A 45-year-old construction worker presents to the emergency department after falling from scaffolding and landing on his outstretched right arm. He reports severe shoulder pain and inability to move his arm. Which assessment finding would be most indicative of an anterior shoulder dislocation?

Anterior shoulder dislocation typically presents with loss of normal rounded shoulder contour and the arm held in slight abduction and external rotation. Other findings like flattened deltoid or acromioclavicular deformity are more characteristic of other injuries.

Understanding the Mechanism and Classic Presentation

When a patient falls onto an outstretched arm, the humeral head is forced anteriorly out of the glenoid fossa. In an anterior shoulder dislocation, the humeral head ends up positioned in front of the joint. This specific displacement dictates the characteristic arm posture and visible loss of the shoulder’s normal contour. The arm becomes locked in a position that minimizes tension on the injured anterior capsule and surrounding structures.

Why Option 3 is Correct

The finding most indicative of an anterior shoulder dislocation is a loss of the normal rounded shoulder contour with the arm held in slight abduction and external rotation. The displaced humeral head leaves a void under the acromion, creating a flattened, squared-off appearance laterally. The patient instinctively holds the arm slightly away from the body (abduction) and rotated outward because any attempt to adduct or internally rotate the arm stretches the anterior capsule and causes severe pain and muscle spasm. This is the classic presentation taught for recognizing anterior dislocations in a clinical setting.

Analysis of Incorrect Options

Option 1 describes a flattened deltoid contour with the arm held in internal rotation and adduction. This posture is the hallmark of a posterior shoulder dislocation. As noted in the literature on posterior instability, the mechanism often involves repetitive loading in flexion, adduction, and internal rotation, and the arm is typically locked in this position ^[2]. It is not the presentation for an anterior dislocation.

Option 2 mentions a prominent acromion process with the arm in external rotation. While a prominent acromion can be seen in an anterior dislocation due to the humeral head’s absence, the arm position is not specific enough. The key element missing is the slight abduction; the arm is not just externally rotated but held away from the body in a combined position.

Option 4 points to a visible deformity at the acromioclavicular (AC) joint with the arm adducted. This finding is characteristic of an AC joint separation, not a glenohumeral dislocation. The injury mechanism and the location of the deformity are distinct from the global loss of shoulder contour seen in a glenohumeral dislocation.

Connecting Physical Findings to Underlying Pathoanatomy

The physical assessment findings directly reflect the underlying structural damage. An anterior dislocation is frequently associated with a Bankart lesion (a tear of the anterior-inferior glenoid labrum) and a Hill-Sachs lesion (a compression fracture of the posterolateral humeral head). These pathoanatomic features are often detected on MRI even in subtle cases of instability . The inability to move the arm and the fixed, painful posture are a result of muscle guarding and the mechanical block created by the displaced humeral head against the anterior glenoid rim. Recognizing the specific arm position—slight abduction and external rotation—is critical for a rapid clinical diagnosis, which should then be confirmed radiographically before any reduction is attempted.