(Radiographics. 1999;19:685-705.)
© RSNA, 1999
Rotator Cuff: Evaluation with US and MR Imaging1
Cameron J. Seibold, MD,
Thomas A. Mallisee, MD,
Scott J. Erickson, MD,
Melbourne D. Boynton, MD,
William G. Raasch, MD and
Michael E. Timins, MD
1 From the Departments of Radiology (C.J.S., T.A.M., S.J.E., M.E.T.) and Orthopaedics (W.G.R.), Froedtert Memorial Lutheran Hospital, 9200 W Wisconsin Ave, Milwaukee, WI; and Vermont Orthopaedic Clinic, Rutland (M.D.B.). Recipient of a Cum Laude award for a scientific exhibit at the 1997 RSNA scientific assembly. Received May 13, 1998; revision requested June 5 and received August 13; accepted August 20. Address reprint requests to S.J.E.
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Abstract
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Magnetic resonance (MR) and ultrasound (US) imaging are currently touted for assessment of rotator cuff disease. Optimum clinical imaging techniques include use of (a) a 1.5-T MR imaging unit with small planar coils, proton-density–weighted and T2-weighted fast spin-echo sequences, and 10–12-cm fields of view (yielding 400–470 x 500–625-µm in-plane spatial resolution) and (b) a state-of-the-art commercial US unit with insonation frequencies of 9–13 MHz (yielding 200–400-µm axial and lateral resolution). Proper diagnosis requires familiarity with normal anatomic characteristics and imaging pitfalls. Care must be taken to avoid sonographic tendon anisotropy and MR imaging magic angle effects, which can be misinterpreted as rotator cuff tear. At MR imaging, a complete cuff tear typically appears as either a hyperintense defect or a tendinous avulsion that extends from the bursal to the articular side of the cuff; a partial cuff tear typically appears as a focal hyperintense region that contacts only one surface of the cuff. Complete and partial tears manifest with a wide spectrum of findings at US. MR imaging and US are effective for evaluating rotator cuff injuries, with high reported accuracies for detection of complete tears but more disparate results for detection of partial tears.
Index Terms: Shoulder, injuries, 414.4813 • Shoulder, MR, 414.12141 • Shoulder, US, 414.1298 • Tendons, injuries, 414.4813
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INTRODUCTION
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Both magnetic resonance (MR) imaging and ultrasound (US) are widely used for the evaluation of pathologic conditions of the rotator cuff and essentially obviate conventional arthrography. The first article about the use of US in the assessment of the rotator cuff was published in 1979 (1); that for MR imaging, 1986 (2). Technical improvements, coupled with advances in the understanding of anatomic and pathologic characteristics of the rotator cuff, have resulted in the maturation of these two modalities. Despite encouraging results, however, diagnostic difficulties may be attributed to technical restrictions, interpreter skill, or both. MR imaging, while more universally accepted, may be limited in the depiction of partial cuff tears. US, although accurate in the hands of experienced imagers, proves challenging for many beginners.
In this article, we first review techniques for MR and sonographic imaging. Second, we present pertinent anatomic characteristics of the rotator cuff constituents, as well as other intimately related structures. Third, a more detailed analysis of tendinous, fibrocartilaginous, and cartilaginous histologic characteristics is provided. Fourth, common imaging pitfalls are demonstrated. Last, the pathogenesis of rotator cuff disease and correlative imaging findings are presented.
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IMAGING TECHNIQUE
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MR Imaging
During a routine examination, patients are supine. In our opinion, the shoulder is best imaged in external rotation, because this anatomic position optimally orients the supraspinatus tendon parallel and perpendicular to the oblique coronal and oblique sagittal imaging planes, respectively.
Selection of the proper local coil is of paramount importance. We prefer to use a small, 5
-inch curved planar coil because it provides optimal coverage for most patients. Alternative coils include flexible (conformable) and phased-array designs.
The optimal imaging protocol is a matter of debate, determined both by personal preference and available technology. Nevertheless, it is generally accepted that the examination must (a) include multiple imaging planes, (b) provide optimal spatial and contrast resolution, and (c) be capable of completion within a reasonably short period. Low-resolution technique may compromise diagnostic accuracy, whereas unnecessarily long examinations both reduce patient throughput and potentially result in motion-related artifacts with attendant degradation in image quality. Intraarticular injection of dilute gadolinium contrast agent will improve diagnostic yield but converts a noninvasive procedure into an invasive one and requires active physician participation, which effectively restricts availability to "daytime" hours.
