Fig. 3.27C ) are areas of skin ulceration and necrosis of underlying tissues caused by prolonged compression of tissues against a bone, for example, in bedridden, immobile elderly individuals with numerous morbidities. The lesions are caused by mechanical pressure and local ischemia.
Fig. 3.27D ) affect the lower extremities, particularly the feet. Tissue necrosis and failure to heal are the result of small vessel disease causing ischemia, neuropathy, systemic metabolic abnormalities, and secondary infections. Histologically, these lesions are characterized by epithelial ulceration (
Fig. 3.27E ) and extensive granulation tissue in the underlying dermis (
Fig. 3.27F
In some cases failure of healing may lead to dehiscence (wound rupture). Although not common, this occurs most frequently after abdominal surgery and is a result of increased abdominal pressure, such as may occur with vomiting, coughing, or ileus.
Excessive formation of the components of the repair process can give rise to hypertrophic scars and keloids.
The accumulation of excessive amounts of collagen may result in a raised scar known as a hypertrophic scar. These often grow rapidly and contain abundant myofibroblasts, but they tend to regress over several months (
Fig. 3.28A ). Hypertrophic scars generally develop after thermal or traumatic injury that involves the deep layers of the dermis. If the scar tissue grows beyond the boundaries of the http://ebooksmedicine.net
Fig. 3.27 Chronic wounds illustrating defects in wound healing. (A–D) External appearance of skin ulcers. repair and regeneration: mechanisms, signaling, and translation, Sci Transl Med 6:265, 2014.) (A) Venous leg ulcer; (B) arterial ulcer, with more extensive tissue necrosis; (C) diabetic ulcer; and (D) pressure sore. (E–F) Histologic appearance of a diabetic ulcer. (E) ulcer crater; (F) chronic inflammation and granulation (From Eming SA, Margin P, Tomic-Canic M: Wound tissue.
original wound and does not regress, it is called a keloid (
Fig. 3.28B ). Certain individuals seem to be predisposed to keloid formation, particularly those of African descent.
Exuberant granulation is another deviation in wound healing characterized by the formation of excessive amounts of granulation tissue, which protrudes above the level of the surrounding skin and blocks reepithelialization (this process has been called, with more literary fervor, proud flesh). Excessive granulation must be removed by cautery or surgical excision to permit restoration of epithelial continuity. Rarely, incisional scars or traumatic injuries may be followed by exuberant proliferation of fibroblasts and other connective tissue elements that may, in fact, recur after excision. Called desmoids, or aggressive fibromatoses, these neoplasms lie at the gray zone of benign and malignant low-grade tumors.
Wound contraction is an important part of the normal healing process. An exaggeration of this process gives rise to contracture and results in deformities of the wound and the surrounding tissues. Contractures are particularly prone to develop on the palms, the soles, and the anterior aspect of the thorax. Contractures are commonly seen after serious burns and can compromise the movement of joints.
The term fibrosis is used to denote the excessive deposition of collagen and other ECM components in a tissue.
The terms scar and fibrosis may be used interchangeably, but fibrosis most often refers to the abnormal deposition of collagen that occurs in internal organs in chronic diseases. The basic mechanisms of fibrosis are the same as those of scar formation in the skin during tissue repair. Fibrosis is a pathologic process induced by persistent injurious stimuli such as chronic infections and immunologic reactions, and is typically associated with loss of tissue (
Fig.