AO Classification


The AO classification (= Working Group for Osteosynthesis Questions), also known as the classification, was introduced in order to provide a clear description of fractures. This classification is valid worldwide and serves as a basis for standardized bone fracture treatment. This makes it possible to describe fractures (fractures) in a standardized way and thus to treat them in a standardized way.


The Arbeitsgemeinschaft Osteosynthesefragen (AO-Classification) was founded in 1958 by 13 surgeons and orthopaedic surgeons. Maurice E., Martin, Robert Schneider and Hans Willenegger took over the management of the AO-Classification. The headquarters of the AO is in Davos (Switzerland).

In 1984, the working group was reorganized in the form of a non-profit foundation. Today, the Association for Osteosynthesis Questions comprises about 5000 members and has become an important network between surgeons. The AO has set itself the task of promoting and standardising medical progress in operative bone fracture treatment of diseases of the musculoskeletal system.

For this reason, the AO classification was introduced to describe bone fractures. The AO- classification consists of a 5-digit alphanumeric code. This code describes the exact location and severity of the fracture in question.

If, in addition to the bone fracture, soft tissue damage, skin or vessel damage is also present, other codes are used. Special codes are also used for foot and hand fractures as well as fractures in childhood. The AO- classification is mainly used for fractures of the long tubular bones (e.g. femur).

In order to be able to use the AO- classification in a standardised way, different numbers are assigned to the body regions and injury patterns: The most frequent use of the AO- classification is on the upper arm (humerus) = 1, forearm (radius = radius, ulna = ulna)= 2, thigh (femur) = 3 and lower leg (tibia = shinbone, fibula = fibula)= 4. The body region is in first place in the code. All other bones of the body are also numbered consecutively and can therefore be described with the AO- classification.

However, this is mainly used for the above-mentioned bones, which is why only these are specifically listed here. Within a body region, the fracture must be precisely located. A distinction is made between the bone end close to the body (= proximal) = 1, the bone shaft (diaphyseal) = 2 and the end farthest from the body (= distal) = 3.

The inner and outer malleoli form an exception and are coded with the number 4. The localization is in second place in the code. In addition, the fractures must be classified according to their severity, prognosis and the difficulty of their treatment.

Shaft fractures are divided into three groups: A= simple fracture, B= wedge fracture, C= complex fracture. If the fracture affects the joint, this fracture is also divided into 3 groups: A= outside the joint space (extra-articular), B= partial (partial) joint fracture, C= complete joint fracture. The severity of the fracture is indicated in the third position of the code. In addition, the severity of the fracture is generally coded as light= 1, medium= 2 or severe= 3.