IMAGING OF THE SHOULDER

Anatomic Part	Abnormality	Preferred MR Imaging Plane
Acromion	Type or configuration of undersurface (Types I, II, III, IV)	Sagittal; 1–2 images peripheral to AC joint
	Anterior down-sloping	Sagittal
	Lateral down-sloping	Coronal
	Enthesiophyte formation	Sagittal/coronal
	Os acromiale “double AC joint” sign	Axial—primary Sagittal/coronal—secondary
Acromioclavicular (AC) joint	Osteoarthritis (mass effect on underlying cuff)	Coronal/sagittal
	AC joint separation (grades I, II, III)	Coronal
	Osteolysis of distal clavicle	Coronal
Coracoacromial ligament	Thickening	Sagittal
Coracoacromial ligament	Calcification	Sagittal

Acromion

The anterolateral aspect of the acromion plays the most crucial role in extrinsic compression of the rotator cuff; therefore, any acromial configuration that narrows the subacromial space anterolaterally places a person at risk for the clinical syndrome of impingement. Five aspects of the acromial shape and morphology should be evaluated on each shoulder MR examination.

1. The morphology of the undersurface of the acromion should be described as follows: a type I acromion demonstrates a flat undersurface; type II acromion, a gentle undersurface curvature; type III acromion, an anterior hook; and type IV acromion, a convex undersurface (Fig. 7-1). Acromial types II and III are associated with an increased risk of impingement, and this association was first demonstrated using the scapular Y radiograph (Fig. 7-2). To accurately type the undersurface of the acromion on MRI, evaluate the first or second image lateral to the AC joint on the oblique sagittal imaging plane (Fig. 7-3). The major pitfall with regard to typing the acromion on MRI is the use of the incorrect image or imaging plane. Evaluation of the acromion on a sagittal image that is too far central or peripheral or use of the coronal imaging plane results in an inaccurate typing of the acromion. Currently, there is decreased emphasis in the orthopedic community on the role of acromial morphology (acromial type) with regard to the clinical syndrome of impingement. The other acromial variations listed in the following text are thought to play a more significant role with regard to impingement.

2. Anterior down-sloping (Fig. 7-4). Anterior tilt (down-sloping) of the scapula is associated with kyphosis and can result in narrowing of the clinical syndrome of impingement.

3. Lateral down-sloping (Fig. 7-5).

4. Spur formation off of the undersurface of the acromion (Fig. 7-6A). The deltoid tendon slip attaches to the lateral aspect of the acromion. A common pitfall is to misinterpret the deltoid tendon slip as an acromial spur on MRI. The two can be easily differentiated because the deltoid tendon slip (Fig. 7-6B) appears dark on all pulse sequences, whereas a spur contains fatty marrow elements and demonstrates fat signal intensity on all pulse sequences (bright on T1).

5. Unfused os acromiale (Fig. 7-7).

Figure 7-1 Acromial configuration. The acromial type is determined by the configuration of the undersurface of the acromion. Acromial types II (gentle undersurface curvature) and III (anterior hook) place an individual at an increased risk for the clinical syndrome of impingement.

Figure 7-2 Acromial typing using radiography. A, Type I acromion. Scapular Y view of the shoulder demonstrates the undersurface of the acromion to be flat. B, Type II acromion. Scapular Y view of the shoulder demonstrates a curved undersurface of the acromion.

Figure 7-3 Acromial typing using MRI. A, Type I acromion, flat undersurface. B, Type II acromion, curved undersurface. C, Type III acromion, hooked acromion. D, Type IV acromion, convex undersurface.

Figure 7-4 Anterior down-sloping of the acromion. A, Anterior down-sloping narrows the acromial-humeral distance and can be a cause of rotator cuff impingement. Evaluation for anterior down-sloping is performed using the sagittal MRI plane. Anterior down-sloping describes the axis of the acromion in the sagittal plane and is independent of the configuration of the undersurface of the acromion as described in Figures 7-1 and 7-2. B, Normal acromial axis in the sagittal imaging plane.

