AO/OTA Fracture Classification
The alphanumeric code structure
Every AO/OTA fracture code follows a hierarchy: bone–segment–type–group–subgroup. A complete code such as 32-B2.1 reads: femoral shaft (3–2), wedge fracture (B), bending wedge (group 2), with the subgroup (.1) specifying the pattern further.
- Element
- Bone
- Values
- 1–9
- Example
- 3 = Femur
- Element
- Segment
- Values
- 1 proximal, 2 shaft, 3 distal (4 malleolar)
- Example
- 2 = Shaft
- Element
- Type
- Values
- A, B or C (severity rises)
- Example
- B = Wedge
- Element
- Group
- Values
- 1, 2 or 3
- Example
- 2 = Bending wedge
- Element
- Subgroup
- Values
- .1, .2 or .3
- Example
- .1 = Specific pattern
Bone — Segment — Type — Group — SubgroupReading the code
Hook:State bone, segment and type as a minimum (e.g. 32-B); group/subgroup earn the extra marks. The hyphen after the first two digits is the type separator.
Bone numbers and segments
- Bone
- Humerus
- Key segments
- 11 proximal, 12 shaft, 13 distal
- Exam-relevant examples
- 11-B surgical neck, 13-C intercondylar
- Bone
- Radius/Ulna
- Key segments
- 21 proximal, 22 shaft, 23 distal
- Exam-relevant examples
- 22-A1 ulna shaft, 23 distal radius
- Bone
- Femur
- Key segments
- 31 proximal, 32 shaft, 33 distal
- Exam-relevant examples
- 31-B intertrochanteric, 32-A simple shaft, 33-C supracondylar
- Bone
- Tibia/Fibula
- Key segments
- 41 proximal, 42 shaft, 43 distal, 44 malleolar
- Exam-relevant examples
- 41-B tibial plateau, 42-C segmental, 43-C pilon, 44 ankle
- Bone
- Spine, pelvis, hand, foot, etc.
- Key segments
- Added/expanded in the 2007 and 2018 compendia
- Exam-relevant examples
- Pelvic ring, acetabulum, calcaneus, clavicle/scapula
- Location
- Proximal
- Boundary
- Metaphysis/epiphysis of the proximal end
- Note
- Proximal femur includes trochanters and neck
- Location
- Shaft (diaphyseal)
- Boundary
- Between the proximal and distal metaphyses
- Note
- The longest segment in most bones
- Location
- Distal
- Boundary
- Metaphysis/epiphysis of the distal end
- Note
- Distal humerus includes the supracondylar region
- Location
- Malleolar
- Boundary
- Ankle/malleolar segment
- Note
- Unique to the tibia/fibula group
Fracture types A, B and C
The type letter carries the most clinical weight, and its meaning differs between diaphyseal and articular segments — the single highest-yield exam point.
- Diaphyseal (segment 2)
- Simple (two fragments)
- Articular (segments 1 & 3)
- Extra-articular
- Severity
- Lowest energy
- Diaphyseal (segment 2)
- Wedge (butterfly fragment)
- Articular (segments 1 & 3)
- Partial articular (part of the joint intact)
- Severity
- Intermediate
- Diaphyseal (segment 2)
- Complex (segmental/multifragmentary)
- Articular (segments 1 & 3)
- Complete articular (joint surface detached from the shaft)
- Severity
- Highest energy
SWC for Shaft, EPC for EndsType meanings — shaft vs joint
Hook:SWC at the Shaft, EPC at the Ends - same letters, different meaning, same A-to-C severity climb.
Type C articular fractures involve complete dissociation of the joint surface from the metaphysis — high-energy injuries that need a CT for surgical planning. Missing the intra-articular extent leads to inadequate fixation and post-traumatic arthritis. Obtain a CT for any B or C articular fracture.
The group (1–3) refines morphology within a type (e.g. femoral shaft 32-A: A1 spiral, A2 oblique, A3 transverse) and the subgroup (.1–.3) specifies geometry/location. But inter-observer reliability falls as you go deeper: it is highest at the bone/segment level, good at the type (A/B/C) level, and declines at group and subgroup. In an exam you are rarely pushed beyond type — the type level is both the most reproducible and the most clinically useful tier.
The A/B/C type is not academic - it points to the mechanical strategy, via the AO principles (anatomic reduction, especially of the articular surface; stable fixation appropriate to the fracture; preservation of the blood supply/biology; early mobilisation). The pivotal concept is two kinds of stability:
- Absolute stability - interfragmentary compression (lag screw, compression plate) abolishes motion → primary (direct) bone healing with no visible callus. Indicated for articular fractures (anatomic joint reduction is mandatory) and for simple patterns that can be anatomically reduced.
- Relative stability - controlled micromotion (bridge plate, intramedullary nail, external fixator) → secondary healing through callus. Indicated for complex/multifragmentary diaphyseal (and metaphyseal) fractures, where preserving biology beats anatomically reducing every fragment.
So a type-C articular fracture is typically treated by anatomically reducing the joint with absolute stability while bridging the metaphyseal/diaphyseal comminution with relative stability - the "articular-absolute, metaphyseal-relative" combined construct.
Clinical utility, limitations & modern context
- 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.
