Shoulder and Arm

Thomas H. Berquist

Jeffrey J. Peterson

INTRODUCTION

There are numerous clinical problems relating to the shoulder, arm, and brachial plexus. The brachial plexus is partially discussed in Chapter 5; however, a more detailed discussion of brachial plexus lesions is covered in this chapter. There is some overlap in clinical syndromes relating to the shoulder, upper extremity, and brachial plexus, but for purposes of discussion, each area will be considered separately.

Most patients with shoulder pathology present with pain and/or pain with reduced range of motion. For many years, shoulder evaluation has been based on clinical data as well as findings from routine radiographs.¹^,²^,³ Routine radiographs are still an important part of the workup of patients with shoulder pain, as subtle changes in both the bone and soft tissues can lead to appropriate selection of additional imaging techniques.⁴ The most recent ACR Appropriateness Criteria rank radiographs 9 (scale 1-9) for patients with acute shoulder pain.³ Additional studies are recommended on the basis of the patient’s age, history, and suspected condition. For example, when initial radiographs are equivocal or normal, MRI is ranked 9 as the optimal study. In this setting CT and ultrasound (US) are ranked 5, or less optimal. In other settings such as a patient with suspected bursitis or rotator cuff tear, US and MRI are considered equal and are both ranked at 9.³ We will incorporate the ACR Appropriateness Criteria in specific sections of the chapter as they apply.

In the past, the brachial plexus was evaluated with myelography and CT. However, the complex anatomy, along with the inability to clearly demonstrate all neurovascular structures resulted in suboptimal studies in many cases.⁵^,⁶^,⁷^,⁸ Examination of the brachial plexus using MRI can be accomplished in the axial, sagittal, coronal, and oblique planes, which allows more complete evaluation of neurovascular structures as they exit the paraspinal tissues and extend into the axillary and shoulder region.⁵^,⁶^,⁷^,⁸ The ACR Appropriateness Criteria rank MRI, including the appropriate anatomic region (cervical spine, upper chest, shoulder) above other techniques for evaluation of the brachial plexus.⁹

This chapter will discuss techniques, anatomy, and applications for evaluation of the shoulder, brachial plexus, and upper arm. Techniques vary depending upon the clinical setting. Anatomy in sagittal, coronal, and axial planes, especially with regard to the course of the neural structures, will be stressed.

TECHNIQUES

Techniques for MR examination of the shoulder, brachial plexus region, and arm vary depending upon the clinical symptoms. For purposes of discussion, it is best to consider the shoulder, arm, and the brachial plexus regions separately, as there are significant differences in the methods for MR examination (Table 9.1).

Table 9.1 MR Examinations of the Shoulder, Arm, and Brachial Plexus at 1.5 T

	Pulse Sequence	Slice Thickness/gap	Field of View (cm)	Matrix	Excitations (NEX)	Image Time
Shoulder
Three-plane scout	Fl 15/5	3 1 cm/no skip	24	256 × 192	1	16 s
Axial	SE 634/16	4 mm/skip 0.5 mm	14 (12-16)	256 × 256	1	3 min 39 s
Axial	GRE 613/19, FA 20°	4 mm/0 skip	14 (12-16)	256 × 256	1	2 min 43 s
Oblique coronal	TSE PD 2,000/19 ET 5	4 mm/skip 0.5 mm	14 (12-16)	256 × 256	1	3 min
Oblique coronal	TSE 3,500/91 2F5 ET 11	4 mm/skip 0.5 mm	14 (12-16)	256 × 256	1	4 min 22 s
Oblique sagittal	TSE PD with FS 3,050/26 ET 5	4 mm/skip 0.5 mm	14 (12-16)	256 × 256	2	3 min
Arthrography
Three-plane scout	Fl 15/5	3 1 cm	24	256 × 192	1	16 s
Axial	SE 500/12 with FS ET 1	4 mm/skip 0.5 mm	14	256 × 256	1	3 min 16 s
Sagittal	SE 544/12 with FS ET 1	4 mm/skip 0.5 mm	14	256 × 256	1	3 min 33 s
Coronal oblique	SE 525/12 with FS ET 1	4 mm/skip 0.5 mm	14	256 × 256	1	3 min 25 s
Coronal oblique	TSE PD 2,000/19 ET 5	4 mm/skip 0.5 mm	14	256 × 256	1	3 min 30 s
Coronal oblique	TSE T2 4,140/19 with FS, ET 11	4 mm/skip 0.5 mm	14	256 × 256	1	3 min 20 s
ABER	SE 500/12 with FS ET 1	4 mm/skip 0.5 mm	14	256 × 256	1	3 min 16 s
Arm
Three-plane scout	Fl 15/5	3 1 cm/no skip	30-48	256 × 192	1	16 s
Axial	TSE PD 3,050/26 ET 4	0.5-1 cm/0.5-1.0 mm skip	˜24	156 × 256	1	3 min
Axial	TSE T2 3,500/91 ET 11	0.5-1 cm/0.5-1.0 mm skip	˜24	256 × 256	1	4 min 22 s
Axial, coronal, or sagittal	SE 634/23	0.5-1 cm/0.5-1.0 mm skip for axial; 3-5 mm/0 skip for coronal or sagittal of humerus	˜24	256 × 256	1	3 min 39 s
Brachial Plexus
Three-plane scout	Fl 15/5	3 1 cm/0 skip	30-48	256 × 256	1	16 s
Coronal	TSE 400/17 ET 1	1.5 mm/0 skip	36	256 × 256	1	5 min 16 s
Axial (right and left)	SE 419/17	5 mm/1.5 mm skip	18	256 × 256	2	4 min 47 s × 2 if bilateral
Sagittal (right and left)	SE 500/13	5 mm/1.5 mm skip	22	256 × 256	1	3 min 40 s × 2 if bilateral
Axial bilateral	TIR 5950/99/TI160	5 mm/1.5 mm skip	36	256 × 256	1	4 min 5 s
Coronal bilateral	TIR 5950/99/TI160	5 mm/1.5 mm skip	36	256 × 256	1	4 min 5 s
SE, spin echo; TSE, turbo spin-echo; Fl, flash; FS, fat suppression; PD, proton density; T2, T2-weighted; ABER, abduction-external rotation; TIR, turbo inversion recovery.

Figure 9.1 Axial gradient-echo image of the shoulder at 3.0 T with the arm positioned with the hand palm up. Note the position of the bicipital groove arrow. If internally rotated the groove would move medially (medial arrowhead) and if too externally rotated it would move laterally (lateral arrowhead).

Glenohumeral and Acromioclavicular Joints

MRI of the shoulder can be accomplished with different field strengths and magnet configurations. Today, there are conventional closed bore systems, open systems, and extremity magnets. Imaging can be accomplished at varying field strengths as well. In our practice we most commonly use 1.5 T units, though 3.0 T is also available.¹⁰^,¹¹ Obtaining quality MR images of the upper extremity in a closed bore unit can be more challenging than evaluation of the lower extremity. Patient size can limit positioning to a certain extent. Larger patients may have to be rotated slightly. Positioning for shoulder imaging should assure patient comfort and avoid motion artifact.¹^,²^,¹²^,¹³ Low-field open gantries are less confining and provide more flexibility for patient positioning.

The arm is at the side in most cases, and a neutral to slightly externally rotated position is preferred (Fig. 9.1).²^,¹²^,¹³ Towels or bolsters can be used to improve comfort and reduce motion. Full external rotation makes soft tissue structures, specifically the labrum, easier to evaluate, but this position may be uncomfortable or difficult to maintain, resulting in motion artifact.¹⁴^,¹⁵^,¹⁶^,¹⁷ We prefer to tailor our examination to the comfort of the patient but avoid internal rotation whenever possible.² Internal rotation causes overlap of the supraspinatus and infraspinatus, which may mimic a lesion.¹⁶^,¹⁸ Tirman et al.¹⁸ advocated abduction and external rotation (arm above the head) to more easily evaluate partial tears. This position may also improve detection of labral pathology.¹⁸^,¹⁹^,²⁰^,²¹ This technique is not often used with conventional MRI in our practice, but is reserved for MR arthrography. Patients frequently complain of shoulder pain, and problems with motion artifact occur with greater frequency than when the arm is at the side.²^,²² Up to one-fourth of patients are unable to tolerate this position.²^,¹⁹^,²²^,²³

Figure 9.2 Phased array shoulder coil. (Courtesy of Siemens Medical Systems, Erlangen, Germany.)

New software and coil (Fig. 9.2) techniques have greatly enhanced the ability of MRI examinations to more optimally evaluate patients with shoulder symptoms. A dedicated phased array shoulder coil (Fig. 9.2) is placed around the shoulder to be examined.²^,²³ New coil technology continues to evolve at 1.5 and 3.0 T.²⁴

Our routine shoulder examination begins with a three-plane scout image [15/5, 24 cm field of view (FOV), 256 × 192 matrix, and one acquisition] that can be obtained in 16 seconds (Table 9.1).

Axial images should include from above the acromioclavicular joint through the axillary region (Fig. 9.3). Two sequences are obtained using 4-mm-thick sections, 256 × 256 or 256 × 192 matrix, a 14-cm FOV, and one acquisition. The first sequence is T1-weighted spin-echo (634/16) and the second a gradient-echo (GRE) [613/19/flip angle (FA) 20°] sequence. The latter is useful for labral evaluation and the reduced image time can also decrease motion artifact.²^,²²

Axial images or scout images are used to select the oblique coronal plane along the scapula or supraspinatus and perpendicular to the glenohumeral articulation (Figs. 9.4 and 9.5).²⁵^,²⁶^,²⁷ Using the same general parameters, we obtain fast spin-echo (FSE) proton density [2,000/19, echo time (ET) 5] and T2-weighted (3,500/90, ET 7) images. Fat suppression is used with the T2-weighted FSE series. Use of both proton density and T2-weighted images provides excellent anatomic detail and allows joint fluid (high signal intensity on T2-weighted images) abnormalities in the glenoid labrum and rotator cuff to be more easily
appreciated and classified. Signal intensity in the rotator cuff increases from proton density to T2-weighted images when partial or complete cuff tears are present. Signal intensity does not increase significantly in areas of tendinosis on T2-weighted sequences.²⁵^,²⁶^,²⁷

Figure 9.3 A: Coronal proton density weighted image demonstrating the region of interest covered with 14 cm field of view MRI. The area from above the acromioclavicular joint to below the axillary recess should be included. B: Axial SE 2,000/80 image degraded by motion artifact. C: Multiplanar gradient-echo (700/31, flip angle 25°) image in the same patient. There is no motion artifact.

