The glenohumeral joint has a unique anatomic configuration

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1 Imaging of Glenohumeral Instability Timothy G. Sanders, MD,*, Michael Zlatkin, MD,*, and Justin Montgomery, MD The glenohumeral joint has a unique anatomic configuration designed to maximize the positioning of the opposable thumb in 3-dimensional space but by its very design is prone to instability. Indeed, the glenohumeral joint is the most commonly dislocated joint in the body. 1 In order for the shoulder to function properly, the large articular surface of the ball-like humeral head must remain centered within the small shallow articular fossa of the glenoid throughout its entire range-of-motion. The analogy of a golf ball sitting atop a tee has been used to describe the precariousness of the humeral head-glenoid fossa articulation. A complex arrangement of soft-tissue structures helps maintain the stability of the joint and injury or anatomic variation of any of these structures can result in instability. Glenohumeral instability, which is defined as the symptomatic displacement of the humeral head out of the glenoid fossa, can be described as subluxation (partial loss of joint congruity) or dislocation (a complete loss of contact between the articular surfaces of the humeral head and the glenoid fossa). Glenohumeral instability is often classified as either unidirectional or multidirectional with unidirectional instability further classified according to direction to include anterior, posterior, inferior, or superior. Most dislocations occur in an anterior direction (approximately 95%) and typically occur because of a fall on the outstretched arm. Posterior dislocations are the next most common with superior (luxatio erecta) and inferior dislocations only rarely occurring. Certain sporting activities such as weightlifting and throwing also predispose to instability without frank dislocation. Weightlifters often present with posterior instability while throwers present with complex shoulder injuries, such as internal impingement or glenohumeral internal rotation deficit disorder (GIRD), which are associated with various forms of instability. *National musculoskeletal Imaging (NMSI), Deerfield Beach, FL. Department of Radiology, University of Kentucky, Chandler Medical Center, 800 Rose St, Lexington, KY. University of Miami, School of Medicine, Miami, FL. Address reprint requests to Timothy G. Sanders, MD, Department of Radiology, University of Kentucky, Chandler Medical Center, 800 Rose St, Lexington, KY. radmantgs@cs.com Normal Glenohumeral Joint Anatomy Radiographic Anatomy Radiographs are often the first imaging study performed in a patient with symptoms of glenohumeral instability or previous dislocation. The shoulder-girdle is anatomically complex and as a result numerous radiographic techniques have been developed to fully evaluate the shoulder. 2,3 The anteroposterior (AP) radiograph is obtained with the beam directed in a true AP direction relative to the body. This view results in overlap of the glenoid rim and the humeral head as the glenohumeral joint is tilted approximately 40 relative to the body. The AP view results in relatively uniform distribution of soft-tissue density across the shoulder, providing excellent anatomic detail and as such is the most useful view to evaluate the entire shoulder-girdle after trauma. It is relatively simple to detect anterior dislocation of the humeral head on the AP view as the humeral head will be positioned in a medial and inferior location. Posterior dislocations, however, can be easily overlooked on this view. The glenohumeral True (Grashey) view differs from the standard AP view as this view is obtained with the beam tilted relative to the body, resulting in a image that is obtained as a true AP view of the glenohumeral joint allowing for improved evaluation of the glenohumeral joint space. This view will demonstrate subtle superior or inferior migration of the humeral head as well as narrowing of the glenohumeral joint. In suspected cases of anterior or posterior dislocation, an axillary view or scapular Y view can be very helpful in demonstrating anterior or posterior humeral head subluxation or dislocation. Numerous variations of the axillary view exist. One variation, that is, the West Point View, was developed to improve detection of a small osseous Bankart lesion of the anterior-inferior glenoid rim. The Striker Notch View nicely depicts the posterolateral aspect of the humeral head and is excellent for depicting a Hill-Sachs deformity. Magnetic Resonance Imaging Anatomy Magnetic resonance (MR) imaging is an excellent method of evaluating both the osseous and soft-tissue structures in the setting of glenohumeral instability or previous dislocation. The soft-tissue stabilizers are often categorized as static sta X/10/$-see front matter 2010 Elsevier Inc. All rights reserved. doi: /j.ro

2 Imaging of glenohumeral instability 161 bilizers (labrum, capsule, and glenohumeral ligaments) and dynamic stabilizers (rotator cuff and the long head of the biceps tendon). The labrum serves to deepen the glenoid fossa, thereby improving glenohumeral stability. It is composed of fibrocartilage and thus appears dark on most MR pulse sequences. The adjacent articular cartilage is composed of hyaline cartilage and is typically of brighter signal intensity on MR imaging. Conventional MR imaging is usually adequate for evaluation of the glenoid labrum, but the addition of intra-articular gadolinium may help in the detection of subtle or nondisplaced tears of the labrum. While all 3 imaging planes can be of value in detecting tears of the glenoid labrum, the axial imaging plane is the primary plane when evaluating the anterior and posterior labrum, while the coronal imaging plane is the primary plane for evaluation of the superior labrum (Fig. 1). The normal labrum is usually triangular in appearance but may occasionally demonstrate blunting or rounding of the free edge. 4 The capsule and glenohumeral ligaments (representing bandlike thickenings of the capsule) also contribute significantly to glenohumeral stability. 5 Disruption of the capsule as seen in the HAGL (humeral avulsion of the glenohumeral ligament) lesion or reverse HAGL lesion is a known source or instability. 6,7 The superior glenohumeral ligament is probably the least important of the 3 ligaments with regard to glenohumeral stability. It helps prevent inferior subluxation of the humeral head when the arm is positioned in 0 of abduction. It is best depicted in the axial imaging plane paralleling the coracoid process as it courses across the anterosuperior aspect of the glenohumeral joint. The superior glenohumeral ligament in conjunction with the coracohumeral ligament forms the roof of the rotator interval. The middle glenohumeral ligament (MGHL) is the most variable of the 3 ligaments ranging from a thin wispy-appearing structure to a thick cordlike structure. It plays only a minor role with regard to stability of the glenohumeral joint preventing external rotation of the humeral head when the arm is abducted between 45 and 60. The MGHL originates adjacent to the superior glenoid tubercle and courses obliquely across the anterior aspect of the glenohumeral joint, deep to the subscapularis muscle and superficial to the anterior glenoid labrum, blending with the deep fibers of the subscapularis tendon before its insertion on the lesser tuberosity of the humeral head. The inferior glenohumeral ligament (IGL) is the most important of the 3 ligaments with regard to glenohumeral stability and plays a major role in preventing anterior subluxation of the humeral head during maximum abduction and external rotation of the arm. The IGL is composed of 3 separate components, including the anterior band, posterior band, and the axillary pouch. 5 Normal Anatomic Variants Several anatomic variations exist that can mimic an abnormality of the labrum on MR imaging. These include cartilage undermining, the superior labral recess, the sublabral foramen, and the Buford Complex. On MR imaging, the hyaline articular cartilage is intermediate in signal intensity while the Figure 1 Normal labrum, MR arthrographic appearance. Axial (A) and coronal (B) MR arthrographic images of the shoulder demonstrate a normal-appearing labrum (short arrows). The articular (hyaline) cartilage (arrow heads) is intermediate in signal intensity. Cartilage undermining (long arrow) is noted as a normal anatomic variant in the superior quadrant. fibrocartilage of the labrum demonstrates low signal intensity. The articular cartilage often undermines the low signal fibrocartilage of the labrum and can mimic a tear. Articular cartilage undermining of the labrum is usually smooth and tapering in appearance and extends only partially beneath the labrum, and can be differentiated from a tear which is usually irregular in appearance and typically demonstrates brighter T2-signal on conventional MR or contrast signal intensity on MR arthrography (Fig. 1B). A tear can be seen