Figure 7-5 Lateral down-sloping of the acromion. A, Lateral down-sloping narrows the acromial-humeral distance and can be a cause of impingement. Evaluation for lateral down-sloping is performed using the coronal imaging plane. Lateral down-sloping of the acromion describes the axis of the acromion in the coronal plane and is independent of the configuration of the undersurface of the acromion. B, Normal acromial axis in the coronal imaging plane.

Figure 7-6 Acromial enthesiophyte. A, Coronal T1-weighted image demonstrates an enthesiophyte extending off of the undersurface of the lateral aspect of the acromion, which can be a source of impingement. Note that an enthesiophyte contains fatty marrow elements and follows marrow signal intensity on MRI. B, The Deltoid tendon slip can mimic an acromial enthesiophyte on MRI, but can be easily differentiated from an enthesiophyte because it contains no marrow elements and appears dark on all pulse sequences.

Figure 7-7 Os acromiale. Unfused os acromiale on axillary view radiograph (A) axial CT slice (B). The synchondrosis of the unfused os acromiale is best seen on axial images and is depicted as a linear lucency extending across the short axis of the acromion. Sclerosis is often seen on both sides of the unfused synchondrosis. AC, acromioclavicular.

The acromion develops from several separate ossification centers, which typically fuse by 22 to 25 years of age. Failure of one of these ossification centers to fuse can lead to an unstable os acromiale with osteophytic lipping at the level of the synchondrosis. An unstable, unfused ossification center can act as a fulcrum or hinge and can displace in a downward direction during contraction of the deltoid muscle, narrowing the subacromial distance and leading to impingement of the rotator cuff. Failure to recognize and address an unfused os acromiale at the time of subacromial decompression and rotator cuff repair is a known cause of failed cuff repair. Arthroscopic removal of small fragments (less than 4 mm) is recommended because it does not disrupt the attachment of the deltoid muscle or alter its function. Larger fragments are surgically fused rather than resected to prevent weakness of the shoulder during abduction.

The MR appearance of an unfused os acromiale is variable depending on which ossification center fails to fuse. The unfused os acromiale is best depicted on axial images (Fig. 7-8). However, the most superior axial image obtained on MRI of the shoulder often begins below the level of the acromion; as a result, if one relies solely on the axial images, an unfused os acromiale may be overlooked. Therefore, the oblique sagittal and oblique coronal images should also be evaluated for an unfused os acromiale. An unfused os acromiale on sagittal or coronal images often demonstrates an appearance similar to that of the AC joint and may be misinterpreted as the AC joint. Simultaneous visualization of the AC joint and the unfused os acromiale on the same image gives the appearance of a “double AC joint,” and has been referred to as the double AC joint sign. More often, the unfused os acromiale appears as a second AC joint (Fig. 7-9) on an image two to three slices posterior to the true AC joint. The presence of fluid within the synchondrosis or edema surrounding the synchondrosis seen on MR imaging usually indicates pseudarthrosis or fibrous union and is an indication of probable instability.

Figure 7-8 Os acromiale on MR imaging. Unfused os acromiale on axial (A) and coronal (B) MR images. The presence of fluid signal within the synchondrosis and subcortical cysts on either side of the synchondrosis suggests that the unfused ossification center is unstable and may act as a “hinge” resulting in shoulder impingement. AC, acromioclavicular.

Figure 7-9 Double acromioclavicular-joint. Oblique sagittal images (A) through the level of the normal acromioclavicular (AC) joint. The unfused os acromiale appears as a second or double AC joint on a more peripheral image (B) and can easily be mistaken for the true AC joint.

An unfused os acromiale can occasionally mimic an acromial fracture. The two entities can be differentiated because the accessory ossicle usually appears triangular with sclerotic margins and forms a synchondrosis that is always oriented perpendicular to the long axis of the acromion. An acute fracture, on the other hand, is usually oriented at an oblique angle relative to the long axis of the acromion and demonstrates nonsclerotic irregular margins (Fig. 7-10).

Figure 7-10 Acromiale fracture. Axial CT image demonstrates a minimally displaced fracture of the posterior acromion. The fracture shows an oblique orientation relative to the long axis of the acromion, and the margins of the fracture fragments are nonsclerotic and slightly irregular when compared with an unfused os acromiale (see Figs. 7-7A and B).