Sagittal images are obtained using axial scout images to select sections perpendicular to the coronal plane and aligned with the glenohumeral articulation.²^,²⁸ We obtain 4-mm thick sections from proximal to the spinoglenoid notch to beyond the lateral margin of the humeral head (Fig. 9.6). Using a 14-cm FOV, 256 × 256 or 256 × 192 matrix, and one acquisition, we obtain an FSE proton density (3,050/26, ET 7) sequence.²^,²⁹ Sagittal images are useful for evaluation of the acromioclavicular (AC) joint, acromial configuration, and quantifying the size and location of rotator cuff tears.²^,¹³^,²⁵^,²⁶^,³⁰

In some situations, additional sequences or image planes may be indicated, but the above technique is generally appropriate for most shoulder or glenohumeral joint pathology.²^,¹³^,³¹ In certain cases, subtle marrow abnormalities may be more easily detected using short T1 inversion recovery (STIR) sequences or when the glenoid labrum is not optimally noted with the routine examination, radial gradient recalled echo in steady state (GRASS) interleaved (GRIL) images may be used to further define this anatomy.²^,³²^,³³^,³⁴^,³⁵ Today, suspected labral pathology is almost always evaluated with MR arthrography.

MR Arthrography

MR arthrography is commonly performed today to evaluate articular cartilage, the biceps-labral complex, and capsular ligamentous complex.³⁶^,³⁷^,³⁸^,³⁹^,⁴⁰ Both direct (intra-articular) and indirect (intravenous) approaches have been used.⁴¹^,⁴²^,⁴³^,⁴⁴^,⁴⁵ We prefer the direct approach except for evaluation of synovial enhancement. Direct arthrography results in more homogenous signal intensity throughout the joint fluid and provides
capsular distention that aids delineation of the labroligamentous structures.

Figure 9.4 A: Image planes selected for coronal images along the axis of the supraspinatus muscle (left) or central supraspinatus tendon (right). B: Axial MR image demonstrating the area covered along the axis of the supraspinatus (S).

Figure 9.5 A: Axial 3.0 T image demonstrating the image planes (lines for area covered) for oblique sagittal imaging. B: Sagittal 3.0 T turbo spin-echo fat suppressed image demonstrating a normal supraspinatus muscle and rotator cuff tendons. The acromion is straight.

Figure 9.6 Illustrations of anterior injection sites with the patient supine (A) and slightly rotated (B).

Optimally, patients should be selected for MR arthrography before they present to the MR suite to assure accurate scheduling. In some cases, conventional MR images are obtained initially. If patients are referred for specific indications or when conventional images have been previously obtained, an MR arthrogram may be performed as the only procedure. Injections may be performed using fluoroscopic, ultrasound, open MR monitoring, or palpation of the glenohumeral joint with the arm abducted 45°.⁴⁶^,⁴⁷^,⁴⁸ Recently, Gokalp et al.⁴⁹ found that using an ultrasound-guided posterior approach was fast and comfortable for the patient. We prefer fluoroscopic guidance to assure proper needle position and to monitor the contrast injection.²^,⁴

There are several anterior injection approaches (Fig. 9.7). Selection of injection approach may vary due to the patient’s anatomy or location of suspected pathology.⁵⁰ For the conventional injection, the patient is supine on the fluoroscopic table with the arm externally rotated (Fig. 9.6A). The patient may be rotated with the involved side down to open the joint (Fig. 9.6B). The shoulder is prepared using sterile technique. The injection site is localized directly over the medial margin of the humeral head, at the junction of the middle and lower third of the glenoid (Fig. 9.7). Local anesthetic is injected into the superficial tissues over the site selected for joint entry. We currently use a combination of iodinated contrast medium, diluted gadolinium and long acting anesthetic for diagnostic purposes. The current mixture is 5 mL of iodinated contrast (Reno-60 or Omnipaque 300), 10 mLof anesthetic (Ropivacaine 0.5%, lidocaine 1%), 5 mL of normal saline, and 0.1 to 0.2 mL of gadodiamide or gadopentetate dimeglumine (Omniscan or Magnevist).⁵¹ Using the conventional anterior approach, it is not unusual to cause extravasation in the soft tissues anteriorly (Fig. 9.8).

Figure 9.7 Illustration of injection site for MR arthrography using the anterior inferior approach. (From Berquist TH. Imaging of Orthopedic Trauma, 2nd ed. New York: Raven Press; 1992.)

Figure 9.8 Axial MR arthrogram image demonstrating anterior contrast extravasation (arrows).

Dépelteau et al.⁵² have suggested an alternative anterior approach to avoid contrast extravasation in the anterior supporting structures of the shoulder. This is accomplished by injecting higher on the humeral head (Fig. 9.9). Again, the patient is supine with the arm externally rotated to avoid the long head of the biceps tendon. Anesthetic is injected over the upper medial humeral head close to the articular margin. A direct vertical or slightly medially angulated approach is used. When the needle contacts the humeral head, the intraarticular location is confirmed with iodinated contrast.⁵²

Anatomically, the needle enters between the supraspinatus and subscapularis and may traverse the coracohumeral ligament and potentially the superior glenohumeral ligament.⁵¹^,⁵² The injection site avoids the subscapularis, inferior glenohumeral ligament, and anterior inferior labrum (Fig. 9.9).⁵²

The posterior approach is advocated in certain situations and avoids distortion of anterior supporting structures (Fig. 9.10).⁴⁹^,⁵¹ Many radiologists are less familiar with this technique. However, it can be quite useful in certain instances. The patient is prone, with the involved shoulder
rotated superiorly to align the glenohumeral articular surface. Patient position is supported by bolsters. After sterile preparation, the skin is marked over the lower medial humeral head. Following anesthetic injection, a spinal needle is advanced vertically until it contacts the humeral head. Intra-articular position is confirmed with iodinated contrast.⁵¹^,⁵³

Figure 9.9 Illustration of the typical anterior injection site (A) through the subscapularis and higher (B) in the rotator cuff interval between the subscapularis and supraspinatus.

At our institution, we most commonly use the conventional anterior approach. Regardless of the entry site we confirm needle position using our mixture of contrast media and anesthetic noted above.⁵¹

Other alternative combinations of injection solutions may be utilized.³⁶^,³⁷^,⁴⁴ It has been demonstrated that mixing iodinated contrast with gadolinium is safe and this approach is used routinely.⁵⁴ The syringe and tubing must be checked for air. If air is introduced into the joint, it may mimic a loose body.³⁷ Using fluoroscopic guidance, the shoulder is exercised to distribute the contrast medium. We prefer to obtain fluoroscopic images during and following the injection before moving the patient to the MR suite. The time between injection and MR evaluation should not exceed 90 minutes.⁵⁵ We obtain MR images 30 to 45 minutes following the injection.

Figure 9.10 Illustration of patient positioned for posterior injection.

Lee et al.⁵⁶ have recommended the Grashey view (oblique with beam tangential to the glenohumeral articular) for evaluation of the superior labrum. This may increase the level of confidence when evaluating coronal oblique images. Table 9.1 summarizes pulse sequences and image planes used for MR arthrography. We routinely obtain fat-suppressed T1-weighted axial, sagittal, and coronal oblique images. Oblique coronal images are also obtained using turbo (fast) spin-echo (TSE) proton density and T2-weighted images. In many cases, we also perform the abduction-external rotation view (ABER; elbow flexed with hand behind the head of involved shoulder) (Fig. 9.11). This image series is useful for evaluating the anterior labrum and the extent of partial tears of the rotator cuff.²⁰^,³²^,⁵¹^,⁵⁷^,⁵⁸

Arm

MR evaluation of the arm may be included as a portion of the shoulder examination or performed separately. The patient should be supine. Patients with suspected bone or soft tissue pathology in the arm are usually examined with axial T2-weighted images with either 1-cm or 5-mm slice thicknesses depending upon the area of interest (Table 9.1). Fat-suppression techniques are useful for subtle pathology. Axial TSE proton density and TSE T2-weighted sequences provide an excellent screening examination. This sequence can be followed by T1-weighted axial images if soft tissue pathology is suspected. Comparing similar sections on both T1- and T2-weighted sequences is useful for more accurate lesion characterization. A second T1- or T2-weighted sequence is performed in the coronal or sagittal plane depending upon the suspect pathology and extent of the lesion. When marrow pathology is suspected, sagittal or coronal images should be selected along the plane of the humerus using either T1-weighted spin-echo sequences and STIR or fat-suppressed TSE T2-weighted images to evaluate subtle changes in the marrow. Contrast-enhanced fat-suppressed T1 or TSE T2-weighted sequences in similar planes are commonly added to our examination.¹^,² Table 9.1 lists the different examinations for the shoulder and arm as well as the parameters and examination times.

Brachial Plexus

Examination of the brachial plexus requires a different approach (Table 9.1).²^,⁵⁹ The cervical spine is examined as part of the study (see Chapter 5). Routine cervical spine examination may be done first to exclude any spinal canal
or proximal nerve root abnormalities. These techniques are fully discussed in Chapter 5. Evaluation of the brachial plexus requires images from the mid cervical spine to the humerus, so a large FOV is necessary (Fig. 9.12). A torso coil is commonly employed. If symptoms are unilateral, a smaller off-center FOV can be used. Comparison is helpful, so we often examine both brachial plexus regions simultaneously. The axial images (right, left, or bilateral) are obtained using cardiac and respiratory gating to minimize the artifact from respiratory and cardiac motion. Slice thickness of 5 mm with 1.5-mm skip and an 18- (unilateral) or 36±-(bilateral) cm FOV with 256 × 256 matrix and 1 to 2 acquisitions are commonly used. Axial images are usually obtained from the mid cervical level (C3-C4) to the mid humerus, which allows the lower neck, shoulder, and upper arm to be completely included in the FOV (Fig. 9.13).

