Wound Healing
By Dr Ann Blackburne
Wound management in equine veterinary practice accounts for a significant component of practice time. Veterinarians regularly attend wounded horses in the field, and many serious wounds are referred to equine hospitals for management.
Equine wounds are managed in three different ways: Primary closure, Delayed primary closure or Second intention healing.
(A) Primary closure involves apposing all tissues and suturing full thickness skin soon after injury. In order for a wound to be a candidate for this type of repair the wound must be minimally contaminated, reasonably fresh, with adequate blood supply and the skin edges must be able to be apposed without excessive tension. Primary closure results in the best cosmetic result and most rapid return to full function.
(B) Delayed primary closure is reserved for heavily contaminated or infected wounds that need to be cleaned up over several days through a process of repeat debridement, lavage, and with antibiotics. A wound can be a candidate for a primary closure days later providing the tissues can still be apposed without excessive tension. This is generally most successful in areas where there is available loose skin that can be mobilised to allow closure. Wounds on a horse’s lower limb result in rapid retraction of the skin, and delaying closure significantly reduces the opportunity for skin edge apposition. However partial closure of these wounds is acceptable and results in more rapid resolution than second intention healing.
(C) Second intention healing occurs when a wound is left open to heal. An equine wound is left to heal in this way if apposition of the wound edges can not be achieved due to extensive loss of tissue or because of skin retraction. Second intention healing is also best in some infected wounds or when repair over areas of excessive motion will doom the repair to fail. Second intention healing results in the least favourable cosmetic result and the slowest return to function.
It is necessary to have some knowledge of the normal processes and stages of wound healing so that management of an open wound maximises the speed of healing and minimises the resultant scar. There are four stages of wound healing: inflammatory, debridement, cellular/tissue proliferation, and wound maturation.
Stage 1: During the inflammatory stage various inflammatory cells and mediators are drawn to the wound to start the healing process. The aim of good equine wound management is to minimise the duration of this stage as a prolonged inflammatory stage will result in a delayed healing.
Stage 2: Adequate debridement of the wound is essential as failure to remove the devitalised and necrotic tissue will result in chronic infection and wound healing will not progress. The aim of equine wound management is to assist this natural process by physical (surgical) or chemical (topical application of creams etc) debridement of the wound.
Stage 3: During the tissue proliferation stage the wound bed fills in rapidly with pink and vascular granulation tissue. Once the wound deficit has filled with granulation tissue contraction of the wound edges over the bed of granulation tissue begins to bring the skin edges together. Epithelialisation is the process of proliferation of the skin edges and is seen as a pale coloured rim around the wound margin. The aim of equine wound management is to ensure that these processes are occurring within a normal time frame. Failure of this to happen may indicate an unhealthy wound. If the granulation tissue is excessive and exuberant “proud flesh” contraction of the wound will cease. If the granulation tissue becomes dark coloured and starts to produce excessive exudate then infection is likely. Failure of the wound to contract adequately with excessive epithelialisation will result in an unsightly scar.
Stage 4: The final process of maturation results in an increase in strength of the wounded area. This is necessary for return to function and to minimise re -injury at this site.
Management of an acute wound
1. Assessment: Firstly it is essential to have a veterinarian thoroughly assess the extent of the wound. Sedation is often necessary to enable a complete and safe examination. Wearing gloves is mandatory so as to not further contaminate the wound. The aim of this process is to assess the depth of the wound considering that synovial structures such as joints and tendon sheaths could be involved. Tendons could be lacerated and extensive bone involvement is not uncommon in lower limb wounds of the horse. Head wounds are not uncommonly complicated by fractures and abdominal and chest wounds may have penetrated the body cavity. Complicating factors such as these may necessitate referral to an equine hospital.
The extent of tissue loss, the vascularity of remaining tissues, the ability to appose skin edges and the degree of contamination are all considered and a decision whether to primarily close or to leave open to heal by second intention is made. It is always recommended to manage a wound with primary closure if possible. Closure results in the best cosmetic outcome and the fastest healing time.
2. Wound preparation: It is essential to avoid the use of antiseptics in the wound as this will be damaging to cells and tissues. Local anaesthetic is usually infiltrated into the tissues.
