Minimizing serious fractures of the racehorse fetlock

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Minimising serious fractures of the racehorse fetlockLink to EVJ article:https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.13273VA Colgate, PHL Ramzan and CM Marr.In March 2020, a symposium was held in Newmarket, UK, aiming to devise measures which could be used internationally to reduce the risk of catastrophic fracture associated with the fetlock joint. The meeting was supported by the Gerald Leigh Charitable Trust, the Beaufort Cottage Charitable Trust and the Jockey Club with additional contributions from a number of industry stakeholders. On the first day a panel of international experts made up of academic professors, Chris Whitton (Melbourne, Australia), Sue Stover (Davis, California), Chris Kawcak (Colorado), Tim Parkin (Glasgow) and Peter Muir (Wisconsin); experienced racehorse clinicians, Ryan Carpenter (Santa Anita) and Peter Ramzan (Newmarket); imaging experts, Sarah Powell (Newmarket) and Mathieu Spriet (Davis, California); and vets with experience in racing regulatory bodies, Scott Palmer (New York) and Chris Riggs (Hong Kong) joined forces to discuss risk assessment protocols, particularly those based on imaging features which might indicate increased risk of imminent fracture. This was followed by a wider discussion with a diverse invited audience of veterinary and industry stakeholders on how our current knowledge of fracture pathophysiology and risk factors for injury could be used to target risk assessment protocols. A report of the workshop outcomes was recently published in Equine Veterinary Journal.<< EVJ logo near here>>The importance of risk reductionWith the ethics of the racing industry now in the public spotlight, there is recognition that together veterinary and horseracing professionals must strive to realise an improvement in equine injury rates. Intervention through risk profiling programmes, primarily based on training and racing metrics, has a proven track record; and the success of a racing risk management program in New York gives evidence that intervention can and will be successful.The fetlock of the thoroughbred racehorse is subjected to very great loads during fast work and racing, and over the course of a training career this can result in cumulative changes in the bone underlying the articular cartilage (‘subchondral’ bone) that causes lameness and may in some circumstances lead to fracture. Fracture propagation involving the bones of the fetlock (cannon, pastern or proximal sesamoid bones) during fast work or racing can have catastrophic consequences, and while serious musculoskeletal injuries are a rare event when measured against race starts, there are obviously welfare and public interest imperatives to reduce the risk to racehorses even further. The dilemma that faces researchers and clinicians is that ‘fatigue’ injuries of the subchondral bone at some sites within the fetlock can be tolerated by many racehorses in training while others develop pathology that tips over into serious fracture. Differentiating horses at imminent risk of raceday fracture from those that are ‘safe’ to run has not proven particularly easy based on clinical grounds to date, and advances in diagnostic imaging offer great promise.Profiling to inform risk assessmentRisk profiling examines the nature and levels of threat faced by an individual and seeks to define the likelihood of adverse events occurring. Catastrophic fracture is usually the end result of repetitive loading, but currently there are no techniques that can accurately determine that a bone is becoming fatigued until some degree of structural failure has actually occurred. However, diagnostic imaging has clear potential to provide information about pathological changes which indicate the early stages of structural damage.Previous research has identified a plethora of epidemiological factors associated with increased risk of serious catastrophic musculoskeletal injury on the racetrack. These can be distilled into race, horse and management-related risk factors that could be combined in statistical models to enable identification of individual horses that may be at increased risk of injury.In North America, the Equine Injury Database compiles fatal and non-fatal injury information for thoroughbred racing in North America. Since 2009, equine fatalities are down 23%; and important risk factors for injury have been identified, and this work has driven ongoing improvement.The problem with all statistics-based models created so far for prediction of racehorse injury is that they have limited predictive ability due to the low prevalence of racetrack catastrophic events. If an event is very rare, and a predictive tool is not entirely accurate, many horses will be incorrectly flagged up as at increased risk. At the Newmarket Fetlock workshop, Prof Tim Parkin shared his work on a model which was based on data from over 2 million race starts and almost 4 million workout starts. Despite the large amount of data used to formulate the model, Tim Parkin suggested that if we had to choose between two horses starting in a race, this model would only correctly identify the horse about to sustain a fracture 65% of the time. Furthermore, the low prevalence of catastrophic injury means it will always be difficult to predict, regardless of which diagnostic procedure is employed.Where do the solutions lie?One possible strategy to overcome the inherent challenge of predicting a rare event involves serial testing. Essentially with this approach, a sequence of tests is carried out to refine sub-populations of interest and thus improve the predictive ability of the specific tests applied. An additional consideration in the design of any such practical profiling system would have to be the ability to speedily come to a decision. For example, starting with a model based on racing and training metrics such as number of starts and length of lay-off periods, as well as information about the risk associated with any particular track or racing jurisdiction, entries could be screened to separate those that are not considered to be at increased risk of injury from a smaller sub-group of horses that warrant further evaluation and will progress to Phase 2. The second phase of screening would be something relatively simple. Although not yet available, there is hope that blood tests for bone biomarkers or genetic profiles could be used to further distil horses into a second sub-group. This second sub-group might then be subjected to more detailed veterinary examination, and from that a third sub-group, involving a very small and manageable number of horses flagged as potentially at increased risk, would undergo advanced imaging. The results of such diagnostic imaging would then allow vets to make evidence-based decisions on whether or not there is sufficient concern to prompt withdrawal of an individual from a specific race from a health and welfare perspective. Of course there are other considerations which limit the feasibility of such a system, including availability of diagnostic equipment and whether or not imaging can be quickly and safely performed without use of sedation or other drugs, which are prohibited near to a race start.Diagnostic techniques for fetlock injury risk profilingCurrently there is no clear consensus on the interpretation of images from all diagnostic imaging modalities, and important areas of uncertainty exist. Although a range of imaging modalities are available, each has its own strengths and weaknesses, and advances in technology currently outstrip our accumulation of published evidence on which to base interpretation of the images obtained.