Acromial Configuration.

Bigliani and colleagues described three predominant shapes of the acromion based on scapular Y view radiographs of the shoulder and believed the shape was an important factor contributing to impingement. A type I acromion has a flat undersurface. Type II acromion has a concave undersurface with the inferior acromial cortex parallel to the cortex of the underlying humeral head. Type III acromion has an inferiorly projecting anterior hook that narrows the space between the acromion and the humerus. It was thought that type II and III acromions had an increased incidence of cuff pathology, but this has not borne out in practice. Our surgeons do not inquire about the shape of the acromion, and acromion type is no longer considered to be an important factor relating to cuff pathology.

Acromial Slope.

The slope of the acromion can be evaluated on sagittal and coronal oblique images. Normally, the lateral aspect of the acromion is oriented nearly horizontal or slopes downward posteriorly on sagittal oblique images. An anteriorly downsloping acromion occurs when the inferior cortex of the anterior acromion is located more caudally than the inferior cortex of the posterior aspect of the acromion ( Fig. 10-12 ). Another abnormal slope to the acromion consists of an inferolateral tilt, which can be detected on coronal oblique images. An inferolateral tilt or slope occurs when the most lateral portion of the acromion is tilted inferiorly relative to the clavicle ( Fig. 10-13 ). Sloping of the acromion in either direction increases the risk for impingement from mechanical trauma to the underlying distal supraspinatus tendon ( Fig. 10-14 ).

Anterior-sloping acromion. T1 sagittal oblique image of the shoulder. The acromion is downward sloping anteriorly (anterior is to the left), which is believed by many to be a cause of impingement.

Acromial slope: inferolateral tilt. T1 coronal oblique image of the shoulder. The acromion (A) tilts inferiorly relative to the horizontal clavicle (C). This narrows the space between the humeral head and the acromion where the supraspinatus tendon and the subacromial/subdeltoid bursa exist, increasing the risk of impingement.

Acromial orientation. FSE T2 oblique coronal images showing the relationship of the acromion to the distal clavicle in three different shoulders. A , Horizontal; this is the normal orientation. B , Low-lying; the acromion is inferior to the distal clavicle. C , Inferolateral; the acromion is laterally sloping in relation to the clavicle. The low-lying and inferolateral positions are thought to be associated with impingement.

Acromial Position.

Normally, the inferior cortex of the acromion is in line with the inferior cortex of the clavicle on coronal oblique views. A low-lying acromion exists when its inferior cortex is positioned below the inferior cortex of the clavicle (see Fig. 10-11 ). This positioning causes narrowing of the acromiohumeral space, which may predispose to impingement.

Os Acromiale.

Os acromiale is an accessory ossification center of the acromion that is normally fused by 25 years of age. An unfused os acromiale after this age may be seen in 15% of the population; its presence is associated with an increased incidence of impingement and rotator cuff tears, presumably because the os is mobile and decreases the space in the coracoacromial arch with motion. The os acromiale is best identified on axial images, although it can be seen in all imaging planes ( Fig. 10-15 ).

Os acromiale. T2 ^* axial image of the shoulder. A cut through the level of the acromioclavicular joint shows a separate os acromiale (OA) that did not fuse in this patient. This predisposes to impingement. There are degenerative, high signal cysts between the os and the adjacent scapula.

Acromioclavicular Joint Degenerative Changes.

Degenerative joint disease of the acromioclavicular joint may manifest as inferiorly projecting osteophytes, fibrous overgrowth of the capsule, or both ( Fig. 10-16 ). , On radiographs, osteophytes are underestimated, and fibrous overgrowth is not depicted at all; MRI directly shows the presence, severity, and extent of these structures that may cause impingement. Obliteration of the fat between the supraspinatus muscle or tendon and the overlying acromioclavicular joint and indentation of the supraspinatus tendon or muscle by abnormalities of the acromioclavicular joint are indications that the degenerative changes are considerable and may contribute to impingement.