The images in this article were obtained with a 1.5-T Signa MR imaging unit (GE Medical Systems, Milwaukee, Wis). Our clinical imaging protocol consisted of four diagnostic sequences and required approximately 19 minutes of imaging time (Table). Fat suppression was used with three of the sequences to increase the conspicuity of both rotator cuff and labral pathologic conditions (3,4). An in-plane spatial resolution of 400 x 500 µm was achieved with the oblique coronal proton-density–weighted sequence, which involves use of a 10-cm field of view and a 256 x 192 matrix. The images obtained in healthy volunteers, shown in this presentation, were primarily obtained with the use of a small 3-inch planar coil with 6- to 8-cm fields of view, which affords in-plane spatial resolution on the order of 230–310 x 310–420 µm.
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US IMAGING
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Proper positioning of the patient is critical for the performance of shoulder sonography. Patients ideally undergo imaging while seated on a stool, which allows the imager to scan the shoulder with the arm placed behind the back, thereby increasing the extent of rotator cuff visualization lateral to the shadowing acromion (Fig 1) (van Holsbeeck M, oral communication, 1997). Turrin et al (5) recently reported an additional technique in which the patient is positioned supine with the pronated arm hanging free at the side. The rotator cuff is routinely imaged in the coronal (longitudinal) and transverse planes.
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Figure 1a. Patient positioning with MR and US imaging. (a) Oblique coronal proton-density–weighted MR image obtained with the arm at the side and in neutral position shows relatively limited exposure of the rotator cuff (arrows in a–d) lateral to the acromion (A in a–d). (b) US image obtained with the arm at the side, palm up, similarly demonstrates relatively limited exposure of the cuff lateral to the shadowing acromion. (c) Oblique coronal proton-density–weighted MR image obtained with the arm in external rotation shows greatly increased exposure of the cuff lateral to the acromion. (d) Extended-field-of-view US image obtained with the hand positioned behind the back demonstrates increased exposure of the cuff lateral to the shadowing acromion.
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Figure 1b. Patient positioning with MR and US imaging. (a) Oblique coronal proton-density–weighted MR image obtained with the arm at the side and in neutral position shows relatively limited exposure of the rotator cuff (arrows in a–d) lateral to the acromion (A in a–d). (b) US image obtained with the arm at the side, palm up, similarly demonstrates relatively limited exposure of the cuff lateral to the shadowing acromion. (c) Oblique coronal proton-density–weighted MR image obtained with the arm in external rotation shows greatly increased exposure of the cuff lateral to the acromion. (d) Extended-field-of-view US image obtained with the hand positioned behind the back demonstrates increased exposure of the cuff lateral to the shadowing acromion.
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Figure 1c. Patient positioning with MR and US imaging. (a) Oblique coronal proton-density–weighted MR image obtained with the arm at the side and in neutral position shows relatively limited exposure of the rotator cuff (arrows in a–d) lateral to the acromion (A in a–d). (b) US image obtained with the arm at the side, palm up, similarly demonstrates relatively limited exposure of the cuff lateral to the shadowing acromion. (c) Oblique coronal proton-density–weighted MR image obtained with the arm in external rotation shows greatly increased exposure of the cuff lateral to the acromion. (d) Extended-field-of-view US image obtained with the hand positioned behind the back demonstrates increased exposure of the cuff lateral to the shadowing acromion.
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Figure 1d. Patient positioning with MR and US imaging. (a) Oblique coronal proton-density–weighted MR image obtained with the arm at the side and in neutral position shows relatively limited exposure of the rotator cuff (arrows in a–d) lateral to the acromion (A in a–d). (b) US image obtained with the arm at the side, palm up, similarly demonstrates relatively limited exposure of the cuff lateral to the shadowing acromion. (c) Oblique coronal proton-density–weighted MR image obtained with the arm in external rotation shows greatly increased exposure of the cuff lateral to the acromion. (d) Extended-field-of-view US image obtained with the hand positioned behind the back demonstrates increased exposure of the cuff lateral to the shadowing acromion.
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Shoulder sonography was initially performed with the use of 5–7.5 MHz transducers. Currently, transducers with frequencies in the range of 9–13 MHz have become standard, which allow in-plane spatial resolution in the range of 200–400 µm and section thickness of 0.5–1.0 mm.
It is important to restrict imaging depth to the minimum required to visualize the particular structure of interest. Failure to do so results in unnecessary reduction of frame rate. The minimal field of view is substantially smaller than that achievable with the use of standard clinical MR imaging protocols (Fig 2). Other parameters that need to be optimized include dynamic range, which is somewhat analogous to the window and level used for display of computed tomographic and MR images, and edge enhancement, which similarly affects image contrast and may be manipulated concurrently with dynamic range. Elective reduction of the frame rate permits more data lines to be acquired per unit time, thereby improving lateral resolution. A relatively recent technologic advance, extended field of view (Siescape; Siemens Medical Systems, Iselin, NJ), allows the imager to obtain panoramic, high-resolution images (6).