Acromioclavicular Joint

Abnormalities of the AC joint are common and are often detected while imaging the shoulder. Osteoarthritis is a very common entity in patients over 40 years of age, whereas in the young athletic individual, post-traumatic AC joint separation and osteolysis of the distal clavicle are common entities. These conditions occasionally mimic one another, but can usually be differentiated on the basis of history and imaging findings (Table 7-2).

Table 7-2 Acromioclavicular (AC) Joint Osteoarthritis versus Osteolysis Distal Clavicle

	Osteoarthritis	Post-traumatic Osteolysis
Patient	Typically > 40	Young athlete
Symptoms	Minimally symptomatic or asymptomatic	Painful
Etiology	No history of trauma	Acute trauma: Repetitive microtrauma (weight-lifters)
Location of involvement	Both sides of joint	Isolated to distal clavicle
Radiographic findings	Joint space narrowing Osteophyte formation Subchondral cyst/sclerosis	Acute Soft tissue swelling Demineralization distal clavicle Loss cortical margin distal clavicle AC joint appears widened Chronic Reconstitution distal clavicle Subchondral sclerosis/cysts
MR findings	Joint space narrowing Osteophyte formation Subchondral cyst/sclerosis Capsular hypertrophy Joint effusion	Marrow edema distal clavicle Capsular hypertrophy Adjacent soft tissue edema Loss of dark cortical line AC joint widening

Osteoarthritis of the AC joint typically involves both sides of the joint, and imaging findings include capsular hypertrophy, joint effusion, and adjacent soft tissue edema. Osteophyte formation, subchondral marrow signal change, and subchondral cyst formation can also occur on both sides of the joint. The differential diagnosis for marrow edema on both sides of the AC joint includes recent AC joint separation, osteoarthritis, and inflammatory arthritis. Osteoarthritis of the AC joint is often asymptomatic or minimally symptomatic when compared with post-traumatic osteolysis of the distal clavicle, which can be quite painful. Osteoarthritis of the AC joint can be a source of extrinsic impingement on the rotator cuff; however, the AC joint does not play as crucial a role as does the anterior acromion with regard to impingement. The portion of the cuff immediately underlying the AC joint is less rigidly confined than the portion of the cuff underlying the anterior acromion, and degenerative osteoarthritis of the AC joint may result in the appearance of mass effect on the underlying cuff on MRI when there are no clinical symptoms of impingement. Degenerative changes of the AC joint with its associated mass effect on the underlying cuff are best appreciated on sagittal and coronal MR images (Fig. 7-11). Large joint effusion and pericapsular edema are signs that are often associated with synovitis of the AC joint and are associated with an increased incidence of symptomatic AC joint osteoarthritis.

Figure 7-11 Osteoarthritis of the acromioclavicular (AC) joint. Osteoarthritis of the AC joint is usually best detected on coronal MR images. On this coronal T1-weighted image, there is capsular hypertrophy, loss of the normal cortex of the distal clavicle and adjacent acromion, and an inferiorly directed osteophyte that results in mass effect on the underlying rotator cuff.

Post-traumatic osteolysis of the distal clavicle is a painful condition that typically occurs after mild to moderate acute trauma to the AC joint or after repetitive microtrauma, as seen in weight lifters and other athletes who experience repetitive stress to the AC joint. Clinically, osteolysis differs from osteoarthritis in that it usually occurs in the young athlete and results in moderate to severe pain, whereas osteoarthritis occurs in the older patient population with no history of trauma or repetitive stress and is minimally painful or asymptomatic.

Initial radiographs in post-traumatic osteolysis demonstrate soft tissue swelling of the AC joint, demineralization, and loss of the cortical margin of the distal clavicle (Fig. 7-12A). The AC joint may appear widened during the acute phase, but over time the distal clavicle usually reconstitutes, at least in part. Chronic changes of subchondral sclerosis and subchondral cystic change isolated to the distal clavicle are common. Early in post-traumatic osteolysis, MRI demonstrates marrow edema isolated to the distal 1 to 3 cm of the clavicle and may demonstrate loss of the normal cortical black line. Findings of a small joint effusion, capsular hypertrophy, and pericapsular edema of the AC joint are often present. Late findings may include widening of the AC joint, mild capsular hypertrophy, and cortical irregularity or subchondral sclerosis involving the distal tip of the clavicle (Fig. 7-12B).