<< box 1 near here>>Interpretation is easy when the imaging modality shows an unequivocal fracture such as a short fissure in a cannon bone. Here the decision is simple: the horse has a fracture and must stop exercising. Many cases, however, demonstrate less clearly defined changes that may be associated with bone fatigue injury.Currently radiography remains the most important imaging modality in fetlock bone risk assessment. With wide availability and the knowledge gained by more advanced imaging techniques refining the most appropriate projections to use; radiography represents a relatively untapped resource that through education of primary care vets could immediately have a profound impact on injury mitigation. The most suitable projection with which to detect prodromal condylar fracture pathology in the equine distal limb is the flexed dorsopalmar (forelimb) or plantarodorsal (hindlimb) projection. On this projection, focal radiolucency in the parasagittal groove, whether well or poorly defined, with or without increased radio-opacity in the surrounding bone, should be considered representative of fracture pathology unless evidence from other diagnostic imaging modalities demonstrates otherwise.<< fig 1 near here>>Computed Tomography (CT) excels at identification of structural changes and is better than radiography at showing very small fissures in the bone. However, additional research is needed to determine specific criteria for interpretation of the significance of small lesions in the parasagittal groove with respect to imminent risk of serious injury. There are good indications that fissure lesion size and proximal sesamoid bone volumetric measurements have the potential to be useful criteria for prediction of condylar and proximal sesamoid bone fractures respectively. With technological advancement, it is likely that CT will be more widely used in quantitative risk analysis in the future.<< fig 2 near here>>Magnetic Resonance Imaging (MRI) has the ability to detect alterations in the fluid content of bones, which allows assessment of acute, active changes. Indeed standing, low-field MRI has been shown to be capable of detecting bone abnormalities not readily identifiable on radiography and has been successfully used for injury mitigation in racehorse practice for some time. However, when used for evaluation of cartilage and subchondral bone lesions, there is a relatively high likelihood of false positive results.Positron Emission Tomography (PET) is a relatively new technique in the veterinary field which relies on similar principles to scintigraphy and provides information on how bone is functioning, enabling it to differentiate between active and inactive structural damage. Early results suggest PET is extremely sensitive, but as with MRI and CT, there is an urgent need to determine the relevance of imaging abnormalities detected in the identification and prediction of individuals at increased risk of serious fetlock injury.Lessons to be learnt from human sports medicineA presentation on the programmes carried out on elite human athletes from Dr Rod Jaques, Director of Medical Services at the English Institute of Sport (EIS), put into sharp focus both the progress equine racecourse veterinary safety assessments have made but also the direction future efforts must take. In elite sports overseen by the EIS, there is a predetermined pathway from diagnosis of any medical condition to management of the condition identified and return of the athlete to competition. The entire pathway is implemented by independent bodies to ensure protocols are followed and athletes fully informed of the consequences of abnormal findings prior to participation.Whilst veterinary assessment and regulatory pathways are in place in many racing jurisdictions globally, transparency about the process and standardisation across countries is lacking. For optimal assessment and accurate identification of horses which are and are not fit to run, there is a need for participation and respect amongst all stakeholders, underpinned by effective education and communication between parties so that trust is built. The workshop participants agreed that primary care vets should be encouraged to share pertinent veterinary history, where deemed necessary, and within the limits of client confidentiality. This maximises information available to racecourse veterinary assessment teams and assists them in making decisions in the interests of equine welfare. Equally, owners, trainers and other stakeholders must understand their obligation to comply with the risk assessment process if they wish to enter a horse in a race. They must also respect the decisions made by regulatory vets and appreciate that these decisions are formulated based on the information and findings available at a specific point in time. Confidence in the pre-race risk assessment process will increase with greater transparency, improved communication and evidence-based decision making.Workshop outcomesIt is clear that further research is needed to enhance knowledge in areas that will advance catastrophic fracture prevention through identification of horses with high immediate risk. The workshop identified several key areas where action is needed:The workshop members have contacted veterinary associations internationally to provide training resources to help improve standards in radiography.More effort is needed to educate horsemen on how serious fatigue injury develops progressively. Identification of early signs will provide the opportunity for prevention of further progression through appropriate modification of athletic activity.In light of the current lag between technological advancements in diagnostic imaging and knowledge of the significance of lesions identified, there is a need to share anonymised medical data as a research tool.Finally, it is clear that advanced diagnostic imaging in particular is a fast-moving field, and periodic revision of recommendations will be required in the future.

