Mechanical tension on a wound has been identified as a leading cause for hypertrophic scar formation.

When a normal wound heals, the body produces new collagen fibres at a rate which balances the breakdown of old collagen. Hypertrophic scars are red and thick and may be itchy or painful. They do not extend beyond the boundary of the original wound, but may continue to thicken for up to six months. They usually improve over one or two years, but may cause distress due to their appearance or the intensity of the itching; they can also restrict movement if they are located close to a joint.

Some people have an inherited tendency to this type of scarring, for example, those with Ehlers–Danlos syndrome, classic type. It is not possible to completely prevent hypertrophic scars, so those with a history of them should inform their doctor or surgeon if they need surgery. Scar therapies may speed up the process of change from a hypertrophic scar to a flatter, paler one.

A hypertrophic scar is a cutaneous condition characterized by deposits of excessive amounts of collagen which gives rise to a raised scar, but not to the degree observed with keloids. Like keloids, they form most often at the sites of pimples, body piercings, cuts and burns. They often contain nerves and blood vessels. They generally develop after thermal or traumatic injury that involves the deep layers of the dermis and express high levels of TGF-β.

Histologically, keloids are fibrotic tumors characterized by a collection of atypical fibroblasts with excessive deposition of extracellular matrix components, especially collagen, fibronectin, elastin, and proteoglycans. Generally, they contain relatively acellular centers and thick, abundant collagen bundles that form nodules in the deep dermal portion of the lesion. Keloids present a therapeutic challenge that must be addressed, as these lesions can cause significant pain, (itching), and physical disfigurement. They may not improve in appearance over time and can limit mobility if located over a joint.

Keloids affect both sexes equally, although the incidence in young female patients has been reported to be higher than in young males, probably reflecting the greater frequency of earlobe piercing among women.

The frequency of occurrence is 15 times higher in highly pigmented people. African descendant people have increased risk of keloid occurrences.

Most people, especially Africans and African Americans, have a positive family history of keloid disorder. Development of keloids among twins also lends credibility to existence of a genetic susceptibility to develop keloids. Marneros et al. (1) reported four sets of identical twins with keloids; Ramakrishnan et al. also described a pair of twins who developed keloids at the same time after vaccination. Case series have reported clinically severe forms of keloids in individuals with a positive family history and black African ethnic origin.

Pseudomelanoma (also known as a "recurrent melanocytic nevus", and "recurrent nevus") is a cutaneous condition in which melantic skin lesions clinically resemble a superficial spreading melanoma at the site of a recent shave removal of a melanocytic nevus.

Lipohypertrophy is a medical term that refers to a lump under the skin caused by accumulation of extra fat at the site of many subcutaneous injections of insulin. It may be unsightly, mildly painful, and may change the timing or completeness of insulin action. It is a common, minor, chronic complication of diabetes mellitus.

Typical injection site hypertrophy is several inches or cm across, smoothly rounded, and somewhat firmer than ordinary subcutaneous fat. There may be some scar tissue as well, but the major component is adipose tissue, as insulin exerts a hypertrophic effect on adipose cells. To avoid lipohypertrophy, persons with diabetes mellitus who inject insulin daily for an extended period of time are advised to "rotate" their injections among several areas (usually upper, outer arms, outer thighs, abdomen below and around the umbilicus, and the upper parts of the buttocks). Rotation charts are often provided as part of diabetes education to help prevent lipohypertrophy.

Lipohypertrophy usually will gradually disappear over months if injections in the area are avoided.

It is a common misconception that the lump is largely scar tissue, as injection site hypertrophy is much rarer and milder with injections of other hormones and medications which lack the specific ability of insulin to stimulate adipose hypertrophy.

In a sense, the "opposite" of injection site lipohypertrophy is injection site lipoatrophy, in which the subcutaneous fat around an injected area "melts away" over a few weeks or months, leaving unsightly, well-demarcated depressions in the skin. The mechanism of this local lipoatrophy is not understood and may involve autoimmunity or local inflammation.

