DR Chris Whitton, Head of the Equine Centre at the Melbourne Veterinary School, discussed bone injuries, the latest knowledge of how and why they occur and how best to reduce the risk of fractures, chips and POD (Palmar Osteochondral Disease).

As part of the Equine Limb Injury Prevention Program (funded by Racing Victoria, the Victorian State Government and the University) a computational model of the forelimb, and mathematical models of bone behaviour are being developed to allow better understanding of how bone reacts to the stresses of training and racing.

Bone fatigue injuries such as stress fractures, small cracks and focal injuries in the subchondral bone (the layer of bone just below the cartilage in a joint) are common at the base of the cannon bone of racehorses. They occur because of the accumulation of damage to the bone from repetitive high-intensity exercise.

BONE ADAPTATION

The horse is well equipped to adapt his body to stresses placed upon it, if he is given time and the correct conditions to do so. Trainers and handlers can make use of this natural process to correctly and safely adapt the horse’s body to the stresses of racing. Bone adapts by modelling (laying down new bone) to increase the total volume of bone in response to stress and repairs itself by remodelling (resorbing damaged bone and replacing it with new bone). The stimulus that causes the bone to model is speed work (breezing) but time must be given between bouts of speed work to allow the bone to repair and rebuild.

It only requires a small volume of high speed work (as little as two furlongs), twice a week, to cause bone to adapt. Correct bone modelling will lead to an increase in bone shaft radius and increased density of the bone ends which increases the resistance to fracture.

If the horse is rested the bone will react to the decreased stress and will resorb more bone, leading to a decrease in bone volume and strength (de-adaptation) but this rest period will enable repair of micro damage. The correct balance of stress and recovery is required within the training program to allow the bones to adapt and strengthen, and to repair and recover from repeated bouts of stress. Care must be taken when bringing a horse back into work following a break as bone volume and strength will have been reduced.

SUBCHONDRAL BONE INJURIES

Most bone fatigue injuries occur in the first few layers of the subchondral bone as this is where the highest strains and loads are found. The limb model showed that the joint surface load on the fetlock when galloping can be up to four tonnes, a stark reminder of the pressure on the equine limb. The joints with the highest loads are the fetlock and the knee.

Subchondral bone injury is caused by repeated high loads (cyclic loading) which leads to bone fatigue and then damage and injury. Any material subjected to repeated cyclic loads will eventually break. The greater the stresses and strains applied to the subchondral bone, the quicker it will reach breaking point.

The most dramatic and often fatal, bone fatigue injuries are fractures. Although they occur spontaneously, there is nearly always existing micro damage present at the site of the fracture. It is rare for an acute trauma to be the sole cause of a fracture, it is more often the final straw added to a build-up of micro damage. As more stress is applied to areas of micro damage, it can propagate vertically to cause fractures, or horizontally to cause POD (Palmar Osteochondral Disease) where the overlying cartilage of the subchondral bone becomes damaged.

Conor Moore’s Ebony Topnotch works through raised trot poles which encourage him to use his core muscles to lift and strengthen his back, and improves his overall flexibility, suppleness and balance. Raised poles quickly show if a horse has a weakness or stiffness in his body and can then be used to improve that. Photo Debby Ewing

FACTORS AFFECTING BONE LOADS

The limb model has shown what affects subchondral bone loads and offers pointers as to how to reduce these loads:

  • The centre of pressure of the foot: good shoeing and correct foot balance can pull back the centre of pressure of the foot by about 2cms and this can reduce fetlock loads by 10%
  • Long toe, low heel conformation increases not just the load on the bones but also the strain on tendons and ligaments. Good shoeing will correct this.
  • Speed: the faster the horse goes the greater the ground reaction force and hyperextension of the fetlock and this increases the load.
  • Weight: the horse’s body weight and the additional weight of rider and tack increases the load – every extra kg carried by the horse = 4.5 x greater load on the fetlock.
  • Damaged joint surface: conditions such as osteoarthritis and POD which damage the articular surface will increase the load on the undamaged joint surface.
  • Track surface: there is substantial data showing differences in injury rates in horses racing on different track surfaces. However, direct evidence of the effect of track surface on limb loads is limited. Increasing the depth of the upper layer of the surface, and reducing the stiffness of it, reduces fetlock extension, which reduces the load.