Figure 9.11 MR arthrogram. Scout images for the axial (A), oblique coronal (B), oblique sagittal (C), and abduction-external rotation (ABER) view (D). Normal axial (E-G), coronal (H, I), sagittal (J-L), and ABER (M) images.

There is considerable fat along the course of the neurovascular bundle of the brachial plexus. Therefore, sagittal T1-weighted images (Fig. 9.14) provide optimal evaluation of the nerve roots as they exit the spinal foramina and extend into the soft tissues of the neck and arm.⁶ The nerve roots appear as small, low-signal-intensity structures with this sequence and are accompanied by the larger arteries and veins as they extend peripherally. Soft tissue masses in the brachial plexus region are seen as low intensity areas and are clearly separated from the fat around the neurovascular
bundle. In most situations, the axial and sagittal images provide an adequate screening examination for brachial plexus pathology. We typically add coronal and axial turbo inversion recovery (TIR) images (Fig. 9.15). Post-contrast fat-suppressed T1-weighted images are useful for evaluation of neural inflammation and characterizing adjacent masses or soft tissue abnormalities. Technical details are discussed in depth in the clinical applications section of this chapter.

Figure 9.11 (continued)

Figure 9.12 Illustration of area studied with axial (A) and sagittal (B) imaging of the brachial plexus.

ANATOMY

It is important to have a thorough knowledge of anatomy in the commonly used conventional (Figs. 9.16,8.17,9.18) MR image planes and those used for MR arthrography (Fig. 9.11).

Osseous Anatomy

The shoulder is comprised of three bony structures—the clavicle, scapula, and humerus.⁶⁰^,⁶¹ The glenohumeral joint is a ball and socket joint. The humeral head is four times larger than the glenoid fossa of the scapula. This permits significant range of motion but also results in an increased susceptibility to instability. The majority of the muscles acting on the humerus are for adduction. Therefore, the clavicle and the sternoclavicular articulation provide important support in maintaining the muscular efficiency of the shoulder.²^,⁴^,⁶¹

There are two main articulations in the shoulder region, the AC joint and the glenohumeral joint (Figs. 9.17 and 9.19). The acromioclavicular articulation is a synovial joint formed by the capsule about the clavicle and acromion. In some cases, the joint is divided by a small articular disk. Motion at this articulation is limited to slight gliding movements between the scapula and clavicle. Supporting structures of the acromion of the scapula, the distal clavicle, and coracoid include the acromioclavicular ligament and the coracoacromial ligament, which extends from the coracoid process to the undersurface of the acromion just distal to the AC joint. The coracoclavicular ligament is divided into the coronoid and trapezoid bands (Fig. 9.20). These ligaments prevent upward displacement of the clavicle by the muscle forces of the trapezius and sternocleidomastoid muscles.⁴^,⁶¹

The glenohumeral articulation is formed by the shallow glenoid cavity, which is surrounded by a cartilaginous labrum (Figs. 9.11 and 9.21).⁴^,³⁴ The labrum is composed of fibrocartilage similar to the meniscus in the knee and is, therefore, seen as a triangular darkor low-intensity structure on MR images (Figs. 9.11 and 9.16). The labrum is somewhat blunted or rounded posteriorly and generally appears more triangular and sharper anteriorly.⁶²^,⁶³^,⁶⁴ The capsule of the shoulder is lined with synovial membrane that arises from the margin of the glenoid labrum and extends around the head of the humerus anteriorly and posteriorly, where it attaches at about the level of the physeal line or anatomic neck.⁶¹^,⁶⁵ The capsule, therefore, usually closely approximates the margins of the articular cartilage of the humeral head (Fig. 9.22). There are variations in the anterior capsular attachment that may play a role in recurrent dislocations. Type I attaches in or near the labrum (Fig. 9.22). Types II and III attach to the scapula more proximally. Capsules that attach more medially (type III) either predispose to, or are the result of, recurrent dislocations.⁶⁶^,⁶⁷ Similar changes may be seen posteriorly when there is posterior instability.² The synovial membrane continues between the greater and lesser tuberosities, forming a sheath for the long head of the biceps tendon. This sheath extends for variable lengths into the upper arm. Extension of the tendon sheath distal to the groove should not be confused with disruption.³⁶ The synovium also extends through a small defect in the capsule anteriorly to form the subscapular or subcoracoid bursa (Fig. 9.24).⁴^,⁶⁶^,⁶⁷^,⁶⁸^,⁶⁹

The capsule of the shoulder is supported by several fibrous capsular ligaments. The most consistent of these ligaments is the coracohumeral ligament (Fig. 9.20), which is a strong band arising from the most lateral edge of the coracoid process and extending over the superior aspect of the shoulder to attach to the greater tuberosity. The glenohumeral ligaments vary in thickness and may be difficult to appreciate on conventional MR images (Fig. 9.24).⁴⁰ The inferior glenohumeral ligament is usually the most obvious; it extends from the middle anterior margin of the glenoid labrum to the lower medial aspect of the humeral neck. The middle glenohumeral ligament is attached somewhat superior to this, extending from both the labrum and the coracoid attaching to the anterior aspect of the lesser tuberosity (Fig. 9.24).⁶¹^,⁶⁵^,⁶⁹^,⁷⁰ The superior glenohumeral ligament arises at the same level as the middle glenohumeral ligament and extends in a parallel manner to the middle
glenohumeral ligament. The transverse ligament extends across the greater and lesser tuberosities, enclosing the synovial sheath and long head of the biceps tendon.⁶⁹^,⁷⁰^,⁷¹

Figure 9.13 Axial images of the brachial plexus region with anatomic levels indicated. A: Axial image through the lower cervical region. B: Axial image at the base of the neck. C: Axial image through the upper humeral head. D: Axial image through the glenohumeral level. E: Axial image through the upper arm and axilla.

Muscular Anatomy

There are numerous muscles that act upon the scapula, shoulder, and glenohumeral articulation. The most important of these, from a clinical standpoint, are deltoid and rotator cuff group. The deltoid is a large muscle covering the shoulder superficially that arises from the lateral third of the clavicle, the acromion, and the spine of the scapula (Figs. 9.16,8.17,9.18, and 9.25). The fibers of the deltoid all converge distally and laterally to insert on the deltoid tuberosity on the lateral aspect of the upper humerus (Fig. 9.25). The deltoid serves as a powerful abductor of the humerus (Table 9.2).⁶¹^,⁶⁹

The supraspinatus muscle (Figs. 9.16,8.17,9.18, and 9.25) is a critical muscle and tendon unit in evaluating patients
with rotator cuff tears.⁴^,¹²^,⁶⁴^,⁷² The supraspinatus muscle arises from the supraspinous fossa of the scapula and is covered by the trapezius in its proximal portion. As the muscle extends peripherally, it passes under the acromion, coracoclavicular ligament, and AC joint (Fig. 9.25) to insert on the most superior of the three facets of the greater tuberosity. Tendon of the supraspinatus is broad, covering the top of the shoulder and blending with the capsule superiorly. The primary function of the supraspinatus is to assist the deltoid in abduction of the humerus (Table 9.2).⁶¹^,⁶⁹^,⁷³^,⁷⁴

Figure 9.13 (continued)

Figure 9.14 Sagittal images of the brachial plexus region with anatomic levels indicated. A: Sagittal image through the facets and intervertebral foramina. B: Sagittal image through the carotid artery. C: Sagittal image through the lateral neck. D: Sagittal image through the lateral chest.

The infraspinatus arises from the infraspinous fossa of the scapula (Fig. 9.25B). As it passes laterally, it is often
separated from the scapula by a bursa that sometimes communicates with the shoulder joint (Table 9.3). The infraspinatus is separated from the supraspinatus by the scapular spine (Figs. 9.16,8.17,9.18). Its tendon forms the upper posterior portion of the rotator cuff and inserts in the greater tuberosity posterior and inferior to the supraspinatus tendon.⁶⁹^,⁷²^,⁷⁵^,⁷⁶^,⁷⁷^,⁷⁸

Figure 9.14 (continued)

The teres minor arises from the middle half of the lateral scapular border, extends in an oblique upward direction to insert as a large flat tendon on the most posterior and inferior of the three facets of the greater tuberosity (Figs. 9.18 and 9.25B). Like the infraspinatus, the teres minor is primarily an external rotator of the humerus.⁶⁹

Along with the supraspinatus, infraspinatus, and teres minor, the subscapularis makes up the fourth of the rotator cuff muscles.⁴^,⁶⁹^,⁷² The subscapularis arises from the anterior subscapular surface of the scapula with its fibers converging and extending laterally to insert in a broad tendon or band along with the capsule on the lesser tuberosity of the humerus and the crest below this tuberosity (Figs. 9.16,8.17,9.18). This large, triangle-shaped muscle forms the posterior wall of the axilla with the axillary vessels and brachial plexus
passing across and anterior to the muscle (Fig. 9.26). The subscapularis bursa or recess lies between the muscle and the neck of the scapula and can communicate with the shoulder joint (Table 9.3). The primary function of the subscapularis is internal rotation of the humerus (Table 9.2).⁶¹^,⁶⁹

Figure 9.15 Coronal images of neck and shoulders with anatomic levels indicated. A: Coronal image through the descending aorta. B: Coronal image through the cervical spine. C: Coronal image through the neurovascular region.

Figure 9.16 Axial MR anatomy of the shoulder with illustration of anatomic levels. A: Axial image through the acromioclavicular joint. B: Axial image through the supraspinatus. C: Axial image through the upper glenoid and coracoid. D: Axial image through the humeral head and glenoid demonstrating the normal labrum. E: Axial image through the lower glenoid.

Figure 9.16 (continued)

Figure 9.17 Coronal MR images of the shoulder with illustration of the section levels. A: Coronal image through the humeral head and acromioclavicular joint. B: Coronal image through the glenohumeral joint.

