THIS important study which builds on a programme of orthopaedic research developed by the HBLB over the last two decades has provided much needed evidence to underpin the interpretation of     MRI images and may help trainers identify horses at risk before a full fracture occurs.     The HBLB's Liverpool bone and fracture studies   One of the Horserace Betting Levy Board's five key strategic research priorities is to improve methods of identification, management, and prevention of musculoskeletal disease and injury in racehorses. The HBLB has recently launched website at rachorsehealth.hblb.org.uk to provide trainers with up-to-date information on their wide-ranging research on Thoroughbred health and disease. Although     research on equine veterinary science is conducted in numerous centres across the globe, a substantial amount of the information about racing injuries has been collected in the UK as a direct result of research funding from the HBLB.     Data on racecourse fatalities has been collected in the UK since the 1970s. Musculoskeletal injuries, in particular bone fractures, are the leading causes of racecourse death. To better understand the causes of these injuries, two large consecutive studies were performed at the University of Liverpool between 1998 and 2003 investigating risk factors for fatal distal limb fractures during racing.     This involved incredible logistical challenges and the cooperation of large numbers of racecourse vets and staff together with trainers and owners. But it proved to be an extremely wor thwhile collaborative effort leading to     invaluable conclusions on the causes and prevention of racing injuries.     The researchers identified all cases of fractures occurring at UK racecourses in this period and collected bone samples from both the affected and unaffected opposite limbs for further analysis. The studies not only provided a wealth of information on racing injures at the time but also an archive of specimens was created and is still being used for cutting edge research today.     Which fractures are most common?     Flat turf racing is the safest with an estimated 0.4 fatal fractures for every 1,000 starts while National Hunt Flat racing has been associated with the highest risk for fatal fracture at a rate of 2.2 per 1,000 starts. Lateral condylar fracture is the most common type overall, accounting for 45% of all fatal limb fractures in the UK and this is the most     common fracture type across National Hunt racing.     In Flat racing, lateral condylar fractures are also all too common but pastern fractures on turf and proximal sesamoid bone fractures on all weather surfaces are more prevalent. The exact reasons why different racing formats, surfaces, and distances lead to different forms of fracture are still being teased apart.     For lateral condylar fractures specifically, risk factors include a lack of fast work, first year of training, and racing first as three- or four-year-olds compared to two-year-olds. When the circumstances of the race in which the lateral condylar fracture occurs have been examined, firm going, longer distances, larger fields, and taking part in amateur jockey races have all been linked to a higher prevalence of this fracture type.     Precursors to fracture     The condyles are the round prominences at the end of a bone that form part of the joint with the adjacent bone. In the cannon bones, there are two condyles, one on either side and thus they are named the lateral (outer) and medial (inner) condyles. Both lateral and medial condylar factures can occur and they can affect both the fore and hind limbs. Lateral condylar fractures form in a specific area of the bone and it has been shown that in this site, prior to fracture, bone can become very dense and hypermineralised. As a result, calcium crystals are deposited in the bone and there is a loss of collagen.     Collagen is a key component of bone and other tissues that provides bendiness and the ability to withstand impact. Loss of collagen and build-up of dense and brittle tissue puts this area of the bone at risk of fracture. In some horses the fracture is so catastrophic that euthanasia is necessary.    Fortunately, in many affected horses, the fractures can be repaired by placing screws across the fracture line either under anaesthesia or, for the least complex configurations, under standing sedation (see Stand and deliver – A    important step forwards in equine fracture repair:   European Trainer Issue 39  ). Nevertheless, even in the least severe cases, lateral condylar fractures can be career threatening.     Equine MRI     MRI, or more correctly magnetic resonance imaging, is an imaging technique with important differences from traditional x-ray. MRI does not use radiation but rather a strong magnet aligned with the body that causes atoms within the body's cells to move subtly, and this change in orientation of the atoms is detected by the scanner and analysed to create a two-dimensional image as though the body was being cut into slices. From one scan, multiple images are reconstructed to show both the long and short axes of the region under examination and multiple very fine slices are created. In this way, MRI reveals incredible detail of the internal structure of bone and soft tissues.     For humans, an MRI study generally involves a session in a claustrophobia-inducing tunnel-shaped machine. This sort of MRI equipment can be used in horses but requires a general anaesthetic. The development of open magnets that can be fitted around the lower limbs of a horse has brought MRI technology into the hands of equine veterinarians, and the technique can now be performed very easily in the standing horse, albeit usually with the aid of sedative drugs.     MRI and lateral condylar fractures     The MRI study on lateral condylar fractures was performed by an international consortium of researchers from UK Veterinary Schools in Glasgow and Liverpool, Newmarket's Animal     Health Trust, and scientists from Colorado State University. Using archived material collected previously on UK racecourses during the HBLB Liverpool Bone and Fracture study, Dr Tim Parkin of the University of Glasgow coordinated the research team.     The objectives were twofold: firstly the features of bone shape and internal structure in cannon bones from horses that had fractured in a race were compared to normal cannon bones from racehorses that died for other unrelated reasons. Secondly, the researchers sought to determine if there were inherent differences in the affected and unaffected bones, predisposing the bone to fracture, which could be measured in the living horse and used as a marker to 'flag up' any individual horse at being at risk of fracture.     The results proved to be extremely important. By comparing normal cannon bones with fractured cannon bones and cannon bones from the horses' uninjured opposite limbs, it was established that areas of super-dense bone were forming in the cannon bones prior to the occurrence of fracture. These areas of microdamage were often triangular in shape and when fractures occurred these typically ran across the triangles of brittle bone.     Carolyne Tranquille, author of the recent report in Equine Veterinary Journal, concluded that MRI is able to detect cartilage and bone changes associated with lateral condylar fractures and that the results of the study might in future allow at-risk horses to be identified. There were some important caveats: the bones were examined after death and in some cases had been in storage for some time. The storage process might have enhanced the changes     visible with MRI. Also, the study provided a simple snapshot in time and, by its nature, could not demonstrate the pathway of progression of microdamage towards catastrophic fracture. Finally, although the largest of its kind, the study involved only 49 horses with fractures and much more work is needed to fully understand how the cannon bone can become weakened and remodelled and more importantly, how this process can be arrested to reduce the risk of fracture in racehorses.     Can this study help racehorses today?   Some, but not all, horses that sustain a lateral condylar fracture will have episodes of detectable unsoundness prior to fracture. Tranquille and Parkin's HBLB-funded study shows that in individuals in which lameness can be localised to the fetlock and lower cannon bone region, consideration should be given to adding MRI to the conventional investigations such as fetlock x-rays that are used routinely in equine veterinary practice today. MRI is available at numerous specialist centres and there is no doubt that it has shown great potential for early diagnosis of bone and soft tissue conditions in horses.     It is important to note, however, that although lateral condylar fractures are common they are by no means the only form of fracture that racehorses suffer, and for some horses in which incipient fracture is suspected a more comprehensive whole body scintigraphic (see Nuclear Scanning – avoiding catastrophe!   European Trainer Issue 22  ) bone scan is more appropriate.
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