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 there 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 SDFT tendonitis and SL desmitis is 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 1990s that equine tendons reached a 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 repeatedly 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 could compromise the lateral and medial digital arteries and veins.
- 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 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 unrestricted 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 tress and can cause skin contact sensitisation, 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: Small weights applied to the lower limb can have a significant effect on energy expenditure and gait. This effect is greater than if the weight is applied closer to the trunk/body -the pendulum effect. Some boots are made of highly absorbent materials which absorb sweat and water (from wet grass, ground or from passing through water).
- Increasing concussive trauma: Boots that have metal strike plates, particularly over the back to protect tendons, may protect against penetration and cutting but may increase the transmission of concussion force into soft tissues leading to internal bruising.
- Pressure: Boots (or bandages) applied incorrectly may cause inflammation of underlying soft tissues such as muscle and tendon and ligament. Smith (2010) used a pressure mapping device (TekScan) to measure the pressure under a variety of boots and bandages on a static limb. The highest recorded contact pressure was reported for one of the protective boots tested (46kPa) and the highest localised pressure was recorded under the bandage (154kPa) (Smith 2010). Morlock et al. (1994) measured the pressure and forces under 6 different bandages on the lower forelimb of Thoroughbred horses during galloping on a treadmill and measured very high peak pressures (14.4 Ncm-2) and peak forces (290 N). Pressure and force were also observed to increase as the horses became fatigued. Morlock et al. (1997) subsequently investigated the pressures generated under a range of cohesive bandages applied by racehorse trainers to an artificial horse limb equipped with pressure sensors. The types of bandage included high modulus latex, two different low modulus latex bandages, a medium modulus laminate non-latex. When wrapped by a machine, the maximal pressures during maximum flexion of the artificial horse limb were greatest under the high modulus latex bandage (5.7 N/cm) and greater than all other bandages (3.5 N/cm). Trainers applied the high modulus latex bandages tighter than any of the other bandages and the authors concluded that the higher pressures under this type of bandage were due to its material characteristics. They also concluded that differences between the other bandages were not due to material differences but due to the unwinding force. “If the unwinding force is higher than the force required to extend the bandage to 50% intercept length, trainers will probably wrap tighter as suggested and desired.” Thus, material characteristics and unwinding force can influence how tightly caregivers apply bandages to horses’ limbs yet it is not clear to what extent these factors are recognised or controlled by bandage manufacturers.
So there are clearly many important considerations when deciding whether or not to use leg protection in the form of boots on racehorses. However, whilst boots are not as commonly used in racing, bandaging is much more common. But what if any advantages do bandages offer over an appropriately designed protective boot? Firstly, bandages offer little in the way of protection from either penetration or blunt trauma which can lead to internal injury. Secondly, they contribute significantly to heating up the limb. In one study, the skin temperature under bandages and boots at rest were only around 1°C higher than the temperature of the bare leg. However, after exercise whilst the bare leg temperature had not changed, the skin temperature under the tendon boot had increased by an average of 5°C and under the bandage by an average of 10°C (Westermann et al. 2014). Thus, not surprisingly, bandages insulated the leg significantly more than a tendon boot. The final consideration is the belief that bandaging offers support to the limb and in particular, to the tendons. In the context of the way bandaging is most commonly used in training and racing the amount of support offered to the tendons is likely to be negligible and the disadvantages in terms of injury through compression, particularly of the SDFT behind the fetlock, will far outweigh the advantages. You can test this by heavily strapping an ankle over your Achilles tendon and running.
Boots or bandages or nothing? Ideally the choice of what if any leg protection to use should be based on an assessment of each individual horse. A horse that interferes with itself in front or from behind on a regular basis is probably at high risk of sustaining a significant injury at some stage and is likely suffering from repeated low grade blunt trauma that is damaging the tendon. Whilst bandaging is popular, it should be considered that it offers minimal if any support to the lower limbs, offers little in the way of protection from either penetration/cutting or blunt trauma, may lead to high pressure on the SDFT during galloping and insulates significantly more than a well-designed lightweight protective boot. If you do decide the use of protective boots is indicated, choosing which make/model is incredibly difficult. From my experience of testing boots it’s almost impossible to gauge the protection a boot will offer simply from appearance. Opening up the boots with a sharp knife can help gain some insight but really the only way to know how well a boot will perform is a controlled drop test with a recognised testing rig. Although this type of service is widely available it’s unfortunate that the majority of manufacturers still do not use them when developing their products. Fortunately some do and I personally would like to see a recognised standard that boots would be required to meet in the same way as standards for riders’ skull caps and body protectors.
If boots are used, they must fit properly, not be overtightened, as like bandages they do not support and can lead to injury, they should be put on as late as possible and be taken off as soon as possible. There is good evidence that heat contributes to chronic tendon injury and so whether or not a horse wears boots or bandages, cooling down the lower legs quickly and effectively following training or racing should also be a part of good management for tendon health!