Figure 7-12 Osteolysis of the distal clavicle. Radiograph of the acromioclavicular (AC) joint (A) demonstrates mild soft tissue swelling superiorly. There is slight widening of the AC joint, loss of the normal cortical white line of the distal clavicle and mild subchondral cyst formation. The changes are much more pronounced on the clavicular side of the joint. Sequential coronal T2-weighted images (B) through the AC joint reveal marked marrow edema isolated to the distal aspect of the clavicle with surrounding soft tissue edema, mild periostitis, and a small joint effusion. Note that the signal intensity within the adjacent acromion is normal.

After trauma to the shoulder, the AC joint should also be evaluated for evidence of fracture and AC joint separation. A fracture is seen on MRI as a dark line on both T1- and T2-weighted images with surrounding marrow edema. Separation of the AC joint is graded on a three-point scale (Fig. 7-13).

Figure 7-13 Grade III acromioclavicular (AC) joint separation. There is complete disruption of the AC joint capsule and the coracoclavicular ligaments and elevation of the distal clavicle.

Grade I injury: Mild strain of the AC joint; the AC and coracoclavicular (CC) ligaments remain intact. Radiographs of the shoulder are normal, and MRI shows only mild pericapsular edema of the AC joint. The symptoms are mild, treatment is conservative, and the patient usually recovers spontaneously.

Grade II injury: Moderate strain of the AC joint: it is associated with disruption of the AC joint capsule; the CC ligaments remain intact. Radiographs demonstrate slight elevation of the distal clavicle, whereas MRI demonstrates pericapsular AC joint edema in addition to distal elevation of the clavicle. With a grade II injury, the AC joint capsule is disrupted whereas the CC ligaments remain intact. Treatment is conservative and recovery is usually spontaneous.

Grade III injury: Severe injury of the AC joint; both the AC capsule and the CC ligaments are disrupted. Radiographs show complete dislocation of the AC joint with marked elevation of the distal clavicle, and MRI also shows pericapsular edema and disruption of the AC joint capsule and the CC ligaments. Complete AC joint separation (grade III injury) may result in scapular droop and may be a source of extrinsic impingement of the rotator cuff.

Coracoacromial Ligament

The coracoacromial ligament is a stout and sturdy ligament that forms a portion of the osseous outlet and acromion, covering the anterior fibers of the supraspinatus tendon and the rotator interval as it extends from the coracoid process anteriorly to the acromion posteriorly. The ligament is best visualized on oblique sagittal MR images and is usually no more than 2 to 3 mm-thickness (Fig. 7-14A). Thickening, hypertrophy, or calcification of the coracoacromial ligament can result in extrinsic impingement on the anterior portion of the rotator cuff (Fig. 7-14B). A thickened ligament is usually treated with either debridement or release at the time of subacromial decompression. In the younger patient, partial debridement of the ligament is preferable because this helps to prevent superior migration of the humeral head, whereas in the older patient the ligament is usually completely released.

Figure 7-14 Coracoacromial ligament. The normal coracoacromial ligament is smooth-appearing and less than 2 to 3 mm thick and is best visualized on oblique sagittal MR images (A) as it extends from the coracoid process to the acromion. B, Note marked thickening of the coracoacromial ligament near its acromial attachment. Thickening or nodularity of the ligament can result in impingement of the anterior rotator cuff and treatment is debridement or surgical release at the time of subacromial decompression.

Coracohumeral Impingement (Subcoracoid Impingement)

Coracohumeral impingement is an uncommon cause of rotator cuff impingement that results from the entrapment of the subscapularis tendon within a narrowed coracohumeral space. The normal coracohumeral distance is approximately 11 mm as depicted on axial MR images. A coracohumeral distance of less than 7 mm may result in entrapment of the subscapularis tendon between the humeral head and the coracoid process, eventually leading to a tear of the tendon (Fig. 7-15). In the case of coracohumeral impingement, successful surgical management requires not only repair of the subscapularis tendon but also correction of the narrowed coracohumeral distance. An isolated tear of the subscapularis tendon should prompt an evaluation of the coracohumeral distance to avoid missing this underlying cause of impingement.

