Equine Wounds: Triage to Treatment

Exposed bone Exposed or denuded bone is a common complication of wounds of the distal aspect of the limb. Exposed cortical bone in which the periosteum has been removed, is prone to desiccation of the superficial layers of the cortex, which may result in infectious superficial osteitis and sequestrum formation. Exposed bone within a wound can delay wound healing directly if the bone becomes infected, or indirectly because its rigid structure can delay the formation of granulation tissue and wound contraction. Distal limb avulsion wounds with exposed bone increase in wound size for 14 to 21 days. Wound expansion is due predominantly to the distraction forces applied across the wound during the inflammatory and debridement stages of wound healing, and the lack of a granulation tissue bed in the center of the wound to neutralize the tensile forces exerted on the wound margins from the surrounding skin. Wounds with a small amount of exposed bone, or wounds without exposed bone, expand for a shorter period because less time is required for granulation tissue to seal the wound. Larger wounds with exposed bone take longer to form a granulation bed and subsequently wound contraction is postponed. Periosteal insults from blunt trauma, tendon/joint capsule strain, surgical manipulation, or laceration/degloving injuries may result in extensive periosteal exostosis. Injuries involving bones in horses stimulate more periosteal new bone growth than similar wounds in other species and ponies. More extensive periosteal reaction in young compared to adult horses has been attributed to a more active osteoblastic activity of the periosteum in young horses. The extensive periosteal new bone growth seen in adult horses is poorly understood. Deferred collagen lysis compared to other species may be a contributing factor. The more extensive periosteal new bone formation in horses compared to ponies is alleged to be the result of a slower onset and longer duration of the periosteal response and prolonged extensive limb swelling in horses, as compared to ponies. Despite the common occurrence of exposed bone associated with trauma to the distal aspect of the limb, there has been little investigation into methods of stimulating coverage of granulation tissue over exposed bone in horses. Granulation tissue development is a very important role in second-intention healing because it provides a barrier to infection and mechanical trauma for the underlying tissues. Healthy granulation tissue is resistant to infection and provides a moist surface for epithelialization. The delay in wound healing caused by exposed bone has prompted the search for different methods to promote granulation tissue coverage of bone in other species. Head trauma, thermal injury, and surgical oncology often results in exposed bone of the cranium in humans. In these cases the outer cortex of the uncovered portion of the cranium is fenestrated with drill holes, burrs, or lasers to expose the medullary cavity from which granulation tissue grows to cover the exposed bone. Similarly, exposed cortices of long bones in humans have been fenestrated with drill holes to promote granulation tissue formation. It has been suggested that the drill holes promote healing by allowing osteogenic factors from the medullary cavity access to the wound, or by the enhancement of healing of bone and soft tissue by a nonspecific response known as “the regional acceleratory phenomenon”. Cortical fenestration combined with drugs that promote topical granulation tissue may accelerate granulation tissue coverage compared to control wounds, but further investigation is needed. Cortical fenestration of 1.6 mm drill holes in the cortex of the second metacarpal bone in experimentally created wounds in dogs resulted in clot formation over the bone that promoted granulation tissue formation and may have protected the bone’s outer layers from desiccation. The effects of cortical fenestration with 3.2 mm drill holes were evaluated in experimentally created wounds of the distal aspect of the limb of horses. Cortical fenestrated wounds became covered with granulation tissue earlier than control wounds, and fenestration had no significant effect on sequestrum formation. The granulation tissue growing directly from the bone surface also contributed to granulation tissue formation. If the wounds are not large (< 6 x 6 cm) it may be difficult to realize a significant contribution from the granulation tissue growing from the cortical fenestration sites alone. Cortical fenestration may also be advantageous if it is used with other methods of promoting granulation tissue. Splinting of the limb is usually not necessary for the recovery from general anesthesia unless there are associated traumatic injuries to the limb that would suggest instability.