3 162 T.G. Sanders, M. Zlatkin, and J. Montgomery extending either partially or completely beneath the labrum. Articular cartilage undermining can be seen in all 4 quadrants of the glenoid. 8,9 The superior labral recess (Fig. 2A) is simply a potential space that extends beneath the superior labrum and occurs only in the superior quadrant of the glenoid. 10 This anatomic variant is best depicted on coronal MR images as a smooth, tapering fluid or contrast collection extending deep to the superior labrum. The recess extends only partially beneath the labrum and there is no associated displacement or irregularity of the labrum or abnormal signal extending into the substance of the labrum. The sublabral foramen (Fig. 2B) describes an anatomic variation in which the anterosuperior labrum is actually detached from the underlying osseous glenoid and this variation occurs only in the anterior superior quadrant of the glenoid. A detachment of the superior labrum isolated to the anterosuperior quadrant likely represents a normal anatomic variant rather a labral tear. 10 The Buford Complex (Fig. 2C) describes an absent or diminutiveappearing labrum in the anterosuperior quadrant combined with a thickened MGHL. The thick bandlike appearing glenohumeral ligament can mimic a torn detached anterior labrum. 11,12 Abduction External Rotation Imaging Anatomy Abduction external rotation (ABER) imaging is performed by placing the arm in maximum abduction and external rotation at the time of MR imaging. This is accomplished by placing the hand behind the head with the patient in the supine position (Fig. 3A). A dual-coil configuration is used. This technique requires repositioning of the patient after obtaining all the standard MR imaging plane and is typically used only during the performance of MR arthrography. Use of the ABER imaging technique usually adds between 7 and 10 minutes of time to the overall length of the study. 13 Patients with significant glenohumeral joint instability may experience significant apprehension (pain and feeling of impending subluxation during abduction and external rotation) and may be unable to complete this portion of the examination. Scout images are performed in the coronal imaging plane. The images are then prescribed off the coronal scout with the ABER images paralleling the long axis of the humeral shaft (Fig. 3B). The ABER position stretches the anterior band of the IGL taut and places tension on the anterior labrum (Fig. 3C). This technique will improve detection of nondisplaced tears of the anterior-inferior glenoid labrum (Perthes type Bankart variant) and is also helpful in detecting partial thick articular-sided tears of the rotator cuff. 14 Anterior Instability An anterior subluxation event or anterior dislocation most often results from a fall on the outstretched hand with the arm in slight abduction and external rotation but may occasionally result from a direct blow to the anterior shoulder. As Figure 2 Normal variants of the superior labrum. Coronal MR arthrographic image of the shoulder (A) demonstrates a smooth tapering contrast collection extending deep to the superior labrum representing the superior labral recess (long arrow). Normal-appearing labrum (short arrow). Axial MR arthrographic image of the shoulder (B) demonstrates the sublabral foramen (long arrow). Normal superior labrum (short arrow). Middle glenohumeral ligament (MGHL) (arrowhead). Axial MR arthrographic image of the shoulder (C) demonstrates the Buford Complex. Absent anterosuperior labrum (short arrow). Thick cordlike MGHL (long arrow).

4 Imaging of glenohumeral instability 163 many as 95% of all dislocations occur in the anterior direction and are not associated with a specific sport. In the younger patient (age 35 years) the most common lesion associated with an anterior subluxation event is a Bankart lesion (injury of the anterior-inferior labrum) or one of the Bankart variants discussed below. 15 In the older patient population (age 35 years) the rotator cuff becomes the weak link rather than the labrum and patients in this older age group experiencing a first-time dislocation are more likely to sustain a tear or avulsion of the rotator cuff tendon or possibly an avulsion fracture of the greater tuberosity. 16 In the patient presenting with an anteriorly dislocated humeral head, the arm is often fixed in slight abduction and external rotation. The humeral head may be palpated in an anteromedial position, beneath the coracoid process. Following reduction, an untreated Bankart lesion will lead to recurrent instability rates that approach 80%-90%. The patient often complains of pain and a feeling of apprehension during abduction and external rotation of the upper arm. Associated injuries include an impaction fracture of the posterosuperior humeral head (Hill-Sachs lesion), osseous or cartilaginous injury of the anterior-inferior glenoid rim, or possibly a stretching-type injury (neuropraxia) of the axillary nerve. In the patient with recurrent subluxation or continued signs of apprehension, the treatment of choice is surgical repair of the specific lesion of instability. Radiographic Findings Associated With Anterior Instability An anterior dislocation of the humeral head is easily detected on conventional radiographs. The AP radiograph will demonstrate inferior and medial displacement of the humeral head into the subcoracoid location and an anterior dislocation is often referred to as a subcoracoid dislocation (Fig. 4A). The axillary and Scapular Y views will also demonstrate anterior dislocation, but are rarely necessary to establish the diagnosis. Once the humeral head is reduced, signs of previous dislocation include flattening of the posterolateral aspect of the humeral head (Hill-Sachs lesion) or a fracture of the anterior-inferior glenoid rim (osseus Bankart lesion) (Fig. 4B and C). The Hill-Sachs lesion is best depicted on the Stryker notch view or on an AP radiograph with the humeral head in internal rotation. 17 The osseous Bankart lesion is best depicted on the West Point view (modified axillary view). 18 In older patients, an avulsion injury of the greater tuberosity may indicate a previous anterior dislocation. Figure 3 Abduction external rotation (ABER) imaging. (A) Normal patient positioning for ABER imaging. The patient is supine, hand placed behind head with arm in maximum abduction external rotation. A dual-coil configuration used. (B) A coronal scout is obtained and the ABER images are prescribed off the scout images along the long axis of the humeral shaft. (C) ABER image shows the taut anterior band of the inferior glenohumeral ligament (short arrows) and the normal-appearing anterior inferior labrum (long arrow). (Color version of figure is available online.) Computed Tomography Findings Associated With Anterior Instability Computed tomography (CT) imaging is helpful in detecting and delineating the extent of osseous involvement. In particular, sagittal reconstruction images through the glenoid fossa will demonstrate the size and number of fragments of an osseous Bankart lesion. The size of the osseous fragment should be reported along with the number of osseous fragments and the extent of displacement of fragments. In addition, an estimation regarding the percentage of the face of the

