The shoulder.

Abstract

The shoulder is a combination of joints including the glenohumeral and acromioclavicular joints. The glenohumeral joint has the greatest range of motion of any joint in the body and, because of its unique design, is subject to dislocation more often than any other joint in the body. The arrangements of muscles, tendons, and ligaments around the shoulder account for the range of motion and protection against inherent dislocation. Knowledge of the anatomy is crucial in understanding the pathology and mechanism of injury, which will aid the clinician in treatment of the patient. As discussed with other joints, utility of fat suppression and fluid-sensitive sequences is helpful in identifying areas of edema or abnormal fluid collections. Imaging in oblique sagittal, oblique coronal, and axial planes is critical for evaluating the anatomy around the shoulder joint. Because the labrum is such a small structure, the selected field of view is key in being able to evaluate this structure. Generally 12 to 14 cm is reasonable.1,2 Oblique sagittal imaging allows for evaluation of the rotator cuff muscles, and when considering sequence design, T1-weighted imaging will give an idea of muscle bulk as well as evidence of atrophy. The bone marrow can also be evaluated for abnormal signal intensity. Fluid-sensitive sequences as mentioned previously will allow for identification of abnormal fluid collections as well as foci of edema. Occasionally, an entire muscle belly may demonstrate abnormal T2 signal indicating pathology. The pattern of abnormal signal can lead to specific diagnoses in some cases. The acromioclavicular joint is also well imaged in the oblique sagittal plane. Oblique coronal imaging is aligned along the plane of the supraspinatus. A fluid-sensitive sequence should be performed in this plane allowing for ease of diagnosis of muscle and tendon pathology as it relates to the rotator cuff. The deltoid muscle and acromioclavicular joint are also well visualized. The small structure of the superior labrum is also visualized in the oblique coronal plane. Finally, the articular cartilage within the glenohumeral joint is nicely visualized in this plane, and an echo time (TE) that is midrange (40–60 milliseconds) is more apt to demonstrate the articular cartilage distinctly from the underlying cortical bone. The axial images allow for visualization of the anterior and posterior labrum, biceps tendon, and subscapularis tendons. Evaluation of the coracohumeral interval is also readily appreciated in this plane of imaging. Evaluation of the articular cartilage can also be accomplished in this plane, keeping in mind that an intermediate-weighted TE will allow for discrimination of the labrum and articular cartilage as they will demonstrate different signal characteristics. The longer the TE, the more the cartilage will resemble the labrum, and areas of pathology may be more challenging to evaluate. Magnetic resonance (MR) arthrography with intra-articular gadolinium contrast injection can assist in evaluation of labral and capsular injuries.3 Because labral pathology predominates in younger patients, MR arthrography is generally reserved for evaluation of patients with clinical instability or in patients younger than 40 years presenting with shoulder pain.4 For rotator cuff pathology, in most circumstances, noncontrast MR imaging (MRI) is sufficient for diagnosis. This chapter will focus on anatomy and pathology as it relates to sports medicine. Understanding the anatomy cannot be underestimated when interpreting a shoulder MRI. Patterns of injury yield clues to mechanism of injury that assists with treatment plan. ROTATOR CUFF The rotator cuff is composed of 4 tendons—supraspinatus, infraspinatus, teres minor, and subscapularis. The tendons of the cuff interdigitate with each other as they attach to the greater and lesser tuberosity of the humerus. These tendons do not have a synovial sheath or a paratenon. Therefore, tenosynovitis is not a finding in the rotator cuff tendons. The supraspinatus tendon is located deep to the undersurface of the acromion and superficial to the humeral head. If the undersurface of the acromion is prominent or irregular or the space between the humeral head and acromion is narrowed, the supraspinatus tendon can become trapped or continually irritated. This may cause irritation of the overlying subacromial-subdeltoid bursa leading to inflammation and a bursitis. While the diagnosis of rotator cuff “impingement” is made on clinical examination, there are MRI findings that can support the diagnosis. These findings include a narrow space within the subacromial location and increased T2 signal in the region of the bursa. Impingement can be related to acromial development or acquired spurs at the inferior margin5,6 (Fig. 1). Overall shape of the acromion is no longer considered to be significant. However, an acromion that slopes downward laterally is