Also known as "scoop", "scallop", or "shave" excisional biopsy, or "shave excision". A trend has occurred in dermatology over the last 10 years with the advocacy of a deep shave excision of a pigmented lesion An author published the result of this method and advocated it as better than standard excision and less time consuming. The added economic benefit is that many surgeons bill the procedure as an excision, rather than a shave biopsy. This save the added time for hemostasis, instruments, and suture cost. The great disadvantage, seen years later is the numerous scallop scars, and a very difficult to deal with lesions called a "recurrent melanocytic nevus". What has happened is that many "shave" excisions does not adequately penetrate the dermis or subcutanous fat enough to include the entire melanocytic lesion. Residual melanocytes regrow into the scar. The combination of scarring, inflammation, blood vessels, and atypical pigmented streaks seen in these recurrent nevus gives the perfect dermatoscopic picture of a melanoma. When a second physicians re-examine the patient, he or she has no choice but to recommend the reexcision of the scar. If one does not have access to the original pathology report, it is impossible to tell a recurring nevus from a severely dysplastic nevus or a melanoma. As the procedure is widely practiced, it is not unusual to see a patient with dozens of scallop scars, with as many as 20% of the scars showing residual pigmentation. The second issue with the shave excision is fat herniation, iatrogenic anetoderma, and hypertrophic scarring. As the deep shave excision either completely remove the full thickness of the dermis or greatly diminishing the dermal thickness, subcutanous fat can herniate outward or pucker the skin out in an unattractive way. In areas prone to friction, this can result in pain, itching, or hypertrophic scarring.

Atrophia Maculosa Varioliformis Cutis (AMVC) is a condition involving spontaneous scarring, specifically depressed scars on the face occurring over a period of months to years. It appears to only affect children and young adults, is considered to be quite rare, normally occurs on the cheeks, temple area and forehead, and is not well understood nor presently treatable. Case reports indicate the scars deepen over time but remain relatively superficial, and with the frequency of new scar appearance diminishing over time.

AMVC is quite difficult to diagnose, for reasons including the depressed box and ice pick scars being very similar to that caused by Acne vulgaris. A confident diagnosis can be made if such scars recently appeared without present acne and without a history of acne. Otherwise the correct diagnosis is usually not made, and even doing so provides little benefit as there is no treatment. It has been suggested in case reports that the condition, although rare, is likely underreported.

Pseudopolyps are projecting masses of scar tissue that develop from granulation tissue during the healing phase in repeated cycle of ulceration (especially in inflammatory bowel disease). Inflammatory tissue without malignant potential, pseudopolyps may, according to Joffe (1977), represent either regenerating mucosal islands between areas of ulceration, edematous polypoid tags or granulation tissue covered by epithelium. There are reported cases when localized giant pseudopolyposis resulted in intestinal obstruction.

Residual mucosal islands between ulcerated and denuded areas of mucosa may have a polypoid appearance and are referred to as pseudopolyps. Polyposis syndromes, such as familial adenomatous polyposis, could give rise to a similar appearance on imaging, although the clinical presentation would differ from that of inflammatory pseudopolyposis.

Numerous, confluent ulcerations with bulging of the edematous residual mucosa determine a cobblestone appearance at endoscopy.

Epidermoid cysts commonly result from implantation of epidermis into the dermis, as in trauma or surgery. They can also be caused by a blocked pore adjacent to a body piercing. They are also seen in Gardner's syndrome and Nevoid Basal Cell Carcinoma Syndrome on the head and neck. They can be infected by bacteria and form a pimple-like shape.

Burn scar contracture refers to the tightening of the skin after a second or third degree burn. When skin is burned, the surrounding skin begins to pull together, resulting in a contracture. It needs to be treated as soon as possible because the scar can result in restriction of movement around the injured area.

Many conditions affect the human integumentary system—the organ system covering the entire surface of the body and composed of skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment. The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue. The two main types of human skin are: glabrous skin, the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, the hairs occur in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle. In the embryo, the epidermis, hair, and glands form from the ectoderm, which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.

The epidermis is the most superficial layer of skin, a squamous epithelium with several strata: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Nourishment is provided to these layers by diffusion from the dermis, since the epidermis is without direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Of these, keratinocytes are the major component, constituting roughly 95 percent of the epidermis. This stratified squamous epithelium is maintained by cell division within the stratum basale, in which differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where cells are continually shed from the surface. In normal skin, the rate of production equals the rate of loss; about two weeks are needed for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional two weeks to cross the stratum corneum.

The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary dermis and the reticular dermis. The superficial papillary dermis with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. Structural components of the dermis are collagen, elastic fibers, and ground substance. Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. The dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which are connected by vertical communicating vessels. The function of blood vessels within the dermis is fourfold: to supply nutrition, to regulate temperature, to modulate inflammation, and to participate in wound healing.

The subcutaneous tissue is a layer of fat between the dermis and underlying fascia. This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus. The main cellular component of this tissue is the adipocyte, or fat cell. The structure of this tissue is composed of septal (i.e. linear strands) and lobular compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.

Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), etiology (skin conditions resulting from physical factors), and so on. Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow). Diagnosis of many conditions often also requires a skin biopsy which yields histologic information that can be correlated with the clinical presentation and any laboratory data.

Peyronie's disease can be a physically and psychologically devastating disease. While most men will continue to be able to have sexual relations, they are likely to experience some degree of deformity and erectile dysfunction in the wake of the disease process. It is not uncommon for men afflicted with Peyronie's disease to exhibit depression or withdrawal from their sexual partners.

PDP occurs more frequently in men than in women (ratio around 7:1). Moreover, men suffer from more severe symptoms (see table 1). African American people are affected to a higher extent.

Table 1. Distribution of different forms of PDP among 201 reported affected men and women (167 men and 34 women).

An epidermoid cyst is a benign cyst usually found on the skin. The cyst develops out of ectodermal tissue. Histologically, it is made of a thin layer of squamous epithelium.

PDP is a rare genetic disease. At least 204 cases of PDP have been reported. The precise incidence and prevalence of PDP are still unknown. A prevalence of 0.16% was suggested by Jajic et Jajic.

Burn scar contractures do not go away on their own, although may improve with the passage of time, with occupationaltherapy and physiotherapy, and with splinting. If persistent the person may need the contracture to be surgically released. Techniques may include local skin flaps (z-plasty) or skin grafting (full thickness or split thickness). There are also pharmacy and drug-store treatments that can be used to help scar maturation, especially silicone gel treatments. Prevention of contracture formation is key. For instance, in the case of a burned hand one would splint the hand and wrap each finger individually. In the instance of burns on the neck, hyperextension of the neck (i.e. no use of pillows) should be maintained during the healing process. Carbon dioxide laser therapy is now also used to aid in the loosening of surrounding skin, although is yet to form as part of an official global rehabilitation program.

Parry–Romberg syndrome appears to occur randomly and for unknown reasons. Prevalence is higher in females than males, with a ratio of roughly 3:2. The condition is observed on the left side of the face about as often as on the right side.

Elephantiasis nostras is a cutaneous condition, a final hypertrophic fibrosis following longstanding chronic lymphangitis.

Treatment consists of antibiotics, elevation of the affected limb, and compression. For persons with elephantiasis nostras who are overweight or obese, weight loss is recommended. Oral retinoids have been used to treat the cutaneous manifestations of the disease.

A tufted angioma (also known as an "Acquired tufted angioma," "Angioblastoma," "Angioblastoma of Nakagawa," "Hypertrophic hemangioma," "Progressive capillary hemangioma," and "Tufted hemangioma") usually develops in infancy or early childhood on the neck and upper trunk, and is an ill-defined, dull red macule with a mottled appearance, varying from 2 to 5 cm in diameter.

The underlying cause of Peyronie's disease is not well understood, but is thought to be caused by trauma or injury to the penis usually through sexual intercourse or physical activity, although many patients are often unaware of any traumatic event or injury.

In pathology, hypertrophic decidual vasculopathy, abbreviated HDV, is the histomorphologic correlate of gestational hypertension, as may be seen in intrauterine growth restriction (IUGR) and HELLP syndrome.

The name of the condition describes its appearance under the microscope; the smooth muscle of the decidual (or maternal) blood vessels is hypertrophic, i.e. the muscle part of the blood vessels feeding the placenta is larger due to cellular enlargement.

The fact that some people affected with this disease have circulating antinuclear antibodies in their serum supports the theory that Parry–Romberg syndrome may be an autoimmune disease, specifically a variant of localized scleroderma. Several instances have been reported where more than one member of a family has been affected, prompting speculation of an autosomal dominant inheritance pattern. However, there has also been at least one report of monozygotic twins in which only one of the twins was affected, casting doubt on this theory. Various other theories about the cause and pathogenesis have been suggested, including alterations in the peripheral sympathetic nervous system (perhaps as a result of trauma or infection involving the cervical plexus or the sympathetic trunk), as the literature reported it following sympathectomy, disorders in migration of cranial neural crest cells, or chronic cell-mediated inflammatory process of the blood vessels. It is likely that the disease results from different mechanisms in different people, with all of these factors potentially being involved.

Risk factors for the development of acne, other than genetics, have not been conclusively identified. Possible secondary contributors include hormones, infections, diet and stress. Studies investigating the impact of smoking on the incidence and severity of acne have been inconclusive. Sunlight and cleanliness are not associated with acne.