Surgical technique – wounds

Healing

With a sound understanding of the healing process, you will be able to undertake effective wound repair and minimise complications. The process of healing is described as a series of phases, however it is important to realise that physiologically these phases overlap even though they are considered separately here.

Inflammatory phase

a) Haemorrhage:

Blood vessels constrict and a thrombosis forms to slow haemorrhage, after a few minutes blood vessels relax, various cell mediators are released and a clot is formed. If this clot remains undisturbed it will provide a framework for the elements of repair.

b) Inflammation:

The mediators released from the damaged cells cause a local accumulation of ‘inflammatory cells’. These inflammatory cells and other blood borne factors ensure the process remains localised, clear up infective debris and secrete locally active growth factors to stimulate the process of healing.

c) Primary wound contracture:

Local fibroblasts contract to decrease the surface area of the wound. The outward signs that may be observed during this acute inflammatory phase are:

• Heat
• Redness
• Swelling
• Pain

Repair

If the body has been able to control infection (and in the context of surgical wounds this is more likely if aseptic surgical techniques are used) inflammation subsides and the process will progress to the repair phase.

a) Epithelialisation:

The cells in the bottom layer of skin at the edges of the wound migrate under the scab to cover the defect. In a moist, protected environment this occurs within 12-24 hours.

b) Granulation:

Capillaries and fibroblasts beneath the epithelial layer multiply and migrate inward. This forms granulation tissue, which is firm and pink in appearance and resistant to infection. This tissue bed supplies oxygen for epithelialisation, provides collagen and plays a part in wound contracture.

Contracture

The granulation tissue pulls the skin margins inwards (20 wound contracture) as it matures into fibrous tissue. This contracture ceases when wound edges are apposed or the tension in surrounding skin is equal to that of the fibroblasts (approximately 5-9 days after wound formation). This process is beneficial as it decreases the area to be covered by epithelialization, however complications can arise at this stage. Some examples of these are:

• Scar formation.
• Web effect: contracture causes a webbing effect in areas that would normally be highly mobile and this may limit joint movement.
• Deformity: contracture deforms or (partially) occludes a body opening/hollow organ.
• Incomplete closure: too much tension is present as the wound contracts because of its size and site leading to incomplete closure.

Maturation

As fibroblasts continue to lay down collagen the granulation tissue is transformed into fibrous tissue and the collagen fibres along the lines of tension become thicker whilst others recede. A balance is established between collagen being laid down and the recession of non-functioning collagen at approximately 3 weeks following the initial injury. Maturation and remodelling continues for months and wound strength can continue to increase for up to 2 years. However, the scar will always remain weaker than the surrounding tissues.

Factors influencing the healing process

Having an understanding of the healing process, and being aware that it is affected by a large number of factors helps the investigator to prevent complications arising and to assess their potential causes if they do occur. The following list illustrates some of the factors that influence the speed and success of healing:

• Presence of alien tissue or a foreign body
• Disturbances in local blood supply
• Infection
• Environmental temperature
• Wound tension
• Tissue trauma
• Region of the body and tissue type
• Age
• Nutritional status
• Vitamin deficiency (specifically C, A and E)
• Pre-existing disease
• Corticosteroids either released by the animal in response to stressful situations or administered as medication
• Chemotherapy or radiation therapy

Wound Classification

Wounds are classified by the extent to which they have become contaminated. The four categories are:

• Clean wound
• Clean contaminated
• Contaminated
• Dirty

The majority of surgical wounds should be “clean” wounds – no infection is encountered during surgery, there is no compromise in aseptic technique and no hollow organs (eg gastro-intestinal tract) are opened. In some instances “clean-contaminated” wounds will occur if there is a minor break in aseptic technique, a hollow organ is opened with minimum spillage or the respiratory or reproductive tract is opened. “Contaminated” wounds should be encountered extremely rarely where gross spillage from a hollow organ or a major break in aseptic technique has occurred. “Dirty” wounds should not be encountered in the context of experimental animals unless it is an intentional component of the study, or the animal has incurred accidental traumatic injury.

Wound Closure

The purpose of wound closure is to bring wound edges together and support them during healing, whilst eliminating potential dead space. There are three options for wound closure; primary closure, delayed primary closure, secondary closure.

Definitions

• Primary closure: union of accurately apposed edges of a wound with an irreducible minimum of granulation tissue. Healing by this means is known as healing by ‘first intention’.
• Delayed primary closure: union of a wound that is closed surgically several days after the wound occurs because it is too contaminated to close at first (an option following a traumatic injury). This allows ‘healing by third intention’.
• Secondary closure: union of a wound by adhesion of granulating surfaces; this can be achieved with or without surgical closure. If surgical closure is selected, the wound is left open for several days until a granulation bed is established and the wound edges are then trimmed and closed. This allows ‘healing by second intention’. Healing by second intention without surgical intervention involves leaving the wound open and managing it carefully to prevent infection whilst allowing normal healing processes to resolve the skin defect.

It is very unlikely that any option other than primary closure would be an appropriate course of action in the case of an experimental animal. Unless the technique was a component of a study or the animal was undergoing veterinary treatment. Only primary closure will be discussed in these notes. If a wound is ‘contaminated’ primary closure is unlikely to be a suitable method of wound management and other sources of information (and veterinary assistance) should be sought to ensure the correct course of action is followed.

Primary closure

The incision is closed immediately and completely following the surgical procedure, with strict adherence to aseptic principles. Clean wounds are closed in this way. Clean-contaminated wounds can be converted to clean wounds by meticulous removal of contaminated tissues, coupled with copious dilution and washing away of the infective material using sterile, isotonic fluids in large volumes, under some pressure (known as lavage). Once the wound has been lavaged it may then be closed primarily. Occasionally, fresh ‘contaminated’ wounds can be converted to ‘clean’ wounds, but it is wise to seek veterinary advice if this course of action is being considered.