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Figure 2a. Comparison of the minimal field of view achievable with MR imaging versus that with US. (a) On the oblique coronal MR image, the minimal field of view extends medial to the glenoid cavity. Note the shelflike insertion site (arrows). (b) On the coronal (longitudinal) US image, field-of-view coverage extends to approximately the midarticular aspect of the humeral head, and the shelflike insertion is well depicted (arrows). Spatial resolution is superior to that of MR imaging.
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Figure 2b. Comparison of the minimal field of view achievable with MR imaging versus that with US. (a) On the oblique coronal MR image, the minimal field of view extends medial to the glenoid cavity. Note the shelflike insertion site (arrows). (b) On the coronal (longitudinal) US image, field-of-view coverage extends to approximately the midarticular aspect of the humeral head, and the shelflike insertion is well depicted (arrows). Spatial resolution is superior to that of MR imaging.
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GROSS ANATOMIC CHARACTERISTICS
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Coracoacromial Arch, Subacromial-Subdeltoid Bursa, and Deltoid Muscle
The coracoacromial ligament courses from the lateral aspect of the coracoid process to the anterior edge and undersurface of the acromion (Fig 3) (7). The subacromial-subdeltoid bursa is an expansive structure interposed between the rotator cuff and the overlying acromion and deltoid muscle (Fig 3). At US, the subacromial-subdeltoid bursal and peribursal space appear as a hyperechoic band split by a hypoechoic layer (Fig 4) (5). The multipennate deltoid muscle inserts onto the deltoid tuberosity of the humerus and appears less echogenic than the underlying rotator cuff tendon (Fig 4).
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Figure 3. Figures 3, 4. (3) Drawing shows the coracoacromial arch, deltoid muscle, rotator cuff, and intervening subacromial-subdeltoid bursa. (4) Coronal (longitudinal) US image shows the hypoechoic subacromial-subdeltoid bursa (solid straight arrows) interposed between hyperechoic layers. The adjacent deltoid muscle (solid curved arrows) and the echogenic rotator cuff (open arrows) are also seen.
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Figure 4. Figures 3, 4. (3) Drawing shows the coracoacromial arch, deltoid muscle, rotator cuff, and intervening subacromial-subdeltoid bursa. (4) Coronal (longitudinal) US image shows the hypoechoic subacromial-subdeltoid bursa (solid straight arrows) interposed between hyperechoic layers. The adjacent deltoid muscle (solid curved arrows) and the echogenic rotator cuff (open arrows) are also seen.
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Rotator Cuff
The four constituents of the rotator cuff include the supraspinatus, infraspinatus, teres minor, and subscapularis tendons (Figs 3, 5). The supraspinatus muscle courses anterolaterally, at an angle of approximately 45° with respect to the coronal plane, to its insertion on the most superior of the three facets of the greater tuberosity—the "shelf" (Figs 2, 4). The imaging appearance is complex. In the midportion of the supraspinatus tendon, two discrete tendons are seen (5,8) (Fig 6), and at its insertion there are extensive intra- and intertendinous interdigitations as manifested by a complex appearance at MR imaging (Fig 7). The infraspinatus and supraspinatus tendons join about 15 mm proximal to their insertions onto the greater tuberosity (Figs 3, 5). The teres minor muscle inserts onto the inferior aspect of the posterior portion of the greater tuberosity (Fig 5). The tendinous bands of the multipennate subscapularis muscle condense laterally into a single tendon in the superior two-thirds of the muscle (Figs 3, 5). The inferior third remains muscular to the level of its insertion onto the lesser tuberosity (9). The subscapularis tendon is best visualized with the arm externally rotated (Fig 8). Sagittal MR and US images reveal a complex appearance due to its multipennate structure and interdigitation (Fig 9).
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Figure 6a. Two tendinous components of the supraspinatus tendon. (a) Oblique sagittal proton-density–weighted MR image shows the anterior "cylindric" (solid straight arrows) and posterior "flat" (curved arrows) contributions of the supraspinatus tendon. The fibers of the infraspinatus tendon (open arrows) interdigitate with the supraspinatus tendon. (b) Corresponding US image shows the cylindric (straight arrows) and flat (curved arrows) contributions of the supraspinatus tendon.
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Figure 6b. Two tendinous components of the supraspinatus tendon. (a) Oblique sagittal proton-density–weighted MR image shows the anterior "cylindric" (solid straight arrows) and posterior "flat" (curved arrows) contributions of the supraspinatus tendon. The fibers of the infraspinatus tendon (open arrows) interdigitate with the supraspinatus tendon. (b) Corresponding US image shows the cylindric (straight arrows) and flat (curved arrows) contributions of the supraspinatus tendon.
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Figure 7a. Peri-insertional rotator cuff complexity. (a) Oblique sagittal proton-density–weighted MR image shows the complex appearance of the merging cuff tendons (arrows). This appearance may be observed on clinical images in which high-resolution technique is used. (b) Transverse US image shows the complex "fibrillar" appearance of the rotator cuff (straight arrows) between the overlying deltoid muscle (D) and the underlying humeral head (curved arrows).
