## Contusion: Mechanism and Pathophysiology ### Definition and Core Feature **Key Point:** A contusion (bruise) is a blunt force injury characterized by rupture of blood vessels with hemorrhage into surrounding tissues, while the overlying skin remains intact. ### Pathophysiology of Contusion **High-Yield:** The hallmark of a contusion is **intact skin overlying hemorrhage**. This distinguishes it from lacerations (which breach skin) and abrasions (which remove epidermis). ### Mechanism 1. Blunt force applied to skin and underlying tissues 2. Compressive and shearing forces rupture capillaries and small vessels 3. Blood extravasates into dermis and subcutaneous tissue 4. Overlying epidermis remains unbroken 5. Visible discoloration (ecchymosis) develops as hemoglobin is metabolized ### Color Changes Over Time (Forensic Dating) | Timeline | Color | Mechanism | |----------|-------|----------| | **0–2 days** | Red, blue, purple | Oxyhemoglobin, deoxyhemoglobin | | **3–5 days** | Green | Biliverdin formation | | **5–7 days** | Yellow-brown | Bilirubin formation | | **7–10 days** | Brown, fading | Hemosiderin | | **>2 weeks** | Resolved | Complete reabsorption | **Clinical Pearl:** Contusions are common in assault cases and accidental trauma. Multiple contusions in different stages of healing suggest repeated trauma over time — a red flag for abuse. ### Distinction from Other Injuries | Injury | Skin Integrity | Bleeding | Depth | |--------|---|---|---| | **Contusion** | Intact | Subcutaneous hemorrhage | Dermis + subcutaneous | | **Abrasion** | Disrupted (removed) | Minimal capillary ooze | Epidermis + superficial dermis | | **Laceration** | Disrupted (torn) | Profuse | Full thickness | **Mnemonic:** **C**ontusion = **C**losed (skin intact), **C**ollection of blood underneath.
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