Acromioclavicular joint degenerative changes. A , T1 coronal oblique image of the shoulder. There is degenerative joint disease of the acromioclavicular joint with osteophytes projecting inferiorly ( arrow ). The supraspinatus is completely torn, with the tendon ( arrowhead ) retracted medially. B , T1 coronal oblique image of the shoulder (different patient than in A ). There is fibrous overgrowth of the capsule and osteophytes of the acromioclavicular joint, impinging on and indenting the top of the supraspinatus muscle and tendon ( arrow ). The distal tendon is mildly abnormal with high signal and thickening from degeneration and partial tears.

Coracoacromial Ligament.

Focal thickening of the coracoacromial ligament may be observed on sagittal oblique MRI, but has never been shown to be related to impingement. Evaluation of the coracoacromial ligament is not part of our interpretation of shoulder MRI.

Post-traumatic Deformity.

As a result of hypertrophic callus formation or malalignment of fracture fragments that involve the bones adjacent to the coracoacromial arch, there may be narrowing of the coracoacromial arch with resultant impingement, although this is uncommon.

Instability.

Glenohumeral degenerative changes can be caused by shoulder instability, and instability can be a contributing factor to impingement. Instability and impingement are two conditions that often coexist; this is discussed later in this chapter.

Muscle Overdevelopment.

Supraspinatus muscle enlargement, as seen in weightlifters, swimmers, and other athletes, may induce impingement even if the coracoacromial arch and acromiohumeral intervals are normal because the large muscle decreases the space available for the tendon to glide between the structures of the coracoacromial arch. A deformity (indentation) of the superior surface of the supraspinatus muscle caused by the acromioclavicular joint is the only abnormality seen with MRI.

Tendons.

The supraspinatus tendon is affected far more commonly than other tendons in the shoulder. Impingement on the supraspinatus tendon from any source can cause tendon degeneration and partial-thickness or full-thickness tears. Conversely, the same tendon abnormalities may exist without evidence of structural or mechanical causes of impingement. Most partial-thickness tears of the supraspinatus tendon occur on the undersurface (articular) aspect of the tendon, rather than on the superior (bursal) surface where abnormalities in the coracoacromial arch would logically be expected to cause early partial tears. The proximal long head of the biceps tendon may be affected by impingement in the same ways as the supraspinatus tendon because of its similar location and course just beneath the supraspinatus tendon within the shoulder joint.

Degenerative Osseous Cysts.

Although the supraspinatus tendon is the structure most often affected by impingement, certain associated osseous findings are very common as well. Cortical hypertrophy, sclerosis, and small degenerative cysts in the greater tuberosity often are present and may precede MRI evidence of abnormalities within the tendons. The osseous degenerative changes have been found histologically to be associated with microtears of the adjacent tendon.

Subacromial/Subdeltoid Bursitis.

Irritation of the subacromial/subdeltoid bursa can occur from impingement. When this occurs, the bursa becomes distended with fluid and is painful (see Fig. 10-11 ). Normally, there is no fluid, or only a trace of detectable fluid, in this bursa. Bursal fluid is common in the presence of rotator cuff tears. Bursal fluid cannot be seen without a T2W sequence, so this sequence is needed even when an MR arthrogram is performed.

Degeneration and Partial Tendon Tears.

Degeneration and partial tendon tears generally are indistinguishable from one another on T1W images, where they appear as focal or diffuse regions of intratendinous intermediate signal intensity, which is one of the reasons we no longer acquire T1W images in the coronal plane. If these areas maintain the same signal intensity as muscle on T2W images, they are most consistent with tendon degeneration; if these areas become high signal intensity, similar to fluid on T2W images, they represent partial tendon tears. It frequently is difficult to distinguish between degeneration or partial tendon tears, and in such a situation the general term tendinopathy may be used to describe the abnormalities. Tendon degeneration and partial tears often coexist.

Magic angle phenomenon has similar signal characteristics as tendon partial tears or degeneration on short TE images. The findings, however, are focal instead of diffuse, are at a specific location (1 cm from the insertion of the supraspinatus tendon on the greater tuberosity) and, most importantly, disappear on long TE images. , In addition, the signal changes are not associated with thinning, thickening, or irregularity of the tendon.

