Modern Study Review (AO/OTA 2018 Compendium)
High-Yield Summary
The distal femur (AO 33) includes the metaphyseal and articular regions just proximal to the knee joint. The AO classification stratifies fractures by articular involvement and complexity, guiding the choice between absolute stability (anatomic reduction and compression) for articular fractures and relative stability (bridge plating or nailing) for extra-articular or simple metaphyseal patterns. This classification is critical for predicting joint congruity restoration, fixation strategy, and prognosis, especially regarding post-traumatic arthritis and non-union risk.
Anatomy & Coding Foundation
- AO Code: 33 = Femur, Distal segment
- Segment: 3 = Distal femur (metaphysis and articular surface)
- Morphology Types:
- A: Extra-articular fractures (metaphyseal only)
- B: Partial articular fractures (involving part of the joint surface)
- C: Complete articular fractures (articular surface and metaphysis fractured separately)
Classification Details
| AO Type | Subtype | Description | Injury Mechanism | Stability Profile | Clinical Significance |
| A | 33-A1 | Apophyseal avulsion | Low-energy avulsion forces | Stable metaphyseal fragment | Usually non-articular, often non-operative |
| 33-A2 | Simple metaphyseal fracture | Direct trauma, bending | Generally stable | May require fixation if displaced | |
| 33-A3 | Comminuted metaphyseal fracture | High-energy trauma | Unstable | Requires bridge plating or nailing | |
| B | 33-B1 | Sagittal fracture of lateral condyle | Varus/valgus stress | Partial articular involvement | Absolute stability needed for articular congruity |
| 33-B2 | Medial condyle partial articular | Varus stress | Partial articular involvement | Requires precise reduction | |
| 33-B3 | Frontal plane partial articular | Axial load with rotation | Partial articular involvement | Absolute stability for joint surface | |
| C | 33-C1 | Simple articular + simple metaphysis | High-energy axial load | Unstable | Requires ORIF with absolute stability |
| 33-C2 | Simple articular + comminuted metaphysis | High-energy trauma | Unstable | Bridge plating or combined fixation | |
| 33-C3 | Comminuted articular + metaphysis | Severe trauma | Highly unstable | Complex ORIF, often staged procedures |
Management Logic
- Operative Indications:
- Type A: Displaced fractures, especially comminuted (A3), require fixation (locking plates or nails).
- Type B: All partial articular fractures require anatomic reduction and absolute stability (lag screws, compression plating).
- Type C: Complete articular fractures mandate ORIF with absolute stability to restore joint congruity and metaphyseal alignment.
- Fixation Principles:
- Absolute Stability: Required for all articular fractures (Types B and C) to prevent post-traumatic arthritis. Achieved by lag screws and compression plating.
- Relative Stability: Acceptable for extra-articular simple or comminuted metaphyseal fractures (Type A2, A3), using bridge plating or intramedullary nailing to preserve biology and promote callus formation.
Prognostic Factors
- Type A: Generally good prognosis if alignment is restored; comminution increases risk of delayed union.
- Type B: Risk of post-traumatic arthritis if articular congruity is not restored; partial articular involvement requires precise reduction.
- Type C: Highest risk of complications including non-union and arthritis; complexity correlates with poorer outcomes and longer rehabilitation.
Classic Clinical Notes
33-Femur Distal
- A-Extra-articular
- 33-A-1 apophyseal avulsion
- 33-A-2 metaphyseal-simple
- 33-A-3 metaphyseal-comminuted
- B-Partial articular
- 33-B-1 sagittal lateral condyle
- 33-B-2 medial condyle
- 33-B-3 frontal plane
- C-Complete articular
- 33-C-1 simple articular + simple metaphysis
- 33-C-2 simple articular + comminuted metaphysis
- 33-C-3 comminuted articular
Last Updated on January 25, 2026 by Christian Veillette
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