The medial portion of the humeral head overlaps with the lateral aspect of the glenoid on AP shoulder radiographs, since the glenohumeral joint is anatomically 35–40° oblique to the coronal plane of the patient (Fig. 14.1). In some cases, the humeral head may project slightly lower or slightly higher than the center of the glenoid. Because the humerus is anterior to the glenoid, if the patient is tilted back when the image is taken, the humeral head may appear high relative to the glenoid, whereas if the patient is tilted forward, it may appear slightly low. The distance between the humeral head and acromion should be evaluated. If the humeral head is superiorly subluxed such that the acromiohumeral distance is less than 7 mm, a rotator cuff tear should be suspected. Because the Grashey view is a “true AP” projection of the glenohumeral joint, there should not be any overlap of the humeral head and glenoid on this view. Overlap of these structures on the Grashey view implies subluxation or dislocation of the humeral head. Finally, when reviewing shoulder radiographs, clinicians must also assess the clavicle, scapula, and ribs for fractures and other lesions, as well as the visualized portions of the lungs for any potential pathologies.

Plain radiography is used to diagnose many common shoulder pathologies, including fractures of the humerus, clavicle, and scapula. Proximal humerus fractures are the third most common type of fragility fracture, accounting for nearly 6% of all adult fractures [6, 7]. As the median age of the world’s population increases, the incidence of this fracture type has also risen [8]. These fractures and fractures of the mid-humerus present few challenges in radiographic interpretation and thus do not usually require further examinations. These fractures present as a lucency and cortical disruption with variable degrees of angulation, impaction, and displacement on plain radiographs (Fig. 14.5). Determining the degrees of angulation and rotation of the fragments may require full-length images of the humerus that include the shoulder and elbow.

Most clavicular fractures are clinically apparent and occur in the midportion or the distal third of the clavicle. In addition, acromioclavicular (AC) joint separation, which is a common traumatic or sports injury, is easily assessed with radiography. The normal AC joint space usually measures <5 mm, and normal coracoclavicular distance is <11–13 mm. Widening of any of these spaces must be considered as a potential separation. AC joint separation is classified into six subgroups based primarily on the distal clavicular angle and degree of the displacement [9]. Some recommend obtaining additional radiographs while hanging weights from the patient’s wrists and comparing these images with images of the unaffected side to detect nondisplaced AC joint injuries.

Fractures of the scapula are relatively rare, although they can occur as the result of a severe, direct blow [10]. Because the scapula is a thin bone, fractures of the body of the scapula may be difficult to appreciate. The Velpeau and West Point variants of the axillary view may be useful for evaluation of the scapular spine and acromion, especially for patients with reverse shoulder arthroplasties who are at risk for fracture (Fig. 14.6). When there is any uncertainly regarding the presence or type of fracture on radiography, a computed tomography (CT) scan may be useful.

Shoulder dislocations are readily diagnosed by radiography. Anterior dislocation of the humeral head accounts for more than 95% of shoulder dislocations. On the AP projection, the displaced humeral head will be inferiorly and medially displaced, overlapping with the glenoid neck and lying inferior to the coracoid (Fig. 14.7). Impaction of the humeral head on the anterior-inferior edge of the glenoid produces a deformity in the posterolateral portion of the humeral head, the Hill-Sachs deformity, which is best seen on the AP view with the arm internally rotated after reduction of the dislocation. There is often an injury of the anterior inferior glenoid rim, as well; this injury, known as a Bankart lesion, may involve the labrum only or both the labrum and the underlying bone. When there is a bony component, the West Point or axillary view may be diagnostic. When only the soft tissue of the glenoid labrum is involved, magnetic resonance (MR) or CT arthrography will be needed for imaging diagnosis.

Posterior shoulder dislocations are uncommon and more difficult than anterior dislocations to diagnose on a standard AP view of the shoulder. On normal shoulder radiographs using the AP view, there is an overlap of the humeral head and the glenoid with a relatively narrow anterior glenohumeral joint space visible. Radiographs following posterior shoulder dislocation show widening of the glenohumeral joint space; additionally, the humeral head may not appear round because of extreme internal rotation. On the Grashey view, there will be an abnormal overlap of the humeral head and glenoid. The axillary and Y views will clearly show the posterior dislocation (Fig. 14.8).

Initial evaluation of shoulder arthritis is frequently performed with radiography. Degenerative or post-traumatic osteoarthritis in the shoulder, as with other joints, is frequently associated with osteophyte formation, subarticular sclerosis, subarticular cysts, and joint space narrowing. As the arthritis progresses, there can be loss of the bone stock of the glenoid with alteration of the version of the glenoid face. When planning for shoulder arthroplasty, evaluation of the glenoid version is critical for proper placement of the glenoid component; CT is often performed for this purpose.

With septic arthritis of the shoulder, radiographs are typically normal in the early stages, although soft tissue swelling or inferior displacement of the humeral head due to effusion may be seen. With more chronic septic arthritis, radiographs may show decreased bone density, joint space narrowing, and bony destruction. When a septic joint is clinically suspected, joint aspiration should be considered.

