Commonest shoulder dislocation
**Core Concept**
The commonest shoulder dislocation is an injury to the shoulder joint, specifically an anterior dislocation of the humerus from the glenohumeral joint. This type of dislocation accounts for approximately 97% of all shoulder dislocations.
**Why the Correct Answer is Right**
The mechanism of injury typically involves a fall on an outstretched hand or a direct blow to the shoulder, causing the humerus to dislocate anteriorly. The labrum, a cartilaginous structure surrounding the glenohumeral joint, may be torn or damaged during the dislocation. The anterior capsule and labrum are often stretched or torn, leading to instability and recurrent dislocations. The anterior dislocation is more common due to the greater mobility and laxity of the anterior capsule and ligaments.
**Why Each Wrong Option is Incorrect**
**Option A:** Posterior dislocation is less common, accounting for approximately 3% of all shoulder dislocations, and typically occurs in the setting of seizures, electrical shock, or other conditions that cause forceful adduction and internal rotation of the arm.
**Option B:** Inferior dislocation is even less common and usually occurs in the setting of a severe shoulder injury or a pathologic condition affecting the shoulder joint.
**Option C:** Superior dislocation is extremely rare and usually occurs in the setting of a severe trauma or a specific underlying condition.
**Clinical Pearl / High-Yield Fact**
The "empty can" sign, where the arm appears deformed and the humeral head is displaced anteriorly, is a classic radiographic finding in anterior shoulder dislocations.
**Correct Answer: A. Anterior dislocation of the humerus from the glenohumeral joint.**