5 164 T.G. Sanders, M. Zlatkin, and J. Montgomery glenoid that is involved should be reported (using the sagittal imaging plane). Extensive bony deficiency of the anteriorinferior glenoid rim has been referred to as the inverted pear appearance (Fig. 5), and bone graft material may be required as a part of the reconstruction if the bony deficiency is greater than 30% of the face of the glenoid. 19 MR Findings Associated With Anterior Instability Conventional MR imaging and direct MR arthrography are both excellent techniques for detecting the extent of softtissue injury after an anterior subluxation event. Direct MR arthrography has the added benefit of creating joint distention, which may increase the conspicuity of a subtle or nondisplaced labral lesion, which will fill with high signal intensity gadolinium. The presence of gadolinium in the joint also allows the use of T1-weighted imaging to evaluate the labrum, which is a higher signal-to-noise image than the T2 images typically required to detect labral pathology when using conventional MR imaging. The main disadvantage of direct MR arthrography is that it is minimally invasive and requires the presence of a physician to perform the intraarticular injection. The use of direct MR arthrography may be especially beneficial in the evaluation of subtle lesions that can occur in the overhead athlete. Numerous Bankart variants have been described, each demonstrating a slightly different MR appearance with some variations requiring a slightly different surgical repair technique. Therefore, knowledge of the various lesions and an Figure 4 Radiographic findings associated with anterior dislocation. AP radiograph (A) shows anterior dislocation of the humeral head (long arrow) relative to the glenoid fossa (short arrow). The humeral head is positioned medial and inferior in position. West-Point view (B) shows a small osseous Bankart lesion. True AP view of the glenohumeral joint (C) shows a small osseous Bankart lesion (short arrow) and a large Hill-Sachs defect (long arrow). Figure 5 Osseous defect of the glenoid rim CT imaging. Sagittal CT reconstruction through the glenoid fossa demonstrates a large osseous defect (arrows), indicating a Bankart lesion after anterior dislocation. The inverted pear appearance of the glenoid fossa indicates significant bone loss likely requiring bone graft to regain postoperative stability of the glenohumeral joint.

6 Imaging of glenohumeral instability 165 accurate description of the instability lesion can be beneficial in obtaining optimal surgical results. Fibrous Bankart Lesion A tear of the anterior-inferior glenoid labrum. This is the most common lesion after an anterior subluxation event. 15 The labral fragment is often displaced and stripped away from the adjacent glenoid. The axial imaging plane is the primary plane for detecting Bankart lesion and MR imaging may demonstrate an irregular fluid or contrast collection extending into the substance of or deep to the labrum with or without abnormal morphology of the labrum. 20,21 The adjacent medial scapular periosteum is often stripped from the adjacent bone or possibly completely disrupted, resulting in a displaced labral fragment (Fig. 6). The location and extent of a labral tear should be described using either the quadrants of the glenoid as reference points or using the numbers of the face of the clock as reference points. The coronal imaging plane may also demonstrate fluid or contrast extending into the substance of the anterior-inferior labrum and this has been described as the double axillary pouch sign (Fig. 7). Perthes Lesion A nondisplaced tear of the anterior-inferior glenoid labrum. 22 The medial scapular periosteum remains intact in this Bankart variant holding the labrum in near anatomic position (Fig. 8A). A chronic Perthes lesion may resynovialize or scar back down in place making this lesion difficult to detect on conventional MR imaging. This is the 1 lesion in which direct MR arthrography with stress ABER imaging may be particularly useful in detecting an otherwise occult lesion of instability 14 (Fig. 8B). Figure 7 Double axillary pouch sign, anterior labral tear in the coronal imaging plane. Coronal MR arthrographic image (A) shows the normal-appearing anterior-inferior labrum (short arrow). Coronal MR arthrographic image (B) shows the double axillary pouch sign, a contrast collection (long arrow) located between the glenoid rim and torn anterior labrum (short arrow). Figure 6 Bankart lesion, MR imaging. Axial T2-weighted image through the glenohumeral joint reveals a tear (long arrow) of the anterior inferior labrum (short arrow). Intermediate signal is seen extending completely beneath the labrum indicating a tear and complete detachment of the labrum. Anterior Labroligamentous Periosteal Sleeve Avulsion Lesion Anterior labroligamentous periosteal sleeve avulsion injury (medialized Bankart lesion) (Fig. 9) is a variant of the Bankart lesion in which the medial scapular periosteum remains intact and pulls the torn labral fragment in a medial direction. 23 These lesions are easily detected in the acute setting, but may be more difficult to detect in the chronic setting when the medialized labral fragment has scarred down to the adjacent

7 166 T.G. Sanders, M. Zlatkin, and J. Montgomery images, while the sagittal imaging plane is usually best suited for demonstrating the size and extent of the lesion (Fig. 10). Marrow edema of the glenoid as seen on MR imaging is usually a very minor component of the abnormality. CT imaging, especially in the sagittal imaging plane can be helpful in depicting the extent of osseous abnormality and in detecting the number of fracture fragments. The face of the glenoid as viewed in the sagittal imaging plane has been described as having a pear-shaped appearance. Loss of significant bone stock along the anterior-inferior margin has been described as the inverted pear appearance of the glenoid (Fig. 5). This is important to describe as a bony deficiency of more than 30% of the face of the glenoid will likely require a bone graft procedure to regain normal stability in the postoperative setting. 19 Glenolabral Articular Disruption Lesion Glenolabral articular disruption 24 lesion most often results from an impaction of the humeral head against the face of the glenoid, resulting in a nondisplaced tear of the anterior-inferior labrum with an associated chondral injury of the inferior glenoid (Fig. 11). 25 Often there is no history of a frank dislocation, and the patient typically presents with shoulder pain but only minimal signs of instability on physical examination. Surgical treatment includes labral repair or debridement coupled with debridement of the chondral lesion. Humeral Avulsion of the Glenohumeral Ligament Lesion Humeral avulsion of the glenohumeral ligament 26,27 lesion results from an anterior dislocation or subluxation event and results in a tear or avulsion of the anterior band of the inferior Figure 8 Perthes lesion (nondisplaced labral tear). Axial (A) and ABER (B) MR arthrographic images demonstrate a collection of contrast (long arrow), extending partially beneath the anterior inferior labrum (short arrow). The medial scapular periosteum (arrow heads) remains intact holding the labrum in near anatomic position. glenoid. In the chronic setting, these lesions may require additional surgical debridement of the fibrosis and scar tissue before completing the Bankart repair. Osseous Bankart Lesion Fracture of the anterior-inferior glenoid rim that accompanies a tear of the glenoid labrum. These lesions may be difficult to detect on both radiographs and MR imaging. On MR imaging, the cortex of the anterior glenoid margin should be thoroughly evaluated in all 3 imaging planes. Disruption or irregularity of the cortex may be seen on axial and coronal Figure 9 ALPSA lesion (anterior labroligamentous periosteal sleeve avulsion), medialized Bankart. Axial MR arthrographic image demonstrates the torn anterior-inferior labrum (long arrow) pulled in a medial direction by an intact medial scapular periosteum (short arrow).