Although radiography is not primarily performed for this purpose, radiographic images may be abnormal in the setting of rotator cuff disease. Calcific tendinosis of the rotator cuff (i.e., the deposition of calcific crystals such as hydroxyapatite within an abnormal tendon) can be readily diagnosed on plain radiographs. Typically, this condition presents as amorphous white densities at the greater tuberosity at the insertion of the affected tendon (Fig. 14.9). With large, retracted tears of rotator cuff tendons, the humeral head may migrate superiorly with resultant decentering of the humeral head on the glenoid, thus narrowing the distance between the humeral head and acromion (i.e., the acromiohumeral distance). With time, secondary glenohumeral osteoarthritis, also known as rotator cuff arthropathy, may develop (Fig. 14.10); this condition is suggestive of an irreparable rotator cuff [11].

Bone or soft tissue neoplasms of the shoulder may be initially evaluated or incidentally found on radiography. The proximal humerus is the third most common site for primary bone tumors and soft tissue tumors, with an incidence of approximately 1.8 in 100,000 [12–16]; it is also one of the most common sites of osteosarcoma in children [17]. As with other bone tumor sites, the degree of bone destruction and fracture risk in the shoulder can be estimated with radiographs. However, advanced imaging techniques should be used for further evaluation of potential bone destruction and for identification of soft tissue masses. When an incidental finding of a bone lesion (usually an enchondroma) is observed on radiographs, the images should be compared with results from previous imaging studies to determine the biological nature of the abnormality. When a benign lesion is suspected, follow-up radiography is indicated. If an aggressive lesion is suspected on radiographs, MR imaging should be considered.

Although this chapter focuses primarily on preoperative shoulder imaging, there are some important postoperative complications that can be readily evaluated on radiography. Plain radiography is routinely used after shoulder arthroplasty to evaluate implant positioning and baseline appearance for help with future assessment, should symptoms arise. Loosening of an arthroplasty component appears as progressively widening radiolucencies at the bone-implant or cement-bone interface, although plain radiography can sometimes underestimate radiolucent lines [18]. In such cases, CT offers improved sensitivity, especially when metal artifact reduction techniques are implemented. With an infected implant, periosteal reaction may be seen. Scapular notching after reverse total shoulder arthroplasty (i.e., erosion of the scapular neck from impaction of the humeral component) usually occurs within the first few months after surgery. The incidence of scapular notching ranges from 44 to 96% [19, 20], and this condition ranges from grade 1 to 4 in severity (with grade 4 potentially leading to glenosphere loosening) based on radiographic findings. The occurrence of scapular notching may require revision surgery. Therefore, radiographs demonstrating bone loss at the inferior scapular neck should be carefully assessed in patients who have undergone reverse total shoulder arthroplasty (Fig. 14.11). Heterotopic ossification in the triceps origin is common following reverse shoulder arthroplasty (Fig. 14.12). Heterotopic ossification usually does not progress after the initial postoperative period. It usually has no effect on functional movement of the shoulder and usually does not require treatment. This new bone can mimic scapular notching. However, notching will show loss of glenoid bone, whereas heterotopic ossification is added bone; in addition, notching and heterotopic ossification may coexist [21].

14.3 CT

CT is commonly used in orthopedic imaging to assess cortical bone, trabecular bone, and joint surfaces in patients with fractures, arthritis, shoulder instability, advanced rotator cuff disease, tumors, or infection; however, soft tissue abnormalities are less well visualized by CT than by MR imaging. Because CT is most often obtained with isotropic voxels, 2D multiplanar and 3D reformatted images can be readily created (Fig. 14.13). CT arthrography, which is obtained by injecting iodinated contrast into the shoulder joint before CT imaging is performed, can provide additional information about the articular cartilage, labrum, and rotator cuff. CT arthrography is often used as an alternative for patients who are unable to undergo shoulder MR imaging. CT offers greater spatial resolution than MR imaging, whereas MR imaging offers higher image contrast for soft tissue abnormalities and can demonstrate edema-like signal in the bone marrow [22].

CT can be more effective than radiography in showing the spatial relationship of fracture fragments in complex fractures of the humerus and scapula [23] (Fig. 14.14). Often, radiography is limited in these cases by patient positioning and superimposition of the fracture fragments. Preoperative planning with CT before fracture reduction or in cases of unreducible or recurrent dislocation may be useful. One study of patients with shoulder instability found that preoperative identification and measurement of bony Bankart fragments of the glenoid and Hill-Sachs impaction of the humeral head can be difficult with radiography, leading to challenges in surgical decision-making [24]. Therefore, CT should be considered in the treatment algorithm for accurate quantification of bone loss to prevent a high rate of recurrent shoulder instability. As previously discussed, for patients with severe glenohumeral osteoarthritis, preoperative measurement of the glenoid version with CT is helpful for shoulder arthroplasty planning.