Figure 9.17 (continued)

Figure 9.18 Sagittal MR images of the shoulder with level of section indicated. A: Sagittal image through the scapular spine. B: Sagittal image through the coracoid. C: Sagittal image through the glenoid articular surface. D: Sagittal image through the humeral head and acromioclavicular joint. E: Sagittal image through the lateral humeral head.

Figure 9.18 (continued)

Figure 9.19 Illustration of the shoulder, demonstrating the glenohumeral and acromioclavicular joints and surrounding structures.

The teres major (Fig. 9.25B) arises on the dorsal aspect of the scapula from its lower lateral border and extends anteriorly to insert below the subscapularis on the medial aspect of the intertubercular groove. The teres major is an additional internal rotator of the humerus and when acting with the latissimus dorsi, it also serves as an extensor and adductor of the humerus. The muscle is innervated by the subscapular nerve, which receives its branches from the C5-C6 levels.⁶⁹^,⁷²

The extrinsic muscles of the shoulder arise from the vertebral column or thoracic cage and attach to the scapula or humerus (Fig. 9.25B). Two of these muscles, the trapezius and latissimus dorsi, completely cover the deep musculature of the back as they course to insert in the shoulder region (Fig. 9.25B). The trapezius has a long, broad origin from the upper superior nuchal line of the occipital bone, ligamentum nuchae, and upper thoracic spinous processes (Figs. 9.17 and 9.25B). The upper fibers insert on the posterior superior aspect of the distal clavicle while the middle fibers insert more inferiorly on the border of the acromion and spine of the scapula. The lower fibers insert on a more clearly defined tendon at the base of the scapular spine. There is commonly a small bursa between the tendon of the lower fibers and the spine of the scapula. The trapezius serves as a retractor and an elevator of the scapula. It is innervated by the accessory spinal nerve and roots from C3 and C4.⁶¹^,⁶⁹

Figure 9.20 Illustration of the ligaments of the shoulder.

Figure 9.21 Illustration of the shoulder, demonstrating the glenoid articular surface, labrum, and supporting structures. Radial image planes and labral configuration are demonstrated.

Figure 9.22 Capsular attachments. A: Axial MR arthrogram image of the shoulder demonstrating the capsular attachments. There is a type I attachment anteriorly and more medial type II attachment posteriorly with capsular distention. B: Axial MR arthrogram image demonstrating a type I posterior attachment and type III anterior attachment (arrow).

The latissimus dorsi is a second broad-based muscle taking its origin from the lower six thoracic spinous processes and the spinous processes of the lumbar and upper sacral vertebrae (Fig. 9.25B). Superiorly, the muscle origin is covered by the lower fibers of the trapezius. This broad, triangular muscle extends superiorly to its insertion along the medial wall of the intertubercular or bicipital groove. There is a bursa between the tendons of latissimus dorsi and the teres major that, when inflamed, can be seen as an area of well-defined high intensity on MR images. The chief actions of the latissimus dorsi are adduction, internal rotation, and extension of the humerus. The muscle is supplied by the thoracodorsal nerve with roots from C6 through C8 (Table 9.2).⁶¹^,⁶⁹

Figure 9.23 Sagittal MR arthrogram shows the subcoracoid bursa (arrow).

The levator scapulae (Fig. 9.25B) originates from the posterior tubercles of the upper first through fourth transverse processes of the cervical spine. The muscle then passes in an oblique posteroinferior direction to insert on the superior angle of the medial scapular border. It is deep to the sternocleidomastoid superiorlyandtrapezius inferiorly. The main function of this muscle is to elevate the scapular border. The muscle is innervated by the deep cervical plexus from the third and fourth roots (Table 9.2).⁶¹^,⁶⁹

The rhomboid muscles (Fig. 9.25) are often divided into minor and major groups; however, in reality, especially on MR images, the muscle is difficult to separate into two groups. The muscles take a broad origin from the ligamentum nuchae and spinous processes of C2 through T5 to pass laterally and insert on the posterior medial aspect of the scapula near the base of the scapular spine. The chief functions of these muscles are to retract the scapula and they are innervated by the dorsoscapular nerve, which is primarily derived from the C5 root (Table 9.2).⁶¹

The serratus anterior (Fig. 9.25B) is a large, flat muscle that covers the thoracic cage laterally and acts primarily on the scapula. It arises from the outer surfaces of the proximal eight or nine ribs and extends around the lateral aspect of the thoracic cage to insert on the anterior medial aspect of the scapula. The chief function of this muscle is to protract or draw the scapula anteriorly. The muscle is supplied by the long thoracic nerve that typically arises from cervical nerves (Table 9.2).⁶¹

Three muscles constitute the musculature of the pectoral region. These include the pectoralis major, pectoralis minor, and subclavius muscles (Figs. 9.16 and 9.25A). The
pectoralis major is a large, triangular, flat muscle covering much of the upper thorax. The pectoralis major has an extensive origin from the medial inferior aspect of the clavicle, the lateral margin of the sternum, and the chondral junctions of the upper ribs. The fibers of this muscle converge to form a broad tendon that passes just anterior to coracobrachialis and biceps brachii. The muscle then passes deep to the anterior margin of the deltoid to insert on the crest of the greater tubercle or lateral aspect of the intertubercular groove. The pectoralis major is a strong adductor of the humerus and is innervated by two nerves, the lateral and medial (anterior thoracic) nerves (Table 9.2).⁶¹^,⁶⁹

Figure 9.24 Illustration of the glenohumeral ligaments (A) and their relationship to the coracohumeral ligament (B).

Figure 9.25 Illustrations of the extrinsic shoulder muscles from anterior (A) and posterior (B).

Table 9.2 Muscles of the Shoulder and Upper Arm

Muscles	Origin	Insertion	Action	Innervation
Intrinsic
Deltoid	Lateral clavicle, acromion, scapular spine	Deltoid tuberosity humerus	Abductor of humerus	Axillary nerve (C5, C6)
Supraspinatus	Supraspinous fossa of scapula	Greater tuberosity superiorly	Abductor of humerus	Suprascapular nerve (C5, C6)
Infraspinatus	Infraspinous fossa of scapula	Posterior inferior greater tuberosity	Lateral rotation humerus	Suprascapular nerve (C5, C6)
Teres minor	Lateral midscapular border	Lateral inferior facet, greater tuberosity	External rotator of humerus	Axillary nerve (C5, C6)
Subscapularis	Suprascapular fossa	Lesser tuberosity	Internal rotator of humerus	Subscapular nerves (C5-C7)
Teres major	Inferior lateral scapula	Medial intertubercular groove	Internal rotation, adduction, extensor of humerus	Subscapular nerve (C5-C6)
Extrinsic
Trapezius	Ligamentum nuchae, thoracic spinous processes	Distal clavicle, acromion, scapular spine	Retractor and elevator of scapula	Spinal accessory and C2, C3
Latissimus dorsi	Spinous processes T6-T12, lumbar, upper sacrum	Medial intertubercular groove	Adductor, internal rotator, and extensor of humerus	Thoracodorsal nerve (C6-C8)
Levator scapulae	Posterior tubercles, C1-C4 transverse processes	Upper medial scapula	Elevates medial scapula	Cervical plexus (C3-C4)
Rhomboideus
Major	C2-T5 spinous processes	Posterior medial scapula	Retractor of scapula	Dorsal scapular nerve (C5)
Minor	Ligamentum nuchae C7 and T1 spinous processes	Posterior medial scapula at base of scapular spine
Serratus anterior	Anterior ribs 1-9	Anterior medial scapula	Protractor, anterior drawing of scapula	Long thoracic nerve (C5-C7)
Pectoral Region
Pectoralis major	Inferomedial clavicle, sternum, and costochondral junctions	Lateral intertubercular groove	Adductor of humerus	Medial and lateral anterior thoracic nerves (C5-T1)
Pectoralis minor	Anterior ribs 2-5	Coracoid of scapula	Depress angle of scapula	Medial pectoral nerve (C8-T1)
Subclavius	Anteromedial rib 1	Midinferior clavicle	Stabilize sternoclavicular joint	Subclavian nerve
From Carter BL, Morehead J, Walpert JM, et al. Cross-sectional Anatomy: Computed Tomography and Ultrasound Correlation. New York: Appleton-Century Crofts; 1977; Iannotti JP, Gabriel JP, Schneck SL, et al. The normal glenohumeral relationship. J Bone Joint Surg. 1992;74A:491-500; and Rosse C, Rosse PG. Hollinsheads Textbook of Anatomy. Philadelphia, PA: Lippincott-Raven; 1997.

The pectoralis minor is deep to the pectoralis major and arises from the anterior aspects of the second through fifth ribs. The muscle passes in a superior and oblique direction
to insert on the coracoid process of the scapula. Its chief action is to depress the angle of the scapula. Nerve supply is via the medial pectoral nerve (C8-T1) (Table 9.2).⁶¹

Table 9.3 Shoulder Bursae

Bursa	Location	Normal Joint Communication
Subscapular	Between subscapulosus tendon and capsule	Yes
Infraspinatus	Between capsule and infraspinatus tendon	Inconsistent
Subdeltoid	Between deltoid and rotator cuff	No
Subacromial	Between acromion and rotator cuff (usually contiguous with subdeltoid)	No
Supra-acromial Subcoracoid	Superior to acromion between coracoid and subscapularis (may be contiguous with subacromial)	No—20% of cases
Coracoclavicular	Between the coronoid and trapezius fascicles of the coracoclavicular ligament	No
Coracobrachialis	Between coracobrachialis and subscapularis	No
Latissimus dorsi	Between latissimus and teres major	No
Teres major	Between teres and humeral insertion	No
Pectoralis major	Between pectoralis and humeral insertion	No
From references 2, 53, 68.

Figure 9.26 A and B: Coronal T1-weighted images through the anterior shoulder, demonstrating the subscapularis and adjacent neurovascular structures.

