If we do not know how each phase of healing progresses, when it starts and ends, and the intricacies of the healing mechanisms of individual tissues, we cannot treat our patients correctly and promote healing. It is imperative for us to have a thorough working knowledge of how tissues heal, their various timelines for healing, and the differences in how they heal. Muscle, bone, tendons, ligaments and cartilage all heal according to a different timeline.
A thorough understanding of tissue healing enables us to treat appropriately; to know whether to stimulate or suppress inflammation, to ensure sufficient rest and minimal stress during repair and what level of progressive stress to add during remodelling.
We will start with the How and move on to the When.
Three phases of healing
There are three phases of healing: The inflammatory phase, the reparative phase, and the remodelling or maturation phase. These phases are all slightly different for different tissue.
The inflammatory phase
Inflammation is the first phase of wound healing and is essential for healing to occur; if the inflammatory phase is too short or too long, healing will be impaired.
The vascular response
The inflammatory phase kicks off with a vascular response that creates homeostasis, or a balanced state. If a wound is formed, blood vessels are disrupted, which leads to bleeding and the presence of blood between cells. Blood platelets will react with collagen to cause a coagulation cascade, resulting in the formation of a fibrin network. The fibrin network will support the blood plug, as well as the next step in the inflammatory process.
Cellular infiltration
Using the fibrin network as support, cellular infiltration will be the main activity in the wound for the next two to three days. The first cells to enter the wound area are neutrophils, which make an appearance six hours after the wound is created. Their primary purpose is to clean or debride the wound and prevent contamination. The next cells to appear, within 24 to 48 hours, are macrophages, which play a role in phagocytosis, debridement, matrix synthesis regulation, cell recruitment and activation, and angiogenesis. Macrophages are essential to transition the wound to the next phase of healing – reparative.
The transition
Wounds need to transition from inflammation to the reparative phase after two to three days. This transition phase is important when choosing which modalities we use for treatment. We don’t want to suppress the inflammatory phase, but in some tissues, we will benefit the body if we can speed it up. Tissues like tendons and ligaments have a very limited inflammatory phase, and we’ll want to be very cautious about anything that might suppress or shorten this phase. Inflammation that continues past the acute phase of an injury can also be a problem – chronic inflammation needs to be addressed as it will lead to further injury and breakdown of the injury site.
Reparative phase
The reparative or proliferation phase starts off with fibroblast proliferation and migration within three days of injury and is followed by matrix synthesis.
The Matrix
Matrix is made up of collagen, elastin and proteoglycans. Proteoglycan synthesis starts after about two weeks. In the following weeks, matrix synthesis slowly starts to increase the tensile strength of the area. As the collagen and fibroblast scaffold forms, new capillaries grow into the area until a normal oxygen level is achieved in the tissue. This tissue is generally recognized as granulation tissue, made up of a dense network of macrophages, fibroblasts and blood vessels.
Protection
During this phase, it is essential to protect the tissue from any stress that might widen the wound, as that would delay healing and cause scarring, which reduces tensile strength in the tissue. We need to be especially cautious with any therapeutic exercise at this point, including hydrotherapy.
Remodelling phase
In the remodelling phase, collagen fibres progressively align themselves parallel to the lines of stress and strain. As they align, crosslinks form between them to create a stable formation. This process is essential if the tissue is to regain its original tensile strength, making rehabilitation during this phase critical. If the incorrect stresses are placed on the tissue, fibrous tissue might dominate this phase and tensile strength will never be regained.
Veterinary Rehabilitation
Our therapeutic exercise becomes so important in the remodelling phase. Rehab is more than just therapeutic exercise. It is imperative that we know and understand how each of our therapeutic modalities impacts tissue and healing so that we can use the modalities correctly and at the appropriate phase of healing. This includes everything from the application of heat and ice, to massage, to therapeutic ultrasound and laser, and all the modalities in between. All of these modalities have different effects on tissues, depending on which type of tissue we’re dealing with. If we don’t know the specifics of how our modalities impact the various tissues at different phases of healing, we may well end up delaying or impairing healing.
Additional resources for your practice
Now that we have an idea of how wound healing progresses, we can move on to the when, and the specific differences between bone, muscle, tendon, ligament and cartilage.
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