3. Wound debridement: Debridement is the process of removing foreign material, bacteria and devitalised or necrotic tissue from the wound and should be performed by a veterinarian. Wounds in horses are frequently heavily contaminated with dirt, plant material, timber and hair. Antibiotics and antiseptics will fail to effectively kill bacteria that reside in necrotic tissue that remains in a wound due to inadequate debridement. It is essential that the process of debridement does not result in further trauma to the remaining tissues as this will have a deleterious effect on wound healing.
4. Repair
a) Primary closure: If primary closure is attempted then the wound margins need to be apposed with minimal tension. Excessive tension on the primary suture line results in vascular damage with suture pull through and failure. Tension relieving sutures are commonly used to facilitate skin apposition while minimising tension on the primary suture line. A properly applied firm bandage is also a very useful way to minimise dead space where serum and blood can accumulate on an equine distal limb wound. Sutures are removed between 10 and 14 days after surgery, and tension relieving sutures may be removed earlier. Skin staples are also commonly used in equine wound repair particularly on the head. Antibiotics are nearly always indicated, except following repair of surgical incisions. Anti–inflammatory medication, usually phenylbutazone is commonly used and both tetanus antitoxin and toxoid are recommended.
b) Second intention healing: When equine wounds are left open to heal there is always the temptation to remove any loose redundant tissue. Unless the tissue is not viable it is best to leave it in place. It can always be removed at a later time. Bandaging the wound should always be considered. The advantages of bandaging include less environmental and bacterial contamination, protection from flies that can transmit parasites and sarcoids and prevention of proud flesh. It has been shown that wound healing progresses more rapidly with better cosmetic end results when bandaging is used properly. Higher temperatures and moisture from wound exudate are proven to be beneficial to wound healing. Generally most dressing changes occur every two to five days. Systemic antibiotics are indicated for severe wound infection or if the wound is close in proximity to a synovial structure. Non-steroidal anti-inflammatory drugs are often used initially and tetanus antitoxin and toxoid are always indicated.
Proud Flesh
Exuberant or excessive granulation tissue in a healing wound of a horse is known as “proud flesh”. This is a complication of wound healing as wound contraction can not occur and the wound does not heal. Proud flesh most commonly occurs in equine limb wounds and is most likely because of mobility. Proud flesh is highly vascular and lacks sensory nerves. Consequently the most effective way to deal with proud flesh is to have a veterinarian cut it off. Considerable bleeding can occur and a tourniquet is sometimes necessary. Pressure bandaging is sufficient to control the haemorrhage. Once the granulation tissue bed is back flush with the skin edges the wound continues to heal. Proud flesh can often be prevented with firm bandaging. Various caustic agents are used to eat the tissue back; all of these products have a deleterious effect on the normal healing cells and tissues and so should be avoided if possible. However sometimes these products are required to control proud flesh. This should only be done on the recommendation of a veterinarian. Astringent products available include lotagen, yellow lotion, and copper sulphate. If used these should not be applied to the freshly epithelialised margins of the wound, the least concentration and frequency of application necessary to treat proud flesh is recommended. Corticosteroid creams are also used to treat proud flesh, but can slow wound healing.
Immobilization with casting
Casting the limb below the knee or hock or just over the hoof and pastern is used in equine wound management to accelerate healing by immobilisation. Hoof or boot casts are commonly used for deep heel lacerations. Casting to below the hock or knee is refers to as half limb casting and this is generally reserved for the hospital setting. Casting is used after extensive distal limb repair for immobilization to reduce the likelihood of wound break down. Limb casting is often used in flexor tendon laceration management. Limb casting can also be used to promote rapid granulation of a wound where movement would otherwise result in a very slow healing process. The most significant complication of limb casting is cast related pressure rubs that can potentially be full thickness involving tendons and joints.
Wounds of the head
Wounds to the horses head region are common. All head wounds should be examined for possible skull fractures as the bone is thin and fractures are not uncommon. Eye involvement should also be ruled out. The blood supply to the skin of the head is excellent and primary repair is frequently possible.