developmentally predisposed to causing outlet impingement. Often in this situation the anterolateral acromial undersurface acquires a broad, flat spur resembling an “elephant’s foot.” Other subacromial spurs develop at the anterior acromion, at the insertion site of the coracoacromial ligament. Another source for impingement is an os acromiale. During development, the anterior portion of the acromion may fail to fuse, resulting in an ossicle with a fibrous synchondrosis to the remaining acromion. The anterior fibers of the deltoid muscle deflect the ossicle into the rotator cuff, leading to impingement. In later stages, the synchondrosis can break, with intervening fluid signaling transformation into a pseudarthrosis.FIGURE 1: Acromial factors associated with impingement of the rotator cuff. A, Coronal oblique T1-weighted image shows lateral down-sloping of the acromion with a broad, flat lateral subacromial spur (arrow) that can be associated with impingement. B, Axial proton-density fat-suppressed image shows an os acromiale (long arrow) attached with a synchondrosis (arrowheads) to the scapula. Note anterior fibers of the deltoid muscle (short arrows) attaching to the ossicle.The normal appearance of the rotator cuff tendons is low signal on T1- and T2-weighted imaging sequences. Diffusely increased signal on T1- and T2-weighted images (but lower T2 signal than fluid) is termed tendinopathy or tendinosis presuming a pathologic diagnosis of degeneration of the tendon5–7 (Fig. 2). The term tendinitis has fallen out of favor because it implies an inflammatory etiology. Foci of black signal may be seen in the tendons or bursa representing calcific tendinosis/bursitis.FIGURE 2: Rotator cuff tendinosis. A, T1-weighted coronal oblique image shows diffuse increase in signal within the supraspinatus tendon (arrows) representing tendinosis. B, T2-weighted coronal oblique image demonstrates a thin rim of fluid signal extending along the subacromial/subdeltoid bursa (short arrows) representing bursitis. Diffuse intermediate signal (arrowheads) extends through the tendon with associated thickening, consistent with tendinosis. A cyst is present in the greater tuberosity (long arrow). C, Coronal oblique T2-weighted fat-suppressed image reveals a focus of low signal (arrows) in the subacromial-subdeltoid bursa representing calcific bursitis.When the signal within a tendon is isointense to fluid on T2-weighted images, it is reported as a tear.5–7 The dimensions of the tear should be measured in anteroposterior and medial-lateral dimension. Rotator cuff tears are described in terms of full thickness (ie, extending from the bursal surface to undersurface), interstitial (along the central tendon fibers with preservation of bursal and articular surfaces), or partial thickness (partially extending through the tendon substance: isolated to the bursal or articular surface). Bursal-sided partial-thickness tears are less common than undersurface tears. Tears at the far anterior aspect of the supraspinatus may be difficult to identify on oblique coronal images and may be best seen on axial or oblique sagittal images (Fig. 3).FIGURE 3: Rotator cuff partial-thickness tears. A, Coronal oblique T2-weighted fat-suppressed image shows an undersurface (articular surface) partial-thickness tear of the supraspinatus tendon (arrows). Note intact superficial fibers (arrowheads). B, Coronal oblique T2-weighted fat-suppressed image shows a bursal surface partial-thickness supraspinatus tear (arrow). Note intact deep fibers (arrowhead). C, Interstitial tear of the supraspinatus (arrow) on coronal oblique T2-weighted fat-suppressed image, with intact deep and superficial fibers (arrowheads).A full-thickness tear is diagnosed by recognizing a gap in the tendon with fluid extending through the entire “footprint” of the rotator cuff insertion, extending through the deep and superficial fibers5–7 (Fig. 4). It is important to describe the tendon quality (ie, whether the tendon is thickened, thinned, frayed) and whether the myotendinous junction (normally positioned at 12 to 1 o’clock on the humeral head) is retracted. Muscle fatty infiltration may also be recognized on T1-weighted images allowing for determination of chronicity of the tendon tear. Large size of the tear, poor tendon quality, retraction, and muscle atrophy are all bad prognostic indicators for surgical repair. The supraspinatus is the most commonly torn rotator cuff tendon. Often when the tear increases in size, it will extend posteriorly to involve the infraspinatus. The teres minor tendon rarely tears; however, muscle atrophy may occur idiopathically in the teres minor muscle. Subscapularis tears are also fairly common, often presenting as interstitial tears allowing for subluxation or dislocation of the long head of the biceps tendon.7–9FIGURE 4: Rotator cuff full-thickness tear. Coronal oblique T2-weighted fat-suppressed image through the supraspinatus shows a full-thickness