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Figure 7b. Peri-insertional rotator cuff complexity. (a) Oblique sagittal proton-density–weighted MR image shows the complex appearance of the merging cuff tendons (arrows). This appearance may be observed on clinical images in which high-resolution technique is used. (b) Transverse US image shows the complex "fibrillar" appearance of the rotator cuff (straight arrows) between the overlying deltoid muscle (D) and the underlying humeral head (curved arrows).
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Figure 8a. Subscapularis tendon in external rotation. (a) Axial proton-density–weighted MR image shows the subscapularis tendon (straight black arrows), coracoid process (curved arrow), and tendinous insertion (white arrow). (b) Transverse US image shows the subscapularis tendon (straight solid arrows), proximal muscle (curved arrows), shadowing coracoid process (open arrows), and insertion site (arrowheads).
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Figure 8b. Subscapularis tendon in external rotation. (a) Axial proton-density–weighted MR image shows the subscapularis tendon (straight black arrows), coracoid process (curved arrow), and tendinous insertion (white arrow). (b) Transverse US image shows the subscapularis tendon (straight solid arrows), proximal muscle (curved arrows), shadowing coracoid process (open arrows), and insertion site (arrowheads).
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Figure 9a. Subscapularis insertion. (a) Oblique sagittal proton-density–weighted MR image shows the complex insertion site of the subscapularis tendon (straight arrows). The appearance of the distal supraspinatus and infraspinatus tendons is similar (curved arrows). (b) Sagittal (cross-sectional) US image shows similar heterogeneity of the subscapularis tendon near the insertion site on the lesser tuberosity (arrows).
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Figure 9b. Subscapularis insertion. (a) Oblique sagittal proton-density–weighted MR image shows the complex insertion site of the subscapularis tendon (straight arrows). The appearance of the distal supraspinatus and infraspinatus tendons is similar (curved arrows). (b) Sagittal (cross-sectional) US image shows similar heterogeneity of the subscapularis tendon near the insertion site on the lesser tuberosity (arrows).
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Rotator Interval and Long Bicipital (Biceps) Tendon
The rotator interval is a triangular region formed as the supraspinatus and subscapularis tendons course around the base of the coracoid process (Figs 10–12). The base of the triangle is represented by the coracoid process, the apex is located at the level of the transverse humeral ligament, and the superior and inferior sides are formed by the supraspinatus and subscapularis tendon edges, respectively. The fibers of the coracohumeral ligament and deep capsule blend together to form the "roof" of the rotator interval and have multiple insertion sites, including both tuberosities on either side of the bicipital groove, the borders of the adjacent supraspinatus and subscapularis tendons, and the transverse humeral ligament (10,11).
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Figure 12a. Rotator interval. (a, b) Oblique sagittal proton-density–weighted MR image (a) and oblique sagittal fat-suppressed T1-weighted MR image obtained after intraarticular injection of dilute gadolinium (b) show the biceps tendon (straight arrow in a) coursing within the rotator interval deep to the capsule and coracohumeral ligament (curved solid arrows), anterior to the supraspinatus tendon (open arrows), and superior to the subscapularis tendon (arrowheads). (c) Transverse US image of the rotator interval shows the biceps tendon coursing between the supraspinatus (SUPRA) and subscapularis (SUBSCAP) tendons. The overlying capsule and coracohumeral ligament are visible (arrows).
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Figure 12b. Rotator interval. (a, b) Oblique sagittal proton-density–weighted MR image (a) and oblique sagittal fat-suppressed T1-weighted MR image obtained after intraarticular injection of dilute gadolinium (b) show the biceps tendon (straight arrow in a) coursing within the rotator interval deep to the capsule and coracohumeral ligament (curved solid arrows), anterior to the supraspinatus tendon (open arrows), and superior to the subscapularis tendon (arrowheads). (c) Transverse US image of the rotator interval shows the biceps tendon coursing between the supraspinatus (SUPRA) and subscapularis (SUBSCAP) tendons. The overlying capsule and coracohumeral ligament are visible (arrows).
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Figure 12c. Rotator interval. (a, b) Oblique sagittal proton-density–weighted MR image (a) and oblique sagittal fat-suppressed T1-weighted MR image obtained after intraarticular injection of dilute gadolinium (b) show the biceps tendon (straight arrow in a) coursing within the rotator interval deep to the capsule and coracohumeral ligament (curved solid arrows), anterior to the supraspinatus tendon (open arrows), and superior to the subscapularis tendon (arrowheads). (c) Transverse US image of the rotator interval shows the biceps tendon coursing between the supraspinatus (SUPRA) and subscapularis (SUBSCAP) tendons. The overlying capsule and coracohumeral ligament are visible (arrows).
