Types of Acne Scars :

There are two general types of acne scars, defined by tissue response to inflammation: (1) scars caused by increased tissue formation, and (2) scars caused by loss of tissue.

Scars Caused by Increased Tissue Formation
The scars caused by increased tissue formation are called keloids or hypertrophic scars. The word hypertrophy means "enlargement" or "overgrowth." Both hypertrophic and keloid scars are associated with excessive amounts of the cell substance collagen. Overproduction of collagen is a response of skin cells to injury. The excess collagen becomes piled up in fibrous masses, resulting in a characteristic firm, smooth, usually irregularly-shaped scar. The photo shows a typical severe acne keloid:

(Photo used with permission of the American Academy of Dermatology
National Library of Dermatologic Teaching Slides)

The typical keloid or hypertrophic scar is 1 to 2 millimeters in diameter, but some may be 1 centimeter or larger. Keloid scars tend to "run in families"—that is, abnormal growth of scar tissue is more likely to occur in susceptible people, who often are people with relatives who have similar types of scars.

Hypertrophic and keloid scars persist for years, but may diminish in size over time.

Scars Caused by Loss of Tissue
Acne scars associated with loss of tissue—similar to scars that result from chicken pox—are more common than keloids and hypertrophic scars. Scars associated with loss of tissue are:

Ice-pick scars usually occur on the cheek. They are usually small, with a somewhat jagged edge and steep sides—like wounds from an ice pick. Ice-pick scars may be shallow or deep, and may be hard or soft to the touch. Soft scars can be improved by stretching the skin; hard ice-pick scars cannot be stretched out.

Depressed fibrotic scars are usually quite large, with sharp edges and steep sides. The base of these scars is firm to the touch. Ice-pick scars may evolve into depressed fibrotic scars over time.

Soft scars, superficial or deep are soft to the touch. They have gently sloping rolled edges that merge with normal skin. They are usually small, and either circular or linear in shape.

Atrophic macules are usually fairly small when they occur on the face, but may be a centimeter or larger on the body. They are soft, often with a slightly wrinkled base, and may be bluish in appearance due to blood vessels lying just under the scar. Over time, these scars change from bluish to ivory white in color in white-skinned people, and become much less obvious.

Follicular macular atrophy is more likely to occur on the chest or back of a person with acne. These are small, white, soft lesions, often barely raised above the surface of the skin—somewhat like whiteheads that didn’t fully develop. This condition is sometimes also called "perifollicular elastolysis." The lesions may persist for months to years.