The subclavius is a small muscle that takes its origin from the junction of the first rib and its articular cartilage and passes upward and laterally to insert on the inferior aspect of the midclavicle. This muscle is covered by the pectoralis major and is innervated by a small branch of the subclavian nerve. It is often seen as a small, intermediate-density structure beneath the clavicle on the sagittal images (Fig. 9.19) and should not be confused with a soft tissue mass (Table 9.2).⁶¹^,⁶⁹

Bursae

There are numerous bursae about the shoulder. It is important to understand their location and appearance to prevent confusing a bursa with other pathology (Fig. 9.23).⁶⁸ Table 9.3 summarizes these bursae and their locations. It is important to note that the subscapular bursa normally communicates with the shoulder joint. The infraspinatus bursa inconsistently communicates with the shoulder joint, and the remaining bursae should not, in normal patients, communicate with the glenohumeral joint.³⁶^,⁶¹^,⁶⁸^,⁶⁹ Most important of these bursae are the subdeltoid and subacromial bursae (Fig. 9.19), which are normally contiguous, do not communicate with the shoulder joint, but can be inflamed. Inflammation does not generally occur in the absence of impingement and rotator cuff tear.³⁶^,⁶⁹

Figure 9.27 Illustration of the brachial plexus and branches.

Neurovascular Anatomy

Because of its complexity and significant clinical implications, the brachial plexus is discussed separately from the arterial and venous anatomy of the shoulder and upper arm. The brachial plexus (Fig. 9.27) arises from the anterior rami of the fifth, sixth, seventh, and eighth cervical and first thoracic nerves. In some situations, C4 also forms a portion of the brachial plexus. In most situations, the C5 and C6 roots form the upper, C7 the middle, and C8 and T1 the lower trunks of the brachial plexus. Each trunk then divides to form anterior and posterior divisions. The anterior divisions of the upper and middle trunks form the lateral cord (Fig. 9.27). The anterior division of the lower trunk continues as the medial cord. The posterior divisions of the upper, middle, and lower trunks unite to form the posterior cord (Fig. 9.27). The branches arising from these cords are demonstrated in Figure 9.27. Sagittal MR images usually clearly define the nerve roots as they exit the intervertebral foramen (Fig. 9.14). As one passes peripherally, it is more difficult to identify the individual cords and branches of the brachial plexus. However, there are certain relationships that can be easily identified and followed on MR images.⁶^,⁵⁹^,⁶⁹

In the lower neck, the brachial plexus lies above the subclavian artery, except for the lower trunk, which lies posterior to this vessel. This relationship is maintained as the brachial plexus crosses the first rib and enters the axilla. Upon entering the axillary region, the brachial plexus is more closely grouped to the axillary artery and vein
(Figs. 9.14 and 9.15). At this level, the lateral cord and a major portion of the posterior cord lie lateral and superior to the axillary artery. The lower trunk of the medial cord is posterior to the artery, with the major continuation of the other groups in close approximation to the vessel (Figs. 9.15 and 9.27). On sagittal images, this forms a closely approximated group of low-intensity structures (Fig. 9.14).⁶^,⁵⁹^,⁶¹^,⁶⁹

Figure 9.28 Illustration of the vascular anatomy in the axillary region.

Figure 9.29 Coronal MR angiogram of the neck and shoulder.

Figure 9.28 demonstrates the major vascular supply of the shoulder. The subclavian artery, along with the transverse cervical and subscapular arteries, is generally clearly demonstrated on MR images. These are most easily demonstrated in the coronal plane (Fig. 9.15). The smaller branch vessels are more difficult to identify on conventional spin-echo sequences.⁷⁹ However, new angiographic techniques clearly define the major branch vessels, including the circumflex vessels, thoracodorsal artery, and other branch vessels around the glenohumeral articulation (Fig. 9.29). The nerve branches of the brachial plexus generally closely follow the courses of the subclavian and axillary artery branches (Fig. 9.14).⁵⁹^,⁶¹^,⁶⁹

PITFALLS

Pitfalls in MRI and MR arthrography may be related to hardware or software artifacts, motion and/or flow artifacts, and normal anatomic variants and examiner experience (Fig. 9.23).¹^,²^,⁸¹^,⁸²^,⁸³^,⁸⁴ Additional problems occur with partial volume effects and improper positioning.²^,³⁵^,³⁶^,⁸⁵^,⁸⁶^,⁸⁷

Technique

Technical factors may include choice of imaging parameters, injection technique and approaches in MR arthrography, and patient positioning.²^,⁵⁰^,⁸¹^,⁸⁸ Motion and flow artifacts
occur less frequently in the shoulder compared to the elbow and wrist. Motion artifacts may be related to pain, tremor, respiratory motion, or forced external or abduction-external rotation, which may be difficult positions to maintain (Fig. 9.30).²^,⁸⁵ Motion artifacts can be reduced by increasing patient comfort, positioning, and using faster sequences to reduce imaging time.²^,¹² Flow artifacts are a less significant problem. Artifact occurs in the phase encoding direction. Therefore, phase encoding directions should be selected on the basis of the suspected pathology. A transverse phase encoding should be selected for rotator cuff disease (Fig. 9.31) to reduce confusion that may occur when flow artifact is in the superior-inferior direction that would pass through the rotator cuff.²^,⁸⁹

Figure 9.30 Axial T2-weighted (A and B) and gradient-echo (C and D) images in a patient with a rotator cuff tear (arrows). There is motion artifact that degrades image quality on conventional spin-echo sequences A and B. No significant artifact is evident on the gradient-echo sequences C and D.

Nonuniform signal intensity or fat suppression also can cause confusion when interpreting MR images. The former is not common today with new phased array shoulder coils compared to circular or flat coils used previously (Fig. 9.2). Nonuniform fat suppression can result in spurious high signal intensity due to marrow or subcutaneous fat. This is a particular problem with FSE T2-weighted sequences. Metal may also cause nonuniform fat suppression.²^,⁸¹ Carroll and Helms⁸¹ have recommended the use of inversion recovery sequences in these situations.

Erickson et al. described the magic angle phenomenon (Fig. 9.32) that can be seen with tendons oriented approximately 55° to the magnetic field (B₀).⁸⁷^,⁹⁰^,⁹¹ This phenomenon occurs with short TE pulse sequences and may cause increased signal in the supraspinatus in the region of the critical zone.²^,⁸⁷^,⁹⁰ This is not a problem with T2-weighted (long TE) sequences. Therefore, if one compares short and long TE images, the signal intensity will return to normal. Also, changing the position of the arm alters the location of the signal intensity changes confirming that it is magic angle phenomenon.⁸⁷

Errors in arthrographic techniques can also create pitfalls. This may be related to examiner experience. For
example, Theodoropoulos et al.⁸⁴ demonstrated significant performance differences comparing musculoskeletal trained radiologists and general radiologists for technique and interpretation. Improper injection of contrast can result in extravasation (Fig. 9.8), which can be confused with pathology.⁵⁰^,⁸¹^,⁸³^,⁸⁴ Inadvertent injection of air bubbles may cause areas of low signal intensity on MR arthrograms that mimic loose bodies. “Blooming” may cause the air bubble to appear larger due to magnetic susceptibility artifact. This phenomenon is more common on GRE images.⁸¹

Figure 9.31 Coronal T2-weighted fast spin-echo image with fat suppression. The flow artifact (arrows) runs transversely.

Improper positioning may result in apparent signal intensity abnormality in the rotator cuff or capsule. Davis et al.⁸⁵ demonstrated abnormal signal intensity in the rotator cuff when the arm is internally rotated. This apparent abnormal signal intensity is due to overlap or narrowing of the supraspinatus and infraspinatus. Internal rotation may also cause overlap of anterior capsule and soft tissue structures, which can result in perceived labral or capsular abnormalities. The degree of internal or external rotation can be determined by the position of the biceptal groove on axial images (Fig. 9.33).⁸¹

Figure 9.32 Coronal SE 500/11 sequence demonstrating increased signal (white arrow) in the supraspinatus that is angled at 55° to the magnet bore (white line).

Figure 9.33 Axial gradient-echo image (A) with the bicipital groove (arrow) position indicating slight internal rotation. Axial MR arthrogram (B) shows more external rotation with the groove (arrow) more lateral and a stretched subscapularis.

Figure 9.34 50-year-old male. Turbo spin-echo proton density (A) and T2-weighted (B) images demonstrate an area of intermediate signal intensity near the insertion (arrow) in A. The signal intensity does not increase on the T2-weighted sequence (arrow in B).

Anatomic

Anatomic pitfalls and variants have been described by several authors and contribute significantly to difficulties in interpreting MR images of the shoulder.¹⁰^,²¹^,³³^,⁵⁰^,⁹⁵^,¹⁰²^,¹⁰³ The most frequently encountered pitfalls involve variations in or adjacent to the rotator cuff, labral configuration, marrow patterns, and fluid collections.³³^,⁹⁵^,¹⁰³^,¹⁰⁴

Rotator Cuff

Normal tendons have uniformly low signal intensity.²^,¹²^,⁶⁴^,⁸¹ Unfortunately, in many adults there is an area of increased signal intensity (intermediate intensity compared with low signal intensity) in the supraspinatus tendon about 1 cm proximal to its insertion. This area of increased signal intensity corresponds to the critical zone (hypovascular area in the cuff). The signal intensity increase is usually 5 to 10 mm and round or oval in appearance.¹² Several theories have been proposed for this signal intensity change. Abnormal signal may represent tendon degeneration.⁹² In cadaver studies, Kjellen et al.⁹³ found that similar abnormal signal intensity represented mucoid degeneration. Though signal intensity was increased on proton density images, it did not increase further on a T2-weighted spin-echo sequence, as do rotator cuff tears (Fig. 9.34).⁹³

Vahlensieck et al.¹⁰⁹ noted similar increased signal intensity in 85% of normal patients. Explanations for these signal intensity changes included mucoid degeneration, positioning artifacts and partial volume effects, and magic angle effect.⁹⁰^,⁹⁵ Liou et al.⁹⁶ noted intermediate signal intensity in most shoulders, abnormal features in the subdeltoid fat in 95%, and degenerative changes in the AC joint in 48% of asymptomatic patients.