A partial tear on the undersurface (joint surface) of the tendon fills with high signal intensity gadolinium solution on MR arthrograms ( Fig. 10-17 ). Partial-thickness tears on the upper (bursal) surface of the tendon need to be evaluated similar to regular shoulder MRI without arthrography because gadolinium cannot enter the tear in that location. Partial-thickness tears generally start on the undersurface of the distal end of the supraspinatus tendon; the inferior layers may retract medially, whereas the superior layers remain intact.

Partial supraspinatus tear. A and B , FSE T2 coronal oblique ( A ) and FSE sagittal oblique ( B ) images of the shoulder. Thinning of the undersurface of the supraspinatus tendon can be seen in both planes ( arrows ), indicative of a partial articular-sided cuff tear.

The most common partial tear, which is the most common cuff tear of all kinds, is one in which the insertional fibers of the cuff on the greater tuberosity are disrupted from the bone ( Fig. 10-18 ). This has been termed a rim rent tear. It was first described by an orthopedic surgeon, Codman, in 1934. It was found in about 10% of cases by Tuite and coworkers, but we believe it is seen more commonly in our practice. In a series of 200 consecutive shoulder MRI studies, we found 117 cuff tears, 70 (60%) of which were partial cuff tears. Only 2 of the 70 were bursal-sided tears. Bursal-sided tears are much less common than joint-sided tears. Of the 70 partial tears, 49 (42%) were rim rent tears.

Partial supraspinatus tear (rim rent tear). A , T1 fatsuppressed, coronal oblique image of the shoulder. The broad footprint of the normal insertion of the supraspinatus tendon onto the greater tuberosity is interrupted with fluid ( arrow ), which indicates a partial articular-sided cuff tear. B , T1 fat-suppressed, sagittal oblique image of the shoulder. Sagittal image confirms fluid disrupting the cuff insertion ( arrow ).

We had overlooked many of these tears on our original interpretations primarily because we did not appreciate the significance of internal rotation of the humerus. The anteriormost fibers of the supraspinatus tendon insert just lateral to the bicipital groove and can be overlooked easily on the oblique coronal images if the shoulder is internally rotated ( Fig. 10-19 ). If one finds the biceps on the most anterior coronal images, the more anterior fibers of the cuff can be seen just adjacent to the bicipital groove ( Fig. 10-20 ). When these fibers are disrupted, the tear can be seen as an interruption of the normal tissue adjacent to the bicipital groove ( Fig. 10-21 ). The tear can be confirmed on sagittal oblique images, but because the biceps is so close to the tear it is often difficult to tell if fluid in the cuff is from partial volume averaging with fluid in the bicipital groove (see Fig. 10-21C ).

Schematic showing effect of internal rotation. This axial drawing shows how with external rotation ( right ) an oblique coronal image goes through the length of the supraspinatus and shows the insertion of the tendon just lateral to the bicipital groove, whereas if the patient is internally rotated ( left ), the same oblique coronal slice misses the anterior portion of the supraspinatus adjacent to the bicipital groove.

Normal cuff with internal rotation. A , FSE T2 coronal oblique image of the shoulder. The normal, broad footprint of the supraspinatus inserting onto the greater tuberosity appears normal on this image ( arrowheads ). B , FSE T2 coronal oblique image of the shoulder, just anterior to the image in A . The biceps can be seen exiting the bicipital groove ( arrowheads ), and the normal insertion of the leading edge of the supraspinatus tendon can be seen just lateral to the bicipital groove ( arrow ).

Rim rent tear hidden by internal rotation. A , FSE T2 coronal oblique image of the shoulder. The normal, broad footprint of the supraspinatus inserting onto the greater tuberosity appears normal on this image. B , Two images anterior to that, at the point where the biceps exits the bicipital groove ( arrowheads ), the greater tuberosity just lateral to the bicipital groove ( arrow ) is bare with no supraspinatus tendon fibers seen. At surgery, this was found to be a high-grade partial articular-sided tear. C , FSE T2 sagittal oblique image of the shoulder. Fluid can be seen on the anterior greater tuberosity where the anterior fibers of the supraspinatus tendon are lifted off ( arrow ). This can easily be mistaken for fluid in the adjacent bicipital groove, which is seen on this image just anterior to the humeral head.

