AO Classification 43 – Tibia/Fibula Malleoli

Modern Study Review (AI-Generated)

High-Yield Summary

The AO/OTA 43 classification addresses fractures of the tibia and fibula malleoli, critical for ankle stability and syndesmotic integrity. It stratifies injuries by their relationship to the syndesmosis—infra-, trans-, and suprasyndesmotic—guiding the need for syndesmotic fixation and influencing surgical approach. Accurate classification informs the choice between absolute stability (e.g., medial malleolus fixation) and relative stability (e.g., fibular plating), directly impacting functional outcomes and risk of post-traumatic arthritis.

Anatomy & Coding Foundation

AO Code: 43 (Distal Tibia and Fibula, Malleolar region)

• Segments:
• 44-A: Infrasyndesmotic (below syndesmosis)
• 44-B: Transsyndesmotic (at syndesmosis level)
• 44-C: Suprasyndesmotic (above syndesmosis)

Morphology Types:

• A: Infrasyndesmotic fractures (generally stable, extra-articular or simple medial malleolus)
• B: Transsyndesmotic fractures (involve syndesmosis, potentially unstable)
• C: Suprasyndesmotic fractures (high fibular fractures, unstable, often with syndesmotic disruption)

Classification Details

Management Logic

Operative Indications:

• 44-A: Usually non-operative unless displaced medial malleolus or posterior fragment threatens stability.
• 44-B: ORIF indicated if medial lesion present or syndesmosis unstable; syndesmotic fixation may be required.
• 44-C: ORIF of fibula with syndesmotic fixation mandatory; proximal fibular fractures (Maisonneuve) require thorough evaluation and fixation of medial structures.

Fixation Principles:

• Absolute Stability: Medial malleolus fixation (lag screws or tension band) to restore ankle mortise congruity.
• Relative Stability: Fibular plating (bridge plating) to restore length and alignment, allowing biological healing.
• Syndesmotic fixation (screws or suture-buttons) to restore syndesmotic stability in B and C types.

Prognostic Factors

• Type A: Generally good prognosis with low non-union and arthritis risk if stable.
• Type B: Increased risk of chronic instability and post-traumatic arthritis if syndesmosis not adequately addressed.
• Type C: Highest risk of complications including non-union, chronic instability, and arthritis; requires meticulous fixation and syndesmotic repair.

Classic Clinical Notes

43-Tibia/Fibula Malleoli

• A – Infrasyndesmotic lesion
• 44-A-1 isolated
• 44-A-2 medial malleolus fracture
• 44-A-3 posteromedial fracture

• B – Transsyndesmotic fibular fracture
• 44-B-1 isolated
• 44-B-2 with medial lesion
• 44-B-3 with medial lesion

• C – Suprasyndesmotic lesion
• 44-C-1 fibular diaphyseal simple
• 44-C-2 fibular diaphyseal comminuted
• 44-C-3 proximal fibular lesion

Last Updated on January 25, 2026 by Christian Veillette