Science‐in‐brief: What is needed to prevent tendon injury in equine athletes? A conversation between researchers and industry stakeholders

Superficial digital flexor tendon (SDFT) injury affects up to 30% of Thoroughbred racehorses and other high-level equine athletes and is both exercise and age related; high incidence, prolonged periods out of work, high retirement rates and animal welfare implications are continuing significant problems for equine industries 1, 2. The SDFT is an energy-storing structure essential for efficient high-speed locomotion, with narrow mechanical safety margins. Our understanding of pathophysiology has advanced significantly, but preventative measures have not been developed. Most injuries occur during athletic activity following undefined periods of matrix microdamage accumulation that is not repaired (and possibly directly contributed to) by tendon fibroblasts and other endogenous cellular populations. Detection and/or prevention of the earliest phases of pathology are likely to have a greater effect than improvements in therapy, given that tendon is a slowly healing and poorly regenerative tissue. This workshop was held in October 2012 at the University of Glasgow, UK to bring a key group of scientists from different disciplines (including cell biology, pathology, bioengineering and physics) together with industry end-users, including veterinarians and trainers. We had a particular focus on the Thoroughbred racehorse and the cell biology of normal and injured tendons. The workshop was funded by the Biotechnology and Biological Sciences Research Council (BBSRC) under the International Workshops scheme. The following is a summary of the discussion points and review of key concepts, including indications of areas in which significant progress is required. It is recognised that racehorse trainers differ significantly in their approaches, attitudes and situations, as is reflected by variation between them in incidences of tendon injury 1, 2. This is a complex issue, but a large component of the variation relates to the type of racing (e.g. National Hunt vs. flat racing), including the ages and ‘types’ of horses involved and, potentially, the progression and/or nature of the pathology itself. In general, J. Givens (Thoroughbred racehorse trainer and veterinary graduate) felt that trainers would be far more interested in measures to prevent SDFT injury than to treat it, due to the significant effects on athletic careers. This is because there is clear recognition in the industry of the binary nature of this condition, i.e. a tendon cannot be ‘a little bit’ damaged; however, the situation may be different for owners using horses for recreational purposes/companionship. Any preventative measure that would significantly reduce injury risk (e.g. by 70%) would be likely to be adopted. It also needs to be kept in mind that the SDFT is not the only injury-prone structure to be concerned with, and that there are significant commercial pressures to develop horses and allow them to achieve their athletic limits. Many factors that might be important in determining injury risk have not been studied in detail, or the information has been conflicting. These factors include track surface conditions, the speed achieved (higher being more risky), the level of ‘fitness’ of a horse and its conformation. Data on relationships between track surface condition and/or type and racehorse injuries are incomplete and often conflicting. This is, in part, due to the very complex interactions between surface type (e.g. wood chip, waxed), the time taken for it to ‘bed in’, its age, amount of usage, amounts of organic matter that become incorporated (into artificial surfaces), the level of maintenance (which can be highly variable) and ambient conditions (e.g. temperature, humidity) 3. It is possible that if the training surface is highly consistent, it will not be seen to play an important role in differences in SDFT injury risk (between horses). However, racing itself occurs on turf in the UK at least (and some horses train on it), and its condition is enormously variable. We do have some information, e.g. a higher risk of tendon injury in the UK has been associated with firmer racing surfaces and with racing in the summer 4, 5. Slipperiness is another factor that may be more important on turf racetracks, where water is applied to achieve consistent ‘going’ conditions and is not really replicating rainfall (i.e. creating variable surfaces). Where synthetic surfaces are used during training or racing, high or low temperatures can alter these surfaces and the associated racehorse speeds (e.g. by thermal transformation of wax) 6. Additionally, we know that the SDFT core becomes hyperthermic during galloping due to loss of some energy as heat during elastic recoil 7. Insulation of the limb might add to this thermal burden. Core hyperthermia of the SDFT in relationship to environmental temperatures and limb insulation has not been measured. In fact, a large number of very complex horse and management factors interact with external conditions. It is extremely difficult to relate tendon injury risk to previous training and racing history, because better horses tend to race less frequently (4–5 races per year), while the less ‘talented’ could race up to once a week. Simple comparisons of ‘good’ and ‘average’ horses are difficult, because a horse that is racing