomes fibrosed and resynovialized in this abnormal position. It can look surprisingly normal at arthroscopy and on MR imaging, although it is a cause of chronic instability. ^22,25

The Perthes lesion, another Bankart variant, also has persistent attachment of the stripped scapular periosteum to the torn labroligamentous complex (figure 4). Unlike the ALPSA lesion, the stripped periosteum does not become rolled up, generally allowing the torn labroligamentous structure to reapproximate its normal position. Like the ALPSA lesion, once resynovialized, the Perthes lesion may appear deceptively normal at surgery and on MR arthrography. ²²

Superior labral anterior-posterior tears --SLAP tears were first described by Snyder et al ²⁶ in a series of 27 patients who presented with shoulder pain and/or joint clicking. This series divided SLAP tears into four categories based on arthroscopic findings: type I (11%), fraying of the free edge of the superior labrum; type II (41%), avulsion of the labral-bicipital complex from the superior glenoid; type III (33%), bucket handle tear of the superior labrum; and type IV (15%), bucket handle tear of the superior labrum with extension into the biceps tendon. ²⁶ At least three less common variations of SLAP tears have subsequently been described in the literature. ²⁷ These variations generally represent more extensive tears.

Two mechanisms of injury have been shown for SLAP lesions. The first mechanism involves a compressive force from a fall on an outstretched arm. The second mechanism involves traction on the arm, either as a result of a sudden pull or secondary to repetitive overhead use, such as in baseball pitchers, swimmers, and tennis and volleyball players. ²⁸

When isolated to the labrum, SLAP tears are not unstable in the classical sense. However, they may mimic instability by presenting with anterior shoulder pain, clicking, and occasionally mechanical locking of the joint. ²² When SLAP lesions involve the biceps anchor, stress maneuvers on physical exam may reveal instability. Surgical reattachment of the superior labrum is indicated when it is associated with avulsion of the biceps anchor. Isolated superior labral tears are treated with surgical debridement alone. ²⁸

SLAP lesions can be identified on MR images as abnormalities in signal and morphology involving the labrum and possibly the biceps anchor (figure 6). Tuite et al ²⁹ reported a sensitivity of 65% and a specificity of 84% in the detection of SLAP tears utilizing conventional (nonarthrographic) MR imaging. With MR arthrography, Bencardino et al ²⁸ observed 89% sensitivity and 91% specificity in the detection of SLAP tears confirmed at arthroscopy.

HAGL lesions --The acronym HAGL represents humeral avulsion of the glenohumeral ligament. Like other causes of anterior instability, this uncommon lesion occurs in the setting of anterior dislocation, although it typically occurs in a patient whose first dislocation occurs after the age of 40. Tirman et al ³⁰ described the MRI findings of seven patients with HAGL lesions. They found heterogeneity or frank disruption of the anterior capsule at its humeral insertion (6/7 patients), tears of the subscapularis tendon (6/7), dislocation of the biceps tendon (4/7), and Hill-Sachs deformities (4/7). MR arthrography demonstrated extravasation of contrast through the capsular defect in two of seven patients. ³⁰ The BHAGL lesion is a HAGL lesion in which a bone fragment has avulsed from the humeral insertion of the inferior glenohumeral ligament. ⁵

GLAD lesions --The GLAD (glenolabral articular disruption) lesion is not a cause of instability; however, it is related to the anterior instability lesions in terms of mechanism, clinical presentation, and findings on MR imaging. At arthroscopy, the lesion is identified as a superficial anterior inferior labral tear seen in association with an anterior inferior glenoid articular cartilage injury. This infrequent cause of anterior shoulder pain results from forced adduction from the abducted and externally rotated position. The treatment of choice is arthroscopic debridement. ³¹ This subtle lesion is best visualized by MR arthrography. ²²

Normal variants and other imaging pitfalls

Knowledge of the multiple variations of the normal labral-capsular-ligamentous complex is important to avoid the misdiagnosis of pathologic abnormalities. One such variant is the Buford complex which consists of a cord-like middle glenohumeral ligament and an absent anterior-superior labrum (figure 7). ³² This entity may be mistaken for a pathologic lesion by both the radiologist and the arthroscopist. Recognition of the Buford complex on MRI can avoid the false-positive diagnosis of a glenoid labral tear, possibly avoiding unnecessary surgery. ^32,33 This variant was detected on arthroscopy in 1.5% of cases ³² and on MR imaging in 2% of cases. ³⁴

Another variant of the anterosuperior labrum is a normal focal labral detachment seen anterior to the bicipital anchor. This variant, termed a sublabral hole or foramen, allows fluid to accumulate between the detached labrum and the underlying glenoid (figure 8). The sublabral foramen is seen in 7% to 12% of the normal population and can be mistaken for an anterior labral tear. ^18,34

Linear high signal, mimicking a tear, may be seen adjacent to the labrum at the insertions of the glenohumeral ligaments. Care should be taken to follow these ligaments to their insertions in order to avoid this diagnostic pitfall. ^18,34 Classically, a triangular shape is said to characterize the normal morphology of the anterior-superior labrum; however, normal labral morphology is variable. Young asymptomatic patients have been shown to have cleaved (11%) or notched (3%) anterior labra, distinct from the sites of ligament insertions. ¹⁶ It is uncertain whether these findings represent subclinical tears or normal variants. ^18,34

The magic angle phenomenon has been shown to cause areas of increased T1 and proton density signal in the posterosuperior and anteroinferior labrum. However, this is easily distinguished from a true labral tear as the abnormal signal will not approach that of fluid on the T2-weighted images. ¹⁸ A final potential pitfall is caused by undercutting of the labrum by the hyperintense transitional zone between the fibrous labrum and the hyaline cartilage covering the bony glenoid. This anatomy can also mimic a labral tear. ^6,18,34

Conclusion

Anterior shoulder instability is a complex problem encountered frequently by both radiologists and their referring clinicians. MR imaging of the shoulder plays an important role in surgical planning and clarifies diagnostically difficult scenarios by demonstrating both intra- and extra-articular anatomy and pathology. Optimization of MR techniques, including the use of arthrography, will increase the likelihood of accurate diagnosis. Additionally, knowledge of ana-tomic variants and other imaging pitfalls is essential to successfully discern pathology of the labral-capsular-
ligamentous complex.

Acknowledgements

The author thanks Clyde A. Helms, MD, for reviewing the manuscript and providing both images and insight. The author is also grateful for the technical assistance of Scott Faber. Special thanks is extended to Theodore A. Dorsay, MD for his generosity in providing MR images.