8 Imaging of glenohumeral instability 167 band of the inferior glenohumeral ligament can be seen at the expected humeral attachment site. In the acute setting, there will be adjacent soft-tissue edema and when performing direct MR arthrography, contrast will often be seen leaking through the capsular defect. Batter s Shoulder This term has been used to describe a subluxation event of the glenohumeral joint that occurs when a baseball batter swings and a misses a pitch. When a powerful batter swings a bat and misses the ball, tremendous forces are transmitted across the glenohumeral joint and can result in a subluxation event of the leading shoulder, (usually the nondominant or nonthrowing shoulder for a baseball player, unless the player is a switch-hitter). We have seen numerous cases in our practice with this exact mechanism of injury leading to a first-time subluxation event resulting in immediate onset of disabling shoulder pain. We have seen both anterior and posterior subluxation events occurring with this exact mechanism of injury and in each case resulting in either a Bankart or reverse Bankart variant, often with an associated Hill-Sachs or reverse Hill-Sachs lesion, typically resulting in disabling pain and instability of the involved glenohumeral joint (Figs. 13 and 14). Surgical repair is usually required for the patient to return to a preinjury level of play. This mechanism of injury has been reported in several professional baseball players and is often a season-ending injury. Figure 10 Osseous Bankart lesion. Axial T2-weighted image (A) and coronal T1-weighted image (B) demonstrate a minimally displaced osseous Bankart lesion (short arrow). The axial image shows an area of cortical step-off (long arrow), while the coronal image demonstrates a low signal intensity line indicating the fracture through the glenoid. glenohumeral ligament from the level of its humeral attachment (Fig. 12). There is no age predilection for this lesion. This lesion is difficult to detect using the standard arthroscopy portals, and it is therefore very important to alert the surgeon to the possibility of this lesion preoperatively so the potential lesion can be adequately evaluated at the time of surgery. A missed HAGL lesion has been reported as a common cause for a failed shoulder reconstruction after anterior dislocation. On MR imaging, a disruption of the anterior Figure 11 GLAD lesion (glenolabral articular disruption). Axial T2- weighted MR image with fat saturation demonstrates a nondisplaced tear of the anterior-inferior glenoid labrum with a small adjacent full-thickness chondral defect involving the anterior-inferior glenoid articular surface (arrow).

9 168 T.G. Sanders, M. Zlatkin, and J. Montgomery with a misdiagnosis rate that approaches 50%. In the proper clinical setting, a high index of suspicion is paramount in establishing the correct diagnosis. Classically, posterior dislocations are associated with seizure or electrical shock; however, posterior instability without frank dislocation has also been shown to be associated with numerous sporting activities. The typical patient with posterior instability is male, with age years who participate in an overhead activity, such as a throwing, weight- Figure 12 HAGL lesion (humeral avulsion of the glenohumeral ligament). Coronal MR arthrographic image shows a complete avulsion of the anterior band of the inferior glenohumeral ligament (long arrow) from the humeral neck. Contrast (short arrow) is seen extending through the capsular defect into the adjacent soft tissues. Lesions of Instability in the Older Patient With a First Time Dislocation While the Bankart variants are the most common lesions occurring in young patients after a first-time dislocation, first-time dislocators aged more than 35 years rarely present with a Bankart lesion. In the older patient population, the rotator cuff becomes the weak link and patients usually present with either a tear or avulsion of the supraspinatus tendon, avulsion of the subscapularis tendon, or an avulsion fracture of the greater tuberosity. 16 Tears of the supraspinatus or subscapularis tendons are treated surgically, while a nondisplaced avulsion fracture of the greater tuberosity is usually treated nonsurgically. Radiographs usually depict avulsion fractures of the greater tuberosity, but these lesions may occasionally be occult radiographically. In these cases, MR imaging will differentiate a surgical rotator cuff lesion from a nonsurgical occult nondisplaced fracture of the greater tuberosity. Even in the setting of an avulsion fracture, MR imaging will sometimes demonstrate a concurrent rotator cuff lesion that will benefit from surgical repair. Posterior Instability Posterior glenohumeral subluxation events occur less commonly than anterior subluxation events, accounting for approximately 3% of all glenohumeral dislocations. 28 Posterior subluxation of the humeral head most often occurs because of a posteriorly directed force with the arm flexed, adducted, and maximally internally rotated. Posterior dislocations are commonly missed at the time of initial clinical presentation Figure 13 Batter s shoulder. Axial (A) and coronal (B) MR arthrographic images of a professional short stop who experienced acute onset of pain of his loading shoulder after a swing and a miss at a pitch. Follow-up MR arthrography revealed a large osseous Bankart lesion (arrows), indicating an anterior subluxation event.

10 Imaging of glenohumeral instability 169 A patient with an acute posterior shoulder dislocation presents with pain and has a tendency to hold the shoulder in internal rotation and adduction. On physical examination there may be loss of the normal rounded contour of the anterior shoulder and a slight posterior bulge. With posterior glenohumeral instability there can be voluntary posterior subluxation of the shoulder and pain when positioning the arm in the vulnerable position of flexion, internal rotation, and adduction. Figure 14 Batter s shoulder. (A) A 26-year-old baseball player heard a pop and experienced immediate onset of pain when he swung at and missed a pitch. T2-weighted axial image shows a reverse Bankart lesion (tear posterior labrum) (short arrow) and a small reverse Hills-Sachs lesion (long arrow) with subcortical marrow edema, indicating a posterior subluxation event. (B) An 18-year-old baseball player heard a pop and experienced immediate onset of pain after a swing and a miss at a pitch. Axial T2-weighted MR image shows a reverse Bankart lesion (short arrow), and a reverse Hill-Sachs contusion (long arrow) indicating a posterior subluxation event. The anterior-inferior labrum was normal. Arrowheads indicate the normal MGHL. lifting, gymnastics, swimming, or racquet sports. Posterior instability can also be seen in individuals who participate in repetitive contact sports, such as football linemen who experience recurring trauma with the arm in the vulnerable position of flexion, adduction, and internal rotation. 29 Radiographic Findings Associated With Posterior Instability While the presence of an anterior dislocation is usually quite obvious on standard radiographic views of the shoulder, radiographic findings in the patient with posterior dislocation can be much more subtle and easily overlooked on AP radiographs, often delaying the diagnosis of posterior dislocation. Standard radiographic projections of the glenohumeral joint after acute trauma should include the AP view, and a lateral view, such as the axillary or scapular Y view, to adequately evaluate for evidence of a posterior dislocation. 2 During posterior dislocation, the humeral head most often dislocates straight posteriorly, which can make diagnosis on an AP radiograph of the shoulder rather difficult. There are several subtle radiographic findings, which when present on the AP radiograph should raise concern for a possible posterior dislocation. The first clue is that the humeral head will appear to be in the same position on both the internal and external rotation views of the shoulder. This occurs because the humeral head is locked in internal rotation at the time of posterior dislocation and the patient is unable to externally rotate the humeral head. Another subtle clue may be the slight lateral positioning of the humeral head relative to the glenoid resulting in a slight gap on the AP radiograph referred to as the rim or empty notch sign (widening of the glenohumeral joint 6 mm). Alternatively, the humeral head may be slightly medially positioned resulting in an overlap of the medial cortex of the humeral head and the glenoid fossa. Finally, a posterior dislocation can result in an impaction of the anterior aspect of the humeral head against the posterior glenoid rim, resulting in a reverse Hill-Sachs impaction fracture. On the AP radiograph, this will appear as a sclerotic vertical line paralleling the medial humeral head cortex referred to as the trough line sign (Fig. 15A). All of these AP radiographic signs are subtle and unreliable; therefore, it is crucial to obtain a lateral radiograph if posterior subluxation is suspected clinically or on the basis of radiographic findings on the AP view. 2 Options include an axillary view or scapular Y view and posterior dislocation is obvious on these views as the humeral head will be sitting posterior to the glenoid (Fig. 15B). CT Findings Associated With Posterior Instability CT imaging can be very helpful in defining the extent of osseous involvement after a posterior subluxation event. Flattening of the anterior medial aspect of the humeral head indicates a reverse Hill-Sachs impaction type fracture, while a