We believe that the most common cuff tear is a partial-thickness tear at the insertion of the cuff fibers onto the greater tuberosity, many of which progress to a fullthickness tear. These tears can occur anteriorly on the greater tuberosity involving the supraspinatus, or posteriorly on the greater tuberosity involving the infraspinatus. These joint-sided or articular-sided tears are much more common than bursal-sided partial tears. Tears at the so-called critical zone, 1 to 1.5 cm proximal to the tendon insertion, are not as common as previously believed.

Detection of tendon abnormalities before the development of a full-thickness tear is important because the process may be arrested with conservative therapy, débridement, and decompression surgery. A full-thickness tear, in addition to pain, results in limited active movements of abduction and requires more involved surgery than that performed for a partial tear. MRI is reportedly less sensitive for detecting partial-thickness tears of the supraspinatus tendon than full-thickness tears. The specificity is in the range of 90%, but the sensitivity of standard MRI for detecting partial-thickness tears is reported to be 35% to 90%; these figures are significantly improved with MR arthrography.

Full-Thickness Tears.

Direct evidence of a full-thickness tear by conventional MRI (not MR arthrography) consists of discontinuity of the tendon with high signal intensity fluid traversing the gap between the tendon fragments from the articular to the bursal surfaces of the tendon on T2W sequences ( Fig. 10-22 ). MR arthrography shows high signal gadolinium on T1W images in the glenohumeral joint and in the subacromial/subdeltoid bursa with discontinuity of the tendon and gadolinium filling the gap between disrupted tendon fragments ( Fig. 10-23 ). MRI also can show the size of the tear, the degree of retraction of fragments, the quality of the remaining tendon fragments, and if there is associated muscle atrophy or osseous abnormalities; these features should be commented on in the report. Muscle atrophy is seen as high signal intensity within the muscle on T1W images ( Fig. 10-24 ), whether in the sagittal or the coronal plane.

Complete supraspinatus tear. FSE T2 oblique coronal image of the shoulder. A full-thickness gap is seen in the supraspinatus tendon ( arrow ) at the critical zone.

Complete supraspinatus tear. A , T1 fat-suppressed coronal oblique shoulder MR arthrogram. A small full-thickness tear ( arrow ) of the distal supraspinatus tendon is seen with contrast enhancement in the subacromial/subdeltoid bursa ( arrowhead ). B , T1 fat-suppressed sagittal oblique shoulder MR arthrogram. The defect in the supraspinatus tendon is easily seen ( arrow ); contrast enhancement also is noted in the overlying bursa.

Complete supraspinatus tear with secondary signs. T1 coronal oblique image of the shoulder. The end of the torn supraspinatus tendon is seen ( open arrow ). There are secondary signs of a tendon tear with medial retraction of the musculotendinous junction ( solid arrows ), atrophy of the muscle with fatty infiltration, and a decreased acromiohumeral interval. There are degenerative changes of the acromioclavicular and glenohumeral joints and a subacromial spur.

A supraspinatus tendon tear usually is located distally in the supraspinatus tendon, either near its attachment to the greater tuberosity or in the critical zone of the tendon located about 1 cm proximal to its insertion. Tears usually start in the anterior portion of the distal supraspinatus tendon as rim rent tears and propagate posteriorly. Complete disruption of fibers in the craniocaudal direction with communication between the joint and bursa indicates a full-thickness tear, even if it has not separated the tendons completely in the anteroposterior direction.

Disruption of the supraspinatus tendon is not always clearly evident or easy to diagnose by standard MRI. Granulation tissue or debris may obscure the tear, the tear may be very small, or it may be located far anteriorly, all of which make the diagnosis much more difficult. These difficulties can be alleviated with MR arthrography, which provides much more information (see Fig. 10-23 ). It is unusual, however, for us to see gadolinium in the subacromialsubdeltoid bursa without seeing a gap in the tendon. One could propose not using gadolinium in the arthrogram solution—only injecting saline for joint distention. We think this is a reasonable approach; it would shorten imaging time considerably and decrease cost, but this hypothesis needs to be tested because the current standard of care for MR shoulder arthrography entails gadolinium.