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The long biceps tendon arises from the superior glenoid labrum or the supraglenoid tubercle or both, courses over the humeral head within the glenohumeral joint, and then curves inferiorly to enter the bicipital groove (Fig 11). The subscapularis and supraspinatus tendons join to form a sheath that surrounds the biceps tendon at the proximal part of the bicipital groove. Since the biceps tendon courses with-in the rotator interval just anterior to the supraspinatus tendon, it provides an important and easily recognizable landmark for both the arthroscopist and imager (Figs 12, 13).
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Figure 13a. Rotator interval. (a) Oblique coronal T2-weighted MR image shows the biceps tendon (straight arrows) coursing just anterior to the supraspinatus tendon and capsule (curved arrows). The interface between these two structures is hyperintense fluid. (b) Coronal (longitudinal) US image shows a normal, hypoechoic fluid interface between the biceps tendon (straight arrows) and the anterior edge of the supraspinatus tendon (curved arrows).
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Figure 13b. Rotator interval. (a) Oblique coronal T2-weighted MR image shows the biceps tendon (straight arrows) coursing just anterior to the supraspinatus tendon and capsule (curved arrows). The interface between these two structures is hyperintense fluid. (b) Coronal (longitudinal) US image shows a normal, hypoechoic fluid interface between the biceps tendon (straight arrows) and the anterior edge of the supraspinatus tendon (curved arrows).
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DETAILED ANATOMIC CHARACTERISTICS
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Tendon Histologic Characteristics
Tendons are composed almost entirely of type 1 collagen (12). Individual collagen fascicles may be seen with the naked eye and measure on the order of 1 mm in diameter.
The rotator cuff is remarkably complex, composed of capsule, ligaments, and the cuff tendons proper (13). The four rotator cuff tendons splay out and interdigitate to form a common, continuous hoodlike insertion on the humerus (13).
Histologic analysis of the supraspinatus and infraspinatus tendons and adjacent structures reveals a complex composed of five layers (Figs 14, 15) (13). The most superficial layer (layer 1) comprises superficial fibers of the coracohumeral ligament. Layer 2 is composed of closely packed, parallel supraspinatus and infraspinatus tendon bundles. Layer 3 is composed of smaller tendon fascicles that intersect one another at an angle of roughly 45°. Layer 4 is composed of predominantly extracapsular loose connective tissue that anteriorly becomes continuous with the deep aspect of the coracohumeral ligament. Layer 5 is composed of the capsule, which, deep to the supraspinatus and infraspinatus tendons, is thickened by a strip of tissue that courses perpendicular to the long axis of the tendon fibers. This region is referred to as the "rotator cable," whereas the thinner cuff tissue lateral to this region is called the "rotator crescent" (Fig 16) (14). This recent anatomic work has potentially great clinicopathologic significance. First, rotator cuff tears are frequently manifested by interfascicular "delamination," which may be attributable to the multilayer composition of the rotator cuff. Second, patients who are "cable dominant," that is, those that possess a well-developed rotator cable, may incur a full-thickness tear of the rotator cuff yet retain much of their range of motion.
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Figure 16a. Rotator cable and crescent. (a) Permission to reprint this figure electronically was denied by the publisher. See print version. (b) Arthroscopic image (lateral on the right, medial on the left) shows the cable (arrows) and the lateral crescent (*). BT = biceps tendon, HH = humeral head.
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Figure 16b. Rotator cable and crescent. (a) Permission to reprint this figure electronically was denied by the publisher. See print version. (b) Arthroscopic image (lateral on the right, medial on the left) shows the cable (arrows) and the lateral crescent (*). BT = biceps tendon, HH = humeral head.
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Unlike most tendons, the rotator cuff lacks both a synovial sheath and surrounding paratenon (loosely associated layer of fibrous tissue). As such, its blood supply is comparatively complex. Several investigators have described a focal, hypovascular "critical zone" within the distal cuff (15,16), whereas others have shown hypervascularity within the rotator cuffs of patients with intact tendons and clinical evidence of impingement (17). Clark and Harryman (13) have shown arterioles to be particularly prevalent in layers 1 and 2 (Fig 15).
Fibrocartilaginous Insertion
Tendons that have epiphyseal attachments, particularly those that course nearly perpendicular to their attachment sites, are characterized by fibrocartilaginous attachments (18). The medial fibers of the supraspinatus tendon course approximately perpendicular to the greater tuberosity, inserting into thick fibrocartilage. The lateral fibers progressively course at more acute angles until, at the extreme lateral margin, the fibers are aligned nearly parallel to their attachment site; here the fibrocartilaginous layer is relatively thin (Fig 17).