11 170 T.G. Sanders, M. Zlatkin, and J. Montgomery fracture along the posterior margin of the glenoid rim denotes a reverse osseous Bankart lesion. CT can also be helpful in the acute or subacute setting demonstrating a persistent locked posterior dislocation or partial subluxation. 2 MR Findings Associated With Posterior Instability MR imaging will demonstrate the reverse Hill-Sachs and reverse Bankart lesions similar to CT imaging, but in addition, can detect the presence or absence of subcortical marrow edema, which will help in determining the acuity of the lesion. MR imaging will also depict associated soft-tissue injuries after posterior dislocation (Fig. 15C). 30 Reverse Bankart Lesion Posterior dislocation is often associated with a tear of the posterior glenoid labrum (Fig. 14). The tear is often nondisplaced and seen on MR imaging as high signal fluid or contrast extending into the substance or deep to the posterior labrum. A labral fragment may also be detached or displaced. A reverse osseous Bankart and or reverse Hill-Sachs lesion may also be present (Fig. 15C). Reverse HAGL Posterior dislocation can also result in a tear of the posterior band of the inferior glenohumeral ligament referred to as the reverse HAGL lesion. The presence of a small osseous avulsion fracture associated with the reverse HAGL lesion is referred to as the reverse bony HAGL (Fig. 16). Reverse HAGL lesions have also been described secondary to repetitive microtrauma in athletes without a frank posterior dislocation. 30,31 Injuries of the teres minor muscle or tendon can occasionally accompany a reverse HAGL lesion. Soft-tissue edema may be seen within the substance of the muscle indicating a grade I muscle strain or contusion, or in severe trauma, a partial or full-thickness avulsion of the teres minor tendon may occur. The presence of an isolated teres minor muscle injury should prompt a thorough search for the presence of a reverse HAGL lesion. Figure 15 Locked posterior dislocation. Thirty-seven years-old fell off son s dirt bike and landed on left arm. AP view of the shoulder (A) obtained in the Emergency Department demonstrates a sclerotic line (arrows) paralleling the medial cortex of the humeral head, representing the trough sign, the radiographic sign indicating a reverse Hill-Sachs lesion. Scapular Y view (B) demonstrates posterior subluxation of the humeral head (long arrow) relative to the center of the glenoid fossa (short arrow). Axial T2-weighted MR image (C) obtained after initial radiographs, shows a posterior locked dislocation of the humeral head with a reverse Hill-Sachs lesion (arrow). Bennett Lesion Glenohumeral instability is a common entity in the overhead throwing athlete. These athletes are predisposed to chronic traction injuries of the posterior band of the inferior glenohumeral ligament that occur during the deceleration phase of throwing. The Bennett lesion is an extra-articular posterior capsular avulsion injury that may be associated with a posterior labral tear, posterior undersurface rotator cuff tear, and posterior subluxation of the humeral head. Calcification occurs within the posterior capsule at the site of avulsion injury. On MR imaging, the Bennett lesion appears as an area of low signal abnormality due to the mineralization within the capsule at the site of avulsion possibly with associated thickening of the capsule or pericapsular edema (Fig. 17). CT and radiography will show a crescentlike area of abnormal mineralization along the posterior margin of the osseous glenoid at the level of the posterior capsular attachment. Patients with a

12 Imaging of glenohumeral instability 171 of throwing. The motion of throwing has been broken down into separate components which include 1. Wind-up phase 2. Early cocking phase 3. Late cocking phase 4. Acceleration phase 5. Deceleration phase 6. Follow-through phase Current understanding of the biomechanics of throwing suggests that the majority of injuries occur either during the late cocking phase with the arm positioned in maximum abduction and external rotation or because of the tremendous forces that are generated across the glenohumeral joint during the acceleration and deceleration phases of throwing. 34,35 The term dead arm has been coined to describe a pathologic condition in which a throwing athlete is unable to continue to throw with the same accuracy or control because of pain or because of a subjective unease in the shoulder. Our understanding of the pathophysiology of the dead arm in throwing athletes has evolved considerably in the past 30 years and while debate and research continues on this topic in the orthopedic community, it is clear that optimal treatment of the various lesions associated with throwing is predicated upon not only an accurate identification of the lesion but also upon a thorough understanding of the pathophysiology leading to these injuries. 34,35 Three theories now dominate the orthopedic community with regard to instability associated with pain in the throwing athlete. These include (1) extrinsic impingement-instability overlap, (2) intrinsic impingement, and (3) GIRD. Figure 16 Reverse HAGL lesion. Coronal (A) and axial (B) MR arthrographic images through the shoulder demonstrate a complete avulsion (long arrows) of the posterior band of the inferior humeral ligament from the humeral neck. Contrast (short arrows) is seen extending through the capsular defect into the adjacent soft tissues along the posterior aspect of the glenohumeral joint. Bennett lesion experience pain during the late cocking phase and acceleration phase of throwing. 32,33 Lesions of Instability Associated With Throwing The biomechanics of throwing have been studies extensively in an attempt to understand and prevent the disabling lesions that are often associated with the repetitive overhead activity Figure 17 Bennett lesion. Axial T2-weighted MR image in an 18- year-old pitcher with shoulder pain demonstrates a thick bandlike area of low signal abnormality (short arrow) indicating an area of ossification, associated with an extra-articular avulsion of the posterior capsule. Pericapsular edema (long arrows) is also noted.