tear (arrows) with fluid signal extending from the joint to the bursa.BICEPS TENDON The biceps tendon is best recognized on the axial images as a low-signal, round or oval structure within the bicipital groove.10,11 It has a tendon sheath that is contiguous with the joint. Therefore, it is not unexpected to identify a distended biceps tendon sheath in the setting of a joint effusion. However, isolated fluid in the tendon sheath without the presence of a joint effusion is suggestive of a biceps tenosynovitis, which can result in anterior shoulder pain (Fig. 5). The long head of the biceps tendon originates from the glenoid rim adjacent to the superior labrum (together known as the “biceps anchor”). It is occasionally torn in the setting of chronic rotator cuff impingement as its location under the supraspinatus tendon makes it vulnerable to injury. The torn tendon may retract to the upper arm, resulting in an “empty biceps groove.” As noted above, subscapularis tears can result in subluxation or dislocation of the tendon from the bicipital groove; initially, the biceps enters the substance of the subscapularis tendon, eventually tearing through the superficial fibers or the deep fibers. In the latter circumstance, the biceps displaces into the anterior glenohumeral joint (Fig. 5).FIGURE 5: Biceps tendon pathology. A, Axial proton-density fat-suppressed image demonstrates prominent fluid signal within the biceps long-head tendon sheath (arrow) representing tenosynovitis. Thickening of the tendon (arrowhead) consistent with tendinosis. B, Subluxation of the long head of the biceps tendon (long arrow) from the bicipital groove (short arrow) into a subscapularis tendon tear (arrowheads) depicted on an axial proton-density fat-suppressed image.Abnormalities of the superior labrum can also involve the attachment of the biceps tendon as an extension of the superior labral tear.10,11 This will be discussed in more detail. GLENOHUMERAL LIGAMENTS, ROTATOR CUFF INTERVAL, AND FROZEN SHOULDER The glenohumeral ligaments represent thickenings of the joint capsule. Three ligaments, the superior, middle, and inferior glenohumeral ligaments, have been described, all at the anterior aspect of the glenohumeral joint12 (Fig. 6). These ligaments form a “Z” configuration with the superior and inferior ligaments projecting horizontally and the middle ligament extending vertically and obliquely along the anterior glenoid rim. There is some variability in the appearance of the ligaments, particularly the middle glenohumeral ligament. The glenohumeral ligaments contribute to the stability of the shoulder in a number of ways. The superior glenohumeral ligament helps stabilize the long head of the biceps tendon.13,14 In coordination with the subscapularis, the middle glenohumeral ligament contributes to anterior stability, whereas the inferior glenohumeral ligament maintains anterior and posterior joint stabilization. Anterior translation is prevented while the shoulder is abducted and externally rotated, and posterior translation is prevented when the shoulder is internally rotated.FIGURE 6: Capsular ligaments and adhesive capsulitis. A, Sagittal oblique T1-weighted MR arthrographic image shows normal anatomy of the capsular ligaments. CHL indicates coracohumeral ligament; SGHL, superior glenohumeral ligament; MGHL, middle glenohumeral ligament; IGHL, inferior glenohumeral ligament (anterior band). B, Sagittal oblique T1-weighted image shows synovial proliferation (arrows) in the rotator cuff interval representing adhesive capsulitis.The rotator cuff interval is the space within the glenohumeral joint anterosuperiorly between the supraspinatus and subscapularis tendons. The horizontal, intra-articular portion of the long head of the biceps tendon passes through the interval before it dives inferiorly within the biceps groove. The intra-articular portion is held in position by the coracohumeral ligament and superior glenohumeral ligament.13,14 Injury to these supporting capsular ligaments, usually associated with rotator cuff tear, can result in synovial proliferation within the interval (seen on MRI as edema and replacement of fat signal in this region, often with mass effect) (Fig. 6).15–17 There may be associated edema of the inferior capsule as well. This condition is known clinically as “frozen shoulder” or “adhesive capsulitis”; patients present with pain and loss of range of motion. Most commonly patients are middle-aged females, but adhesive capsulitis can present in any population, often incited by injury. Treatment generally involves a protracted course of rehabilitation, although joint injection under pressure has been previously used (brisement procedure) with questionable success. LABRUM AND INSTABILITY The labrum is composed of fibrocartilaginous tissue that attaches to the rim of the glenoid of the scapula with 1 function to deepen the glenoid fossa. It also functions as the attachment of the long head