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Figure 17a. Fibrocartilaginous insertion of the supraspinatus tendon. (a) Photograph of a cryomicrotome section obtained from the cadaver of a 37-year-old woman shows the fibrocartilaginous insertion on the "shelf" (straight arrows), as well as the adjacent hyaline cartilage (curved arrows). The fascicles of the overlying supraspinatus tendon curve toward their attachment sites (arrowheads). (b) Permission to reprint this figure electronically was denied by the publisher. See print version. (c) Oblique coronal proton-density–weighted MR image shows the hypointense fibrocartilaginous insertion site (straight arrows) and adjacent hyaline cartilage (curved arrows). (d) Coronal (longitudinal) US image shows the hypoechoic fibrocartilaginous insertion site (straight arrows) as well as the adjacent, hypoechoic hyaline articular cartilage (curved arrows).
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Figure 17b. Fibrocartilaginous insertion of the supraspinatus tendon. (a) Photograph of a cryomicrotome section obtained from the cadaver of a 37-year-old woman shows the fibrocartilaginous insertion on the "shelf" (straight arrows), as well as the adjacent hyaline cartilage (curved arrows). The fascicles of the overlying supraspinatus tendon curve toward their attachment sites (arrowheads). (b) Permission to reprint this figure electronically was denied by the publisher. See print version. (c) Oblique coronal proton-density–weighted MR image shows the hypointense fibrocartilaginous insertion site (straight arrows) and adjacent hyaline cartilage (curved arrows). (d) Coronal (longitudinal) US image shows the hypoechoic fibrocartilaginous insertion site (straight arrows) as well as the adjacent, hypoechoic hyaline articular cartilage (curved arrows).
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Figure 17c. Fibrocartilaginous insertion of the supraspinatus tendon. (a) Photograph of a cryomicrotome section obtained from the cadaver of a 37-year-old woman shows the fibrocartilaginous insertion on the "shelf" (straight arrows), as well as the adjacent hyaline cartilage (curved arrows). The fascicles of the overlying supraspinatus tendon curve toward their attachment sites (arrowheads). (b) Permission to reprint this figure electronically was denied by the publisher. See print version. (c) Oblique coronal proton-density–weighted MR image shows the hypointense fibrocartilaginous insertion site (straight arrows) and adjacent hyaline cartilage (curved arrows). (d) Coronal (longitudinal) US image shows the hypoechoic fibrocartilaginous insertion site (straight arrows) as well as the adjacent, hypoechoic hyaline articular cartilage (curved arrows).
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Figure 17d. Fibrocartilaginous insertion of the supraspinatus tendon. (a) Photograph of a cryomicrotome section obtained from the cadaver of a 37-year-old woman shows the fibrocartilaginous insertion on the "shelf" (straight arrows), as well as the adjacent hyaline cartilage (curved arrows). The fascicles of the overlying supraspinatus tendon curve toward their attachment sites (arrowheads). (b) Permission to reprint this figure electronically was denied by the publisher. See print version. (c) Oblique coronal proton-density–weighted MR image shows the hypointense fibrocartilaginous insertion site (straight arrows) and adjacent hyaline cartilage (curved arrows). (d) Coronal (longitudinal) US image shows the hypoechoic fibrocartilaginous insertion site (straight arrows) as well as the adjacent, hypoechoic hyaline articular cartilage (curved arrows).
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What is the reason for the fibrocartilaginous transition? Some hypothesize that this tissue serves to protect tendon fibers from fraying at bone attachments, "having some of the function of a sesamoid cartilage at the end of a tendon" (18). The fibrocartilaginous insertion of the supraspinatus tendon appears, as a rule, hypointense in comparison with hyaline cartilage at MR imaging and, in our experience, isoechoic with hyaline cartilage at sonography (Fig 17). The appearance, however, may be dependent on the orientation of the tissue in relation to the main magnetic field with MR imaging and the angle of insonation with US.
Hyaline Cartilage
Humeral hyaline cartilage is relatively thin, measuring 1.23 mm, on average, on the basis of a cadaver study performed by Hodler et al (19) and roughly 1–1.5 mm on the basis of our own anecdotal sonographic experience. This tissue appears of intermediate signal intensity on MR images and is hypoechoic on US images (Fig 17). The humeral "bare area" located in the posterolateral aspect of the head is devoid of hyaline cartilage.
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IMAGING PITFALLS
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MR Imaging
Pitfalls of MR imaging include the magic angle effect and normal fiber interdigitation. Tendons that course at or near the magic angle of 55° exhibit markedly augmented signal intensity that is most pronounced with short-echo-time sequences (20). The supraspinatus tendon is particularly vulnerable, since it curves along its course between the musculotendinous junction and its insertion on fibrocartilage (21) (Fig 18). Furthermore, magic angle augmentation of signal intensity within the biceps tendon may be misinterpreted as focal cuff pathologic conditions near the rotator interval (22). The complex arrangement of rotator cuff fibers is frequently manifested by striations that may be mistaken for intrasubstance tears (Figs 6a, 7a).
