A shoulder X-ray is a foundational diagnostic tool that uses small doses of radiation to create images of the dense structures within the joint. It is routinely employed to assess the shoulder girdle following acute trauma, such as a fall or sports injury. The primary purpose is to evaluate the integrity and alignment of the bones and check the joint spaces for immediate damage or degenerative change. It serves as a rapid, accessible method for diagnosing conditions like fractures, dislocations, and arthritis.
Foundational Knowledge: Standard Views and Key Anatomy
Interpreting a shoulder X-ray requires understanding the anatomy and the specific angles used for imaging. The shoulder girdle is composed of three bones: the humerus (upper arm bone), the scapula (shoulder blade), and the clavicle (collarbone). The glenohumeral joint is where the rounded head of the humerus articulates with the shallow socket of the scapula, called the glenoid.
Multiple views are necessary because the shoulder is a three-dimensional, mobile joint with overlapping structures, and a single view can hide pathology. The standard series usually includes the Anteroposterior (AP) view, which provides a frontal image displaying the alignment of the clavicle, scapula, and humerus. The dedicated Scapular Y-view is a lateral projection that profiles the scapula, confirming the direction and presence of a shoulder dislocation.
The Scapular Y-view is named for the Y-shape formed by the scapula’s body, coracoid process, and acromion. In a normal X-ray, the humeral head should be centered in the intersection of this Y-shape. An Axillary view may also be requested, as it visualizes the joint surfaces and confirms a dislocation, particularly when pain prevents positioning for other views.
The Systematic Approach to Reading the X-Ray
A systematic approach is employed to ensure no subtle findings are missed, moving through the image in a consistent pattern. A common method follows the A-B-C-S sequence: Alignment, Bone, Cartilage/Joint Space, and Soft Tissue. This review begins by assessing the alignment of the joint components.
Alignment involves confirming that the humeral head remains centered within the glenoid fossa on the AP view and positioned within the Y on the Scapular Y-view. It is also important to check the alignment of the acromioclavicular (AC) joint, where the clavicle meets the acromion. Ensuring the inferior borders of these two bones line up correctly is necessary, as disruption suggests potential instability or dislocation.
The next step, Bone, requires tracing the smooth, uninterrupted cortical margin, the outer layer of each bone. Any disruption in this contour, such as a break, buckle, or step-off, indicates a fracture line. Bone texture and density are also checked for areas of increased whiteness (sclerosis) or decreased whiteness (osteopenia), which can indicate an old injury or chronic disease.
The Cartilage component focuses on the joint space, which appears as a gap between the bones. Normal joint space should have parallel articular surfaces and a consistent width. Narrowing often suggests cartilage loss from arthritis, while abnormal widening can indicate a joint effusion or a subtle dislocation. Finally, Soft Tissue is examined for changes in the surrounding non-bony structures.
Soft tissue assessment looks for generalized swelling, which appears as increased density or bulging around the joint capsule. The presence of calcification, appearing as small, dense white deposits outside the bone, may suggest conditions like calcific tendinitis. It is also standard practice to check surrounding included structures, such as the upper ribs and lung fields. Associated injuries like a rib fracture or lung collapse (pneumothorax) can occur with trauma.
Recognizing Major Shoulder Injuries
The systematic review identifies common abnormal findings, with shoulder dislocations being one of the immediate concerns. An anterior shoulder dislocation is the most frequent type, representing about 95% of all glenohumeral dislocations. On the AP view, the humeral head is visibly displaced, typically resting below and medial to the glenoid, sometimes appearing under the coracoid process.
In contrast, a posterior shoulder dislocation is often more subtle and can be missed on a standard AP view. A tell-tale sign on the AP film is the “light bulb sign,” where the humeral head appears symmetrically rounded due to fixed internal rotation. The Scapular Y-view is useful here, as it clearly shows the humeral head displaced posteriorly to the glenoid, confirming the diagnosis.
Fractures of the proximal humerus are common, especially in older individuals with osteoporotic bone following a low-energy fall. These breaks most frequently occur at the surgical neck, the narrowed region just below the humeral head. Radiographic evaluation focuses on the degree of displacement, noting if the fracture fragments are separated by more than one centimeter or angulated by more than 45 degrees.
Clavicle fractures are also frequent, with the midshaft being the most common location. Radiographs show the fracture line, and the displacement is often characteristic. The weight of the arm pulls the outer (distal) fragment downward and inward, while the neck muscles pull the inner (medial) fragment upward. A specialized 45-degree cephalic tilt view can better define the extent of this displacement.