Imaging of the shoulder

Transcription

1 ISBN: PPI201402DC ISSN: Imaging of the shoulder ABSTRACT It is difficult to precisely define which imaging method is the gold standard in the evolution of numerous problems that arise in and the glenohumeral joint. In cases of rotator cuff tear, arthrography, computed arthrotomography, magnetic resonance imaging and ultrasonography all hove value. In cases of the shoulder impingement syndrome, magnetic resonance imaging appears to be most ideal in defining the cause of the syndrome. In cases of tendinitis, the same is true. In instances of calcific tendinitis, routine radiography, arthrography, and computed arthrotomography can be employed. Adhesive capsulitis is best evaluated with arthrography. In all instances of shoulder pain, the initial examination should be routine radiography. In cases of rotator cuff tear, shoulder impingement and tendinitis the next examination probably should be magnetic resonance imaging. In instances of instability, computed arthrotomography and magnetic resonance imaging have similar advantages. MR arthrography cans also be employed. In instances of adhesive capsulitis arthrography remains the gold standard. The shoulder is a region of complex anatomy that is evaluated by a number of advanced imaging tech niques or particular importance is the application of computed arthrotomography and magnetic resonance imaging to the analysis of shoulder disorders. Some of the major applications of these two techniques to such problems will be emphasized, although reference will be made to others including routine radiography, con ventional arthrography, computed tomography and ultrasonography. ANATOMY Whether it be the orthopedic surgeon through dissec tion technique or the radiologist through cross-sectional image analysis, physicians should be aware that the anatomy of the shoulder region is complex. Indeed, the number of muscles found in this region are many. With routine radiography, of course, knowing the anatomy of individual muscles in this region was rela tively unimportant; with magnetic resonance imaging, and to a lessor extent, computed arthrotomography, knowledge of this anatomy is pertinent to correct diag nosis. Muscles of the shoulder region can be divided into those connecting the limb to the spine; those of the shoulder itself; and those of the upper arm. In each category, numerous muscles can be identified. The anterior aspect of the shoulder region contains many of these muscles. The subscapularis muscle attaches to the anterior surface of the scapula in a broad site of origin. Just lateral to this near the glenoid is the origin of the long head of the biceps brachii muscle. The short head of the biceps brachii arises along with the coracobrachialis muscle at the tip of the coracoid proc ess. Among the muscle attached to the proximal portion of the humerus are the pectoralis major, latissimus dorsi and teres major muscles. With regard to the posterior shoulder anatomy, the supraspinatus attaches to the upper portion of the scapula, above the spinous process. Beneath this, a large site of origin of the infraspinatus can be observed. Just lateral to this site of attachment exists the attachment site of the teres minor muscle, beneath which are the attachment sites of the teres major and latissimus dorsi muscles. The supraspinatus, infraspinatus and teres minor mus cles attach to the greater tuberosity of the humerus. The muscles of the upper arm include the biceps brachii, coracobrachialis and triceps muscles. Two of these are readily apparent as large anterior structures. The short head of the biceps and coracobrachialis arise from a common origin on the coracoid process. The long head of the biceps arises adjacent to the upper portion of the glenoid cavity and extends through the glenohumeral joint into the bicipital groove. It is surrounded by a synovial sheath. With regard to the posterior muscles of the upper arm, the long head of the triceps attaches to the inferior aspect of the scapula, just medial to the glenoid cavity. The lateral head of the triceps attaches to the upper humerus. These two heads extend in to the arm. As they cross the

2 teres major muscle, three spaces can be identified. The axillary nerve and the posterior circum flex humeral artery pass through the quadrilateral space. The circumflex scapular artery extends through the omotricipical triangle, and a lower triangle allows pas sage of the radial nerve and deep brachial artery. Many ligaments arise from the scapula, clavicle and humerus. The coracoacromial ligament extends from the coracoid process anteriorly to the acromion poste rior, creating a roof above the glenohumeral joint. The coracohumeral ligament arises at the base of the coracoid process and extends to the humerus, attaching near the greater and lesser tuberosities. The coracoclavicular ligament consists of a medial conoid part and a lateral trapezoid part. The superior transverse scapular liga ment creates a foramen on the superior border of the scapula through which the suprascapular nerve passes. The inferior transverse scapular ligament is not invariably present. The components of the glenohumeral joint include the articular cartilage, glenoid labrum, articular capsule and synovial membrane. The articular cartilage of the humeral head is thickest in its central portion, whereas that of the glenoid labrum is thinnest in this region. The articular capsule surrounds the joint and is reinforced by a variety of ligaments that will be discussed subse quently. The synovial membrane exists on the inner aspect of the capsule. The glenoid labrum represents a fibrocartilaginous structure that surrounds the glenoid cavity. Three recesses can be identified in the glenohumeral joint: the subscapular recess, the axillary