The sensitivity and specificity for full-thickness rotator cuff tears by MRI are greater than 90%. MR arthrography improves these figures and increases the confidence with which the diagnoses are made. The ability of MRI to show complete tears of the rotator cuff tendons is significantly better than its ability to show partial-thickness tears.

Tears.

The long head of the biceps tendon is completely torn in about 7% of patients with supraspinatus tendon tears and is abnormal (degeneration or partial tears) in about one third. Its proximity to the supraspinatus tendon makes it vulnerable to forces of impingement identical to forces affecting the supraspinatus tendon. Long head of the biceps tendon tears associated with supraspinatus tendon tears involve the impingement zone just proximal to the bicipital groove, and usually occur in older individuals. The distal tendon fragment and the muscle may retract distally, and an empty bicipital groove may be shown on axial MRIs of the shoulder ( Fig. 10-25 ). Acute tears of the long head of the biceps tendon may occur with severe trauma in young individuals or in older weekend athletes. Acute tears unrelated to impingement generally occur distally in the tendon, near the musculotendinous junction.

Biceps tendon tear. T2 ^* axial image of the shoulder. The bicipital groove ( arrow ) is empty, without evidence of the oval, low signal long head of the biceps tendon, indicating a complete rupture.

Dislocation.

Other abnormalities of the long head of the biceps tendon may occur, unassociated with impingement. Acute trauma can cause avulsion, subluxation, and dislocation of the tendon, all of which need to be repaired surgically. For subluxation or dislocation to exist, there must be disruption of the transverse humeral ligament that normally bridges the lesser and greater tuberosities and holds the long head of the biceps tendon in place. Usually, a tear of the subscapularis tendon also coexists. With dislocation, the long head of the biceps tendon may displace anteromedially, which may or may not be associated with an intact subscapularis tendon. It may dislocate medially within the shoulder joint, which is always associated with a tear of the subscapularis tendon at its attachment to the lesser tuberosity. , Biceps tendon subluxation and dislocation are shown best on axial MRIs, where the bicipital groove is empty, and the low signal round tendon is seen at variable distances medial to the groove, either deep or superficial to the subscapularis tendon ( Fig. 10-26 ).

Dislocated biceps tendon. A , T1 fat-suppressed axial shoulder MR arthrogram. The bicipital groove is empty. The biceps tendon ( arrowhead ) is located over the anterior glenohumeral joint and posterior to the subscapularis tendon ( arrow ), which has been avulsed from its attachment to the lesser tuberosity of the humerus. B , T1 fat-suppressed coronal oblique shoulder MR arthrogram. The biceps tendon ( arrowheads ) is dislocated medially overlying the shoulder joint.

Posterosuperior Impingement ( Box 10-7 ).

Posterosuperior impingement refers to impingement of the supraspinatus tendon, and mainly of the infraspinatus tendon, between the humeral head and the posterior glenoid rim during overhead movements with abduction and external rotation, such as pitching. , Posterosuperior impingement results in abnormalities affecting the rotator cuff, most commonly the infraspinatus tendon, the posterosuperior labrum, and the humeral head at the point of impaction with the posterosuperior glenoid. Patients present with posterior shoulder pain and may have associated anterior shoulder instability. MRI findings include degenerative cysts on the posterior aspect of the humeral head near the insertion of the infraspinatus tendon; fraying, partial, or complete tears of the infraspinatus tendon or supraspinatus tendon; and fraying or tears of the posterior glenoid labrum ( Fig. 10-28 ).

Posterior impingement. A , T1 fat-suppressed coronal oblique shoulder MR arthrogram. There is a large cyst in the posterolateral humeral head ( arrowhead ) that is filled with contrast material at the site of impaction between the humeral head and posterior labrum during overhead movements. There is incomplete fat suppression in this image, with fat remaining high signal laterally. B , T1 fat-suppressed axial shoulder MR arthrogram. The posterior labrum is detached from the adjacent glenoid ( arrow ).