## Delayed Union in Fracture Healing **Key Point:** Delayed union occurs when fracture healing progresses at a slower rate than expected but eventually achieves bony union, typically beyond 3–6 months depending on bone type and location. ### Most Common Cause: Inadequate Immobilization Inadequate immobilization is the **most frequent preventable cause** of delayed union. Excessive micromotion at the fracture site disrupts the formation of a stable callus and prolongs the inflammatory and reparative phases. **High-Yield:** The fracture healing process requires: - Haematoma formation (0–1 week) - Inflammatory phase (1–2 weeks) - Soft callus formation (2–3 weeks) - Hard callus formation (3–12 weeks) - Remodelling phase (months to years) When immobilization is inadequate, progression through these phases is delayed. ### Comparison of Causes of Delayed Union | Cause | Mechanism | Reversibility | Frequency | | --- | --- | --- | --- | | **Inadequate immobilization** | Excessive micromotion disrupts callus | Reversible with proper fixation | Most common | | Infection | Osteomyelitis destroys new bone | Partially reversible | Common | | Excessive gap | Requires bridging callus over large distance | Partially reversible | Moderate | | Vascular injury | Impaired blood supply to fracture ends | Often irreversible | Less common | **Clinical Pearl:** Delayed union is distinguished from non-union: delayed union eventually heals (though slowly), whereas non-union shows no progression toward healing at 6–9 months and requires surgical intervention (e.g., bone grafting, plate fixation). **Tip:** In exam questions, when asked about "delayed union," think of **modifiable factors** — inadequate immobilization tops the list because it is the most common and most easily corrected cause.
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