Figure 7-15 Coracohumeral impingement. Axial T2-weighted MRI shows a narrowed coracohumeral distance resulting in entrapment of the subscapularis tendon and partial-thickness tearing of the tendon near its attachment to the lesser tuberosity.

ROTATOR CUFF

The rotator cuff is composed of four separate muscles and their tendons. Each muscle originates along either the anterior or posterior margin of the scapula and then extends toward and completely covers the humeral head with tendons inserting onto either the greater or lesser tuberosities of the humeral head. The rotator cuff contributes to the motion of the upper extremity and also plays an important role in stabilizing the humeral head within the glenoid fossa during movement of the glenohumeral joint. The rotator cuff is often referred to as an active stabilizer of the rotator cuff.

Rotator Cuff Anatomy

The supraspinatus muscle is the most superior of the muscles; it originates from the medial two thirds of the supraspinatus fossa along the dorsal aspect of the scapula. Its tendon inserts onto the highest facet of the greater tuberosity of the humeral head. The infraspinatus muscle is located posterior and inferior to the supraspinatus muscle arising from the middle two thirds of the infraspinatus fossa. The tendon crosses the glenohumeral joint posteriorly to insert onto the middle facet of the greater tuberosity of the humeral head. The supraspinatus and infraspinatus muscles are both innervated by branches of the suprascapular nerve. The teres minor makes up the most inferior portion of the rotator cuff posteriorly. The muscle originates along the dorsal aspect of the lateral margin of the scapula and courses upward and laterally to insert onto the lowest facet of the greater tuberosity. The teres minor is innervated by the axillary nerve. The subscapularis muscle is a broad triangular-shaped muscle that arises from the ventral aspect of the scapula. Its fibers converge to form a broad multi-slip tendon that inserts onto the lesser tuberosity located on the anterior aspect of the humeral head. The subscapularis muscle is innervated by the upper and lower subscapular nerves.

MR Appearance of the Normal Rotator Cuff

The normal rotator cuff, as just described, is a complex structure, and an accurate assessment of the cuff on MRI requires a thorough understanding of the normal MR appearance of the cuff in all three imaging planes, as well as some knowledge of the common variations and imaging pitfalls that can occur.

The supraspinatus muscle and tendon are best evaluated on the oblique coronal and sagittal imaging planes (Fig. 7-16); the axial images, however, also provide important information regarding the status of the supraspinatus. The normal muscle should completely fill the supraspinatus fossa and should demonstrate intermediate T1 and T2 signal. Without fatty atrophy, the muscle should demonstrate a bulk that is relatively similar to that of the infraspinatus and teres minor muscles, as seen on the oblique sagittal images. The supraspinatus muscle tapers from central to peripheral with the normal musculotendinous junction situated at approximately the 12 o’clock position of the humeral head. The infraspinatus and teres minor muscles are also best evaluated on the oblique sagittal and oblique coronal images, and these two muscles should completely fill the infraspinatus fossa. They also taper peripherally with their musculotendinous junctions located at a level similar to that of the supraspinatus muscle. On coronal images, the supraspinatus tendon can easily be differentiated from the infraspinatus because the supraspinatus tendon is oriented horizontally, whereas the infraspinatus tendon is oblique in orientation.

Figure 7-16 Normal MRI appearance of the supraspinatus tendon. The normal supraspinatus tendon demonstrates low T1 and T2 signal and is oriented in the horizontal plane. The normal musculotendinous junction is located in the 12 o’clock position of the humeral head. Muscle is intermediate signal intensity on both T1 and T2-weighted images.

The subscapularis muscle has a broad origin along the anterior border of the scapula and is best evaluated on the axial MR images. However, the oblique sagittal and oblique coronal sequences are important secondary imaging planes. Unlike the other three muscles, the subscapularis musculoskeletal junction is broad with multiple tendon slips arising out of the muscle and extending peripherally to insert in a broad fashion onto the lesser tuberosity of the humeral head.