of the biceps tendon (superior labrum) as well as the glenohumeral ligaments. The labrum as well as glenohumeral ligaments are normally low in signal on all pulse sequences. The superior labrum and inferior labrum are best evaluated on oblique coronal images (Fig. 7), whereas the anterior labrum and posterior labrum are optimally visualized on the axial images (Fig. 7). Hyaline articular cartilage may be seen interposed between the labrum and the underlying glenoid. The articular cartilage is best identified on intermediate-weighted images separate from the adjacent low signal labrum. When the TE is longer, the articular cartilage becomes lower in signal intensity and is difficult to distinguish from the adjacent labrum.FIGURE 7: Normal labral variation. A, Coronal oblique T2-weighted fat-suppressed image shows a sublabral recess, with a thin, smooth rim of fluid signal (arrow) extending under the superior labrum at the biceps anchor. B, Sublabral foramen on an axial proton-density fat-suppressed image. Note fluid extending under the anterosuperior labrum (arrow). C, Buford complex on an axial T1-weighted fat-suppressed MR arthrographic image. Note a thick, band-like middle glenohumeral ligament (arrow) with absent anterosuperior labrum (arrowhead showing bare glenoid rim).There are a few normal variants that are important to know in order to not misdiagnose labral pathology.18 The labrum is most often triangular in configuration, but may occasionally be round or flattened in appearance¢ the labrum as it attaches to the glenoid may have articular cartilage interposed. The superior labral attachment can be loose, allowing for fluid to extend under the labral substance, known as a sublabral recess. Two additional labral variants are present at the anterosuperior glenoid. A sublabral foramen can be present at the anterosuperior labrum creating a hole (Fig. 7), and the labrum may be absent with compensatory thickening of the middle glenohumeral ligament, known as a Buford complex18–20 (Fig. 7). The labrum may be degenerated, torn, detached, or frayed. The MRI criteria used for diagnosing a labral tear include linear fluid signal within the substance of the labrum or irregularity of the margins, absent or an abnormally small labrum, and detachment of the labrum from the glenoid rim, keeping in mind normal variations described previously.21–24 Detachment of the labrum other than at the superior (sublabral recess) or anterosuperior glenoid (sublabral foramen) is considered a labral detachment and pathologic. The Bankart lesion is the most common injury following an anterior dislocation of the glenohumeral joint. It is a detachment or tear of the anterior inferior labrum from the glenoid (Fig. 8). Dislocation may alternatively be manifested as a fracture of the anteroinferior glenoid (bony Bankart). It is important to note how large the bone injury is. If more than 30% of the rim of the glenoid is involved, the injury is likely to be unstable requiring reduction and fixation. With anterior dislocation, corresponding impaction injury may be identified on the posterosuperior humerus termed a Hill-Sachs defect (Fig. 8). Therefore, recognizing a bone contusion or impaction in this location should force a thorough evaluation of the anterior inferior labrum. The Bankart lesion may be present, however, without an associated Hill-Sachs lesion. A nondisplaced Bankart lesion is termed a Perthes lesion; this type of tear is difficult to detect and may not be visible without MR arthrography. Some have advocated performing a sequence with the arm in abduction-external rotation position in order to place traction on the anteroinferior labrum for this reason.FIGURE 8: Anteroinferior labral tear. A, Axial proton-density fat-suppressed image showing fluid signal under the anteroinferior labrum (arrowhead) representing a Bankart lesion. Note bone bruise (arrow) at the posterior humeral head related to recent dislocation. B, Same patient as A showing a subacute Hill-Sachs defect (arrow) at the posterosuperior aspect of the humeral head with cortical indentation and adjacent bone marrow edema on an axial proton-density fat-suppressed image. C, Different patient with remote history of anterior dislocation. Fracture of the anteroinferior glenoid rim (bony Bankart, long arrow) is depicted on this axial T1-weighted fat-suppressed MR arthrographic image. Also note stripping of the anterior capsule (arrowheads) and a cartilaginous body in the posterior recess (short arrow).A detached labrum with intact periosteum has been termed an anterior labral periosteal sleeve avulsion or ALPSA lesion. This is important to recognize. Because the periosteum remains intact, the labroligamentous complex can retract and medialize, scarring in a position that will lead to persistent instability (Fig. 9). A chronic Bankart injury may resemble an ALPSA injury as scar tissue/fibrosis may develop around the torn labrum resembling a periosteal sleeve