Secondary signs of rotator cuff tears that include an abnormal subdeltoid-subacromial fat plane and fluid in the bursa (Fig. 9.35) can also create pitfalls.²³ Specifically, abnormalities (thinning, irregularity, partial obliteration) in the fat plane are the rule rather than the exception in patients with normal rotator cuffs. Abnormal fat planes are noted in up to 95% of normal patients.⁹⁶ Fluid in the subacromial or
subdeltoid bursa can also be detected in patients without an associated rotator cuff tear.⁷³^,⁹⁷ However, other conditions such as impingement, bursitis, and tendinitis may be associated with fluid in the bursa.⁴⁷ Fat-suppression techniques may be useful to reduce problems caused by chemical shift artifacts that occur at fat-water interfaces.⁷³^,⁹⁷

Figure 9.35 Coronal proton density image with increased signal in the supraspinatus (arrow) and loss of the fat plane (open arrow). There is no rotator cuff tear.

Labral-Capsular Complex

The glenoid labrum is a fibrous extension of the glenoid rim that has an MR appearance similar to the acetabular labrum or knee meniscus, in that it is low signal intensity and typically triangular in shape. This structure is important for maintaining glenohumeral stability.⁹⁸^,⁹⁹^,¹⁰⁰

Anatomic variations in the labrum, labral-biceps complex, and adjacent capsule can create confusion and simulate a labral tear.¹²^,³⁴^,³⁷^,⁸¹^,¹⁰⁰^,¹⁰¹^,¹⁰²^,¹⁰³^,¹⁰⁴ Neumann et al.¹⁰² described multiple labral configurations in asymptomatic shoulders. The most frequent configuration is a triangular anterior (45%) and posterior (73%) labrum. However, in our experience the posterior labrum is frequently more rounded (Fig. 9.36) than the sharper anterior labrum. A rounded appearance (19% anteriorly, 12% posteriorly) (Fig. 9.36) was the second most common appearance reported by Neumann et al.¹⁰² Cleaved (frayed appearance) and notched labra were reported in 15% and 8% of cases, respectively. Flat labra were noted in 6% to 7%, and absence of the labrum was seen anteriorly in 6% and posteriorly in 8%.¹⁰⁵ In our experience, we have not seen an absent labrum in the normal shoulder.

Figure 9.36 Normal variations in the glenoid labrum. A: Illustrations of variations in the anterior labrum at the midglenoid level. a, triangular; b, small but triangular; c, rounded; d, crescentic; e, recess between the labrum and cartilage; f, middle glenohumeral ligament proximal to the labrum; g, small anterior labrum with adjacent thick middle glenohumeral ligament. B: MR arthrogram demonstrating triangular (arrows) anterior and posterior labra. The posterior capsule is distended with a type II or III attachment (open arrow). C: MR arthrogram demonstrating near-complete absence of the anterior labrum (arrow) and a rounded posterior labrum (bracket). D: MR arthrogram depicting rounded anterior and posterior labra. E: MR arthrogram demonstrating a rounded posterior labrum and intermediate signal intensity and deformity of the anterior labrum (arrow) due to a prior Bankart repair.

A normal cleft is usually seen between the articular cartilage and labrum (Fig. 9.36B). This does not transverse the entire length of the labrum and should not be mistaken for a tear.⁸⁶^,⁹⁶ The proximity of the middle glenohumeral ligament to the anterior labrum (Fig. 9.36C) can also be confused with a tear.³⁷^,⁸⁶^,⁹⁶

There are other clinically significant variations in the ligaments, capsule, and labrum that deserve mention in this section. Most common variations occur in the ligaments and capsular attachments.¹²^,³⁷^,¹⁰⁰

The superior glenohumeral ligament originates at the superior glenoid margin anterior to the long head of the biceps tendon. This ligament may originate separately or with the biceps tendon or middle glenohumeral ligament (Fig. 9.37).³⁷^,⁶¹ The superior glenohumeral ligament can be identified in 98% of patients on MR arthrograms.³⁷ Variations in thickness of the superior and middle glenohumeral ligaments are common. The superior glenohumeral ligament is usually thinner than the middle
glenohumeral ligament. When the superior ligament is thicker, the middle glenohumeral ligament may be absent or thinner.¹²^,³⁷^,¹⁰⁰ The superior glenohumeral ligament may be absent in up to 10% of patients.³⁷

Figure 9.36 (continued)

The middle glenohumeral ligament is the most variable. It may be thick or thin and is absent in up to 30% of specimens.³⁷ The middle glenohumeral ligament originates at the anterosuperior labrum (Fig. 9.37), where it may blend with the superior and/or inferior glenohumeral ligament. The ligament blends with the capsule and inserts on the humerus at the base of the greater tuberosity.¹²^,³⁷^,⁶¹ The middle glenohumeral ligament may be thick and associated with absence of the anterior superior labrum (Fig. 9.38). This anatomic configuration is called the Buford complex.¹²^,³⁷^,⁸¹^,¹⁰⁰^,¹⁰⁶ This anatomic variation is seen in about 1.5% of the general population.⁸¹ The Buford complex differs from the sublabral foramen, which consists of a normal anterosuperior labral detachment (Fig. 9.39). The sublabral foramen is present in 8% to 12% of individuals.¹²^,⁸¹^,¹⁰⁶ The middle glenohumeral ligament is not thickened and isolated labral tears in this region are rare.³⁷

The inferior glenohumeral ligament has anterior and posterior bands with an interposed axillary recess. The anterior and posterior bands arise from the inferior labrum anteriorly and posteriorly and extend to the respective regions on the surgical neck of the humerus¹²^,⁶¹ The anterior band is typically thicker than the posterior bands (Fig. 9.37A).³⁷

Figure 9.37 A: Illustration of glenohumeral ligaments, labrum, and capsule seen in the sagittal plane. B: Sagittal MR arthrogram demonstrating the middle glenohumeral ligament (^*), the anterior band of the inferior glenohumeral ligament (white arrow), and the posterior band of the inferior glenohumeral ligament (black arrow).

Duplication of the long head of the biceps tendon may be seen in up to 10% of individuals. This results from a third or fourth head of the biceps brachii muscle.⁶¹^,⁸¹ This creates the appearance of a longitudinal tendon split.⁸¹

Fluid Collections

There is usually a small amount of fluid in the glenohumeral joint in normal subjects. Recht et al.¹⁰⁴ reported only about 2 mL of fluid in normal shoulders. Schweitzer et al.¹⁰⁷ reported that sufficient fluid to be identified as more than a fine line in the joint or bursa should be considered an effusion and is usually associated with degenerative disease or rotator cuff tears.

Figure 9.38 A: Illustration of the Buford complex with absent anterior superior labrum and thickening of the middle glenohumeral ligament. B: Axial MR image demonstrating an absent anterior labrum (white arrow) and thickening of the middle glenohumeral ligament (black arrow).

There are numerous bursae about the shoulder (Table 9.3). Most do not communicate with the joint. Therefore, fluid in these bursae is usually abnormal and indicates
inflammation or communication with the joint via a rotator cuff tear or capsular defect (Figs. 9.19 and 9.40A).¹²^,³⁶^,⁶¹

Figure 9.39 A: Illustration of the anterior labral foramen. B: MR arthrogram demonstrating a sublabral foramen with a small focal area of the anterior labrum (open arrow) that is not adherent to the underlying anterior superior glenoid (white arrow).

Fluid may also be demonstrated in the biceps tendon sheath. Kaplan et al.⁸⁶ reported a small amount of fluid in the tendon sheath in 14 of 15 normal subjects. In addition, a rounded fluid density was noted in 19 of 30 cases. This proved to be the anterolateral branch of the anterior circumflex artery and not fluid.

Figure 9.40 A: Coronal T2-weighted image demonstrating bursitis with distention of the subacromial-subdeltoid bursa (arrows). The rotator cuff is intact. B: Axial T2-weighted image demonstrating fluid completely surrounding the biceps tendon with distention of the tendon sheath (arrows).

Fluid in the biceps tendon sheath should not be considered abnormal unless the tendon is completely surrounded by fluid (Fig. 9.40B).⁸⁷

Fluid in the subcoracoid bursa is commonly seen, as this bursa communicates with the joint in 20% of cases.¹²^,⁶¹^,⁸¹ The subscapularis bursa is seen in 90% of cases (Table 9.3).¹² Fluid may be evident in the subacromial subdeltoid bursae if inflammation is present or following therapeutic
injections. Fluid typically remains up to 48 hours after an injection.⁸¹^,¹⁰⁸

Figure 9.41 Coronal proton density image demonstrating yellow marrow in the epiphysis with mixed red and yellow marrow (predominantly red) in the metaphysis in a 19-year-old male. The marrow pattern in the clavicle is also mixed.

Marrow/Osseous Variants

Variations in red and yellow marrow and other anatomic osseous variants may also create confusion on MR images. There is considerable variation in the amount of red and yellow marrow in the metaphysis (Fig. 9.41). With age, there is generally conversion of red to yellow marrow. Richardson andPatten¹⁰⁹ evaluated marrow patterns on patients 15 to 69 years of age. Each of five regions (glenoid, acromion, epiphysis, metaphysis, and diaphysis) was graded (Fig. 9.42) from 1 to 7, with 1 representing 100% red marrow, 2 to 6 indicating reducing percentages of red marrow, and 7 representing 0% red marrow (all yellow marrow). Marrow was graded as homogeneous (red), geographic, mottled, or all yellow (Fig. 9.43).¹⁰⁹ Marrow patterns changed with age in all five regions in a fairly predictable fashion. However, red marrow can persist beyond the previously reported 15-year age range. For example, subchondral red marrow can be seen in 88% of patients in their 20s and up to 23% of patients in the seventh decade of life. Therefore, signal inhomogeneity in the shoulder may be due to red-yellow marrow variations. Normal marrow patterns can mimic marrow abnormalities. However, errors can be avoided by comparison with the contralateral shoulder (marrow patterns are usually symmetrical). Also, marrow variations are not associated with cortical bone destruction or soft tissue changes.¹⁰⁹^,¹¹⁰^,¹¹¹^,¹¹²

Figure 9.42 Grading system for conversion from red (100% left) to yellow marrow with no residual red marrow in grade 7. (From Richardson ML, Patten RM. Age-related changes in marrow distribution in the shoulder: MR image findings. Radiology. 1994;192:209-215.)

