AO CLASSIFICATION OF LOWER END OF FEMUE
Reading File
Finding Sources
Finding Sources
Searching the Web
AO classification lower end femur distal femur fractures 33A 33B 33C
Reading File
Finding Sources
Reading Web Page
https://www.ncbi.nlm.nih.gov/books/NBK551675
Here is a complete and well-structured breakdown of the AO/OTA Classification of the Lower End of Femur (Distal Femur) - coded 33 in the AO system (Segment 3 = femur, Region 3 = distal):
AO/OTA Classification of the Distal Femur (Code: 33)
The AO/OTA system uses bone-segment numbering: the femur is bone 3, and its distal end is segment 3, giving the prefix 33. Three main types (A, B, C) are each subdivided into three groups (1, 2, 3) based on fracture complexity.
TYPE 33-A: Extra-articular
The joint surface is completely intact; the fracture is entirely in the metaphysis/supracondylar region.
| Subtype | Description |
|---|---|
| A1 | Simple (two-part) - transverse or short oblique |
| A2 | Wedge/butterfly fragment - metaphyseal wedge |
| A3 | Complex/multifragmentary - comminuted, no intact wedge |
The condyles remain intact and connected to the shaft fragment.
TYPE 33-B: Partial Articular (Unicondylar)
Part of the articular surface remains attached to the diaphysis; one condyle is involved but the other maintains continuity with the shaft.
| Subtype | Description |
|---|---|
| B1 | Lateral condyle, sagittal plane (vertical split of lateral condyle) |
| B2 | Medial condyle, sagittal plane (vertical split of medial condyle) |
| B3 | Frontal/coronal plane fracture - "Hoffa fracture" (posterior condyle shear fracture) |
B3 (Hoffa fracture) is particularly important - it is a coronal shear fracture of one or both condyles, frequently missed on plain X-rays and requires CT for diagnosis.
TYPE 33-C: Complete Articular (Bicondylar/Intercondylar)
Both condyles are separated from the diaphysis - the articular surface is completely dissociated from the shaft. This is the most severe type.
| Subtype | Description |
|---|---|
| C1 | Simple articular + simple metaphyseal (classic "Y" or "T" fracture, single intercondylar line, no comminution) |
| C2 | Simple articular + comminuted/multifragmentary metaphyseal (condyles intact but shaft is comminuted) |
| C3 | Comminuted articular + comminuted metaphyseal (both the joint surface and shaft are shattered) |
C3 is the most challenging to treat and carries the worst prognosis.
Summary Table
| Type | Articular Involvement | Complexity |
|---|---|---|
| A1 | None (extra-articular) | Simple |
| A2 | None | Wedge |
| A3 | None | Comminuted |
| B1 | Partial - lateral condyle (sagittal) | - |
| B2 | Partial - medial condyle (sagittal) | - |
| B3 | Partial - coronal (Hoffa) | - |
| C1 | Complete - both condyles | Simple metaphysis |
| C2 | Complete - both condyles | Complex metaphysis |
| C3 | Complete - comminuted condyles | Complex metaphysis |
Key Points
- Severity increases from A → B → C, and from 1 → 2 → 3 within each type
- Type A fractures can sometimes be managed with retrograde/antegrade nailing
- Type B fractures require anatomic reduction of the condyle, often with lag screws ± buttress plate
- Type C fractures require ORIF with distal femoral locking plates (LISS/LCP); C3 is the most technically demanding
- B3 (Hoffa fracture) is easily missed on plain X-ray - CT is mandatory for any suspected condylar fracture
- Gastrocnemius pull causes posterior angulation of the distal fragment in all types
Source: Rockwood and Green's Fractures in Adults, 10th ed. 2025; StatPearls - Distal Femur Fractures; AO Surgery Reference - Distal Femur
This is a shared conversation. Sign in to Orris to start your own chat.