recess, and the bicipital tendon sheath. These are readily identified with routine arthrography. The subscapular recess is located Predominantly beneath the subscapularis muscle and tendon. The axillary recess is present between the upper portions of the humerus and scapula, and the bicipital tendon sheath surrounds the long head of the biceps in the bicipital groove. GLENOHUMERAL JOINT STABILITY It should be recognized that the glenohumeral joint possesses little inherent stability. Several reasons exist for this, including the relatively small size of the glenoid cavity compared to that of the humeral head and the redundant capsule. In particular, the anteroinferior as pect of the capsule is rather weak. Four structures contribute to shoulder stability, however. These struc tures are the rotator cuff, glenoid labrum, glenohumeral ligaments and coracoacromial arch. The muscles of the rotator cuff include the supraspinatus, infraspinatus, teres minor and the subscapularis. These muscles and their corresponding tendons pass in front of, over and behind the humeral head, providing increased stability to the glenohumeral joint and channeling its motion. An intact rotator cuff separates the glenohumeral joint from the subacromial (Subdeltoid) bursa. Hence, during arthrography, communication between these two synovial spaces indicates a full thickness tear of the rotator cuff. Fat lines the subacromial bursa. This peribursal fat can be identified on routine radiography, particularly those done during internal rotation of the shoulder. Obliteration of this fat may be indicative of inflammation of the shoulder, and is compatible with the diagnosis of an acute rupture of the rotator cuff. The second structure providing stability is the glenoid labrum. This consists of fibrocartilage and surrounds the glenoid cavity, thereby deepening the articular surface. The third structure that provides stability to the glenohumeral joint is a group of glenohumeral liga ments. These can be divided in to the superior glenohumeral, middle glenohumeral, and inferior glenohumeral ligaments. They are not constantly present nor are they always of predicable size. These ligaments represent thickenings of the capsule surface that can be identified from the interior of the joint; they are not visible on the external surface of the capsule. Of these three ligaments, the inferior glenohumeral ligament is most constant and thickest. These three glenohumeral ligaments arise close to the anterior surface of the glenoid labrum. They pass to attach to the anatomic neck of the humerus, the lesser tuberosity, or the surgi cal neck, or combinations of these three structures. An opening may exist between the superior and middle glenohumeral ligaments or between the middle and inferior glenohumeral ligaments, or at both locations, allowing communication of the glenohumeral joint with the subscapular recess. It has been suggested that the glenohumeral ligaments are an important component of the shoulder, providing stability to the anterior capsular structures. It is further suggested that absence or deficiency of any or these ligaments may predispose an individual to anterior glenohumeral joint dislocation. The fourth structure providing stability to the glenohumeral joint is the coracoacromial arch. This consists of the coracoid process anteriorly, the acromion posterior, and the intervening coracoacromial liga ment. TECHNIQUE Computed arthrotomography is accomplished using air alone or air mixed with a small amount of contrast material. Following the introduction of approximately 15cc of this mixture, a series of transaxial images is obtained. Generally, these are 3 mm in thickness and spaced each 3mm. Both bone and son tissue windows are employed. With regard to the technique employed for magnetic resonance imaging, this should be tailored to the indi vidual patient. Certain techniques are best utilized to evaluate abnormalities of the rotator cuff, others to evaluate shoulder impingement, and still others for shoulder in stability. In the evaluation or abnormalities of the rotator cuff, the major plane of interest is the coronal (oblique). Thus, one should utilize T1, balanced and T2-weighted images in 2

3 the coronal plane. In addition, a transaxial T1-weighted series or gradient echo sequence is also useful to evalu ate any associated abnormalities of the glenoid labrum. In the analysis of shoulder impingement, the same series of images used for rotator cuff abnormalities can be employed; however, in addition, a sagittal (oblique) T1- weighted series may be useful to evaluate the coracoacromial arch. In the evaluation of shoulder in stability, the major plane of interest is the transaxial plane. Therefore, in this plane, T1-weighted, balanced and T2-weighted se quences are used. In addition, a T1-weighted coronal (oblique) sequence is added to this series gradient echo sequences also may be employed. It is important to recognize that the coronal and sagittal image planes are oriented in the axis of the scapula, not in the axis of the body. Hence, they are coronal oblique and sagittal oblique planes when the body is used as the reference point. These planes can be constructed from initial transaxial localizer images. The importance of appropriate surface coils in the evaluation of the shoulder cannot be overemphasized Currently, several manufactures have designed specific surface coils, one placed on the anterior aspect of the shoulder and one on the posterior aspect of the shoulder, or surrounding the shoulder. NORMAL EXAMINATION The initial anterior image in the coronal (oblique) plane demonstrates the region of the subscapularis muscle and tendon. This tendon can be seen inserting on the lesser tuberosity as a black structure without signal. A slightly more