Non-healing, or difficult to heal wounds in veterinary practice rarely pose an immediate risk to life, however, they can result in prolonged periods of veterinary care which can be very costly to the owner, uncomfortable and stressful for the patient, and potentially may result in euthanasia, or limb amputation in some cases.

There are numerous underlying causes of non-healing wounds and these can generally be divided into wound factors, patient factors and surgeon factors.

Wound Factors

Wound Aetiology

Traumatic wounds may be more difficult to heal, dependent on the amount of damage caused to the surrounding skin. Also, when dealing with shearing injuries to the lower limbs, it is not uncommon for exposed bone to be present. Exposed bone is often encountered in distal limb degloving wounds and inhibits wound contraction and epithelialization. When allowed to heal by second intention, a bed of granulation tissue is required for second-intention healing to progress. During the wound healing process the majority of exposed bone becomes covered with granulation tissue that arises from the viable periosteum or viable soft tissues adjacent to the bone surfaces, but there is always the risk of potential for osteomyelitis or sequestrum formation. One useful technique is to drill a number of small holes into the vascular medullary canal of the bone (forage), which will promote granulation tissue formation. This obviously needs to be done carefully, to ensure the bone is not weakened, resulting in fracture.

Contamination

Wounds are commonly classed as clean, clean contaminated, contaminated, and dirty. It is important to determine the wound type from this perspective order to plan wound management. Only clean or clean-contaminated wounds should be considered for closure, with all other wounds requiring at least a period of open wound management to convert them to a clean-contaminated state, whereby they may be considered for closure.

Infection

When infection is present within a wound, this will cause inhibition of wound healing throughout all stages of the wound healing process. In order to identify infection a deep tissue culture should be obtained, this is preferable to a surface swab, as surface contaminants do not truly reflect infection. Infection is thought to be present in around 3% of all wounds at the time of suture removal, but infection rates are very variable, dependent on the degree of wound contamination. The aetiology of the wound also will affect the likelihood of infection occurring, with bite wounds having a high probability of infection due to the wound and its surrounding tissues creating the ideal environment for bacterial replication, along with poor drainage and a hypoxic environment. There are also surgical causes of infection e.g., duration of the procedure, number of theatre staff, placement of surgical drains, suture material, underlying disease, and the use of propofol, have all been associated with increased incidence in surgical site infections in veterinary patients. When faced with chronic wound infections, where a purulent discharge is present, a multimodal approach to dealing with infection is recommended. The following approach was recommended by Demetriou and Stein when dealing with wound infection:

• Investigate and address any underlying causes or contributing factors, e.g., malnutrition, anaemia, systemic disease
• Maintain an ‘open’ wound environment, while fully exploring any pockets between soft tissue planes using an aseptic technique
• Debride necrotic, devitalised tissue
• Ensure thorough lavage of the affected areas with sterile isotonic fluids is performed
• Obtain deep tissue samples for aerobic and anaerobic bacteriological culture and sensitivity testing
• Commence empirical antibiotics (which can be changed according to culture results)
• Use bandaging to help with debridement (e.g., wet-to-dry bandages, or antimicrobial dressings)
• Continue open wound management until infection is controlled

Wound Location

Wounds over joints present a challenge to healing in that they are subject to tension, compression, or shearing forces. The desired result of wound healing is for the two sides of a wound to heal together. If they are exposed to these forces, however, healing is impaired. Wounds over extension surfaces of joints (e.g., carpus, stifle) are subject to tension when joint flexion pulls wound edges apart. Thus, meticulous closure is necessary. Casting or splinting the joint is necessary to prevent joint flexion for proper healing. Footpads are shock absorbing and spread as weight is applied. Thus, in the presence of an open wound, edges are pushed apart, impeding healing. If sutures are present in the pad, such pad spreading results in sutures tearing through the tissues. It is, therefore, necessary to relieve pressure on footpads to attain adequate healing, especially in large dogs. Various bandaging and splinting techniques have been evaluated as to their efficacy in reducing pressure on digital and metacarpal or metatarsal pads using various forms of foam rubber pads, metal splints, and combinations of these. Wounds in the axillary and inguinal areas may result from forelimb entrapment in a collar, vehicular trauma, burns, neoplasia, and infections. A primary factor in the impaired healing of such wounds is the shearing movement between the two wound surfaces as the animal ambulates. If such wounds have been present for a long period, it is possible that there may be infection with an atypical organism. Thus, a biopsy for culture and sensitivity testing is indicated. Techniques for closing such wounds have included meticulous closure and the use of skin fold flaps, omental pedicle flaps, axial pattern skin flaps, or combinations of these.