Tendons arise out of the four separate muscles and then broaden and flatten peripherally, merging to form a single water-tight unit that inserts onto the tuberosities of the humeral head. The normal tendon demonstrates low signal intensity on all MR pulse sequences. Tendon pathology is usually manifested as increased signal or as a focal area of thickening, thinning, or attenuation, surface irregularity or as an area of discontinuity of the cuff.

Ultrasound Appearance of the Rotator Cuff

A complete description of the technique for ultrasound examination of the rotator cuff is beyond the scope of this chapter. However, knowledge of the normal MR appearance of the cuff can aid in the understanding the normal ultrasound appearance of the cuff. The examination often begins by scanning in the transverse imaging plane along the anterior surface of the shoulder to identify the key landmark of the long head of the biceps tendon within the intertubercular groove covered by the subscapularis tendon. Imaging then proceeds in a systematic manner to evaluate the musculotendinous units of each rotator cuff muscle in both the transverse and longitudinal planes. The normal rotator cuff tendon demonstrates a typical bandlike appearance of medium-level echoes located deep to the deltoid muscle. A thin stripe of bright echoes just superficial to the rotator cuff tendon represents the normal subacromial subdeltoid bursa. Muscle is hypoechoic in appearance. The biceps tendon is seen as an oval-appearing area of bright echoes located within the intertubercular groove. The bony surface of the humerus results in a rim of bright echoes (Fig. 7-17).

Figure 7-17 Normal sonographic appearance of the rotator cuff. A, Transverse scan shows the normal appearance of the biceps tendon within the intertubercular groove covered by the overlying subscapularis muscle. B and C show the normal appearance of the supraspinatus and infraspinatus tendons. D, Longitudinal scan shows the normal appearance of the superior labrum.

Sonographic signs of a rotator cuff tear include:

Nonvisualization of the cuff

Focal defect or absence of the cuff

Discontinuity of the cuff

Tendinosis may manifest as focal or diffuse areas of tendon thickening with altered echogenicity, either increased or decreased. A secondary sign of rotator cuff pathology may include fluid within the subdeltoid bursa (Fig. 7-18).

Figure 7-18 Sonographic appearance of rotator cuff pathology. A, Large tear of the rotator cuff with retraction. B, Small interstitial tear of the supraspinatus tendon. C, Calcific tendinosis of the infraspinatus tendon. D, Subscapularis tear with medial dislocation of the long head of the biceps tendon.

MR Appearance of Rotator Cuff Pathology

The sensitivity and specificity of MRI in the detection of rotator cuff tears range from 88% to 100%. Supraspinatus and infraspinatus pathology is best demonstrated on T2-weighted coronal and sagittal images, whereas subscapularis pathology is best evaluated on axial T2-weighted sequences. Rotator cuff pathology can be classified as discussed in the following text (Table 7-3 and Fig. 7-19).

Table 7-3 MR Appearance of Rotator Cuff Pathology

Cuff Pathology	Appearance on MRI
Normal tendon	Dark on T1 and T2
Tendonopathy	Thickening of tendon
Tendonopathy	Intermediate signal T1/T2
Calcific tendinitis	Globular decreased signal T1/T2 within tendon; often with surrounding soft tissue edema
	Thickened tendon; intermediate signal T1/T2
	“Blooming” artifact on gradient echo
Partial-thickness tear	Fluid signal/gadolinium extending partially through tendon superior to inferior
	Bursal/articular/interstitial
	Associated intramuscular cyst
	No retraction of tendon
Full-thickness tear	Fluid extending completely through tendon top to bottom
	Retraction of tendon
	Gap/discontinuity in tendon
Musculotendinous retraction	Measured as the length of the medial-to-lateral tendon gap
Fatty atrophy	Grade—mild/moderate/severe
	Streaks of high signal on T1—irreversible
	Loss of muscle bulk relative to other rotator cuff muscles on sagittal imaging—reversible

Figure 7-19 Patterns of rotator cuff tear. A, Normal rotator cuff. B, Partial thickness articular sided tear. C, Partial thickness bursal-side tear. D, Partial thickness interstitial tear. E, Partial-thickness articular surface supraspinatus avulsion (PASTA lesion). F, Partial thickness articular-side tear with intramuscular cyst. G, Attritional fraying of the tendon. H, Full-thickness tear. I, Partial-thickness articular sided tear with delamination and retraction of the deeper fibers.