There is a groove in the posterior lateral humeral head which can be confused with a Hill-Sachs lesion. This normal finding is usually located more distally than a Hill-Sachs lesion (Fig. 9.44).¹¹⁰^,¹¹² In our experience, most Hill-Sachs lesions are much larger (Fig. 9.44B).

Another osseous variant that may be related to clinical symptoms is the os acromiale. This is due to a developmental abnormality of ossification involving the anterior acromion.⁸¹^,¹¹³^,¹¹⁴^,¹¹⁵ This is usually evident on all MR image planes but is typically most obvious on axial images (Fig. 9.45). The deltoid may displace the os acromiale resulting in impingement.¹²^,⁸¹^,¹¹⁶

Morgan et al.⁸² described pseudotumor deltoideus, an anatomic variant at the deltoid insertion on the humerus. This variant is a spectrum of changes at the deltoid insertion that may present as a lucent area or with prominence or irregularity in the corticoperiosteal region on radiographs. Bone scans may demonstrate a focal area of increased tracer. There is mixed increased and decreased signal intensity on MR images (Fig. 9.46).⁸²

Miscellaneous Variants

Interpreters of MR images must also become familiar with other artifacts or variants created by technique, positioning, anatomy, and equipment problems. In the shoulder, the
rotator cuff, labrum, fluid changes, and marrow changes are most frequently discussed in the literature as noted above. Other variants, such as tendon attachments (Fig. 9.35A), can be confused with osteophytes.⁸⁶ Confusion can be avoided by comparing MR findings with radiographic features. Other findings such as soft tissue calcifications (Fig. 9.47) provide additional value for radiographic comparison when interpreting MR images.

Figure 9.43 Illustration of marrow pattern categories progressing from homogeneous red to homogeneous yellow. (From Richardson ML, Patten RM. Age-related changes in marrow distribution in the shoulder: MR image findings. Radiology. 1994;192:209-215.)

Figure 9.44 Axial SE 450/15 image of the proximal humerus (A) demonstrating the normal posterolateral groove (arrow). Axial (B) and coronal (C) proton density images demonstrating a Hill-Sachs lesion (arrow) and a rotator cuff tear (open arrow). The levels of the usual Hill-Sachs (HS) lesion and normal groove (G) are indicated with transverse lines to show where they would be seen on axial images.

Figure 9.45 Axial proton density image shows an os acromiale (arrowheads).

Vacuum phenomenon can be noted in the shoulder as well as the knee and other joints. In the shoulder, this finding is particularly common when the arm is positioned in external rotation.¹¹⁷ The MR finding appears to occur most frequently on gradient-echo sequences (Fig. 9.48). Patten¹¹⁷ reported the vacuum phenomenon in up to 20% of cases. Awareness of this artifact is important to avoid misinterpretation of intra-articular air as loose bodies or chondrocalcinosis.¹¹⁷

APPLICATIONS

Routine radiography is an important initial examination for screening patients with shoulder, arm, and suspected brachial plexus lesions.⁴^,¹¹⁸^,¹¹⁹^,¹²⁰ Other techniques, including radionuclide studies, CT, and arthrography, also have an important role in evaluating these patients.⁴^,²¹^,⁵¹^,¹²¹^,¹²² In recent years, there has been increased utilization of ultrasound for screening patients with suspected rotator cuff tears and other abnormalities in the shoulder region.¹²²^,¹²³ Applications for conventional MRI and MR arthrography have expanded significantly since the prior edition. Both 1.5 and 3.0 T systems are commonly used clinically.¹²⁴^,¹²⁵ Because of the clinical presentations and anatomic and examination differences, we will discuss the shoulder and arm separately from brachial plexus examinations.

SHOULDER AND ARM

The majority of patients who are referred for MRI of the shoulder or arm present with pain or restricted range of motion in the shoulder. Most commonly, patients are referred for MRI because of suspected rotator cuff tears or impingement, defects in the glenoid-labrum, or instability, osteonecrosis, biceps tendon abnormalities, other soft tissue or neural injuries, suspected soft tissue or bony neoplasms, and infectious or inflammatory disorders.¹²^,⁶⁴^,¹²⁶^,¹²⁷

Osseous Trauma

Routine radiographs or computed radiography images are usually sufficient for identifying subtle and/or complete fractures and other osseous injuries in the shoulder and humerus. Occasionally, CT is needed to clearly identify the position of the fracture fragments.⁴ MRI also plays a significant role in evaluating isolated osseous trauma. Subtle fractures can be easily identified with MRI. Also, any associated soft tissue injuries are easily detected (Fig. 9.49).⁸⁹^,¹²⁸^,¹²⁹^,¹³⁰^,¹³¹ Osseous injuries are frequently associated with soft tissue injury and/or instability. Hill-Sachs lesions (Fig. 9.50) and other fractures or osteochondral injuries may also provide valuable clues to additional soft tissue pathology and shoulder instability (Fig. 9.51).¹³^,⁸⁹^,¹²⁹^,¹³⁰^,¹³² Both Hill-Sachs and Bankart lesions are commonly associated with anterior dislocations. However, tuberosity fractures may also be evident in 15% of patients with anterior dislocations (Fig. 9.49). Fractures of the greater tuberosity may also follow forced abduction or direct trauma. Symptoms may mimic a rotator cuff tear.¹³³ This injury is often subtle. However, when displaced, greater tuberosity fractures may result in biceps tendon entrapment.⁴^,¹³⁴ Avulsion of the lesser tuberosity along with the capsule (Fig. 9.51) or subscapularis tears are also noted with anterior dislocations.¹⁶

Injuries to the distal clavicle and AC joint are common.¹¹⁴^,¹³⁵^,¹³⁶^,¹³⁷^,¹³⁸^,¹³⁹^,¹⁴⁰ Acromioclavicular dislocations account for 10% of shoulder injuries.¹³⁵ Routine radiographs or stress views are usually adequate for diagnosis and classification.¹⁴¹^,¹⁴²

The classification system for AC joint injuries was developed by Buckholtz and Hickman.¹⁴³ This classification is based upon mechanism of injury and extent of ligament and osseous injury. MR features may provide additional information regarding both soft tissue and osseous injuries.¹³⁵ However, patient age is important since articular and soft tissue degenerative changes are common in older adults and may not be related to clinical symptoms.¹¹⁶^,¹³⁵ Table 9.4 summarizes AC joint injuries and MR features. Keep in mind that MR features are most useful in children and young adults that have not yet developed degenerative changes.

Type I injury is a sprain of the AC ligaments.¹⁴³^,¹⁴⁴ Radiographs are normal. MR images using T2-weighted or STIR sequences may demonstrate increased signal intensity in the periarticular tissues and early marrow edema.¹³ Type II injuries result in disruption of the AC ligaments and
coracoclavicular sprain. Stress radiographs demonstrate subluxation of the joint. MR images demonstrate increased signal in the AC ligaments and marrow edema in the acromion and clavicle. There may also be increased signal intensity in the coracoclavicular ligament.¹³⁵ Axial and sagittal image planes are most useful. Type III injuries result in disruption of the AC and coracoclavicular ligaments. The joint dislocates radiographically. MR images demonstrate increased signal intensity in both ligament complexes (Table 9.4) and widening with varying degrees of joint subluxation or dislocation.¹³⁵^,¹⁴³ The deltoid and trapezius muscles may be detached from the distal clavicle.¹³⁵ All three image planes maybe required toassess the soft tissue and articular changes completely. A fracture at the base of coracoid has also been reported instead of coracoclavicular ligament disruption.¹⁴³ Fractures of the coracoid base are most obvious on sagittal images. Type IV dislocations result in posterior displacement of the clavicle which is most easily appreciated on axial
images.¹³⁵ Sternoclavicular dislocations may also occurwith this injury. Therefore, both medial and lateral ends of the clavicle should be imaged.¹³⁵^,¹⁴³ Type IV dislocations are similar to type III except the trapezius and deltoid muscles are stripped from the ends of the clavicle and acromion. The clavicle is significantly elevated by cephalad traction of the sternocleidomastoid muscle. This feature is best appreciated on coronal images. Type VI dislocations result from severe force from superior to the clavicle with the humerus abducted.¹⁴³ Type IV injuries may be most easily identified on coronal or sagittal images.

Figure 9.46 Pseudotumor deltoideus. A: Coronal T1-weighted image of the right humerus shows an area of fat signal intensity surrounded by low signal intensity (arrow) at the deltoid insertion. Inversion recovery coronal (B) and axial (C) images show low to intermediate signal intensity surrounded by high signal intensity (arrow). (From Morgan H, Damron T, Cohen H, et al. Pseudotumor deltoideus. A previously undescribed variant at the deltoid insertion site. Skeletal Radiol. 2001;30:512-518.)

Figure 9.47 Anteroposterior radiograph demonstrates dense calcification in the supraspinatus tendon.

Figure 9.48 Axial gradient-echo image of the shoulder with linear and globular areas of intra-articular decreased signal intensity (arrows) due to vacuum phenomenon. Joint fluid would be high signal intensity. Loose bodies tend to be located in recesses or areas of articular abnormality.

Figure 9.49 Patient with posttraumatic shoulder pain. Routine radiographs were normal. Axial (A) and coronal (B) T1-weighted and axial (C) and coronal (D) fast spin-echo T2-weighted images demonstrate a fracture of the greater tuberosity with fluid between the fragments on the T2-weighted images C and D.

Figure 9.50 Coronal proton density-weighted image demonstrating a Hill-Sachs lesion (arrow) not evident on routine radiographs.

Figure 9.51 Lesser tuberosity avulsion and tears in the capsule and middle glenohumeral ligament. Axial proton density (A) and gradient-echo (B) images show the displaced lesser tuberosity (arrow) and ligament tear (open arrow).