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Figure 18a. Magic angle effect. (a) Short-echo-time MR image obtained with the arm in neutral position shows a hyperintense focus approximately 1 cm proximal to the rotator cuff insertion (arrows). (b) On an MR image obtained with identical imaging parameters but with lateral flexion at the waist, the hyperintense region now extends to the insertion site (arrows). The change in position resulted in reorientation of the fibers in relation to the main magnetic field, causing a shift in the site of signal augmentation. (Fig 18a and 18b reprinted, with permission, from reference 21.)
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Figure 18b. Magic angle effect. (a) Short-echo-time MR image obtained with the arm in neutral position shows a hyperintense focus approximately 1 cm proximal to the rotator cuff insertion (arrows). (b) On an MR image obtained with identical imaging parameters but with lateral flexion at the waist, the hyperintense region now extends to the insertion site (arrows). The change in position resulted in reorientation of the fibers in relation to the main magnetic field, causing a shift in the site of signal augmentation. (Fig 18a and 18b reprinted, with permission, from reference 21.)
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US Imaging
Pitfalls of US imaging commonly encountered during examination of the rotator cuff are attributable to anisotropy, calcific tendinitis, complex anatomy in the region of the rotator interval, and tendinous interdigitation. Highly ordered tendinous structure is responsible for the observed relationship between echogenicity and insonation angle, a characteristic known as anisotropy (23,24). The rotator cuff appears echogenic when the angle of insonation is perpendicular, or nearly perpendicular, to the surface. Minor deviation from this orientation is manifested by a dramatic reduction in tendon echogenicity that may be mistaken for a tear (Fig 19). Intratendinous deposition of calcium is manifested by both focal echogenicity and acoustical shadowing (Fig 20). Failure to recognize these coexistent findings may result in inadvertent misinterpretation of the hypoechoic, shadowing zone as a rotator cuff tear. The rotator interval, which is normally devoid of cuff tissue, may be mistaken for a rotator cuff defect. The edge of the intraarticular biceps tendon may be mistaken for a torn and retracted cuff tear (Fig 21). The hypoechoic interface between the biceps and adjacent supraspinatus tendons may simulate a longitudinal cuff tear (Fig 13). Heterogeneity of the normal rotator cuff can be normally observed because of the complex interdigitation of fibers from the various contributions (Fig 22).
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Figure 19. Artifactual reduction in tendon echogenicity attributable to tendon anisotropy. Coronal (longitudinal) US image shows hypoechoic appearance of the distal rotator cuff tendon that extends to the insertion site (arrows). Compare this with the appearance of the rotator cuff in Figures 3b, 4, 6, and 21d. The examiner must make a conscious effort to continually reorient the US probe so that it is perpendicular to the direction of cuff fibers; otherwise, anisotropy may be mistaken for a cuff tear.
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Figure 20. Calcific tendinitis. Coronal (longitudinal) US image of the rotator cuff shows an arcuate echogenic focus (straight arrows) with acoustical shadowing (curved arrows). This focus corresponds to an intratendinous calcium deposit that was removed at surgery.
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Figure 21. Biceps tendon pitfall. On an oblique US image of the biceps tendon, the tendon may mimic the appearance of a torn and retracted supraspinatus tendon (arrows). Compare this image with Figure 16b, in which the hypoechoic interface between the anterior edge of the supraspinatus and biceps tendons may be mistaken for a longitudinal tear. The biceps tendon is, as a rule, more echogenic than the rotator cuff tendons (Craig J, oral communication, 1997).
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Figure 22. Heterogeneity of the normal rotator cuff. Coronal (longitudinal) US image of a 16-year-old patient with a history of shoulder instability shows multiple hypoechoic zones (arrows) extending to the fibrocartilaginous insertion site. The cuff was normal at arthroscopy. Heterogeneity of the normal rotator cuff is attributable to complex interdigitation of intra- and intertendinous contributions.
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PATHOLOGIC CHARACTERISTICS
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Pathogenesis and Epidemiology
There is an emerging consensus that rotator cuff injury is probably a multifactor process, in which both extrinsic and intrinsic mechanisms have a role (12). Regardless of pathogenesis, however, it is important to remember that (a) the supraspinatus tendon is primarily involved in most instances, (b) tears usually begin near the rotator interval, and (c) partial tears are much more prevalent on the undersurface (articular) aspect.
Extrinsic Mechanisms.—Acute macrotrauma is an infrequent, yet accepted, mechanism for rotator cuff tear. Repetitive microtrauma, however, is considered by many investigators to be a more relevant factor than acute trauma. According to this theory, stresses on the tendon are manifested by focal "microtears." Codman (25) originally described the "rim rent," which refers to retraction of undersurface rotator cuff fibers from the fibrocartilaginous insertion (Fig 23). Neer (26) has proposed that 95% of rotator cuff injuries occur as a result of subacromial impingement of the cuff by the overlying coracoacromial arch, a theory apparently supported by the clinical success of acromioplasty. Recently, this generally accepted hypothesis has been challenged (27,28). Cadaveric studies have shown that specimens with articular-sided partial tears frequently have no degenerative changes on the undersurface of the acromion, whereas specimens with bursal-sided partial tears frequently have acromial involvement.
