By Dr. David Marlin
Lower leg injuries are extremely common in all types of race and sport horses. Risk factors for traumatic injury from shoes on other legs due to over-reaching or brushing or even from other horses in racing or polo include high speed, jumping and rapid turning. Injuries can also occur from sharp stones being thrown up from the ground. Whilst in sports such as eventing and showjumping, it’s unusual to see horses competing without leg protection at least over the forelimb tendon area; in racing the use of protective boots is much less common. This is also true for polo—a sport which would present a high risk of injury to the lower leg due to balls, sticks, high speed, turning and close proximity to other horses. So is the risk much lower than we might expect, or is their some other reason why boots are not commonly used in racing?
Epidemiological studies of injuries to racehorses in training and racing certainly don’t point to a large risk for interference-type injuries from shoes within or from other horses. In the younger flat racehorse population, fractures remain the greatest concern both in training and racing. Injuries due to forelimb Superficial Digital Flexor Tendons (SDFT) tendonitis and Suspensory Ligament (SL) desmitis are not uncommon, with a prevalence of around 10-20%. However, information on the rate of injuries due to interference in racehorses is generally lacking.
Whilst an injury to the lower forelimb where the skin has been cut and there is clear penetration is easily identified, this is not the only way that tendons may be injured. Blunt force which does not result in obvious superficial injury may still lead to internal bruising and inflammation. Repeated traumatic insults due to interference may therefore still contribute to tendon inflammation. As we now recognise that most tendon injuries are due to chronic inflammation and damage as opposed to isolated accidents, anything that contributes to tendon inflammation is a cause for concern.
The risk to tendons from the heat generated during exercise may be one of the reasons why racing has tended to shy away from the widespread use of boots, except perhaps in the case of individuals suffering repeated or severe injuries. Wilson and Goodship at Bristol Vet School showed in the 1990’s that equine tendons reached temperature of around 45°C during galloping. Tendons are essentially large elastic bands which store and release energy on each stride—one of the adaptations that makes the horse such a supreme athlete. In the same way that if we stretch an ordinary rubber band, it will heat up. Tendons have a poor blood supply, and so the heat accumulates and the tendon increases in temperature during the gallop; the longer and faster the gallop, the higher the temperature. Why is tendon temperature a concern?
Heat: Tendon cells appear to be sensitive to increases in temperature. When isolated tendon cells in culture were heated for 10 minutes at 45°C (113.0°F), around 10% died; but when they were heated to 48°C (118.4°F) for 10 minutes, then around 80% died. Similar results were found in a later study by a group from Japan. Even though the number of live tendon cells in a tendon is low, compared with the elastic matrix that makes up the majority of the tendon, injured or dead cells release inflammatory mediators which in turn can lead to tendon damage. And of course, anything that insulates the leg reduces heat loss and can lead to even higher tendon temperatures. This is likely one of the primary considerations for not using a protective boot, although there are others.
Abrasion/rubbing: Boots (or bandages) that do not fit correctly or that are applied incorrectly may lead to skin abrasion and an increased risk of skin infections. In addition, boots that allow the ingress of surface material between the boot and the skin will likely lead to rubbing.
Restriction of blood flow to and from the foot: Morlock, et al. (1994) observed pressure under bandages applied to the lower limb during galloping which they concluded were high enough to restrict blood flow. In bandages or boots applied over the fetlock and cannon, high pressures due to the method of application, the tightness of the application and the type of material used the lateral and medial digital arteries and veins could be compromised.
Restriction of range of motion: Restricting the range of motion of a joint will change the loading dynamics of the joint. This may be beneficial in the case of a joint that is injured, but this will reduce the extent to which that joint dissipates forces during the loading phase. This may in turn lead to overloading of other limb structures. If the restriction is only on one limb, then this may lead to asymmetry and an increased risk of injury in the un-restricted limb. The effect of various equine boots on range of motion both in vitro and in vivo has been reported (Balch, et al. 1998; Kicker, et al. 2004).
Contact dermatitis: Boots and bandages have the potential to cause contact dermatitis. Neoprene is commonly used in boots, but it has been estimated that around 6% of horses are allergic to neoprene. Rosin (also known as colophony), which is commonly found in the resin of pine and conifer trees, can cause skin contact sensitisation. It is used in neoprene adhesives and may also pose a risk of contact dermatitis in horses. The risk of skin infection is also increased by boots and bandages that do not allow sweat to evaporate and therefore result in hyper-hydration of the skin under the boot or bandage. This results in both an increased susceptibility to mechanical trauma to the skin from friction and an increased risk of infection, particularly by fungi.
Increase in weight….