posterior cut will demonstrate the supraspinatus muscle and tendon. The latter appears as a black line inserting on the greater tuberosity. Above this line one can recognize the relatively bright signal of the peribursal fat. On this image, one can also see the superior and inferior portions of the glenoid labrum which normally are entirely back. The articular carti lage of the humeral head and glenoid cavil y can be seen. One also may appreciate the long head of the biceps tendon as it passes over the top of the humeral head and into the bicipital groove. Slightly more posteriorly in the coronal oblique plane, one sees again the supraspinatus muscle and tendon. Once again, the tendon is usually homogeneously dark in signal characteristics. As one proceeds posteriorly, one begins to image the infraspinatus tendon which passes above and behind the humeral head. On the most posterior cuts, the infraspinatus and teres minor ten dons and muscles can be seen. Beneath the teres minor muscle exists the quadrilateral space. This allows one to determine the position of the teres minor muscle and teres major muscle as well. On many of these coronal cuts, the trapezius and deltoid muscles can also be identified. In the sagittal (oblique) plane, beginning medially, one can identify the glenoid region of the scapula. In this region, the supraspinatus, infraspinatus and teres minor muscles are seen along the posterior surface of the scapula, and the subscapularis muscle is seen anteriorly. Portions of the coracoclavicular ligaments may be appreciated. Slightly more laterally, one images the area of the glenoid fossa and adjacent labrum. The latter appears as a circular region of decreased density about the glenoid fossa. At this level, one may appreciate also the coracohumeral and coracoacromial ligaments. As one proceeds more laterally, one begins to image the humeral head. Around the humeral head, one can appreciate the tendons of the rotator cuff. These consist of the subscapularis tendon in front and the supraspinatus, infraspinatus and teres minor tendons above and behind the humeral head. Additional structures that are imaged in this plane include the bicipital tendon sheath and the coracohumeral ligament. It is the cross-sectorial or transaxial images that are most complex. In evaluating the series of images, four precise levels are of importance: the superior portion of the glenoid fossa; the region of the subscapular recess; the midglenoid fossa; and the inferior fossa. In the superior portion of the glenoid fossa, one can appreciate the superior glenohumeral ligament. This extends from the superior margin of the glenoid cavity to of the humerus. it is intimate with the capsule, superior labrum, and bicipital tendon. It is best appreci ated on images done at the level of the coracoid process. At the level of the subscapular recess, one can note the subscapularis muscle and tendon, extending medially from the scapula to the lesser tuberosity. The recess it self is located predominately behind the tendon but it may spill over in front of the tendon. In some images at this level, one may begin to identify the middle glenohumeral ligament. At the midglenoid fossa, the pattern of anterior capsular attachment can be assessed. Three of capsular insertion have been identified: type I occurs in or near the labrum; type II is slightly medial to the labrum; and type III occurs far medial to the labrum. It is suggested that the type II and certainly, type III pattern of capsular inser tion may predispose an individual to anterior instability. The final cross-sectional level is the region of the inferior glenoid fossa. It is in this area that one can appreciate the strong inferior glenohumeral ligament. A series of computed arthrotomograms in the transaxial plane will illustrate this pertinent anatomy. Once again, beginning from the top of the joint, identified structures include the superior glenohumeral ligament, the subscapular recess, the subscapularis tendon, the glenoid labrum, the middle glenohumeral ligament, and the inferior glenohumeral ligament. 3

4 Similar anatomy is displayed by transaxial MR images. The uppermost image will display the acromioclavicular joint. Slightly lower one will see the supraspinatus muscle extending over the humeral head. Below this, The superior glenohumeral ligament and infraspinatus muscle and tendon become visible. At this level, one can appreciate The bicipital tendon within the bicipital tendon groove. At a slightly lower level, the subs capularis muscle and tendon become apparent. Behind the tendon, one way identify the middle glenohumeral ligament. In The same area, the dark signal of the anterior and posterior portions of the glenoid labrum can be appreciated. The lowest cuts demonstrate the inferior glenohumeral ligament. ABNORMAL EXAMINATION Magnetic resonance imaging and computed arthrotomography are best applied to the evaluation of three specific shoulder problems: rotator cuff abnor malities, causes of the shoulder impingement syn drome, and shoulder instability. With regard to rotator cuff abnormalities, it is the portion of the rotator cuff tendons adjacent to their sites of attachment that are vulnerable to disruption follow ing acute trauma, chronic stress or a systemic inflam matory articular process. Specifically in this region, there is a zone of relative avascularity or a critical zone, best identified within the supraspinatus tendon. It is this tendon that most frequently is ruptured. There are no reliable routine radiographic features of acute tears of The rotator cuff. Certainly, a number of radiographic signs have been identified in those indi viduals who have chronic rotator cuff disruptions, however. These include superior displacement of the hu meral head, narrowing of the