    • RISK FACTORS FOR BONE INJURY

    Once you accept that cyclic loading and repeated stresses and strains on the limbs are the cause of most bone and soft tissue injuries, the risk factors are obvious.

    THE RISK OF FATAL INJURY INCREASES WITH:

  • Longer career duration and greater number of races in career (accumulation of micro damage over time)
  • Greater number of races in last 30 days and having started a race in the previous one to 14 days (rapid accumulation of damage)

    • THE RISK OF JOINT SURFACE INJURY INCREASES WITH:

  • Total lifetime races
  • Multiple racing seasons
  • Shorter time between races
  • Shorter time since last race

    • A racehorse has to train and has to race. Both activities add stresses and strains to his musculoskeletal system, leading to micro damage which can turn to performance affecting injury. Injuries occur when damage repair cannot keep up with damage accumulation.

    To reduce injury risk, repair and recovery time must be given within the weekly training schedule and within the training season.

    TENDON RESPONSE TO TRAINING

    The computer model demonstrated the loads and strains placed upon tendons. As Whitton explained, the equine limb is well designed for speed. The upper limb provides the muscle power, the lower limb is light and contains long tendons that supply the passive spring for movement, much like an athlete’s prosthetic limb.

    Tendons store and release energy. There is about 10cms of stretch in the SDF (Superficial Digital Flexor) tendon, but only about 1 cm of contraction in the upper limb muscles. As a result, the tendons create the greatest share of the forces needed for limb movement. The SDF tendon stores and releases the most energy and comes under particular strain because of the large loads generated by fetlock extension. It has to withstand massive loads: 835 kgs/stride in canter and 1440kgs/stride in gallop.

    SDF tendon loads are affected by the same factors as subchondral bone loads: speed, weight and foot balance. Unlike bone, there is minimal remodelling and limited adaptation of tendons in response to exercise. There is some evidence of increased cross sectional area of the tendon in young horses when they first enter training but not in older horses.

    This demonstrates the advantage of earlier conditioning of racehorses. This window of opportunity is available to condition tendons and ligaments for the rigours of racing. However, because of the tendons’ inability to remodel itself to any great extent, it is particularly important to allow time for the tendons to adapt to the work load being placed upon them in order to reduce injury risk, and to increase work load gradually.

    DO WE HAVE ANY ANSWERS?

    SO how do you successful train a racehorse with minimal risk of injury? Much of the information presented gave scientific backing to what common sense and good horsemanship has known for years: ‘No foot no horse’, ‘don’t leave your race on the gallop’, ‘rest is the best cure’, ‘speed kills’, and so on.

    The reality of training horses is far removed from computer models and statistics. Trainers’ facilities are often decided by the landscape and the finances available. Horses have varying degrees of natural resilience. How much is down to breeding and genetics? Nutrition, early management, trainer and rider experience, the horse’s physique and way of going all play their part.

    RECAP

    There were some answers. Some may be obvious but are worth repeating. Going back to the beginning – the horse has a highly effective respiratory system when it is healthy. Keeping it healthy means eliminating or minimising inflammation and, as discussed in part one, this is down to good management. Bone and tendon loads are both negatively affected by poor foot balance and conformation. The trainer is not a farrier, but he must be able to judge if his horses are well shod. This is a risk factor we can control.

    Training that allows correct and progressive musculoskeletal adaptation is key to giving a horse the best chance of withstanding the rigours of racing. My interest in attending this conference was as part of my own ongoing research into how to reduce injury rates. There are many factors involved and each merits further discussion which a future article will provide. In brief, there is a mass of evidence to suggest that National Hunt horses should start their athletic conditioning earlier.

    The horse’s body is at its most responsive and adaptive in the first two years of its life, providing a window of opportunity to condition bones, joints, tendons and ligaments for their athletic future. Tradition might dictate that National Hunt stores be left at grass until they are three or four. However, everything that is known about musculoskeletal development points towards conditioning horses from a younger age.