avulsion. These lesions can be difficult for an arthroscopist to recognize, and on MRI, the signal intensity of the fibrosis will appear more intermediate in signal intensity than the normal labrum especially on an intermediate-weighted TE axial sequence. Contrast may not extend into this scar tissue on MR arthrography.FIGURE 9: Anterior labrum periosteal sleeve avulsion (ALPSA) lesion. A, Axial T1-weighted fat-suppressed MR arthrographic image showing detached anteroinferior labrum (arrow) with attached scapular periosteum (arrowheads). B, Different patient with remote injury and persistent instability. Axial T1-weighted fat-suppressed MR arthrographic image showing displacement of the anteroinferior labrum (arrows) to the anterior glenoid with intermediate signal. This represents the medialized labrum with scar formation.Injuries to the posterior labrum are not as uncommon as once thought.23,24 The criteria for pathology are the same as those for an anterior inferior labral tear. A posterior dislocation of the shoulder is a result of trauma with the humerus in abduction and internal rotation. This mechanism of injury can be seen in football as well as wrestling and an awkward fall in skiing. Magnetic resonance imaging is more helpful than conventional radiographs in making the diagnosis. Occasionally, the radiograph may look normal, and identification of the abnormality may be overlooked. Magnetic resonance imaging will show the detachment of the posterior labrum and associated glenoid injury (reverse Bankart injury). The humeral injury can also be readily appreciated on MRI (reverse Hill-Sachs). This injury has also been referred to as a “trough” sign (Fig. 10).FIGURE 10: Posterior labral tear. Axial proton-density fat-suppressed image in a patient with recent posterior dislocation. A posterior labral tear (arrow) is noted with bone marrow edema at the anterior humeral head (arrowheads) representing a reverse Hill-Sachs defect (trough sign).A spectrum of findings at the shoulder accompanies athletes involved or previously involved in overhand throwing sports25–31 (Fig. 11). The acromion is one of the last ossification centers to fuse; adolescent pitchers presenting with shoulder pain occasionally show edema at the acromial apophysis representing stress injury. This has been implicated in delayed apophyseal union and development of os acromiale.26 Throwing early in childhood can also result in deformation of the posterior glenoid rim, leading to a rounded appearance simulating glenoid dysplasia. This may represent a useful adaptive phenomenon allowing for increased range of external rotation; however, it can lead to posterior labral tear and cartilage loss with eventual posterior instability. Active pitchers can also suffer from a condition known as “internal impingement.” This consists of injury to the undersurface of the infraspinatus as well as the posterior superior labrum. The pathology is a result of abduction and external rotation of the humeral head within the glenoid accompanying the pitching motion on the “wind-up” or late cocking phase. The soft tissue structures of the infraspinatus and the posterior superior glenoid get trapped between the glenoid and humeral head. Magnetic resonance imaging findings include undersurface partial-thickness tearing of the infraspinatus as well as tearing of the posterior superior labrum. Associated cystic change is often noted at the posterior-superior humeral head. Another condition in active pitchers has been termed glenohumeral internal rotational deficit, presenting with pain on the follow-through phase of throwing. This is related to scarring of the posterior capsule from repeated stretching on follow-through, leading to limitation of internal rotation of the humerus. Magnetic resonance imaging shows thickening of the posterior capsule, in conjunction with other posterior glenoid findings associated with pitching noted above. A late finding can include posterior glenoid ossification (Bennett lesion).FIGURE 11: Magnetic resonance imaging findings in throwing athletes. A, Axial proton-density fat-suppressed image of a 16-year-old high school pitcher with pain during throwing. The acromial apophysis shows edema (arrows) compatible with stress-related apophysitis. B, Axial T1-weighted fat-suppressed MR arthrographic image of a professional baseball pitcher. Note rounding of the posterior glenoid (arrow) associated with posterior labral degeneration and tear (arrowhead). C, Internal impingement in a throwing athlete with pain in late cocking phase. Coronal oblique T1-weighted fat-suppressed MR arthrographic image demonstrates an undersurface partial-thickness tear of the infraspinatus tendon (arrow) as well as a tear of the posterosuperior labrum (arrowhead). D, Professional baseball player with pain while throwing, on follow-through. Glenohumeral internal rotation deficit (GIRD) is demonstrated on this axial proton-density fat-suppressed image