AO Classification – General Principles

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High-Yield Summary

The AO/OTA classification system organizes fractures by bone and segment, providing a standardized framework essential for guiding treatment strategies. It distinguishes fractures by location—proximal, diaphyseal, or distal—and morphology, which directly influences stability and fixation principles. Understanding the classification enables surgeons to determine the need for absolute versus relative stability and select appropriate fixation methods.

Anatomy & Coding Foundation

• AO Code:
• Bone:
• 1 = Humerus
• 2 = Radius/Ulna
• 3 = Femur
• 4 = Tibia/Fibula
• Segment:
• 1 = Proximal
• 2 = Diaphysis
• 3 = Distal

• Morphology Types:
• A: Simple, extra-articular fractures
• B: Wedge or segmental, partial articular fractures
• C: Complex, comminuted, articular fractures

Classification Details

Management Logic

• Operative Indications:
• Type A: Often managed non-operatively if alignment is acceptable; ORIF or intramedullary nailing if unstable or displaced.
• Type B: Usually requires ORIF to restore alignment and partial articular congruity.
• Type C: Requires anatomical reduction and rigid fixation (ORIF) to restore joint surface and stability.

• Fixation Principles:
• Absolute Stability: Indicated for articular fractures (Types B and C) to allow early motion and prevent post-traumatic arthritis.
• Relative Stability: Used for simple diaphyseal fractures (Type A) with bridge plating or intramedullary nailing to promote secondary bone healing.

Prognostic Factors

• Type A: Generally good prognosis with low non-union rates if adequately stabilized.
• Type B: Moderate risk of non-union and malalignment; requires precise fixation.
• Type C: Highest risk of complications including non-union, post-traumatic arthritis, and stiffness due to articular involvement and comminution.

Classic Clinical Notes

General Principles

• Bones:
• 1 – Humerus
• 2 – Radius/Ulna
• 3 – Femur
• 4 – Tibia/Fibula

• Segments:
• 1 – Proximal
• 2 – Diaphysis
• 3 – Distal

• Fracture Morphology:
• A: Simple extra-articular
• 1 – Spiral
• 2 – Oblique
• 3 – Transverse
• B: Segmental partial-articular
• C: Comminuted articular

Last Updated on January 25, 2026 by orthonet