Thursday, June 7, 2007
All wounds heal through the same orderly process. Consider the simple laceration which is a well-studied wound. There are three phases of wound healing: the inflammatory, fibroblastic, and maturation stages.
The Inflammatory phase begins with the injury/laceration which is the stimulus needed to initiate the cellular and vascular response that serve to clean the wound of devitalized tissue and foreign material. The initial changes are vascular--a transient 5-10 minute period of vasoconstriction (aids hemostasis) followed by active vasodilation. Inflammation begins after injury and peaks at three to five days. The wound site swells as the biochemical ingredients needed for healing gather: leukocytes and monocytes for bacterial phagocytosis (wound cleaning) and lysis, fibrinogen for adherence of wound edges, histamine, prostaglandins, and vasoactive substances for hemostasis. All this must occur to prepare the wound for the succeeding phases of healing. Therefore, drugs which limit inflammation, such as non-steroidal anti-inflammatories or steroids, can slow the healing of a wound.
The Fibroblastic Phase begins on day 2 or 3 with fibroblasts moving into the wound along a framework of fibrin fibers. These fibroblasts begin to proliferate and position themselves for their principal task, collagen synthesis. As collagen content increases, the number of synthesizing fibroblasts begin to decrease until the rates of collagen degradation and synthesis are equivalent (collagen homeostasis). This is the time when the wound site strengthens. Sutures may be removed in three to fourteen days, depending on their location. However, the wound's tensile strength and collagen content increase over the next several weeks; collagen turnover within the wound continues indefinitely.
The Maturation Phase (or remodeling phase) begins approximately 3 weeks after injury and lasts the longest. This phase may continue for several years, with concomitant improvements in wound appearance. During this interval collagen synthesis and degradation are accelerated (no net increase in collagen content), large numbers of new capillaries growing into the wound regress and disappear, and collagen fibers initally deposited in a haphazard fashion gradually become more organized and arranged into a pattern determined by local mechanical forces. The formerly indurated, raised, and pruritic scar becomes a softer, less conspicuous scar, while the wound continues to gain tensile strength. As new collagen is deposited during this phase, more stable and permanent cross-links are established.
The tensile strength of a wound is a measurement of its load capacity per unit area. All wounds gain strength at approximately the same rate during the first 14-21 days, but thereafter the curves may diverge significantly according to the tissue involve. In skin, the peak tensile strength is achieved at approximately 60 days after injury. Even given optimal healing conditions, the tensile strength of a wound never reaches that of the original, leveling off at about 80%.
If more information is desired on wound healing, a very good review is the article "Wound Healing: An Overview" written by George Broughton II, M.D., Jeffrey E. Janis, M.D. and Christopher E. Attinger, M.D., published in the Plastic & Reconstructive Surgery Journal Volume 117, Number 7S, June Supplement 2006.