Posttraumatic osteolysis is another condition that may mimic fracture or rotator cuff tears clinically.⁴^,¹⁴⁵ This condition typically follows acute trauma to the AC joint usually due to falling on the point of the shoulder. Osteolysis may also follow repeated microtrauma in laborers, throwing athletes, and individuals involved in weight training.¹³^,¹³⁷^,¹⁴⁶^,¹⁴⁷ Patients present with pain and tenderness over the AC joint and weakness on arm abduction.⁴^,¹⁴⁶ Radiographs demonstrate subtle resorption of the distal clavicle. Recently, Kassarjian et al.¹⁴⁵ demonstrated that subchondral fractures occur commonly in patients with posttraumatic osteolysis. They reviewed 36 patients with posttraumatic osteolysis and found a subchondral line in 31 (86%) suggesting a subchondral fracture. In addition, 89% of patients had fluid in the AC joint and 75% associated cysts or erosions in the distal clavicle.¹⁴⁵

MR images in the coronal, sagittal, and axial planes will demonstrate irregularity of the clavicle with increased signal intensity on T2- and decreased signal on T1-weighted images. Theacromion is relatively spared. There may be fluid in the joint and increased signal intensity in the surrounding soft tissues (Fig. 9.52).¹⁴⁶ Conservative therapy may result in complete recovery with image features returning to normal. Residual chronic changes may remain (Fig. 9.53). If left untreated, extensive joint hypertrophy and instability of the joint may develop (Fig. 9.54). Patients that do not respond to conservative therapy may be treated by resection of the distal clavicle.

Interpretation of AC abnormalities is more complex in older adults. The presence of osteophytes, capsular hypertrophy, and joint space changes, including effusion, may not correlate with clinical symptoms. Jordan et al.¹¹⁶ found some correlation with high signal intensity in the distal clavicle and a trend toward the presence of an effusion and a
positive clinical examination. Other changes did not correlate with clinical findings.¹¹⁶ Schweitzer et al.¹⁴⁸ reported joint effusions in 66% of patients with grade 2 or 3 impingement compared with 12% in normal volunteers.

Table 9.4 Acromioclavicular Joint Injuries

Classification	Description	MR Features (T2 or STIR)
Type I	Acromioclavicular ligament sprain	↑ Signal intensity in periarticular soft tissues
Type II	Acromioclavicular ligament tear with coracoclavicular sprain	Marrow edema, ↑ signal intensity in AC ligaments. Type I coracoclavicular
Type III	Acromioclavicular dislocation, disruption coracoclavicular ligament	↑ Signal intensity both ligaments, joint widening and/or displacement
Type IV	Same as type III, but posterior clavicular dislocation	Same as type III plus posterior clavicle position
Type V	Same as type III, with deltoid and trapezius muscles stripped from the acromion and clavicle	Same as type III with soft tissue changes
Type VI	Inferior clavicle dislocation	Same as type III with clavicle inferior to acromion or coracoid
From references 4, 114, 136, 144.

Figure 9.52 Posttraumatic osteolysis. A: Axial T1-weighted image of the normal acromioclavicular joint. The bone margins are sharp and there is no joint effusion or soft tissue edema. Proton density coronal (B) and T2-weighted sagittal (C) images demonstrate irregularity and abnormal signal in the distal clavicle (arrow) with sparing of the acromion.

Rotator Cuff Tears

One of the most common indications for MRI of shoulder and upper arm trauma is evaluation of soft tissue injuries. Most of these injuries are chronic in nature.

Figure 9.53 Axial T1-weighted image shows residual erosive changes (arrow) in the distal clavicle after traumatic osteolysis.

There are numerous causes of shoulder pain. Most are related to rotator cuff pathology or instability; however, other lesions may also be evaluated with MRI.¹⁴⁶^,¹⁴⁹ One of the most common indications for MRI is evaluation of patients with suspected rotator cuff tear or impingement syndrome.⁴^,¹²^,¹⁵⁰^,¹⁵¹^,¹⁵²^,¹⁵³^,¹⁵⁴^,¹⁵⁵^,¹⁵⁶^,¹⁵⁷^,¹⁵⁸^,¹⁵⁹^,¹⁶⁰^,¹⁶¹^,¹⁶²

The rotator cuff is comprised of the supraspinatus, infraspinatus, teres minor, and suprascapularis tendons. The supraspinatus, infraspinatus, and teres minor insert on the superior, middle, and inferior facets of the greater tuberosity. The subscapularis is the largest and most powerful muscle of the rotator cuff and inserts on the lesser tuberosity, with fibers extending over the bicipital groove to the greater tuberosity and fibers of the inferior third extending to the humeral metaphysis directly inferior to the lesser tuberosity.¹⁵³ This forms the transverse ligament.¹²^,⁶¹ The space between the supraspinatus and subscapularis is the rotator cuff interval.¹⁵⁶^,¹⁶⁰ The coracoid projects into this space medially. Within this space are the medial and lateral bundles of the coracohumeral ligament. Deep to the ligament, the interval contains the superior glenohumeral ligament.¹²^,⁵⁴

Figure 9.54 Chronic untreated traumatic osteolysis with marked joint hypertrophy on this sagittal T1-weighted image.

Table 9.5 Etiologies of Rotator Cuff Tears

Primary extrinsic impingement

Acromial configuration

Lateral/downsloping acromion

Acromioclavicular joint osteophytes or hypertrophy

Acromial osteophytes

Low-lying acromion

Os acromiale

Thickened coracoclavicular ligament

Secondary extrinsic impingement

Instability in throwing athletes

Posterosubglenoid impingement

Throwing athletes

Subcoracoid impingement

Primary cuff degeneration

Ischemia

Trauma

From references 12, 13, 164, 165.

The rotator cuff provides 33% to 50% of the muscle effort required for abduction, and 80% to 90% for external rotation.¹⁶²^,¹⁶³ Multiple etiologies have been implicated in development of rotator cuff tears (Table 9.5). Zlatkin¹² categorized etiologies of rotator cuff tears as extrinsic (impingement, impingement with instability, subcoracoid
impingement, etc.) and primary cuff degeneration, which may be ischemic.

Figure 9.55 Illustration demonstrating the coracoacromial arch and supporting structures of the shoulder. Note the coracoacromial ligament (arrow).

Primary extrinsic impingement occurs when the rotator cuff is entrapped beneath the subacromial arch. The arch is composed of the clavicle, anterior acromion, AC arch, anterior or distal coracoid, and the coracoacromial ligament (Fig. 9.55).¹⁵⁴^,¹⁵⁷^,¹⁶⁴^,¹⁶⁵^,¹⁶⁶^,¹⁶⁷^,¹⁶⁸^,¹⁶⁹^,¹⁷⁰^,¹⁷¹ This includes acromial abnormalities, AC joint osteophytes, and thickening of the coracoacromial ligament. This mechanism for pathophysiology of rotator cuff tears has been most popular over the years.

Neer proposed that the majority (95%) of rotator cuff tears were the result of chronic impingement of the supraspinatus tendon against the acromial arch.²^,¹²^,¹⁶⁴^,¹⁷² Impingement can occur due to abnormalities or variants in the coracoacromial arch. Abnormalities in the osseous, ligamentous, or soft tissues within the arch may cause impingement. The os acromiale is due to failure to fuse of one of three acromial ossification centers.¹⁷³^,¹⁷⁴^,¹⁷⁵ The os acromiale may be one or more infused segments. The incidence has been reported to be among 1% to 15% of the general population. Sammarco¹⁷⁵ evaluated 2,367 cadaver specimens and reported the incidence of os acromiale to be 0.8%. The variant was bilateral in 33%. Others have reported bilateral involvement in up to 60%.¹⁷⁴ Os acromiale are more common in males than females and in African-Americans than whites.¹⁷⁵ This variant, along with abnormalities in the osseous and soft tissue structures of the arch, can result in impingement of the rotator cuff (Fig. 9.56).¹²^,¹³^,¹¹⁵^,¹⁶⁹^,¹⁷⁶^,¹⁷⁷ Acromial configuration
has also been implicated in impingement (Fig. 9.57) and rotator cuff tears.¹¹⁵^,¹⁶⁸^,¹⁷⁸^,¹⁷⁹ A type 1 acromial configuration is straight; type 2, curved; type 3, hooked; and type 4 has a convex inferior surface (Fig. 9.57).²¹^,³¹ Anterior or lateral angulation of the acromion may also cause impingement (Fig. 9.58).¹⁷⁸^,¹⁸⁶ Type 1 acromions occur in 18% to 23%; type 2 in 42% to 68%; and type 3 10% to 39% of patients. Type 4 acromia occur in 7% of the general population.¹²^,¹⁶^,¹⁷⁹ Type 3 or hooked acromia are more common in males.¹⁸⁰ Zlatkin and Falchook reported rotator cuff tears in 51% of patients with type 3 acromia, os acromiale, or anterior inferior acromial bone spurs (Fig. 9.59). More recent studies noted type 3 acromia in 70% to 80% of rotator cuff tears. Only 3% of patients with rotator cuff tears had type 1 acromia.¹⁸¹

Figure 9.56 Impingement of the rotator cuff. A: Sagittal T1-weighted image of a normal acromioclavicular joint region. There is fat between the bone and the upper surface (arrows) of the supraspinatus is convex. B: Sagittal T1-weighted image demonstrating degenerative change in the acromioclavicular joint causing mild impingement with a concave upper surface (arrowhead) on the supraspinatus muscle. There is still fat between the muscle and joint. C: Sagittal T1-weighted image demonstrating moderate to marked impingement due to AC joint hypertrophy (arrows). D: Axial proton density-weighted image demonstrating acromioclavicular joint degeneration (arrows) and an os acromiale (open arrows).

Figure 9.57 Illustrations of acromial shapes and orientations as seen on oblique sagittal MR images. A: Flat. B: Curved. C: Hooked. D: Convex inferior surface. E: Sagittal T1-weighted image of a curved (type 2) acromion. F: Sagittal T1-weighted image of a curved, anteriorly angled acromion compressing the supraspinatus.

Consistent categorization of the acromial shape (Figs. 9.57

Only gold members can continue reading. Log In or Register to continue