On MRI, tendinopathy appears as intermediate signal intensity within the substance of the tendon on both T1- and T2-weighted images (slightly less bright than water on T2 images). The tendon may also demonstrate mild to moderate thickening (Fig. 7-20). Histologically, the increased signal represents mucoid degeneration of the tendon.

Figure 7-20 Tendinopathy. T1- (A) and T2-weighted (B) coronal images demonstrate mild thickening and intermediate signal intensity within the substance of the supraspinatus tendon, but no fluid signal intensity on the T2-weighted image. This is consistent with tendinopathy, but no tear. SST, supraspinatus tendon.

Hydroxyapatite deposition disease can occur within the rotator cuff tendon, calcific tendinitis, or within other periarticular soft tissues including the glenohumeral ligaments and adjacent bursa, calcific bursitis (Fig. 7-21). These calcifications are best detected on radiographs but may also be identified on MRI as focal areas of globular decreased signal on both T1- and T2-weighted images. Calcific tendinitis is often associated with intense inflammatory changes of the adjacent soft tissues, resulting in surrounding increased signal on T2-weighted images. On MRI, it may be difficult in some cases to detect the calcification and to distinguish calcific tendinitis from noncalcific tendinitis. Gradient-echo imaging can be helpful in this regard, since “blooming” artifact seen on gradient-echo images exaggerates the size and appearance of the calcification.

Figure 7-21 Calcific tendinitis and bursitis. A, T1-weighted coronal image demonstrates a globular area of low signal intensity within the anterior aspect of the supraspinatus tendon (SST) representing calcific tendinosis. B, Gradient-echo axial image shows “blooming” artifact, or apparent enlargement of the area of calcific tendinosis increasing the conspicuity of the calcific deposition. SST, supraspinatus tendon. C, Coronal T2-weighted image with fat saturation shows a large globular area of low signal surrounded by fluid within the subacromial subdeltoid bursa. SASD, subacromial subdeltoid; SST, supraspinatus tendon.

A partial-thickness tear of the rotator cuff is defined as a tear that extends partially through the thickness of the tendon from superior to inferior. MR imaging demonstrates fluid signal intensity on T2-weighted images extending partially through the thickness of the tendon. The tear may involve the bursal or articular surface, or the interstitial portions of the tendon (Fig. 7-22). A partial-thickness tear may occasionally be difficult to differentiate from tendinopathy, especially when partial healing has occurred with granulation tissue filling the tendon defect. The granulation tissue may demonstrate intermediate signal intensity on T2-weighted images, mimicking tendinopathy.

Figure 7-22 Types of partial-thickness rotator cuff tears. Partial-thickness articular surface tear coronal (A) and sagittal (B) T2-weighted images show fluid signal extending into the articular surface of the supraspinatus tendon (SST). Partial-thickness interstitial tear (C), fluid signal is present within the substance of the tendon but does not extend to either the articular or bursal surfaces. Partial-thickness bursal surface tear (D), fluid signal extends into the bursal surface of the SST but does not extend completely through to involve the articular surface. Coronal (E) and sagittal (F) T2-weighted images demonstrate a partial articular-side SST avulsion or PASTA lesion. The undersurface of the SST is avulsed from the greater tuberosity, but the bursal surface remains intact.

