THE NORMAL HEALING PROCESS
Understanding the normal healing process will facilitate proper use of a given modality. There are three basic phases of wound healing: inflammation, proliferation, and remodeling. The inflammatory response is the body's nonspecific defense mechanism and begins almost immediately following injury. Inflammation can be triggered by a variety of causes, including trauma, disease, invading pathogens, or allergic reactions. Acute inflammation is characterized by varying degrees of redness, warmth, pain, swelling, and loss of function.
During the inflammatory phase, hemostasis occurs first. It begins almost immediately after injury and is responsible for stopping bleeding at the injury site. The most important cellular component is the platelet, which responds to the injured area, adheres to exposed collagen, and forms a clot that stops the bleeding. In addition, platelets play a role in later stages of healing because of the growth factors they produce. After the bleeding stops, edema continues to accumulate in the injured area due to extravasation, which is movement of fluid from the blood vessels into the extravascular space. This excess fluid results in swelling, redness, and elevated local temperature. In addition, distention of the local tissues and irritation of the free nerve endings in the injured area can result in pain. Although inflammation is usually perceived as being negative, a healthy inflammatory process is critical to successful healing. The tissue distention creates space for the influx of phagocytic cells and proteins that set the stage for later phases of healing. These cells, specifically neutrophils and macrophages, are responsible for cleaning the wounded area of nonviable material so the proliferative phase can begin.
The inflammatory phase leads to the proliferative phase, in which fibroblasts and keratinocytes predominate. The function of this phase is to repair the defect. It can last several weeks, depending on the injury. Fibroblasts are attracted to the wound by macrophages in the inflammatory phase and typically arrive in the area 48 to 72 hours after injury. Fibroblasts lay down collagen and elastin, which replace the tissue that was damaged in the initial injury or removed during the inflammatory process. If the injury resulted in a break in the skin, keratinocytes will also be important to cover the wound with a new layer of epithelium.
The final stage is remodeling. During this phase, the newly formed collagen matrix is rearranged and continues to gain tensile strength. This stage is by far the longest and can last in excess of 1 year.
Tissue healing can become problematic at any phase. Tissues may remain chronically inflamed or may fail to regenerate as needed to heal the defect. In an effort to maximize healing, biophysical agents have been used in all phases of wound healing. The remainder of this chapter focuses on common physical modalities and their effects on tissue healing. A more encompassing discussion of the healing process is available elsewhere.1