Modern Study Review (AI-Generated)
High-Yield Summary
The AO/OTA 31 classification addresses proximal femur fractures, encompassing extra-articular trochanteric (A), extra-articular neck (B), and articular head (C) fractures. This classification guides surgical decision-making by distinguishing fracture morphology and articular involvement, which directly influences the choice between fixation methods and arthroplasty. Understanding the stability and displacement inherent to each subtype is critical for optimizing outcomes and minimizing complications such as nonunion or avascular necrosis.
Anatomy & Coding Foundation
- AO Code: 31 = Femur Proximal
- Segments:
- 1 = Trochanteric region (extra-articular)
- 2 = Femoral neck (extra-articular)
- 3 = Femoral head (articular)
- Morphology Types:
- A: Extra-articular trochanteric fractures
- B: Extra-articular femoral neck fractures
- C: Articular femoral head fractures
Classification Details
| AO Type | Subtype | Description | Injury Mechanism | Stability Profile |
| 31-A | A1 | Peritrochanteric simple fracture | Low-energy fall, direct trauma | Generally stable |
| A2 | Peritrochanteric multi-fragment | High-energy trauma | Unstable due to comminution | |
| A3 | Intertrochanteric fracture | High-energy trauma | Unstable, often reverse obliquity | |
| 31-B | B1 | Subcapital, minimally displaced | Low-energy fall | Potentially stable |
| B2 | Transcervical fracture | Moderate trauma | Unstable, risk of displacement | |
| B3 | Displaced subcapital fracture | High-energy or displaced low-energy | Unstable, high risk of AVN | |
| 31-C | C1 | Femoral head split (Pipkin type) | High-energy trauma | Intra-articular, unstable |
| C2 | Split with depression | High-energy trauma | Intra-articular, unstable | |
| C3 | Femoral head fracture with neck fracture | High-energy trauma | Complex, unstable |
Management Logic
- Operative Indications:
- 31-A1: Often treated with cephalomedullary nailing or sliding hip screw.
- 31-A2 & A3: Require stable fixation with intramedullary nails; absolute stability is difficult, relative stability via bridge plating or nailing preferred.
- 31-B1: May be treated non-operatively if minimally displaced; otherwise, fixation or arthroplasty considered.
- 31-B2 & B3: Usually require ORIF or arthroplasty depending on patient age and displacement.
- 31-C: Require open reduction and internal fixation (ORIF); arthroplasty considered if head viability compromised.
- Fixation Principles:
- Absolute Stability: Required for femoral neck fractures (B types) to promote union and reduce AVN risk.
- Relative Stability: Applied in trochanteric fractures (A types) to allow controlled impaction and healing.
- Articular Fractures (C types): Require anatomical reduction and absolute stability to restore joint congruity.
Prognostic Factors
- 31-A: Generally good prognosis with stable fixation; comminution (A2, A3) increases risk of fixation failure.
- 31-B: Displacement and fracture location correlate with nonunion and avascular necrosis (AVN) rates; displaced subcapital fractures (B3) have highest risk.
- 31-C: Articular involvement predicts poorer outcomes; risk of post-traumatic arthritis and AVN is high, especially with associated neck fractures (C3).
Classic Clinical Notes
31-Femur Proximal
- A – Extra-articular – Trochanteric
- 31-A-1: Peritrochanteric simple
- 31-A-2: Peritrochanteric multi fragments
- 31-A-3: Intertrochanteric
- B – Extra-articular – Neck Fractures
- 31-B-1: Subcapital – minimally displaced
- 31-B-2: Transcervical
- 31-B-3: Displaced subcapital
- C – Articular – Head Fractures
- 31-C-1: Split (Pipkin)
- 31-C-2: Split with depression
- 31-C-3: With neck fracture
Last Updated on January 25, 2026 by Christian Veillette
Leave a Reply
Want to join the discussion?Feel free to contribute!