AO/OTA Fracture Classification System
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Direct answer
The AO/OTA fracture classification is an alphanumeric code, developed by the AO Foundation and the Orthopaedic Trauma Association, that describes bone, segment and pattern. The first digit identifies the bone, the second the segment as proximal, shaft or distal, and a letter with numbers describes the fracture type, with A, B and C indicating increasing complexity. It standardises communication, supports research and allows outcomes to be compared between centres.
What you must remember
- Origin: developed by the AO Foundation with the Orthopaedic Trauma Association, and revised over time.
- Bone digit: 1 for humerus, 2 for radius and ulna, 3 for femur and 4 for tibia and fibula.
- Segment digit: 1 for proximal, 2 for diaphysis or shaft and 3 for distal, in long bones.
- Letter in diaphyseal segments: A for simple, B for wedge and C for complex or multifragmentary fractures.
- Letter in articular segments: A for extra-articular, B for partial articular and C for complete articular fractures, as in the distal radius, distal femur and distal tibia; the proximal femur and proximal humerus use their own definitions.
- Subgroups: the numbers after the letter refine the pattern, and higher numbers generally signal greater complexity.
- Worked example: 32A is a simple femoral shaft fracture, and 23-C3 is a complete articular distal radius fracture.
Reading a code from left to right
An AO/OTA code is read in a fixed order, from the bone to the detail of the pattern. For example, 23-C3 reads as a complete articular fracture of the distal radius, and the steps below decode it one position at a time.
- Read the bone: the first digit identifies the bone, so 2 means the radius or ulna and 3 means the femur.
- Read the segment: the second digit places the fracture as proximal (1), shaft (2) or distal (3) in a long bone, so 3 here means distal.
- Read the fracture type: the letter grades the pattern, but its meaning depends on whether the segment is articular, so C here means complete articular.
- Read the group and subgroup: the numbers after the letter refine the pattern, so 23-C3 is a higher-complexity variant within the C group.
- Use the code with clinical judgement: the code communicates the injury and supports research, but it does not replace examination, imaging and the soft tissue assessment that guide treatment.
Where the exam tries to catch you
AO/OTA questions usually test the digit positions and the difference between shaft and articular letters.
- Trap: reading the letter A as always meaning simple Correct: in articular segments A means extra-articular, so the simple meaning applies only to diaphyseal segments.
- Trap: taking the first digit as the segment rather than the bone Correct: the first digit identifies the bone, and the second digit gives the segment within it.
- Trap: treating the code as a substitute for management decisions Correct: the classification standardises description and research, while treatment depends on the patient, the fracture and the soft tissues.
Frequently asked questions
What do the digits in an AO/OTA fracture code mean?
The first digit identifies the bone, with 1 for the humerus, 2 for the radius and ulna, 3 for the femur and 4 for the tibia and fibula. The second digit identifies the segment in a long bone as proximal, shaft or distal.
What do the letters A, B and C mean in the AO/OTA classification?
For diaphyseal segments, A, B and C denote simple, wedge and complex fractures in increasing order of complexity. For articular segments, they denote extra-articular, partial articular and complete articular fractures.
Why is the AO/OTA classification used in clinical practice?
The AO/OTA classification standardises fracture description, so surgeons, trauma centres and researchers use the same language. It also helps compare outcomes and plan treatment, although it does not replace clinical assessment.
What does 32A denote in the AO/OTA system?
In the AO/OTA system, 32A denotes a simple fracture of the femoral shaft. The 3 identifies the femur, the 2 identifies the diaphysis or shaft, and the A identifies a simple pattern.