as posterior capsular thickening (arrowhead). Note rounding of the posterior glenoid (arrow), commonly seen in throwing athletes.A posterior labral detachment that may be chronic in nature is that which is associated with weight lifting, particularly the activity of weight lifting, and specifically the bench press. The posterior glenohumeral joint becomes “weight bearing” in the extension maneuver of weight lifting. This leads to increased force along the posterior glenohumeral joint space and laxity at the glenolabral interface. Eventually, the glenoid attachment can weaken, and the labrum can detach. Fluid signal is recognized at the glenolabral interface, and the labrum itself may be small, blunted, or abnormal in signal intensity (Fig. 10). Finally, injuries may also occur at the superior labrum. The superior labrum is particularly unique because of the insertion of the long head of the biceps tendon on the superior labrum. Therefore, injuries to the superior labrum may also involve the biceps anchor as it attaches to the superior labrum. A SLAP (superior labrum anterior to posterior) lesion is a term for a tear that is oriented in an anterior-to-posterior direction and occurs at the attachment site of the biceps tendon32–34 (Fig. 12). The term is slowly going out of favor, especially in older patients who may have insignificant degeneration of the labrum in this location. The literature reports 12 SLAP types based on tear pattern and extent. It is much more important to be descriptive and recognize an abnormality in the biceps labral complex. The initial classification described 4 types. Type 1 represents a labrum with a frayed undersurface. Type 2 is felt to be the most common type of superior labral tear describing a detachment of the superior labrum from the underlying glenoid. Type 3 is defined as a bucket-handle tear of the superior labrum without involvement of the biceps attachment, and type 4 involves the attachment of the long head of the biceps with a bucket-handle tear of the superior labrum. Keep in mind that the sublabral recess can mimic a SLAP tear, and it is important to identify fluid signal extending into the dark triangular labral substance. Superior labral tears are best diagnosed on coronal fluid-sensitive sequences. The diagnosis is made somewhat easier if MR arthrography is performed.FIGURE 12: Superior labrum anterior to posterior (SLAP) tear. Coronal oblique T1-weighted fat-suppressed MR arthrographic image shows contrast entering the substance of the superior labrum (arrow) representing a superior labral tear.In addition to labral pathology, capsular injuries may occur. Tearing of the inferior capsule has been termed an HAGL (humeral avulsion of the glenohumeral ligament) lesion, a somewhat inappropriate term, considering that the tear usually occurs away from the humeral attachment and can be present anywhere along the inferior capsule. On MRI, this injury can be seen as frank discontinuity of the inferior capsule, or extension of fluid (or contrast if an MR arthrogram) from the joint into the axillary recess35,36 (Fig. 13).FIGURE 13: Humeral avulsion of the glenohumeral (HAGL) lesion. Coronal oblique T1-weighted fat-suppressed MR arthrographic image shows injected contrast leaking from the joint through an inferior capsular tear (arrow) into the axilla.This should be distinguished from paralabral cysts–lobulated fluid signal masses adjacent to the glenoid that result from a tear in the labrum. They are important to recognize as their presence ensures underlying labral tear or detachment; frequently, a narrow neck extends to the labral tear. These cysts can be located anywhere around the labrum but are most common posterosuperiorly and inferiorly (Fig. 14). Not infrequently a paralabral cyst arising from a superior labral tear extends into the suprascapular or spinoglenoid notch. Mass effect in this location can lead to compression of the suprascapular nerve that supplies the supraspinatus and infrapsinatus muscle. Mass effect on the nerve can lead to pain simulating a rotator cuff tear. On MRI, neurogenic edema is identified as uniform increased T2 signal throughout the muscle belly, which can be distinguished from muscle injury typically seen as a feathery, heterogeneous pattern of edema. The second most common location for a paralabral cyst is adjacent to the inferior glenoid related to an inferior labral tear. A cyst in this location can cause mass effect on the quadrilateral space and impingement on the axillary nerve that supplies the teres minor and posterior deltoid muscle. Similar to above, denervation patterns in these muscles can be seen on MRI in this situation, known as “quadrilateral space syndrome.” Muscle denervation patterns on MRI presenting without nerve impingement should raise concern for Parsonage-Turner syndrome, an idiopathic brachial plexopathy.FIGURE 14: Paralabral cysts. A, Coronal oblique T2-weighted fat-suppressed image shows a