    There was much discussion about the right period of adaptation to allow a horse returning from rest through to race fitness. Three months is a common suggestion. However, the same mathematical model used by Chris Whitton in his presentation, indicated that a minimum of four months is needed for bones to fully adapt to the stresses of racing for a horse entering training for the first time, and only a few weeks less for a horse coming back into training – without allowing for setbacks.

    TRAINING REGIMES

    Long before computer models, Dr David Nunamaker and trainer Dr John Fisher (father of leading American steeplechase trainer Jack Fisher) devised a training regime to minimise the incidence of sore shins and fractures in flat racehorses. Once young horses were comfortably working over a mile, they started a program of progressive short sprints, twice a week (one-three furlongs) over a 16-week period. As the speed of the sprint was increased, the distance was shortened and then built back up again to avoid over stressing the bones. They produced convincing evidence that this program reduced sore shins and fracture rates. Horses in the regime had stronger, larger diameter bones that were well adapted for racing. It is hard to understand why this program, or variations of it, are not used more widely outside of America. On that evidence five to six months looks a more suitable time frame to allow full skeletal adaptation.

    Longer cumulative canter and gallop distances have been shown to increase the chances of winning. They also increase the risk of injury. Each horse has to be trained to be fit enough but without stressing him to the point of injury. The simple truth is that cyclic loading causes micro damage which can ultimately lead to injury. It only requires small volumes of fast work, twice a week, to stimulate correct bone adaptation. It was suggested that working the horse over a 400-600m distance (at a heartrate of over 150bpm) three times, twice a week will maintain VO2 max and condition the musculoskeletal system for racing.

    This leaves the trainer deciding how much long-distance canter work the horse needs alongside this, and whether he needs additional fast work to be as fit as possible. In the words of one wise trainer, ‘You can only get them fit, you can’t get them fitter than fit’! Greater use of GPS data and heart monitors would allow for more accurate monitoring of work loads.

    In military research in humans, a stress fracture rate of 31% in new recruits, was reduced to 10% by increasing the intensity of the training, but reducing the volume of it, without affecting performance. Research in training yards in Australia found that larger volumes of high speed training made no difference to performance. High intensity, low volume training increases performance. Over-training reduces performance and increases injury risk. No matter what the volume of training is, recovery time remains the most important thing.

    TAKING A WIDER VIEW

    Cyclic loading is the underlying cause of the majority of racehorse injuries. So why not reduce cyclic loading by working and loading the horse’s body in different ways? Introducing flatwork as part of the horse’s weekly training regime is one of the most beneficial ways of doing this. It improves suppleness, balance, and muscle strength, creates and maintains a biomechanically correct way of going which allows the horse to gallop and jump as effectively and efficiently as possible. It also improves jumping technique and allows the rider to feel any early signs of asymmetry or stiffness. In other equine sports, flatwork is the foundation of the equine athletes’ training.

    Racehorses are meant to be elite athletes, yet in many cases we are not taking these steps to help them be as athletic, supple and balanced as possible. These three virtues will help reduce injury risk and improve performance.

    The French model of breaking racehorses at a younger age should be considered. Their National Hunt horses are introduced to jumping several times a week early in their training, helping to condition them to jump and gallop. They start racing at three and are noted for being well balanced and able to look after themselves in front of a fence. It is argued that Irish and English thoroughbreds mature later. Some horses will always need more time but that doesn’t mean that nothing should be done with them. Appropriate early conditioning will give their bodies an athletic advantage, which will stand them in good stead as they mature.

    THE THOROUGHBRED HEALTH NETWORK

    The conference demonstrated that there is plenty of research material available to owners and trainers. Veterinary research papers can be very heavy reading but a relatively new initiative, aimed at making research findings more accessible, is the Thoroughbred Health Network. A registered charity aiming to optimise the health of racehorses and other equines, it is helping to establish a network of like-minded people whose common interest is in minimising the impact of equine injury and disease. The website www.thoroughbredhealthnetwork.co.uk provides free registration and access to summaries of recent research papers concerned with performance-limiting problems.

    BE PROACTIVE

    Accidents and injuries can’t always be prevented but it is important to raise awareness, as this conference has, of some of the risk factors to consider. It is too easy and convenient to simply accept injuries as a hazard of training. There is a need to be proactive rather than reactive, and for a change of attitude from ‘these things happen’ to ‘why did this happen?’