Wounds are frequently encountered in equine practice and come in all shapes and sizes. Many, based on their location or degree of lameness, require immobilisation, radiographs and referral. A sound knowledge of anatomy is essential to fully appreciate factors which may complicate wound healing. In this article, we will look at management of wounds which are not over a synovial structure or over a bone with accompanying severe lameness which would raise suspicion of a fracture.
Type of injury
How the wound was created will influence the degree of soft tissue damage and the risk of subsequent wound infection (Edlich et al., 1988). Kicks and similar crush injuries will be accompanied by contusions which will result in vessel thrombosis making them more prone to infection compared to sharp lacerations or incisions.
Stages of wound healing
It is important to recognise that different parts of the wound can be at different stages in the wound healing process (Figure 1). This will usually be due to one part being stuck in the inflammatory phase and this is the most common reason for the formation of exuberant granulation tissue. During the cell proliferation phase, wound strength starts to increase but progress is slow, reaching 20 percent of normal tissue strength after three weeks, increasing to 50 percent after three months. This should be borne in mind when advising a return to work following wounds in high motion areas.
Exuberant granulation tissue (EGT)
Horses have a slow and inefficient inflammatory response, even compared to ponies whose inflammatory response is demonstrably swifter and more intense (Wilmink et al., 2003). This leads to a protracted inflammatory phase of wound healing with associated persistence of macrophages which signal to fibroblasts to retain their synthetic role rather than differentiating into myofibroblasts. Anything which potentiates inflammation (eg movement, necrotic tissue, foreign bodies, infection) will therefore predispose to EGT. Such factors must be managed appropriately to reduce the formation of EGT.
EGT should be excised to 1mm below the level of the surrounding skin (Figure 2); whilst bleeding is profuse there are no nerve endings within the EGT. This may need to be repeated during the process of healing. A single application of topical corticosteroid (provided underlying causes of EGT have been addressed) can be very effective in reducing the rate of EGT formation. Delayed skin closure and skin grafting techniques can also be useful.
Initial management of wounds
The healing of any wound is influenced most by its initial management. Prolonged irrigation with tap water (for more than 10 minutes) should be avoided as its hypotonicity will result in cellular swelling (Moscati et al., 1998). A water-soluble hydrogel placed on the wound prior to clipping will prevent hair from becoming adhered to the wound surface. Chlorhexidine at 0.05% (25ml of 2% solution in 975ml water) or povidone iodine at 0.1% to 0.2% (10 to 20ml of 10% solution in 1 litre water [weak tea colour]) can be used for wound lavage but higher concentrations are cytotoxic (Sanchez et al., 1988). The last lavage should always be with 0.9% sodium chloride solution.
Perineural analgesia is most effective at removing sensation otherwise infiltration of local anaesthetic (mepivicaine) around the wound is another option. Wound debridement removes contaminated or devitalised tissue reducing the requirement for autolytic debridement (and the associated inflammation). Sharp debridement with a scalpel blade is least traumatic but caution should be exercised as it is not always clear initially whether tissue is viable or not, and knowledge of local anatomy is required. Debridement can be repeated or delayed for three to five days to allow distinction between viable and non-viable tissue.
Skin closure should only be performed if it can be achieved without undue tension (Box 1), with a good blood supply and following scrupulous debridement. Dead space must be addressed; this is most effectively achieved in the field with Penrose drains sutured deep in the wound proximally (with a nylon suture exiting through the skin which can be removed) and exiting at the most dependent part of the wound, ideally though a separate stab incision (Figure 3). This is removed after two to five days; longer placement can generate wound exudate. Other techniques such as meshing the skin (Figure 4) or deep monofilament absorbable sutures can be used to reduce dead space. When required, tension-relieving sutures should be employed; the author prefers the near-far-far-near suture which combines a tension suture, the far portion, and an appositional suture, the near portion. Equally, partial closure can be performed or stents can be applied (Figure 5). All tension-relieving sutures should be perpendicular to the wound so as to not compromise blood supply. Delayed primary closure allows debridement to take place (either surgical or autolytic) alongside a reduction in bacterial contamination with closure performed after three to five days.
Second intention healing
The majority of wounds are left to heal by second intention. This will be the case when primary or delayed primary closure cannot be accomplished. The wound surface should be kept moist and as such the wound exudate is purposely left in contact with the wound bed. The wound exudate provides the necessary cells, is rich in growth and chemotactic factors (enhancing the inflammatory response) and enzymes and has some antimicrobial action.
The author uses a low adherent absorbent dressing on the majority of wounds with cotton wool layers to absorb excess exudate. This can be switched to a semi-occlusive foam dressing once a healthy bed of granulation tissue is present as these provide a moist environment and thermal regulation suitable for epithelialisation. In the presence of highly infected wounds, manuka honey impregnated dressings can be effective (Bischofberger et al., 2012). For wounds healing by second intention initially an occlusive, hydrogel or honey dressing should be used followed by a semi-occlusive foam dressing. The field of wound dressings is huge and a full description of them all is beyond the scope of this article. Nevertheless, be prepared to research and adjust what you use as their different properties can be employed very effectively in different wounds.
Progression of healing
A wound should be regularly monitored to ensure healing is proceeding satisfactorily. Persistent exudate can indicate infection, or the presence of necrotic tissue (eg sequestrum) or foreign body. Other signs of infection include discoloured granulation tissue, oedema, fetid odour, lameness and pain on palpation. A swab for culture and sensitivity should be taken alongside radiography and ultrasound assessment if appropriate. Debridement remains the most effective way of reducing bacterial numbers. Regional perfusion with antibiotics (usually an aminoglycoside) or topical therapy (eg silver sulphadiazine) can also be employed.
Movement is also a contributor to delayed wound healing thus box rest is usually the most appropriate management regime.
Splints or casts should be considered; this can be in the form of a bandage cast. Slipper casts are particularly effective in managing heel bulb lacerations and can be placed under sedation.
If a healthy bed of granulation tissue is present but epithelialisation remains sluggish grafts should be considered. Likewise, large wounds that would struggle to heal by any other means should be considered for grafting. There are two basic types of skin graft: pedicle and free grafts. A pedicle graft remains connected to the donor site and therefore does not rely on a healthy bed of granulation tissue at the recipient site. The author most commonly uses these for facial wounds where a communication exists with the underlying sinuses (Figure 6). Free grafts are completely separated from the donor site and transferred to a healthy bed of granulation tissue at the recipient site where new vascular connections must be established. In practice, punch and pinch grafts are most practical; punch grafts are illustrated in Figure 7. Pinch grafts are usually harvested from the area of neck under the mane. A small cone of skin is elevated using forceps or a hypodermic needle with a bent point and excised using a scalpel blade. The grafts are implanted into pockets created in the recipient granulation bed using a number 15 scalpel blade at an acute angle 3 to 5mm apart. One can expect 50 to 75 percent of grafts to survive (Mackay-Smith and Marks, 1968). Full- and split-thickness skin grafts are much larger and require specialist equipment and expertise so are usually performed in referral hospitals.