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Figure 23. Reproduction of page 101 from E. A. Codman's classic textbook (25) shows partial-thickness undersurface tears of the rotator cuff. Figure 1 demonstrates a subtle, undersurface lesion located at the capsular reflection. Figure 2 shows a more extensive, partial undersurface tear that involves approximately 50% of the cuff thickness ("rim-rent"). Figure 3 shows a yet more extensive partial, undersurface tear with delamination and retraction of the undersurface contribution (straight arrow). Figure 4 shows a lesion confined predominantly to the cuff substance. The secondary osseous changes are most conspicuous in Figures 2 and 3 (curved arrows).
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Intrinsic Mechanisms.—The role of rotator cuff vascularity in the pathogenesis of tears remains a matter of debate. Supporters of the mechanism of cuff degeneration (tendinopathy) cite supportive histologic findings that are commonly observed with cuff pathologic conditions. Degenerative holes ("tears") are seen in 50% of cadavers in postmortem studies (12). Although frequently asymptomatic, these foci may predispose the rotator cuff to tear by means of low-energy mechanisms.
Imaging Findings
MR Imaging.—With MR imaging, a full-thickness tear of the rotator cuff is typically manifested as either a hyperintense defect or a tendinous avulsion that extends from the bursal to the articular side of the cuff. Although usually apparent with all imaging sequences, it is the presence of this finding on T2-weighted images that is most specific. Less common, the tear site is filled predominantly with hypertrophic synovium, granulation tissue, or fibrotic tissue, resulting in an intermediate signal intensity or hypointense focus with all sequences. In this instance, recognition of morphologic changes, the presence of retraction, or both are necessary for correct diagnosis. Large tears do not present a diagnostic challenge, but the size and degree of retraction, status of the musculature, and associated bone findings must be noted in the report (Figs 24, 25). Reported sensitivities and specificities for the diagnosis of full-thickness tears range from 84% to 100% and 93% to 99%, respectively (4,29–33).
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Figure 24. Figures 24, 25. Large, full-thickness tear of the rotator cuff. (24) Oblique coronal T2-weighted MR image shows a large cuff defect, with the edge retracted far medially (white arrows). Fluid is within both the glenohumeral joint and subacromial-subdeltoid bursa (black arrows). The overlying deltoid muscle (D) nearly apposes the humeral head. A large, full-thickness tear of the rotator cuff was verified at surgery. (25) Oblique coronal proton-density–weighted MR image shows the edges of the torn tendon contrasted with high-signal-intensity fluid. There is delamination, with the undersurface fibers (white arrow) retracted further medially than the bursal-sided fibers (black arrows). A large, full-thickness tear of the rotator cuff was verified at surgery.
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Figure 25. Figures 24, 25. Large, full-thickness tear of the rotator cuff. (24) Oblique coronal T2-weighted MR image shows a large cuff defect, with the edge retracted far medially (white arrows). Fluid is within both the glenohumeral joint and subacromial-subdeltoid bursa (black arrows). The overlying deltoid muscle (D) nearly apposes the humeral head. A large, full-thickness tear of the rotator cuff was verified at surgery. (25) Oblique coronal proton-density–weighted MR image shows the edges of the torn tendon contrasted with high-signal-intensity fluid. There is delamination, with the undersurface fibers (white arrow) retracted further medially than the bursal-sided fibers (black arrows). A large, full-thickness tear of the rotator cuff was verified at surgery.
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A partial-thickness tear is typically manifested as a focal hyperintense region that contacts only one surface of the rotator cuff. Interruption of the deep fibers—frequently associated with delamination—is diagnostic of an undersurface (articular-sided) tear (Fig 26). False-positive diagnoses attributable to intrasubstance pathologic conditions (tendinosis), and false-negative diagnoses attributable to nondisplaced, undersurface flaps, however, may occur. MR arthrography with the arm positioned in abduction and external rotation is probably the most sensitive imaging study for the detection of the latter (34). Bursal-sided tears interrupt the superficial (bursal) aspect of the rotator cuff but spare the deep fibers (Fig 27). Reported sensitivities and specificities for partial-thickness tears range from 35% to 92% and 85% to 99%, respectively (3,4,29–33). Reinus et al (3) were able to correctly identify the site of the tear (ie, undersurface or bursal) in only 50% of the cases in which a partial-thickness tear was diagnosed with MR imaging.
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Abstract
INTRODUCTION
