Discussion

Skin grafting in horses:
The nature of many equine wounds is such that full epithelialisation of the tissue bed is one of the most protracted stages of healing. This is especially so in wounds on the distal limb which may have little opportunity for contracture to reduce the overall surface area. In addition at this stage the veterinarian may need to deal with additional problems such as the presence of exuberant granulation tissue. Depending how these are managed there is likely to be a significant cost to the owner. Consequently techniques which are able to shorten the length of the proliferation phase of wound healing can be extremely useful. In addition to providing viable epithelial cells, one often overlooked advantage of skin grafting is the inhibitory effect on granulation tissue formation.
Skin grafting techniques vary from being simple enough to perform in the field with virtually no specialist instruments to those requiring general anaesthesia and an overall equipment outlay of up to GBP20,000. Different techniques will affect overall cosmesis quite markedly with the basic techniques generally resulting in a worse overall appearance; however, this should be considered in conjunction with the overall benefits of more rapid epithelialisation, reduced granulation tissue formation and ultimately faster healing.

Preparation of the wound bed:
This is perhaps the most important aspect of performing successful skin grafts. It is absolutely vital that the recipient bed is in a suitable state as it will need to provide nutrition and oxygen to support the donor tissue. Typically grafts survive initially by a process known as plasma or osmotic imbibition relying on local tissue fluid for support. Subsequently inosculation or secondary vascularisation occurs whereby there is direct blood supply between the 2 tissues. In optimal conditions this can occur in as little as 2 days.
A confluent and healthy bed of granulation tissue is therefore an absolute requirement before grafting takes place, and thus it is important to monitor this process as fibroplasia develops. Any situation that may affect the long-term healing of the wound, such as the formation of sequestra in regions of periosteal exposure or foreign bodies should be dealt with prior to grafting. If excessive granulation tissue is present this should be trimmed back to just below the level of the surrounding skin and bandaged for 24 - 48 h to control haemorrhage. In chronic wounds, the granulation tissue may be fibrous and poorly vascularised and so should be removed as far as possible to allow healthy granulation tissue to form.

Maintaining a healthy graft site:
Whilst the high vascularity and presence of copious inflammatory cells renders granulation tissue extremely resistant to infection, steps to reduce the overall level of bacterial presence at the wound site are very important. Skin grafting should not be attempted if there is any evidence of infection, and at all other times aseptic techniques should be employed. Modern primary dressings, such as those impregnated with ionic silver, can be very beneficial as in certain circumstances they may be either bacteriostatic or even bacteriocidal. As well as causing direct tissue necrosis of the vulnerable skin grafts, bacteria will compete for nutrients as well as produce fibrinolytic enzymes which prevent adherence to the recipient tissue.
Once grafting has taken place it is very important that appropriate coaptation of the wound occurs. As skin grafting typically is performed in the lower limb this will usually consist of a modified Robert-Jones bandage, or occasionally a cast. These perform several important functions which are extremely important to the chances of success. Movement needs to be restricted as far as possible to prevent mechanical disruption of the skin grafts, and also to create good apposition of the grafts. Haemorrhage also needs to be controlled as bleeding might cause the graft to move, or result in a blood clot beneath the graft. Whilst it is always tempting to check the appearance of the wound bed, the dressings should ideally remain in place for 5 - 7 days. It is extremely disheartening to remove a bandage earlier and watch the grafts pull away as the primary dressing is removed!

Skin grafting techniques:
There are essentially 4 grafting techniques applied in the horse, although there is variation as to exactly how each is performed. These are: pinch grafts, punch grafts, sheet grafts and mesh grafts, and in this order vary from most simple to advanced in both complexity and requirement for specific surgical equipment. Each technique will be discussed in greater detail during the presentation. Selection of the donor site is probably less important for successful grafting than appropriate preparation of the wound bed and aftercare; however, it is important that the donor bed is aseptically prepared and in order to improve cosmesis it is recommended to choose an area with the same base hair coat as that which surrounds the wound. The author would typically take pinch grafts from an area covered by the mane for pinch grafts, and skin from the pectoral region or stifle fold for punch grafting although many other sites may be appropriate.