Learning Objective
Explain the mechanism of scar formation, compare hypertrophic and keloid scars, and identify their key clinical differences.
Scar Formation
Definition
Scar formation occurs when tissue repair cannot be achieved by cell regeneration alone. In the setting of severe acute or chronic injury, nonregenerated cells are replaced by connective tissue, resulting in a fibrous scar.
Mechanical properties
- Approximately 70–80% of normal tensile strength is regained by 3 months
- Minimal additional tensile strength is gained after this period
Pathophysiology
- Excess TGF-β signaling promotes aberrant fibroblast activation and excessive collagen deposition
- Associated with abnormal scarring, including hypertrophic scars and keloids
Types of Pathologic Scars
| Feature | Hypertrophic Scar | Keloid Scar |
|---|---|---|
| Collagen synthesis | Increased (↑ type III collagen) | Markedly increased (↑↑↑ type I and III collagen) |
| Collagen organization | Parallel, organized | Disorganized |
| Extent of scar | Confined to the borders of the original wound | Extends beyond wound margins with “clawlike” projections |
| Common locations | Any wound site | Earlobes, face, upper extremities |
| Recurrence after excision | Infrequent | Frequent |
| Predisposition | None | Increased incidence in individuals with darker skin |