Patient Factors

Underlying disease can be considered from two viewpoints: disease arising due to the trauma that caused the wound to be sustained, and pre-existing concurrent disease. Trauma can cause a number of issues which will have a knock-on effect in respect to wound healing times including; shock, hypoperfusion, or dehydration, concurrent organ dysfunction, (e.g., respiratory, renal, gastrointestinal compromise/damage). Patients may also have a concurrent disease process that can hinder wound healing. Hyperadrenocorticism (Cushing’s disease), diabetes mellitus, hypothyroidism, anaemia, are some of the disease processes, which can delay wound healing.

Nutritional Status

Adequate nutritional intake is vital for wound healing to occur, and should be a priority in all critical/trauma patients. A catabolic state, attributable to malnutrition, is a major contributing factor to nonhealing wounds. In this condition, the body does not have the necessary protein and energy sources (fats and carbohydrates); therefore, existing stores of protein are broken down to maintain basal functions. This means, the increased calorific and protein demands for healing are not available, and the wound becomes quiescent. Glucose and protein are important for normal progression of wound healing. Glucose is the primary source of energy for leukocytes and fibroblasts. It is the integral molecule within the ground substance that is laid down by the fibroblasts. Deposition of this is necessary before collagen formation. Thus, glucose deficiency can affect collagen formation and wound strength. Depletion of protein stores can result in attenuated fibroplasia and prolonged healing time. It is vital that patients receive adequate protein levels, as they are necessary for animals undergoing healing. Sufficient protein levels help to prevent oedema and promote increased fibroplasia with increased wound strength. Vitamins may also affect wound healing. Excess vitamin A labilizes lysosomes to enhance inflammation. Because steroids stabilize lysosomes and inhibit wound repair, vitamin A can counteract this negative effect. Likewise, vitamin E stabilizes lysosomes similar to steroids and thus can inhibit healing in large doses. Again, vitamin A can reverse the effects of vitamin E. Vitamin C deficiency can impair healing in that it is necessary for the hydroxylation of proline and lysine in collagen synthesis. Although dogs and cats do not require exogenous sources of vitamin C, there is the possibility that the vital levels of ascorbic acid in the blood may decrease after trauma (i.e., wounding). Once all other factors affecting wound healing have been ruled out there could be an indication for vitamin C supplementation in these animals. Zinc deficiency can result in lack of replication of epithelial cells and fibroblasts, causing a weak wound and lack of epithelialization. At the other extreme, an elevated zinc concentration can inhibit macrophages, decrease phagocytosis, and interfere with collagen cross linking to have a negative effect on healing, and should be considered once other factors have been excluded. Ideally the patient’s nutritional status on admission to the practice should be noted, including a body condition score (BCS) and weight and this should be performed daily during the hospitalization period. If there is concern that the patient is unable to achieve its resting energy requirement (RER) alone, then assisted feeding techniques such as the placement of oesophogostomy or gastrotomy tubes should be commenced. Many of these patients will require general anaesthesia as part of a wound management protocol, e.g., wound debridement, and so the opportunity for the placement of a feeding tube is likely to be available. For patients requiring sedation or anaesthesia over a prolonged period of time, nutritional status may also need to be addressed in terms of prolonged periods of starvation prior to anaesthesia, and in these cases the patient’s calorific requirements should be calculated and compared against the actual calorie intake of the patient.

Surgeon Factors

The surgeon dealing with the wound management of the patient will ultimately have an effect on the likelihood of complications occurring. There are various issues that can be created during wound closure.

Postoperative Haemorrhage and Haematomas

The presence of postoperative haemorrhage, or haematoma formation, may delay wound healing by putting pressure on suture lines, providing a rich environment for infection, and cause discomfort to the patient, thus resulting in self-trauma. In addition to the indirect effect on wound tension, there is experimental evidence that postoperative haematoma formation can directly affect flap survival where reconstructive surgery has been performed. Adherence to Halstead’s principles, including effective haemostasis, can help to prevent this. Minor incisional haemorrhage can be controlled with direct manual pressure for 10 to 15 minutes, but moderate to severe haemorrhage will require further intervention. Conservative management involving placement of pressure bandages, movement restriction, and fluid deficit replacement can be instituted initially if the patient is stable. In more severe cases of haemorrhage, or those not responding to conservative management, surgical intervention may be required to identify the source and provide definitive haemostasis.

