Extension Type: Most common, accounting for more than 90%. When falling, the elbow joint is in a semi-flexed or extended position, with the palm touching the ground. The force is transmitted through the forearm to the distal humerus, pushing the humeral condyle backward. Due to gravity, the humeral shaft is pushed forward, causing a supracondylar fracture of the humerus. The fracture line slopes from the front lower part to the back upper part. The proximal fracture segment often pierces the brachialis muscle, injuring the median nerve and brachial artery. In addition to receiving anterior and posterior forces, the distal humerus can also be subjected to lateral forces, which can be divided into ulnar deviation type and radial deviation type based on the displacement.

    1. Ulnar Deviation Type: The fracture force comes from the anterolateral side of the humeral condyle, pushing the humeral condyle backward and inward during the fracture. The medial bone cortex is compressed, causing some collapse. The anterolateral periosteum is ruptured, while the medial periosteum remains intact. The distal fracture segment is displaced to the ulnar side. Therefore, after reduction, the distal segment is prone to re-displacement to the ulnar side. Even if anatomical reduction is achieved, the medial cortex compression defect may cause medial deviation. The incidence of cubitus varus is highest after ulnar deviation type fractures.

    2. Radial Deviation Type: Opposite to the ulnar deviation type. The radial side bone cortex of the fracture end collapses due to compression. The lateral periosteum remains continuous. The ulnar side periosteum is ruptured, and the distal fracture segment is displaced to the radial side. This type of fracture does not cause severe cubitus valgus even if not completely reduced, but anatomical reduction or overcorrection can also result in cubitus varus deformity.

Flexion Type: Less common. When falling with the elbow in a flexed position, the force impacts the olecranon of the ulna from the back lower part to the front upper part, causing the distal fracture segment to move forward. The fracture line often slopes from the back lower part to the front upper part, opposite to the extension type. Vascular and nerve injuries are rare.

Comminuted Type: More common in adults. This type of fracture often involves intercondylar fractures of the humerus and can be classified into T-type, Y-type, or comminuted fractures based on the shape of the fracture line.