13 172 T.G. Sanders, M. Zlatkin, and J. Montgomery Extrinsic Impingement-Instability Overlap (Secondary Impingement) In 1990, Jobe first put forth the theory that repetitive throwing or overhead activity results in stretching of the anterior capsule and the anterior support structures of the glenohumeral joint resulting in subtle anterosuperior migration of the humeral head during the late-cocking and early-acceleration phases of throwing. 36 This migration of the humeral head results in extrinsic impingement of the rotator cuff against the undersurface of the osseous outlet. Extrinsic impingementinstability overlap syndrome is thought to differ from the standard theory of extrinsic impingement as the impingement results from humeral head migration secondary to instability of the glenohumeral joint rather than resulting from an anatomic variation or a pathologic lesion of the osseous outlet (secondary impingement). Radiographs are usually unremarkable. MR imaging may demonstrate cuff tendinosis involving either the supraspinatus or the infraspinatus tendon and if a rotator cuff tear is present, it is usually small and partial-thickness in nature involving the articular side of the tendon. MR arthrography may be more sensitive in detecting a small partial-thickness rotator cuff tear in the young throwing athlete resulting from secondary impingement. Extrinsic impingement-instability overlap is still widely accepted as a source of pain in the throwing athlete and is usually treated with anterior capsular tightening with a reported 50% return rate to preinjury level of performance after surgery. 34 Internal Impingement Jobe and Walsh later put forth the theory of internal impingement to explain the dead arm syndrome in certain throwing athletes. This theory differs slightly from the extrinsic impingement-instability overlap theory described above. With intrinsic impingement, the primary cause of the lesion is again thought to be anterior capsular stretching resulting from repetitive microtrauma to these structures during the late-cocking and early-acceleration phases of throwing. In internal impingement, however, the lesions rather than resulting from an extrinsic impingement, occur because of the impingement of the undersurface of the posterior cuff between the posterior aspect of the greater tuberosity and the posterosuperior labrum. 34 This theory of internal impingement put forth by Jobe and Walsh further supported the idea of anterior capsular stretching as the primary lesion in throwing athletes. However, the anatomic lesions differed slightly with the lesions of internal impingement primarily seen as tendinosis and partial thickness articular-sided tears of the posterior cuff combined with posterior superior labral tears. Radiographs again are usually normal but subtle nonspecific findings of sclerosis and subchondral cysts of the posterior aspect of the greater tuberosity may be seen on AP radiographs and the Grashey view may demonstrate subtle inferior subluxation of the humeral head associated with anterior capsular laxity. MR imaging typically demonstrates some combination of 4 abnormalities (Fig. 18). These include (1) tendinosis and or partial thickness-articular sided tearing of the posterior cuff, (2) abnormal signal or a frank tear of the Figure 18 Internal impingement. A 24-year-old professional baseball pitcher demonstrates changes in internal impingement. Coronal (A) and axial (B) MR arthrographic images demonstrate fraying, irregularity and a tear of the posterior superior labrum (short arrows), tendinosis and partial thickness articular sided tearing of the infraspinatus tendon (long arrows), and subcortical cystic changes and marrow edema of the greater tuberosity (arrowheads). posterosuperior labrum, (3) marrow edema and or subcortical cystic change within the posterior aspect of the humeral head, and finally (4) internal impingement seen on ABER imaging. The presence of abnormalities of the posterior cuff combined with an abnormality of the superior labrum in the symptomatic overhead athlete appears to be most sensitive with regard to the diagnosis of internal impingement The presence of subcortical marrow changes in the greater

14 Imaging of glenohumeral instability 173 tuberosity is most variable, while the presence of internal impingement of the posterior cuff between the greater tuberosity and the posterosuperior labrum on ABER imaging is the least specific MR finding with regard to internal impingement. These lesions of the posterior cuff and superior labrum can be quite subtle and the use of direct MR arthrography can improve detection of these lesions in the young overhead athlete with symptoms of internal impingement. The theory of internal impingement put forth by Jobe and Walsh further supported the idea that the primary lesion of the dead arm is that of anterior capsular stretching injury resulting from the repetitive microtrauma of throwing. Treatment of internal impingement is again anterior capsular reconstruction or tightening with as many as 50% of overhead athletes returning to preinjury level of pitching after surgery. Glenohumeral Internal Rotation Deficit Disorder GIRD is the most recent theory put forth by Dr Burkhart in the early 1990s regarding the pathophysiology of the dead arm syndrome and still remains controversial in some corners of the orthopedic community. 34,35 This theory suggests that the primary lesion in many overhead athletes is not a stretching of the anterior capsule but rather scarring and fibrosis of the posterior capsule or tightening of the posterior cuff musculature secondary to repetitive overhead activity. This new theory suggests that the primary lesion is one of posterior capsular tightness rather than anterior capsular laxity that lead to the various lesions of the labrum and rotator cuff in the dead arm syndrome. It is theorized that posterior capsular scarring or posterior muscle tightness pulls the humeral head in a posterior direction, shifting the humeral head point of contact on the face of the glenoid in a posterosuperior direction during the late-cocking phase of throwing. This results in a loss of internal rotation of the glenohumeral joint to less than 180 when compared with the nonthrowing arm. In GIRD, it is theorized that the posterior capsular scarring or posterior muscle tightening results from repetitive trauma to the posterior shoulder during the late cocking phase of throwing. This results in a cascade of injuries, beginning with a posterosuperior shift of the point of contact of the humeral head on the glenoid face during the late cocking phase of throwing. Loss of internal rotation of the glenohumeral joint of the throwing arm ensues and as a result, during the late cocking phase of throwing, abnormal twisting rotational forces are applied to the anchor of the long of the biceps tendon and a peel back injury occurs to the posterior superior labrum during the acceleration phase of throwing resulting in a tear of the posterosuperior labrum. These changes in the posterior capsule and posterosuperior labrum are often seen in conjunction with changes of the posterior cuff resulting from internal impingement. Radiographs are usually normal and there are still only spotty reports in the literature regarding MR findings associated with GIRD (Fig. 19). The most commonly reported findings include a focal area of thickening or fibrosis of the posterior capsule immediately adjacent to the glenoid attachment. Also, Figure 19 GIRD (glenohumeral internal rotation deficit disorder). An 18-year-old baseball pitcher with clinical findings of glenohumeral internal rotation deficit disorder. Axial proton-density image (A) demonstrates marked thickening (arrows) of the posterior capsule. (B) Coronal T2-weighted image also shows signs of internal impingement with tendinosis of the infraspinatus tendon (long arrows) and signal alteration within the posterosuperior labrum (short arrow). the changes in internal impingement including abnormalities of the posterior cuff and greater tuberosity are seen. Finally, a peel back or avulsion-type lesion of the posterior superior labrum may be seen. 40 Preventive treatment for GIRD includes posterior capsule/ muscle stretching exercises after each pitching outing, which has been shown to decrease the incidence of GIRD in the highly

