The MRI findings of shoulder impingement syndrome and its associated supraspinatus injury are best seen on oblique coronal MR images that visualize the full length of the supraspinatus muscle belly and tendon (Fig. 6-1B). The normal muscle belly displays moderately low signal intensity. The tendon is visualized as an intermediate–signalintensity structure that blends with the low signal intensity of the superior capsule as it courses to its insertion on the greater tubercle of the humerus. The tendon demonstrates smooth tapering from medial to lateral into its insertion in the greater tuberosity. The inferior aspect of the tendon is delimited below by the moderate signal intensity of the hyaline cartilage on the superior aspect of the humeral head. The superior aspects of both the muscle belly and tendon are delimited by a high–signal-intensity subacromial and subdeltoid fat plane. The normal subacromial-subdeltoid bursa is not specifically visualized because its walls are separated only by monomolecular layers of a synovial-type fluid, but it is situated between the supraspinatus tendon and the fat plane. Above the fat plane, the clavicle, acromioclavicular joint, acromion, and deltoid muscle are demonstrated on different oblique coronal sections.
Although rotator cuff impingement is a clinical diagnosis, MRI can provide direct visualization of the constituents to the coracoacromial arch and their relationship to the supraspinatus (Fig. 6-2A and B). Downward slanting of the acromion in the coronal or the sagittal plane, a thickened coracoacromial ligament or inferior osteophytosis within the acromioclavicular joint can exert mass effect upon the supraspinatus. This has been implied as being in part responsible for chronic tears of the supraspinatus.
FIGURE 6-2. A: Coronal oblique T2-weighted pulse sequence with fat suppression demonstrates downward slanting to the acromion (long arrow ), which is against the supraspinatus tendon (*). Note focal area of increased signal at the myotendinous junction of the SsT (short arrow ). B: Sagittal oblique T2WI with fat suppression demonstrates to a better advantage the inferior slanting to the acromion against the SsT (short arrow ). Note focal tendinosis (long arrow ).
FIGURE 6-3. A: T1-weighted MRI of focal supraspinatus tendinosis demonstrating focal thickening and slight increase signal intensity to the tendon (arrow ). B: CoronalobliqueT2-weightedfat suppressed sequence with increased signal intensity within the area of tendinosis (arrowhead ). There is fluid within the adjacent subdeltoid bursa (long arrow ). C: Sagittal oblique T2-weighted fat suppressed image demonstrates the area of increased signal to be within the anterior superior portion of the rotator cuff representing fibers of the supraspinatus tendon (short arrow ).
Neer stated that 95% of rotator cuff tears are associated with chronic impingement syndrome (7) and described three stages in the progression of rotator cuff injury. These can be visualized by MRI (6–10). Stage 1 is characterized by edema and hemorrhage within the supraspinatus tendon characteristic of an early tendinitis. On MRI, there is focal tendon thickening and diffuse moderate increase in signal intensity within the tendon (Fig. 6-3A–C). In stage 2, Neer described both inflammation and fibrosis within the tendon. MRI shows this as thinning and irregularity of the tendon. Stage 3 is a frank tear of the supraspinatus tendon. On MRI, complete tears are noted by a discontinuity of the tendon with a well-defined focus of high signal intensity on T2-weighted images (Fig. 6-4). The most susceptible area is the critical zone of hypovascularity, located about 1 cm from the insertion (11). With small or partial tears, there is no retraction of the muscle-tendon junction, the subacromial-subdeltoid fat plane is commonly obliterated, and fluid may accumulate in the subacromial-subdeltoid bursa, which becomes hyper- intense on T2-weighted images. There also may be effusion of the shoulder joint, which may extend inferiorly along the tendon sheath about the long head of the biceps. With a complete supraspinatus tendon tear, the muscle belly may retract medially, and atrophy may occur as the tear becomes chronic (Fig. 6-5A,B). Muscle atrophy appears as areas of high signal intensity because of fatty replacement within the muscle belly and decreased muscle mass. Finally, the acromiohumeral interval narrows as the humeral head migrates superiorly, because of the loss of supraspinatus restraint to the deltoid’s tendency to sublux the humerus superiorly during abduction.
FIGURE 6-4. Complete rupture of the supraspinatus tendon is seen in a T2-weighted MRI. There is fluid filling the gap (arrow) and there is retraction to the tendon fibers underneath the acromion.