more often is likely not to be trained as hard. This is additionally complicated by differences between racing and training; speeds at which horses run will often be higher on the racetrack. This means there is a possibility that most SDFT microdamage is accumulated during racing, and also that training exercise may not be sufficiently intense to induce injury resistance (if such adaptation is possible). Other horse factors, including conformation, are difficult to assess objectively, but there are some clues, e.g. SDFT injury risk has been associated with increased metacarpophalangeal angle 8. Factors that seem to be important for human athletes, such as ‘running style’, are difficult to measure or change in horses. Injuries to the SDFT are frequently noticed 6–10 days following racing, the evening after the horse resumes training. It is possible that many lesions are initiated during a race (or an episode of training in which similarly ‘stronger’ exercise is undertaken), but develop further to a grossly observable level with some further activity. Detection relies on personnel feeling for swelling; however, while some gains could be made by providing education, this is a very advanced stage of injury, in which some microdamage has already occurred. A key problem in all research approaches, including those discussed below, is the detection of subclinical pathology in its very earliest stages, which requires: 1) affordable, noninvasive and objective detection methods; and 2) a means of their application by veterinary surgeons, which may require a change in culture such that fees are based on a ‘herd health’ approach. Significant amounts of work would still be required to link factors responsible for lesion development with measures to prevent their further progression or to reverse the pathology (if that is possible). These might include training regimens, optimal surface choices and correct frequency of racing. The implementation of some practices, e.g. application of cold boots following exercise (if that were proven to be preventative), might be impossible for every horse in the training/racing environment. In terms of reversibility of injury, trainers and veterinarians often notice transient SDFT swelling in young horses. We need a better understanding of whether this is a different form of pathology (i.e. not chronic and progressive degeneration), why it subsides and/or if the risk of SDFT rupture is higher later on. Exercise specifically to induce SDFT adaptation has been a research focus, particularly early in life, although there has been debate about sufficiency levels/intensity of exercise provided to experimental animals, including foals and racehorses. Adaptive change has not been found, other than a more rapid growth in SDFT cross-sectional area, but there was also no evidence of pathology; additionally, there was a later occurrence of musculoskeletal injury (in general) once these horses were in the racing environment 9-11. Further study of early exercise would require interaction of researchers with stud farms as well as with the trainers. Once horses are in the yard, there are few data available, and it is not certain when horses should ideally start racing or what happens in response to different exercise regimens. It is possible that tendons cannot adapt at all. This is complicated by high individual variability (e.g. in SDFT cross-sectional area, to which relative contributions of genetics and exercise are not known) and our lack of knowledge regarding the most important elements of exercise. For example, the results of recent work using human subjects suggest that low-volume but high-intensity exercise is sufficient for aerobic fitness, although admittedly, the musculoskeletal effects are not known 12. There may be a delicate balance, because in some studies the total amount of high-speed exercise during training has been associated with increased risk of injury 13. Maintaining a certain frequency of exercise may also be important. This is supported by findings in human athletes, in whom any return to training requires correctly spaced exercise of appropriate intensity, i.e. ‘tendons don't like rest or change’ (Jill Cook, cited in 14). In terms of change, it is important to know whether training on one surface vs. a variety of them is advantageous or not. Other issues include weights carried due to handicapping, jockey weights, tack weights and the more esoteric issue of mental attitude (‘the art part of the art and science of training’). At least some breeders would be likely to use tests that genuinely indicate injury risk. However, much of the variance in the likelihood of SDFT injury is probably not genetic. Biomarkers indicating early pathology would have a far greater appeal for the industry due to the practicality vs. imaging (as frequent scanning of all tendons is not realistic) and the minimally invasive nature of blood sampling. In this respect, one could envision biomarker analysis in the context of a ‘stress test’ (i.e. pre- and post exercise sample comparison), with equine-specific protein arrays and a system such as the Luminex platform 15. In this way, a horse can act as its own control, which is a situation that may of Most of the problems involved in preventative measures relate to this including the for can be a particular issue for example, rest periods are required. possibility in terms of practicality could be