AO typing is a comprehensive classification system for long tubular bone fractures. The basic principle of this classification is to represent each long bone site with a number. Each part of the fracture is divided into three categories, and each category is divided into three subtypes according to the complexity of the fracture pattern; the more posterior the type or subtype, the more difficult the treatment of the fracture and the worse the prognosis. The AO subtype of intertrochanteric fracture of the femur is 31-A. Type A1 is a simple two-part fracture with an intact lateral bone cortex of the greater trochanter and a well-supported medial cortex. There are three subtypes according to the location of fracture line invasion. Type A2 is a comminuted fracture with intact lateral femoral greater trochanter cortices and ruptured medial and posterior cortices in several planes with loss of integrity and support. There are three subtypes according to the number of medial fracture blocks. Type A3 is a rupture of both the medial and lateral cortices of the greater trochanter of the femur, with the fracture line extending from the medial to the lateral lower femur, that is, reversal of the intertrochanteric fracture, and there are three subtypes according to the morphology of the fracture line.