When a scar, instead of becoming soft and pale in the usual manner, becomes red and thickened it is described as being either a hypertrophic scar or a keloid. These terms tend to be used rather indiscriminately, probably because it is difficult to define each with certainty.
The hypertrophic scar is raised above the level of the surrounding skin, rather red initially, but does not encroach on the surrounding normal skin, and shows an eventual tendency to regress spontaneously.
The keloid is a much more florid lesion, grossly elevated, tending to spread and involve the surrounding normal skin. Itching of the involved area is common, its severity matching the degree of activity, sometimes with hyperaesthesia and tenderness to touch. The tendency to spontaneous regression is much less in evidence.
These are the extremes and as such easily recognised, but in reality there is a gradation from the completely quiescent scar through the very mildly hypertrophic scar to the most severe of keloids, and the point at which a hypertrophic scar becomes a keloid is a matter of opinion. The gradation rather suggests that the arbitrary division into keloid and hypertrophic scar is artificial and that the conditions are really a single entity of varying severity.
The name is fortunately of subsidiary importance, for the treatment of both conditions is similar. Virtually nothing is known of the cause.