Mallet Finger (Baseball Finger): This condition occurs when the extensor tendon on the back of the finger (or thumb) is stretched or torn, usually due to a sudden, forceful blow to the tip of the thumb, causing it to extend. It's a common injury in sports and daily activities, making it the most likely diagnosis for a thumb in extension due to trauma.

Thumb Fracture: A fracture of any of the bones in the thumb could lead to abnormal positioning, including extension, especially if the fracture involves the joint or the bone alignment is significantly disrupted.
Ligament Sprain: Severe sprains of the ligaments around the thumb joint can cause instability and lead to an abnormal extension posture of the thumb.
Dislocation of the Thumb: A dislocation, particularly of the metacarpophalangeal (MCP) joint, can result in the thumb being positioned in extension due to the disruption of the normal joint alignment.

Open Fracture or Laceration: Although less common, an open fracture (where the bone pierces the skin) or a deep laceration can lead to infection and other serious complications if not promptly addressed. The presence of an open wound with a thumb in extension post-trauma is critical to identify.
Neurovascular Injury: Damage to the nerves or blood vessels supplying the thumb can have severe and long-lasting consequences, including loss of sensation, movement, or even the need for amputation in extreme cases.

Tendon Laceration: A direct cut to the extensor tendon of the thumb could result in the inability to flex the thumb, leading it to remain in extension. This would typically be associated with a penetrating trauma.
Intrinsic Thumb Muscle Rupture: Rupture of the intrinsic muscles of the thumb (such as the thenar muscles) could potentially lead to weakness and abnormal posturing of the thumb, although this is less common compared to other injuries.