Wound Dehiscence

Dehiscence is defined as the breakdown of a surgically closed wound. Signs of a problem incision may be erythema, oedema, or pain with signs of imminent wound dehiscence including necrosis of the skin edges, extensive cutaneous bruising, the presence of serum beneath the skin, and serosanguinous discharge from the suture line.

The most common causes of wound dehiscence include:

• Excessive tension
• Ischaemic or necrotic wound edges
• Inappropriate suture material or suture pattern selection
• Accumulation of moisture leading to tissue maceration
• Underlying pocket of infection, necrosis, foreign body or neoplasia
• Lack or inappropriate postoperative protection/support (e.g., bandage, Elizabethan collar)
• Premature suture removal
• Delayed healing caused by administration of corticosteroids and other agents

Assessment of the Small Animal Wound

The treatment of small animal wounds will commonly require a staged approach. Definitive treatment will often be delayed whilst emergency treatment of the patient and its wound, and initial stabilisation is performed. Analgesia and effective restraint is often required, until the animal is stable enough for sedation or GA. Wounds can be classified by their cause and the type of tissue damage caused: incisional, abrasion, avulsion (or degloving), shearing, puncture or perforated, and burns. Regardless of the aetiology of the wound, the factor which has the single biggest impact of future healing is the presence of contamination and necrotic tissue.

Once the patient is stable a more thorough evaluation may be carried out. Appropriate chemical restraint may be required for examination. Diagnostic imaging may be used to check for foreign material, penetrating injuries, associated fractures, dislocations, and tendon or ligament damage. A management plan should take into account the wound’s location, size, damage to local structures, and the amount of tissue loss.

Preparation and Lavage

The aim of wound lavage (or irrigation) is to remove loose foreign material and necrotic tissue from the wound, while diluting the bacterial contamination present. In so doing, lavage aims to remove impediments to wound healing, and reduce the risk of infection.

Variations exist in the technique employed for lavage, and in the choice of lavage fluid used. However, some factors are agreed on:

• The greater the volume of lavage fluid, the lesser the risk of infection
• The more contaminated the wound, the greater the volume of fluid required
• Warmed fluids are more comfortable than room temperature fluids
• The earlier the wound is lavaged, the better the removal of bacteria

Lavage fluids are generally administered as a controlled jet directed over the wound surface. The pressure at which the fluid is applied is crucial to achieve the goal of removing impediments to healing; pressures need to be sufficient to dislodge debris and loose tissue, and overcome adhesive forces of bacteria, but excessive pressure will drive bacteria and debris deeper into the wound, and open up previously uncontaminated tissue planes.

Pressures of 8–12 pounds per square inch (psi) are strong enough to overcome adhesive forces of bacteria (Longmire et al. 1987). Pressures greater than 15 psi may cause wound trauma and drive bacteria deeper into wounds. Pressures lower than 4 psi are insufficient to remove surface contamination and bacteria.

In a practical situation, correct pressures can be achieved with a 20 or 30 ml syringe, and a 19 gauge needle or intravenous cannula, this provides an output pressure range of 11–31 psi, but the end pressure of the jet that reaches the wound is probably about 8 psi. The syringe can be rapidly refilled by incorporating a 3-way tap and a giving set attached to a bag of lavage fluids. An alternative technique which achieves similar pressures is to connect a fluid bag to a giving set with a 19-gauge intravenous (IV) cannula on the end, and squeeze the fluid bag with a pressure cuff or pressure bag inflated to 400 mm Hg. The stream of lavage fluid should be directed at 45 degrees to the wound bed to maximise dislodgement of debris.

Other techniques such as lavage with bulb syringes, or manually squeezing punctured fluid bags deliver insufficient pressures and are not as effective at reducing infection.

In human medicine suggestions exist for the volume of lavage fluid deemed necessary based on the size of the presenting wound; volumes of 50–100 ml per centimeter of laceration or per square centimeter of wound have been suggested. In veterinary texts suggestions are made regarding the minimum volume per wound - 500 ml. Required volumes are likely to be higher in veterinary patients due to an increased risk of contamination from hair, environment and patient interference. As mentioned previously, the greater the amount of contamination, the greater the volume of lavage required.

Choice of Lavage Fluid

Controversies exist in the choice of lavage solution, and whether antiseptics should be added to the solution. Solutions need to be non-toxic to tissues, reduce the number of microorganisms, not cause sensitivity reactions, and be widely available and cost effective.