to which some data already to at increased risk of SDFT injury, and focus and time on those are not to training to their however, in system and (e.g. will allow more of the training researchers will need to of these data and Other including and track surface conditions, would also need i.e. in to any data on training vs. SDFT injury that the there will need to be studies that do not with the in training This is an in which trainers would be likely to particularly if the not an increased of blood sampling. researchers could for early injury, there is likely to be in those horses in which the work is being and to alter training in horses where early is The reversibility of early injury is of to the The is on researchers to and this is an of The is to of specifically in the in a of from all of the other tendons and In all biomarker analysis in the will be part of a together from imaging of the There is a issue in this in terms of the implications in of whether subclinical injury should be There would be resistance to of this type of information in many In tendon injuries are to the and the of this was more rapid to with early This to a in from racing due to tendon It might be very for equine welfare and if there was a more this the as to whether prevention research should be in where that attitude and/or greater are the There are significant in our knowledge of what occurs injury-prone SDFT (i.e. the during this is by the external and factors discussed (including exercise intensity, and early change and whether to a certain the pathology is This information is required to develop for preventative both at horse level and in our in the SDFT to swelling will most probably phases of subclinical but do require in terms of understanding the of In terms of the tendon hyperthermia was in the SDFT core in one study but not any further in relationship to exercise and conditions. important factor directly is the level of tendon is not highly to other are of between which is an important of of this energy-storing structure These are by matrix (and amounts of that is and by the and of The is in that i.e. most are not directly to any blood or has frequently been as a factor in and due to of high of which to also of effects of on tendon have been highly i.e. cell vs. high of However, we have not been to and these The of tendon to conditions that occur during (as well as a better of what these conditions also require further if we are to and study These should include of and matrix We for example, that amounts of type (which than the type can in the later of in the SDFT core However, this a significant problem for researchers in terms of what cellular to and to do There are different of between in the and and The is a external in areas where there is no external including the of the The is most and is known to that and injured tendon but we do not know whether or to what this occurs in early subclinical factors are significant of cellular in the environment between are likely to have very different and activity from that a narrow of in the tendon of during tests that the tendon core is not following and this well with an of of in the equine SDFT However, the have not been We for example, that matrix (e.g. alter with exercise in the SDFT and it is possible that this is specifically the is a to be important in and surface in other but it has not been in the have other cell including system (e.g. elements of the system (e.g. and or tendon The of is a and there are other significant including that not large such as every change will be e.g. a tendon core significantly amounts of type can have (and normal However, there are significant to being to the and there have been recent i.e. the which has been used to early pathology in human tendons and post exercise in days post racing has been using horse tendons. 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Other have been used to both tendons and including system or issues can include the effects and of the rupture due to than applied and of under is possible under or The effects of mechanical on tendon are well and the in tendons have been to become more with application of However, there is still a problem when determining which optimal or i.e. this may not with that would occur in and studies by researchers would need to be to between factors and SDFT injury occurrence (e.g. track surface conditions, training and individual horse including the use of and other of early exercise (and mental of foals relative to i.e. with stud Further development of imaging for detection of early pathology in with and but not imaging and of using other training that might be more e.g. and in the of large of SDFT from horses of known training/racing to early pathology this would need to be and include of tests to of early injury be to the study to This could include of horses most at risk in to focus the sampling. of the SDFT environment during including and cellular activity (e.g. by for application to cell culture of a of cell for the and of the effects of factors, including those in cell culture matrix type and nature and injury of the tissue. of cell culture including study of and further of the matrix of data in of and application of The would like to the of this workshop by the number The is not responsible for the or of any information by the Any than should be to the for the

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