Blackburn and Cosman outline these criteria in their landmark article in 1966. The value of these criteria is that they are predictive of clinical outcome. Keloids differ from hypertrophic scars in osmotic pressure, metabolic activity, and collagen turnover as reflected in the local concentrations of sodium, magnesium and calcium respectively. Keloid collagen is thin and irregular and reveals cross-striations by electron microscopy, suggesting that it is immature. Also, keloids have a higher content of water and soluble collagen than normal skin.
Keloids are deficient in lymphatics and associated elastic fibers.
These qualities are true of early hypertrophic scars as well, but after seven months the two diverge as the hypertrophic scars normalize their water and collagen content. The activity of Proline hydroxylase is markedly elevated in both hypertrophic scars and keloids when compared to normal scars. Collagen degradation appears to be normal or elevated. Serum alpha-globulins may explain why keloids accumulate collagen when the relative rates of synthesis and degradation appear normal in vitro. These substances are all known to inhibit the activity of collagenase.