AO Classification 13 – Distal Humerus

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High-Yield Summary

The distal humerus (AO code 13) is a complex anatomical region critical for elbow function, involving the articular surface and metaphyseal bone. The AO/OTA classification stratifies fractures into extra-articular (A), partial articular (B), and complete articular (C) types, guiding surgical decision-making by defining fracture complexity and stability. This classification informs the necessity for absolute stability (anatomic reduction and compression) in articular fractures versus relative stability in extra-articular patterns, directly impacting fixation strategy and prognosis.

Anatomy & Coding Foundation

  • AO Code: 13 (Distal Humerus)
  • Segment:
  • 13-A: Extra-articular
  • 13-B: Partial articular
  • 13-C: Complete articular
  • Morphology Types:
  • A: Extra-articular (no joint involvement)
  • B: Partial articular (fracture involves part of the joint surface)
  • C: Complete articular (fracture involves entire joint surface with metaphyseal involvement)

Classification Details

AO TypeSubtypeDescriptionInjury MechanismStability Profile
13-A13-A1Apophyseal avulsionAvulsion injury, often tractionExtra-articular, stable
13-A2Simple metaphyseal fractureDirect trauma or bendingExtra-articular, stable
13-A3Comminuted metaphyseal fractureHigh-energy traumaExtra-articular, unstable
13-B13-B1Sagittal lateral condyle fractureVarus/valgus stressPartial articular, stable/unstable depending on displacement
13-B2Sagittal medial condyle fractureVarus/valgus stressPartial articular, stable/unstable depending on displacement
13-B3Frontal plane partial articularAxial load with rotationPartial articular, unstable
13-C13-C1Simple articular + simple metaphysisHigh-energy traumaComplete articular, unstable
13-C2Simple articular + comminuted metaphysisHigh-energy traumaComplete articular, unstable
13-C3Comminuted articular fractureHigh-energy traumaComplete articular, highly unstable

Management Logic

  • Operative Indications:
  • 13-A3, 13-B (displaced), and all 13-C fractures require operative fixation due to instability and joint involvement.
  • 13-A1 and 13-A2 may be managed non-operatively if minimally displaced and stable.
  • Fixation Principles:
  • Absolute Stability: Required for articular fractures (13-B and 13-C) to restore joint congruity and allow early motion. Achieved via compression plating or lag screws.
  • Relative Stability: Acceptable for extra-articular simple fractures (13-A2), often treated with bridge plating or intramedullary fixation.
  • Complex comminuted fractures (13-A3, 13-C2, 13-C3) may require combined approaches and dual plating for stable fixation.

Prognostic Factors

  • AO Grade Correlation:
  • Type A fractures generally have good prognosis with low non-union rates if adequately stabilized.
  • Type B fractures carry risk of joint stiffness and post-traumatic arthritis if articular congruity is not restored.
  • Type C fractures have the highest risk of non-union, malunion, and post-traumatic arthritis due to articular comminution and metaphyseal involvement.
  • Early anatomic reduction and stable fixation improve functional outcomes and reduce complications.

Classic Clinical Notes

13-Distal humerus

  • A-Extra-articular
  • 13-A-1 apophyseal avulsion (me)
  • 13-A-2 metaphyseal-simple
  • 13-A-3 metaphyseal-comminuted
  • B-Partial articular
  • 13-B-1 sagittal lateral condyle
  • 13-B-2 sagittal medial condyle
  • 13-B-3 frontal plane
  • C-Complete articular
  • 13-C-1 simple articular + simple metaphysis
  • 13-C-2 simple articular + comminuted metaphysis
  • 13-C-3 comminuted articular

Last Updated on January 25, 2026 by Christian Veillette

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