Scars/ Acne Scarring
A detailed and comprehensive discussion of acne scars starts with causes of scarring, prevention of scarring, types of scars, and treatments for scars.
Before talking about scars, a word about spots that may look like scars but are not scars in the sense that a permanent change has occurred. Even though they are not true scars and disappear in time, they are visible and can cause embarrassment.

Macules or "pseudo-scars" are flat, red or reddish spots that are the final stage of most inflamed acne lesions. After an inflamed acne lesion flattens, a macule may remain to "mark the spot" for up to 6 months. When the macule eventually disappears, no trace of it will remain—unlike a scar.

Post-inflammatory pigmentation
is discoloration of the skin at the site of a healed or healing inflamed acne lesion. It occurs more frequently in darker-skinned people, but occasionally is seen in people with white skin. Early treatment by a dermatologist may minimize the development of post-inflammatory pigmentation. Some post-inflammatory pigmentation may persist for up to 18 months, especially with excessive sun exposure. Chemical peeling may hasten the disappearance of post-inflammatory pigmentation.

Causes of Acne Scars
In the simplest terms, scars form at the site of an injury to tissue. They are the visible reminders of injury and tissue repair. In the case of acne, the injury is caused by the body’s inflammatory response to sebum, bacteria and dead cells in the plugged sebaceous follicle. Two types of true scars exist, as discussed later: (1) depressed areas such as ice-pick scars, and (2) raised thickened tissue such as keloids.

When tissue suffers an injury, the body rushes its repair kit to the injury site. Among the elements of the repair kit are white blood cells and an array of inflammatory molecules that have the task of repairing tissue and fighting infection. However, when their job is done they may leave a somewhat messy repair site in the form of fibrous scar tissue, or eroded tissue.

White blood cells and inflammatory molecules may remain at the site of an active acne lesion for days or even weeks. In people who are susceptible to scarring, the result may be an acne scar. The occurrence and incidence of scarring is still not well understood, however. There is considerable variation in scarring between one person and another, indicating that some people are more prone to scarring than others. Scarring frequently results from severe inflammatory nodulocystic acne that occurs deep in the skin. But, scarring also may arise from more superficial inflamed lesions.

The life history of scars also is not well understood. Some people bear their acne scars for a lifetime with little change in the scars, but in other people the skin undergoes some degree of remodeling and acne scars diminish in size.
People also have differing feelings about acne scars. Scars of more or less the same size that may be psychologically distressing to one person may be accepted by another person as "not too bad." The person who is distressed by scars is more likely to seek treatment to moderate or remove the scars.

Types of Scars/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 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.