15 174 T.G. Sanders, M. Zlatkin, and J. Montgomery Miscellaneous Lesions of Instability Paralabral Cysts Paralabral cysts of the shoulder have been described as analogous to parameniscal cysts of the knee and most often occur in conjunction with a labral tear and demonstrate a high association with glenohumeral joint instability. They typi- Figure 20 Paralabral cyst with compression of the suprascapular nerve. (A) Axial T2-weighted image demonstrates a tear (short arrow) of the posterior labrum with an associated paralabral cyst (long arrow) dissecting into the spinoglenoid notch posteriorly. Sagittal T2-weighted image (B) shows the paralabral cyst (long arrow) dissecting into the spinoglenoid notch posteriorly. There is diffuse edema (short arrows) isolated to the infraspinatus muscle indicating neurogenic edema associated with a compression of the suprascapular nerve at the level of the spinoglenoid notch. competitive overhead athlete. Once a labral lesion occurs, treatment includes superior labrum anterior posterior repair of the peel back lesion rather than debridement and rotator cuff debridement or repair as needed. Early results suggest a 70%-80% return to preinjury level of activity. 34,35 Figure 21 Axillary nerve neuropraxy. Coronal T2-weighted image without fat saturation (A) and coronal STIR (B) images in a patient after anterior dislocation of the shoulder who complains of pain and shoulder weakness. There is diffuse edema present throughout the teres minor (short arrows) and deltoid (long arrows) muscles, indicating neurogenic edema associated with a stretching injury of the axillary nerve.

16 Imaging of glenohumeral instability 175 Axillary Nerve Neuropraxy The axillary nerve is the shortest branch of the brachial plexus and traverses the quadrilateral space adjacent to the posterior humeral circumflex artery. Anterior dislocation of the glenohumeral joint can result in a stretching type injury of the axillary nerve, leading to denervation of the teres minor muscle and less commonly the deltoid muscle. Loss of deltoid muscle function may mimic rotator cuff pathology on physical examination. MR imaging can be very helpful in detecting signs that suggest a previous stretching-type injury of the nerve (Fig. 21). In the acute setting, as in compressive type lesions of the nerve, the denervated muscle will demonstrate diffuse edema, while in the chronic setting, fatty infiltration and atrophy of the muscle will occur. 43 Figure 22 Recurrent Bankart lesion after labral repair. Axial MR arthrographic image through the shoulder demonstrating contrast (long arrow) extending deep to the anterior-inferior labrum indicating a recurrent tear after previous labral repair. Short arrow indicates a suture anchor. Arrowheads indicate a recurrent tear of the anterior scapular periosteum. cally occur because of extrusion of fluid through a tear of the labrum. They can be small and focal or quite large dissecting into various spaces adjacent to the joint. They are best detected on T2-weighted images with fat saturation, this being one of the main reasons that most shoulder protocols include at least 1 T2-weighted sequence, even with shoulder MR arthrography. They may communicate with the joint space, but only rarely fill with contrast during direct MR arthrography. MR imaging usually depicts an oval or multiseptated fluid filled mass on T1- and T2-weighted images. The lesion may dissect into the suprascapular notch or into the spinoglenoid notch resulting in compression of the suprascapular nerve (Fig. 20). Rarely, a paralabral cyst may dissect into the quadrilateral space, resulting in compression of the axillary nerve. Nerve compression can result in denervation of the muscles enervated by the affected nerve. In the acute setting the muscle will demonstrate diffuse edema, at this point the muscle and nerve damage is still considered reversible. In the chronic setting muscle edema is replaced by fatty atrophy/ fatty infiltration of the involved muscle. At this point the nerve and muscle damage is considered irreversible. Compression of the suprascapular nerve at the level of the suprascapular notch results in denervation of both the supraspinatus and infraspinatus muscles, while compression of the nerve at the level of the spinoglenoid notch results in isolated denervation of the infraspinatus muscle, as the branch to the supraspinatus muscle has already been given off. Compression of the axillary nerve generally results in denervation of the teres minor and sometimes the deltoid muscle. 41,42 Surgical Treatment for Glenohumeral Instability Several procedures have been developed using indirect surgical techniques to correct glenohumeral instability. These procedures rather than repair the torn labrum, attempt to prevent recurrent instability through indirect means. Some of these procedures include a staple capsulorrhaphy (DuToit and Roit repair), subscapularis manipulation (Putti Platt and Magnuson Stack procedure) and repositioning of the coracoid process (Bristow procedure). With advances in arthroscopic techniques, these indirect repair procedures are only rarely performed today and are mainly of historical interest, although occasionally a patient will present for imaging who has undergone one of these procedures years ago. Figure 23 Artifact associated with hardware from prior osseous Bankart repair. Axial MR arthrographic image demonstrates marked artifact (arrow) obscuring much of the anatomy of the anterior inferior glenohumeral joint. This artifact is associated with a screw that was used to repair an osseous Bankart lesion.

17 176 T.G. Sanders, M. Zlatkin, and J. Montgomery with a screw, may result in severe artifact obscuring visualization of the labrum on subsequent MR imaging (Fig. 23). In these cases CT arthrography may better delineate the appearance of the postoperative labrum in suspected cases of reinjury. Figure 24 CT imaging appearance after a Laterjet procedure. Axial CT image through the glenohumeral joint after a Laterjet procedure (bone graft procedure to repair an osseous Bankart lesion). A large screw (long arrow) holds the bone graft (short arrow) in place. The size of the screw shows why MR imaging often demonstrates marked susceptibility artifact after repairs of an osseous Bankart lesion. Direct repair techniques usually consist of debridement and or suturing of the torn labrum using suture anchors placed within the glenoid to reattach the torn labral fragment. Osseous Bankart lesions may be repaired using a screw to reaffix the bone fragment or may require use of allograft material to correct a bony deficit of the osseous glenoid. An engaging Hill-Sachs lesion may also require the use of bone graft material to repair a large Hill-Sachs defect. Following labral repair, suture anchor artifact is often seen within the osseous glenoid adjacent to the site of labral repair. In some cases, this artifact may be severe enough to partially obscure the repaired labrum, but in most cases, the artifact is minimal and does not prevent adequate evaluation of the labrum. The normal postoperative labrum may appear blunted or irregular in appearance. MR findings suggesting a recurrent tear include a detached or displaced labral fragment or fluid undermining the labrum (Fig. 22). Recurrent tears may occur in an area of previous repair or may extend beyond the site of repair to involve adjacent labral tissue that was normal at the time surgery. The use of direct MR arthrography may help in the evaluation of the postoperative labrum. 44,45 Surgical repair of an osseous Bankart lesion is usually performed if the original bony defect involves more than 30% of the face of the glenoid. 46 If there is a single large bone fragment, a screw may be adequate to repair the fractured glenoid (Fig. 23). If the bone fragment is comminuted, this may require debridement and placement of graft material (Laterjet procedure) (Fig. 24). Repair of an osseous Bankart lesion Figure 25 Hardware artifact after arthroscopic Bankart repair. Axial MR arthrographic image (A) of a patient with continued pain after arthroscopic repair of a Bankart lesion demonstrates artifact associated with a prior Bankart repair. An area of prominent susceptibility artifact (arrow) is suspicious for a proud suture anchor. The patient was taken to the CT scanner immediately after the MR examination. Axial CT image (B) through the shoulder demonstrates a proud suture anchor (arrow) as the source of continued pain.

