AO/OTA Fracture Classification

Every AO/OTA fracture code follows a hierarchy: bone–segment–type–group–subgroup. A complete code such as 32-B2 reads: femoral shaft (3–2), wedge fracture (B), intact wedge (group 2).

Mind the vintage of the group names. The 2018 compendium renamed the diaphyseal wedge and segmental groups by whether the intercalary fragment is intact or fragmentary, replacing the older 2007 descriptors:

So a humeral shaft fracture with a single intact butterfly fragment is 12-B2 (intact wedge), and a femoral shaft fracture with a comminuted segmental portion is 32-C3 (fragmentary segmental). Quoting "bending wedge" for B2 dates your answer to the previous compendium.

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Bones 1 to 4 are the ones worth committing to memory — they cover the long bones where the segment-and-type logic below applies cleanly, and they are what an examiner will ask you to code. Bones 5 to 9 exist for completeness of the compendium, and in day-to-day practice the spine and pelvis are usually described with their own dedicated systems rather than a 5- or 6- code.

Where the Shaft Ends and the End Segment Begins: the Square Rule

"Between the proximal and distal metaphyses" is a description, not a decision rule, and the boundary decides the second digit of every code you assign. AO defines it geometrically.

Draw a square whose sides equal the widest part of that bone end — the epiphysis/metaphysis at its broadest, measured on the AP view. Anything whose centre lies inside the square belongs to the end segment (1 proximal or 3 distal); anything outside it is the shaft (segment 2). It is deliberately mechanical, so that two surgeons in different countries assign the same digit.

Two exceptions worth knowing, because both are common fractures:

• 31 (proximal femur) does not follow the square: the proximal femoral segment is defined anatomically to include the head, neck and trochanteric region, which extends well beyond any square drawn on the femoral head.
• 44 (malleolar) is the extra segment already noted, carved out of the distal tibia/fibula because ankle fractures behave as a ligamentous-and-bony unit rather than as a distal tibial articular fracture.

A worked example: a transverse fracture at the junction of the middle and distal thirds of the tibia. Draw the square on the widest part of the distal tibial plafond; the fracture centre sits well proximal to it, so this is segment 2 (shaft), not 3 — and it is therefore typed by the shaft meanings (simple/wedge/complex), not the articular ones.

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The type letter carries the most clinical weight, and its meaning differs between diaphyseal and articular segments — the single highest-yield exam point.

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• A universal research language. AO/OTA is the standard for trauma research and registries (NJR, AOANJRR, AO documentation), enabling comparison across centres and countries.
• It complements, not replaces, eponymous systems. Garden (femoral neck), Neer (proximal humerus), Weber (ankle) and Schatzker (tibial plateau) remain the everyday clinical language; AO/OTA adds research precision.
• Reliability is moderate — highest at bone/segment, good at type, lower at group/subgroup; training with reference radiographs improves agreement.
• The 2018 compendium (Meinberg et al.) is current — it refined codes, updated paediatric classification and added universal location/qualifier codes; not every region uses the classic A/B/C pattern (e.g. spine uses its own descriptors).
• Eponymous systems still drive bedside decisions — "Garden 3" instantly conveys management in a way "31-B" does not for most clinicians; the AO/OTA code is a precision supplement.

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Everything below condenses the AO/OTA system for revision and viva practice.