acromiohumeral space, reversal of the normal convexity on the inferior surface of the acromion, and cysts, sclerosis or rarefaction in the humeral head and acromion. It should be recognized, however, that obliteration of the peribursal fat plane may indicate the existence of an acute rotator cuff tear. This is a relatively sensitive but nonspecific finding. Conventional arthrography can be used to diagnose a complete tear or some types of partial tear of the rotator cuff. This is best accomplished utilizing double contrast technique or air alone. In this manner, one can identify not only a tear of the rotator cuff, but the status of the remaining portions or the cuff and the site of disruption. Magnetic resonance imaging can be utilized to evaluate the rotator cuff. There is little difficulty in diagnosing full thickness tears of the rotator cuff. Some difficulty, however, arises in differentiating tendinitis and partial tears of the rotator cuff. In general, tendinitis and partial tears of the rotator cuff are associated with some disruption of the black signal within the normal tendon, close to its attachment to the tuberosity. Areas or intermediate signal in tensity can be identified that may brighten up slightly on tile T2- weighted images. There is no retraction or the supraspinatus muscle, however. In addition the peribursal fat plane generally is intact. Full thickness tears of the rotator cuff are associated with retraction of the muscle-tendon junction. Bright signal can be demonstrated throughout the tendon and there may be fluid both in the joint and in the subacromial bursa. The shoulder impingement syndrome is defined as impingement of the periarticular soft tissues or the shoulder between the greater tuberosity of the humerus and the coracoacromial arch. This is best observed on physical examination during abduction of flexion of the arm. It has been suggested that there are three stages of the shoulder impingement syndrome. The first is noted in individuals below the age of 25 years and consist of edema and hemorrhage within the rotator cuff. The second is observed between the ages of 25 and 40 years and consists principally of fibrosis. The third stage, seen after the age of 40 years, is associated with rotator cuff disruption. There are some investigators who be lieve that most cases of rotator cuff disruption begin as the shoulder impingement syndrome. There are no early radiographic findings oft he shoulder impingement syndrome. Nonspecific sclerosis or eburnation of the greater tuberosity may be apparent in chronic cases. In addition, a subacromial enthesophyte develops at the attachment site of the coracoacromial ligament. This is best seen on anteroposterior radio graphs done with angulation of the x-ray bean. Fluoroscopy and subacromial bursography have been utilized to investigate the shoulder impingement syn drome. Magnetic resonance imaging can demonstrate quite nicely, in the coronal plane, the relationship of the supraspinatus muscle and tendon and the acromion. Normally, the peribursal fat planes can be appreciated between the muscle and the acromion. In addition, there is a rather smooth surface to the superior surf ace of the supraspinatus muscle and tendon. In cases of shoulder impingement, there is narrowing of the distance be tween the acromion and humerus. Associated bicipital tendinitis and disrupt ion of the rotator cuff may be seen. Osteophytes developing at the acromioclavicular joint represent a further cause of the shoulder impingement syndrome. The final condition that is well evaluated with com puted arthrotomography and magnetic resonance imaging is shoulder instability. Such in stability can be classified in a variety of ways, one of which relates to the direction of subluxation or dislocation. In some instances, osseous abnormalities result from a disloca tion. These osseous abnormalities may be identified with routine radiography. An example of this includes the Hill-Sachs and Bankart lesions that may be seen following an anterior glenohumeral dislocation. 4

5 Although the classic Bankart lesion is purely cartilaginous, bone injuries on the anterior surface of the glenoid fossa also can accompany anterior glenohumeral joint dislocations. These are well identi fied with specialized radiographic projections and transaxial computed tomography. When the Bankart lesion is purely cartilaginous, computed arthrotomography is a valuable technique. In such cases, one can identify disruption of the normal smooth anterior and posterior labral surfaces. Similar findings can be appreciated with magnetic resonance imaging. The normal characteristic of the labrum on T1 weighted MR images include a homogeneously b lack and smooth structure. Considerable variability exists in this appearance, however. Tears involving the anterior glenoid labrum will be associated with accumulation of air during computed arthrotomography and with areas of intermediate signal intensity on magnetic resonance images. Similar altera tions accompany tears of the posterior portion of the labrum. In some instances, one can appreciate disruption of the glenohumeral ligaments in combination with altera tions of the labrum. This can be seen with both computed arthrotomography and magnetic resonance imaging. The Hill-Sachs lesion represents a compression fracture on the porterolateral surface of the humeral head. This can be well shown with routine radiography, but it also can be appreciated with computed tomography and magnetic resonance imaging. The best level for demonstration of the Hill-Sachs lesion is in the area of the coracoid process or above this level Autor Donald Resnick, M.D Revista Archivos de Reumatología VOL 5 N 2 /1994 Se publica con autorización de la Sociedad Venezolana de Reumatología venreuma@gmail.com 5