18 Imaging of glenohumeral instability 177 MR Imaging Appearance of Postoperative Complications Postoperative complications after shoulder reconstruction for instability can include displaced or loosening of hardware or suture anchor devices (Figs. 25 and 26). MR arthrography may be helpful in detecting the intra-articular location of displaced hardware, some of which is nonradiopaque, and therefore occult on radiographs. 44 Osteolysis or osteomyelitis can also occur adjacent to hardware resulting in loosening or fluid signal adjacent to the hardware or screws. Postoperative synovitis may result in a large joint effusion and may mimic postoperative infection on MR imaging, which will present with a large joint effusion, synovial thickening, and enhancement with administration of intravenous gadolinium (Fig. 27). Acute chondrolysis of the glenohumeral joint refers to a rapid onset of widespread chondrocyte death occurring over a short interval of time. This is a devastating complication that has been reported with increasing frequency after arthroscopic reconstruction of the glenohumeral joint in young individuals for shoulder instability. The patient typically presents with rapid onset of shoulder discomfort and a painful decreased range of motion, which usually occurs within the first 6-12 months after shoulder reconstruction. Treatment is supportive with the patient often eventually requiring joint arthroplasty for severe glenohumeral osteoarthritis. The exact etiology remains unclear, but there is evidence to suggest that acute chondrolysis may be related to the use of thermal probes or Marcaine pumps at the time of surgery, and most surgeons now avoid the use of these ancillary techniques in an attempt to avoid this devastating complication. Other suggested etiologies include an unknown infectious agent or immune response to bioabsorbable material. Preoperative imaging usually demonstrates normal symmetric joint space on radiographs and normal articular cartilage on MR imaging (Fig. 28A). At the time of onset of symptoms in the postoperative patient, radiographs usually depict severe glenohumeral joint space narrowing and minimal subchondral sclerosis without other signs of osteoarthritis (Fig. Figure 26 Hardware complication after shoulder surgery. Coronal T2-weighted image shows a displaced tack (arrow) within the axillary pouch as a source of continued shoulder pain after surgery. Figure 27 Osteomyelitis and septic arthritis after rotator cuff repair and superior labral debridement. Coronal T1-weighted (A) and T2- weighted (B) images demonstrate marrow edema (short arrows) surrounding a suture anchor in the humeral head and a small complex joint effusion (long arrow) in this patient who developed osteomyelitis and septic arthritis after a rotator cuff repair and superior labral debridement.

19 178 T.G. Sanders, M. Zlatkin, and J. Montgomery 28B). In the acute stages of chondrolysis of the glenohumeral joint, MR imaging shows diffuse chondral loss on both sides of the glenohumeral joint. Minimal subchondral marrow signal changes are noted. There is usually a paucity of joint effusion (Fig. 28C), unlike in a postoperative septic joint which usually presents with a large complex joint effusion. Even in suspected cases of acute chondrolysis, septic arthritis should be considered in the differential diagnosis with appropriate culture of joint fluid. In most cases of acute chondrolysis, no infectious agent is ever identified. 47 Conclusions Glenohumeral instability is a very complex and challenging topic both for the clinician and the imager. Numerous subtle lesions can be detected on radiographs, CT, and MR imaging and a thorough understanding of not only the anatomy of the glenohumeral joint but the pathophysiology of these injuries is essential to adequately diagnose and treat the various lesions of instability. Figure 28 Acute chondrolysis of the glenohumeral joint after Bankart repair. Preoperative AP radiograph of the shoulder (A) demonstrates a normal glenohumeral joint space (arrows). AP radiograph of the shoulder approximately 6 months after surgery (B) demonstrates marked narrowing the glenohumeral joint space (arrows) in this patient who began experiencing progressive onset of pain and decreased range of motion. Axial T2-weighted image (C) shows complete loss of the normal articular cartilage on both sides of the glenohumeral joint (long arrows) with subchondral marrow edema (short arrow). Note the paucity of joint fluid which can be a useful MR imaging sign to help differentiate acute chondrolysis from a postoperative septic arthritis. References 1. Kroener K, Lind T, Jensen J: The epidemiology of shoulder dislocations. Arch Orthop Trauma Surg 108: , Sanders TG, Jersey SL: Conventional radiograph of the shoulder. Semin Roentgenol 40: , Merrill V: Shoulder girdle, in Ballinger PW (ed): Merrill s Atlas of Radiographic Positions and Radiographic Procedures, Vol I (ed 6). St Louis, MO, Mosby, pp , Loredo R, Longo C, Salonen D, et al: Glenoid labrum: MR imaging with histologic correlation. Radiology 196:33-41, Bencardino JT, Beltran J: MR imaging of the glenohumeral ligaments. Radiol Clin North Am 44: , Tirman PF, Steinbach LS, Feller JF, et al: Humeral avulsion of the anterior shoulder stabilizing structures after anterior shoulder dislocation: demonstration by MRI and MR arthrography. Skeletal Radiol 25: , Wolf E, Chang J, Dickson K: Humeral avulsion of the glenohumeral ligaments as a cause of anterior shoulder instability. Arthroscopy 11: , Kaplan PA, Bryans KC, Davick JP, et al: MR imaging of the normal shoulder: variants and pitfalls. Radiology 184: , Legan JM, Burkhard TK, Goff WB II, et al: Tears of the glenoid labrum: MR imaging of 88 arthroscopically confirmed cases. Radiology 179: , Yeh L, Kwak S, Kim Y, et al: Anterior labroligamentous structures of the glenohumeral joint: correlation of MR arthrography and anatomic dissection in cadavers. AJR Am J Roentgenol 171: , Tirman PFJ, Feller JF, Palmer WE, et al: The Buford complex a variation of normal shoulder anatomy: MR arthroscopic imaging features. AJR Am J Roentgenol 166: , Williams MM, Snyder SJ, Buford D: The Buford complex the cordlike middle glenohumeral ligament and absent anterosuperior labrum complex: a normal anatomic capsulolabral variant. Arthroscopy 10: , Cvitanic O, Tirman PJF, Feller JF, et al: Using abduction and external rotation of the shoulder to increase sensitivity of MR arthrography in revealing tears of the glenoid labrum. AJR Am J Roentgenol 169: , Wischer TK, Bredella MA, Genant HK, et al: Perthes lesion (a variant of the Bankart lesion): MR imaging and MR arthrographic findings with surgical correlation. AJR Am J Roentgenol 178: , Bankart A: The pathology and treatment of recurrent dislocation of the shoulder joint. Br J Surg 26:23-29, 1938