A lot of information in veterinary wound management is derived from human medicine, which in turn is often based on studies conducted on wound models in experimental animals.

However, whereas normal saline is used extensively in human wound lavage, and is seen as the ‘standard’ solution, most veterinary texts recommend Hartmann’s solution (Lactated Ringers) as the wound lavage solution of choice. This choice seems to be based on a paper that described the effects of wound lavage solutions on canine fibroblasts in a Petri dish (in vitro). The cells were exposed to phosphate buffered saline, normal saline, tap water, and Hartmann’s solution, for time periods of between 30 seconds and 10 minutes. Tap water damaged the fibroblasts at all time intervals, and normal saline caused cytotoxic effects after 10 minutes. Neither phosphate buffered saline, nor Hartmann’s caused any significant fibroblast injury. No clinical randomised veterinary study exists to confirm the significance of this finding on the healing and infection rates of acute wounds.

While both normal saline and Hartmann’s are isotonic, and evidence suggests lavage should be carried out with a fluid that has similar osmotic pressure to that found in living cells, recent human literature has looked at the use of ordinary drinking water as an alternative lavage solution for acute wounds. Drinking water has the advantage of being readily available and cheap. A study on contaminated musculoskeletal animal wound models showed identical reduction in bacterial counts following lavage with identical volumes of normal saline and drinking water, even in open fractures. While studies in humans have shown no difference in infection rates or healing rates when comparing normal saline to drinking water in the lavage of acute wounds.

The addition of antiseptics to lavage fluid can have cytotoxic effects on important cells involved in healing, such as keratinocytes and fibroblasts. Hydrogen peroxide and povidone-iodine reduce proliferation and migration of fibroblasts in a dose-dependent fashion. Chlorhexadine and silver-containing antiseptics also reduce proliferation at high concentrations, but at lower concentrations they may actually enhance epithelial growth. If chlorhexadine is used in a wound, it should be used at low concentrations (a solution of 0.05%); used at this dilution chlorhexadine causes no significant difference in wound contraction or epithelialisation compared to sterile saline or Hartmann’s in dogs, while achieving 100% bacterial kill rates against Staphylococcus intermedius, a common skin commensal bacteria.

The addition of antiseptics is often used in infected wounds; the aim of lavage is to reduce bacterial levels to a where the immune system can prevent critical colonisation or infection. Lavage of a 6-hour old wound with 9-litres of saline or tap water reduced bacterial counts to 71% of the pre-lavage levels in a contaminated wound model; in an infected wound these remaining levels of bacteria may exceed the critical bioburden and infection may ensue. The use of an antiseptic in the lavage fluid may lower residual bacterial counts by killing as well as diluting.

Addition of antibiotics to lavage solution is not recommended. Antibiotics may cause a sensitivity reaction in the patient, they are unlikely to maintain effective local levels for a suitable length of time, they are costly, and the promotion of resistance is a concern.

Soap or surfactants are commonly used in lavage of open fractures in human patients, where their use has been shown to reduce the complication rate compared with lavage with normal saline. Soaps have lipophilic components which block bacterial cell adhesion in a wound, so assisting their removal.

Soaps may also be of use in chronic wounds where they are more effective at removing adherent denatured proteins, such as dry fibrin and blood compared with saline or Hartmann’s.

Aims of Debridement

Debridement is defined as the removal of damaged tissue or foreign material from a wound. Necrotic tissue will delay wound healing and increase the risk of wound breakdown by acting as a focus for infection. The presence of necrotic tissue also slows wound healing by obstructing re-epithelialisation and wound contraction.

One of the most important considerations in deciding whether, and when, to close a wound are the degree of contamination (bacterial and foreign material) and necrotic tissue present. Decreasing the presence of these impairments to wound healing will enable the reduction of the level of contamination and the presence of necrotic tissue to a point where the closure technique chosen can be safely carried out. This may not be possible in a single procedure; wounds that have extensive contamination and ischaemia may require repeated debridement, and the use of more than one debridement technique, before closure can be considered. The mainstays of debridement in acute wounds are surgical and mechanical debridement.

Conclusion

In wound management, it is vital that veterinary staff have a good understanding of the processes in wound management, along with good knowledge of the actions of wound dressings, so that an appropriate dressing can be match for the specific stage of wound management, in order to optimise wound healing. Wound management is an ever-evolving field so it is essential that staff remain up to date on the novel technologies.

When dealing with a non-healing wound it is important to reassess the patient, and consider the various factors that could be contributing to the problem, in order to create an effective and appropriate wound management plan.