AO Classification of lower end of femur
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The AO classification divides distal femur fractures into three main types (A, B, C) based on the extent of articular involvement. Let me search the medical library for detailed content.
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AO/OTA Classification of Distal Femur Fractures (Segment 33)
The AO/OTA system codes the distal femur as segment 33 (bone 3 = femur, segment 3 = distal). The classification is based on the fundamental principle of articular involvement and fracture complexity, dividing all fractures into three types and nine groups.
Figure: OTA/AO classification of distal femur fractures (43A-C) - Rockwood and Green's Fractures in Adults, 10th ed. 2025
Type A - Extra-articular (33-A)
The articular surface is completely intact. The fracture involves only the metaphysis/diaphysis.
| Group | Code | Description |
|---|---|---|
| A1 | 33-A1 | Simple - single metaphyseal fracture line (transverse or oblique) |
| A2 | 33-A2 | Metaphyseal wedge - butterfly/wedge fragment present, main fragments maintain contact |
| A3 | 33-A3 | Metaphyseal complex (comminuted) - severe comminution of the metaphysis, no contact between main fragments |
Type B - Partial Articular (33-B)
Part of the articular surface is fractured, but the remaining articular segment stays in continuity with the diaphysis. These are unicondylar fractures.
| Group | Code | Description |
|---|---|---|
| B1 | 33-B1 | Lateral condyle, sagittal plane - fracture runs in the sagittal plane splitting the lateral condyle |
| B2 | 33-B2 | Medial condyle, sagittal plane - sagittal split of the medial condyle |
| B3 | 33-B3 | Frontal plane (Hoffa fracture) - fracture runs in the coronal plane, detaching a posterior condylar fragment; often missed on plain X-rays (40% missed rate noted in studies) |
Type C - Complete Articular (33-C)
The articular surface is completely disrupted and separated from the diaphysis. These are bicondylar (intercondylar/T- or Y-shaped) fractures - the most severe type.
| Group | Code | Description |
|---|---|---|
| C1 | 33-C1 | Articular simple + Metaphyseal simple - single intercondylar split (Y or T pattern) with simple metaphyseal component |
| C2 | 33-C2 | Articular simple + Metaphyseal multifragmentary - simple condylar split but comminuted metaphysis |
| C3 | 33-C3 | Multifragmentary articular fracture - comminuted both at the articular level and the metaphysis; most complex, worst prognosis |
Key Principles
- Severity increases from A to C and from 1 to 3 within each type
- Treatment planning: Type A - primarily metaphyseal fixation (plate/nail); Type B - anatomic condylar fixation (lag screws +/- buttress plate); Type C - combined articular reconstruction + metaphyseal bridging
- B3 (Hoffa) fractures are easily missed on AP/lateral X-rays and require CT for identification
- The OTA/AO system does not capture "fracture personality" factors (soft tissue injury, osteoporosis, vascular injury) which also influence management - these must be assessed separately
Coding Structure
Bone (3) - Segment (3) - Type (A/B/C) - Group (1/2/3)
↓
33-A1, 33-B3, 33-C2, etc.
- Rockwood and Green's Fractures in Adults, 10th ed. 2025, Chapter 58 (Classification of Distal Femur Fractures)
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