Eyelid lacerations
Eyelid lacerations should always be repaired. Loss of the eyelid margin results in tears escaping the eye and continually streaming down the face. Scarring of the eyelids can result in hair and lashes impinging on the cornea causing chronic ulceration. Any eye lid trauma potentially could have resulted in eye globe injury so a thorough ocular examination is always indicated.
Penetrating wounds into joints and tendon sheaths
Penetrating wounds of synovial structures are life threatening injuries. Contamination of joints and tendon sheaths will invariably result in infection which can have devastating consequences for the horse. Joint infections rapidly damage the articular cartilage and underlying bone resulting in osteoarthritis. Tendon sheath infections can degrade the tendon structure and can result in adhesions between the tendons and/or tendon sheath. Permanent lameness can result, even after resolution of infection. Infection within synovial structures is difficult to treat and is occasionally unresolvable necessitating euthanasia. Any wound that is located over a horses’ joint or tendon should be treated as serious and as an emergency. Treatment needs to be aggressive and urgent and usually involves general anaesthesia and surgery. Management involves needle flushing or arthroscopy and systemic and regional antibiotics.
Tendon lacerations
Tendon lacerations in horses can be divided into those that occur over the front of the cannon bone involving the extensor tendons, and those over the back of the cannon bone involving the flexor tendons. Extensor tendon lacerations generally carry a favourable prognosis for return to function providing that synovial structures aren’t involved. Flexor tendon lacerations are more serious and the prognosis for return to athletic function is guarded.
Heel lacerations
Heel lacerations are seen commonly. The injury frequently results from hooking the heel in a fence wire and pulling back. Given the proximity to the ground, hoof and pastern wounds are generally heavily contaminated. A heel laceration should always be investigated by a veterinarian for potential penetration into the coffin joint, distal digital tendon sheath or navicular bursa. Involvement of these synovial structures necessitates more aggressive treatments and worsens the prognosis for complete recovery. Deep heel lacerations are often slow to heal due to widening of the wound edges on weight bearing. Boot casts are very useful for accelerated healing, and achieving a superior cosmetic result.
Coronary band wounds
Coronary band puncture wounds are not seen infrequently. These wounds should always be investigated by a veterinarian for foreign material. Often splinters of timber from fence posts or trees complicate these wounds. The coronary band tissue should be preserved at all costs. Damage to or loss of the coronet will result in weak and deformed hoof wall growth.
Wounds to the axilla
Wounds in the axilla or armpit region are often from stake trauma deep into the muscles. Occasionally air is drawn into these wounds when the horse advances its leg. Replacing the leg to a normal standing position seals the wound and the air can not escape. Significant amounts of air can accumulate and it tracks beneath the skin. This is referred to as subcutaneous emphysema and on palpation the distended subcutaneous space can be depressed and a crackling sound is heard. Extensive subcutaneous emphysema can be quite spectacular with large volumes of air tracking from the tail base to the head. Other sources of air need to be considered so penetrating wounds to the chest and trachea need to be ruled out by a veterinarian. There is no way of removing the air mechanically or by drainage but with time it usually resolves.
Habronema and sarcoids
Open wounds can become infested with parasites. Habronema are stomach worms of horses. Occasionally the larvae can be deposited by flies into open wounds. An inflammatory reaction results in response to the larval presence and the granulation tissue bed becomes unhealthy and occasionally itchy. The wound healing process is consequently delayed and the horse may chew at its wounds. This is most prevalent when flies are worse in the warmer months hence the common name summer sores. Ivermectin wormers are used to treat this condition, and often wound debridement and anti inflammatory therapy are necessary also.
Sarcoids are the most common equine skin tumour and it is possible that sarcoids can be transferred by flies to an open wound. This is why it is not uncommon for sarcoids to appear at the site of previous wounds. If there is a horse with sarcoids near by it is important to keep the wound covered or use insecticides to help prevent transmission of sarcoids as well as habronema larvae.
Chronic non-healing wounds
When a wound is not healing within a typical time frame complicating factors need to be considered such as infection, proud flesh and habronema. Non healing wounds over bone need to be investigated with x-ray to look for bone infection or sequestration. Chronic draining tracts need to be investigated with x-ray and ultrasound for foreign material within the wound. Treatment of the underlying problem is necessary for wound healing to progress.
