Posterior shoulder dislocation: This is the most likely diagnosis given the mechanism of injury (falling on the side at 25 mph) and the limited abduction at 45°. Posterior dislocations are often associated with a fall onto an outstretched hand or a direct blow to the shoulder, and they can present with limited external rotation and abduction.

Proximal humerus fracture: A fracture of the proximal humerus could also result from a fall onto the side and could limit abduction due to pain and mechanical instability.
Acromioclavicular joint separation: Although less likely given the specific mention of posterior shoulder pain, an AC joint separation could occur from a fall onto the side and might present with limited abduction due to pain.
Adhesive capsulitis (frozen shoulder): While less likely in the acute setting of a motorcycle accident, if the patient had pre-existing shoulder issues, an acute injury could exacerbate or trigger the onset of adhesive capsulitis, leading to limited range of motion.

Brachial plexus injury: Given the high-speed nature of the accident, a brachial plexus injury is possible and could have severe long-term consequences if missed. Symptoms might include numbness, tingling, or weakness in the arm or hand.
Vascular injury: Any significant trauma to the shoulder region could potentially damage the nearby blood vessels, leading to serious complications if not promptly addressed.

Scapulothoracic dissociation: This is a rare but serious injury where the scapula and the thoracic wall are separated, often due to severe trauma. It could present with significant pain and limited mobility of the shoulder.
Osteonecrosis of the humeral head: While not immediately apparent after an accident, osteonecrosis could be a late complication of a posterior shoulder dislocation or fracture, especially if the blood supply to the humeral head is compromised.