The AO/OTA code grades the bone only - it says nothing about the soft-tissue envelope, which often matters more for timing and outcome. Always pair the bony code with a soft-tissue assessment: the Gustilo-Anderson classification for open fractures and the Tscherne classification for closed soft-tissue injury; the AO compendium itself adds a soft-tissue coding (integument closed/open, muscle-tendon, neurovascular - IC/IO/MT/NV). The point examiners want: a "simple" bony pattern with a severe soft-tissue injury (or compartment syndrome) is NOT a benign fracture - the soft tissues drive the damage-control vs definitive timing as much as the bone code does.
Viva practice
Exam viva
Practise clinical reasoning and management decisions out loud
“A 42-year-old man is involved in a high-speed motor vehicle collision. Radiographs of the right knee show a comminuted supracondylar femoral fracture with the articular surface completely separated from the shaft. CT confirms multifragmentary involvement of the articular surface. Classify this fracture and outline your management.”
“An examiner shows you a radiograph of a tibial shaft fracture with a butterfly fragment and asks you to classify it using the AO/OTA system. They then ask why the AO/OTA system exists when eponymous names are more commonly used clinically.”
Exam & revision
Everything below condenses the AO/OTA system for revision and viva practice.
- Code = Bone–Segment–Type–Group–Subgroup (e.g. 32-B2.1 = femoral shaft, bending wedge).
- A/B/C dual meaning: shaft = simple/wedge/complex; joint = extra-/partial-/complete-articular.
- Severity is ordinal A → C in every segment.
- Always translate the code into clinical language in a viva (33-C3 = comminuted intra-articular distal femur).
- CT any B or C articular fracture before fixation.
- Reliability is best at the type level; it declines at group/subgroup. The 2018 compendium is current.
1-2-3-4 = Humerus, Radius/Ulna, Femur, Tibia/FibulaThe four long bones
Hook:The original Müller system = the four long bones (1-2-3-4); everything else was bolted on by the 2007 and 2018 compendia.
Exam cheat sheet
Code: Bone-Segment-Type-Group-Subgroup
- Bone: 1 humerus, 2 radius/ulna, 3 femur, 4 tibia/fibula (5-9 added later)
- Segment: 1 proximal, 2 shaft, 3 distal (4 malleolar for tibia)
- Type: A/B/C - severity rises A to C in ALL segments
- Group 1/2/3 and subgroup .1/.2/.3 add morphology
Type A/B/C - shaft vs articular
- Diaphyseal (seg 2): A simple, B wedge (butterfly), C complex (segmental)
- Articular (seg 1/3): A extra-articular, B partial articular, C complete articular
- Mnemonic: SWC for Shaft, EPC for Ends
- Type level is the most reliable and clinically relevant tier
Relationship to eponymous systems
- AO/OTA = research/registry standard; eponymous = clinical language
- Garden (femoral neck) ~ 31-B; Weber (ankle) ~ 44; Schatzker (plateau) ~ 41-B
- Always translate an AO/OTA code into clinical language in a viva
- 2018 Meinberg compendium is the current version
Limitations & caveats
- Inter-observer reliability declines from type to group to subgroup
- Not all fractures fit neatly - complex articular patterns can be ambiguous
- Eponymous systems may better guide region-specific decisions
- CT essential before definitive classification of any articular (B/C) fracture
Evidence Base
Fracture and dislocation classification compendium - 2007: Orthopaedic Trauma Association classification, database and outcomes committee
- Republished the OTA classification and unified the AO and OTA systems into a single alphanumeric code, eliminating the prior differences between the two.
- Significantly revised coding for the clavicle/scapula, foot/hand and patella, and expanded dislocations on an anatomic basis.
- Published as a Journal of Orthopaedic Trauma supplement to provide a universal language for coding and classifying fractures.
Fracture and Dislocation Classification Compendium-2018
- The most recent major revision of the AO/OTA classification, refining codes and adding fracture patterns.
- Updated segment definitions and the paediatric fracture classification.
- Added universal fracture-location and qualifier codes for audit and registry use.
Current classification of fractures. Rationale and utility
- An ideal fracture classification should be reliable, reproducible, all-inclusive, mutually exclusive, logical and clinically useful.
- The AO/ASIF classification provides a unified scheme for the whole skeleton but observer agreement drops to unacceptable levels at the group and subgroup levels.
- Type-level (A/B/C) reliability is acceptable, justifying its use as the working tier.
A concept for the validation of fracture classifications
- Most fracture classifications in common use were not developed or validated by rigorous scientific methods.
- Proposed a 3-phase validation concept: expert agreement studies (reliability/accuracy), a multicentre agreement study, then a prospective clinical study of usefulness.
- Classification categories must be reproducible enough to limit misclassification and associated treatment errors.
According to PubMed, the unified AO/OTA nomenclature comes from the Marsh et al. 2007 compendium (DOI) and its current form from the Meinberg et al. 2018 compendium (DOI); the reliability principle (good at type level, poor at group/subgroup) from Martin & Marsh 1997 (Radiol Clin North Am, PMID 9167660); and the validation framework from Audigé et al. 2005 (DOI). All are expert-consensus or narrative sources (Level V) - appropriate for a classification system rather than a treatment comparison. The eponymous-system equivalences are standard clinical knowledge.