posterosuperior paralabral cyst (arrow) extending into the suprascapular notch. Note narrow neck (arrowheads) to the posterosuperior labrum. Although a labral tear is not visible on this image, one is assumed to be present. B, Coronal oblique T2-weighted fat-suppressed image showing an inferior paralabral cyst (arrows). Again, a neck (arrowhead) extends to the inferior labrum where a presumed tear is present. C, Sagittal oblique T2-weighted image shows denervation atrophy of the infraspinatus muscle (arrows) with diffuse edema-like signal related to impingement upon the suprascapular nerve by a cyst (arrowheads) in the spinoglenoid notch.ARTICULAR CARTILAGE An articular cartilage injury may result from an impaction injury and can be isolated in its appearance. By themselves, these lesions may not lead to instability. However, recognition of an articular cartilage injury may assist with treatment planning. The lesion typically results from impaction of the humeral head against the articular surface of the glenoid with the arm in abduction and external rotation. Pain is often the clinical complaint rather than a sense of instability. Shear injuries may also occur to the glenohumeral articular cartilage resulting in delamination defects and development of osteoarthritis over time. The lesion is most easily recognized on axial and occasionally coronal images by recognizing the articular cartilage defect filled with fluid signal37,38 (Fig. 15). It is important to diagnose and address the articular cartilage within the radiology report. Articular cartilage injuries are not isolated to impaction but can be seen coexistent with rotator cuff pathology, and the presence of cartilage lesions may have an impact on surgical planning for cuff repair.FIGURE 15: Glenohumeral cartilage lesions. A, Axial T1-weighted fat-suppressed MR arthrographic image showing a cartilage defect (arrow). Note adjacent labral tear (arrowhead). B, Axial T1-weighted fat-suppressed MR arthrographic image shows a focus of cartilage delamination (arrows) at the posterior aspect of the glenoid.PECTORALIS MUSCLE TEARS Tears of the pectoralis muscle most often occur in weightlifters.39,40 Identification of where the tear occurs affects treatment of the injury. The pectoralis major muscle is composed of 2 heads, the clavicular and sternal, which converge as they approach insertion on the humerus at the bicipital groove. Tears may be partial or complete and may occur in the muscle belly, musculotendinous junction, or at the attachment of the tendon to the humerus. Surgery is generally performed for avulsions at the humerus, whereas nonsurgical treatment is recommended for injuries to the muscle or musculotendinous junction. The axial images as well as far anterior coronal images will show the muscle to best advantage. Tears have the same appearance as in all other muscles. The appearance depends on the age and whether hemorrhage is present. If hemorrhage is present, this can be recognized as increased T1 signal. The location of the fluid signal helps with treatment planning. Recognition of increased T2 signal at the attachment of the humerus rather than within the muscle belly is indicative of an avulsion warranting surgical intervention (Fig. 16).FIGURE 16: Pectoralis major tear. A, Axial proton-density fat-suppressed image of a weightlifter with subacute injury. Edema is seen at the insertion of the pectoralis major tendon (arrowheads) on the proximal humeral shaft representing a tear at the attachment, typically a surgical lesion. Note associated strain of the muscle belly (arrow). B, Axial STIR image of a different patient showing a tear at the myotendinous junction (arrow); this injury is usually treated conservatively. Note intact tendon at the humerus (arrowheads).CLAVICULAR OSTEOLYSIS Weightlifters are especially prone to clavicular osteolysis, seen as resorption of the articular surface of the clavicle at the acromioclavicular joint on radiographs and bone marrow edema on fluid-sensitive MRI sequences. Typically, the osseous changes are confined or predominant on the clavicular side and absent or minimal on the acromial side. Current thinking is that the condition represents a subchondral stress fracture, related to massive force-per-area exerted on the articular surface during overhead lifting41,42 (Fig. 17).FIGURE 17: Distal clavicular osteolysis. Axial T2-weighted fat-suppressed image through the acromioclavicular joint shows bone marrow edema isolated to the distal clavicle (arrow). Note low signal line (arrowhead) representing the subchondral stress fracture.This chapter presented the more commonly found injuries in sports trauma as well as the additional clues during the search pattern that can help to understand the mechanism of injury leading to more timely and appropriate treatment for the patient. Many of these lesions are more readily appreciated by the radiologist at interpretation than can be appreciated at arthroscopy.