One helpful indirect MRI sign that can help in establishing the diagnosis of a partial-thickness rotator cuff tear is the presence of an intramuscular cyst. Joint fluid can track through the defect of a partial-thickness articular surface tear of the rotator cuff tendon and then dissect in a laminar fashion through the tendon and form a cyst within the substance of the rotator cuff muscle. These intramuscular cysts can easily be differentiated from paralabral cysts because they are contained within the fascia of the rotator cuff muscle, best demonstrated on sagittal T2-weighted images. An intramuscular cyst has a high association with partial-thickness tearing of the rotator cuff, just as a paralabral cyst has a high association with a labral tear. The presence of an intramuscular cyst can help differentiate between tendinopathy and a partial-thickness tear of the rotator cuff (Fig. 7-23). Two other imaging tips that can help differentiate between tendinopathy and a partial-thickness articular surface tears of the rotator cuff are the use of direct MR arthrography and ABER (abducted and externally rotated) imaging (with or without intra-articular contrast). Both techniques improve the conspicuity of partial-thickness articular surface tears of the rotator cuff (Fig. 7-24).

Figure 7-23 Partial-thickness tear: intramuscular cyst. A, T2-weighted oblique coronal image demonstrates intermediate signal involving the articular surface of the infraspinatus tendon (IST). By strict MR criteria, this represents tendinopathy and not a tear. B, The next most posterior image in the same series demonstrates a cyst within the infraspinatus muscle. This indicates that fluid is tracking into the muscle through a defect or partial-thickness undersurface tear of the IST and confirms the presence of a tear rather than tendinopathy. This was interpreted as a partial-thickness articular surface tear of the IST and was later confirmed at arthroscopy.

Figure 7-24 Partial-thickness tear: MR arthrography and ABER imaging. A, The presence of intra-articular contrast on this direct MR arthrogram helps to confirm the presence of a subtle articular surface tear of the supraspinatus tendon (SST) on a coronal T1-weighted image. B, The abduction external rotation (ABER) image demonstrates contrast extending into the substance of the muscle and is very helpful in confirming the presence of a subtle partial-thickness articular surface tear.

It is important to provide an accurate description of the extent and location of a partial-thickness tear because this information can have an impact on the decision to operate and can also influence the surgical approach and the type of surgery. A tear that extends more than 70% through the thickness of the tendon from superior to inferior is usually completed by the surgeon and then treated as a full-thickness tear. A tear that extends less than 30% through the thickness of the tendon is usually treated conservatively or by debridement alone (similar to the surgical treatment of tendinopathy). A tear that extends 30% to 70% through the thickness of the tendon is treated with debridement followed by suturing to shorten the “bridge” that is created by the debridement. In all cases, the cause of impingement must be addressed surgically to prevent recurrence of the rotator cuff pathology.

A partial articular-side supraspinatus tendon avulsion or “PASTA” lesion is a subset of partial-thickness tears that has been recently described in the orthopedic literature. The tear represents a partial-thickness articular-side avulsion of the supraspinatus tendon at its most anterior attachment site. This type of tear deserves special attention and should be accurately described on MRI because the recommended treatment for this subset of tendon tears differs from the standard partial-thickness tears previously described. A transtendon suture technique is performed to preserve the intact portion of the tendon while firmly reattaching the torn portion of the tendon to the humeral footprint. On MRI, a small articular-side avulsion is seen as fluid signal extending into the articular surface of the supraspinatus tendon at its anterior attachment site with partial avulsion of the tendon at this level and represents a subset of the articular surface partial-thickness tears (see Fig. 7-22E and F).

A full-thickness tear of the rotator cuff is defined as a tear that extends completely through the thickness of the tendon from superior to inferior. MR demonstrates bright fluid on T2-weighted images extending through the entire thickness of the tendon (Fig. 7-25). There may be retraction of the torn tendon end. Sagittal and axial images can be very helpful in differentiating a partial-thickness from a fullthickness tear, especially when the tear is located at the level of attachment to the humeral head. Small full-thickness tears often involve the most anterior aspect of the supraspinatus tendon immediately adjacent to its attachment to the greater tuberosity. It is important to evaluate the supraspinatus tendon on the most anterior coronal image to avoid missing a small full-thickness tear at the most anterior bone-tendon interface (Fig. 7-26). The extent of tear should be measured and reported in both the anteroposterior and medial-lateral directions. A tear that is larger in the anteroposterior direction is often repaired using a tendon-to-bone suture technique, whereas a tear that is greater in the medial-lateral direction may be amenable to repair using a mattress type tendon-to-tendon repair technique.