Differential Diagnosis for Shoulder Pain in a 25-year-old Baseball Player

Single most likely diagnosis

• Acromioclavicular (AC) joint sprain or separation: The patient's symptoms, including pain in the right anterior shoulder that worsens with active use of the arm, tenderness and tissue texture abnormalities in the AC joint, and restricted gapping of the AC joint with adduction or external rotation of the humerus, are consistent with an AC joint injury. The patient's history of playing baseball, which involves repetitive throwing motions, increases the likelihood of this diagnosis.

Other Likely diagnoses

• Subacromial impingement syndrome: This condition, which involves inflammation of the tendons and bursae under the acromion, can cause pain in the anterior shoulder that worsens with active use of the arm. The patient's symptoms and physical examination findings, such as restricted gapping of the AC joint, could also be consistent with subacromial impingement syndrome.
• Rotator cuff tendinitis or strain: The patient's symptoms, including pain in the anterior shoulder that worsens with active use of the arm, could be consistent with rotator cuff tendinitis or strain. The patient's history of playing baseball, which involves repetitive throwing motions, increases the likelihood of this diagnosis.

Do Not Miss diagnoses

• Fracture of the clavicle or acromion: Although less likely, a fracture of the clavicle or acromion could cause the patient's symptoms and must be ruled out with imaging studies. Missing this diagnosis could lead to delayed treatment and potentially serious complications.
• Osteolysis of the distal clavicle: This condition, which involves resorption of the distal clavicle, can cause pain and tenderness in the AC joint. Although less common, osteolysis of the distal clavicle must be considered in the differential diagnosis.

Rare diagnoses

• Septic arthritis of the AC joint: This condition, which involves infection of the AC joint, is rare but could cause the patient's symptoms. Septic arthritis of the AC joint would require prompt diagnosis and treatment with antibiotics to prevent serious complications.
• Ganglion cyst of the AC joint: A ganglion cyst, which is a benign tumor that arises from the joint capsule, could cause pain and tenderness in the AC joint. Although rare, a ganglion cyst must be considered in the differential diagnosis.

In terms of the clavicular glide most likely restricting the acromion in this patient, the posterior glide is the most likely answer. The posterior glide of the clavicle would restrict the movement of the acromion, consistent with the patient's symptoms and physical examination findings.