By VA Colgate, PHL Ramzan & CM Marr

Minimizing serious fractures of the racehorse fetlock

In March 2020, a symposium was held in Newmarket, UK, aiming to devise measures which could be used internationally to reduce the risk of catastrophic fracture associated with the fetlock joint. The meeting was supported by the Gerald Leigh Charitable Trust, the Beaufort Cottage Charitable Trust and the Jockey Club with additional contributions from a number of industry stakeholders. On the first day a panel of international experts made up of academic professors, Chris Whitton (Melbourne, Australia), Sue Stover (Davis, California), Chris Kawcak (Colorado), Tim Parkin (Glasgow) and Peter Muir (Wisconsin); experienced racehorse clinicians, Ryan Carpenter (Santa Anita) and Peter Ramzan (Newmarket); imaging experts, Sarah Powell (Newmarket) and Mathieu Spriet (Davis, California); and vets with experience in racing regulatory bodies, Scott Palmer (New York) and Chris Riggs (Hong Kong) joined forces to discuss risk assessment protocols, particularly those based on imaging features which might indicate increased risk of imminent fracture. This was followed by a wider discussion with a diverse invited audience of veterinary and industry stakeholders on how our current knowledge of fracture pathophysiology and risk factors for injury could be used to target risk assessment protocols. A report of the workshop outcomes was recently published in Equine Veterinary Journal.

The importance of risk reduction

With the ethics of the racing industry now in the public spotlight, there is recognition that together veterinary and horseracing professionals must strive to realise an improvement in equine injury rates. Intervention through risk profiling programmes, primarily based on training and racing metrics, has a proven track record; and the success of a racing risk management program in New York gives evidence that intervention can and will be successful. 

The fetlock of the thoroughbred racehorse is subjected to very great loads during fast work and racing, and over the course of a training career this can result in cumulative changes in the bone underlying the articular cartilage (‘subchondral’ bone) that causes lameness and may in some circumstances lead to fracture. Fracture propagation involving the bones of the fetlock (cannon, pastern or proximal sesamoid bones) during fast work or racing can have catastrophic consequences, and while serious musculoskeletal injuries are a rare event when measured against race starts, there are obviously welfare and public interest imperatives to reduce the risk to racehorses even further. The dilemma that faces researchers and clinicians is that ‘fatigue’ injuries of the subchondral bone at some sites within the fetlock can be tolerated by many racehorses in training while others develop pathology that tips over into serious fracture. Differentiating horses at imminent risk of raceday fracture from those that are ‘safe’ to run has not proven particularly easy based on clinical grounds to date, and advances in diagnostic imaging offer great promise.

Profiling to inform risk assessment

Risk profiling examines the nature and levels of threat faced by an individual and seeks to define the likelihood of adverse events occurring. Catastrophic fracture is usually the end result of repetitive loading, but currently there are no techniques that can accurately determine that a bone is becoming fatigued until some degree of structural failure has actually occurred. However, diagnostic imaging has clear potential to provide information about pathological changes which indicate the early stages of structural damage. 

Previous research has identified a plethora of epidemiological factors associated with increased risk of serious catastrophic musculoskeletal injury on the racetrack. These can be distilled into race, horse and management-related risk factors that could be combined in statistical models to enable identification of individual horses that may be at increased risk of injury. 

In North America, the Equine Injury Database compiles fatal and non-fatal injury information for thoroughbred racing in North America. Since 2009, equine fatalities are down 23%; and important risk factors for injury have been identified, and this work has driven ongoing improvement.

The problem with all statistics-based models created so far for prediction of racehorse injury is that they have limited predictive ability due to the low prevalence of racetrack catastrophic events. If an event is very rare, and a predictive tool is not entirely accurate, many horses will be incorrectly flagged up as at increased risk. At the Newmarket Fetlock workshop, Prof Tim Parkin shared his work on a model which was based on data from over 2 million race starts and almost 4 million workout starts. Despite the large amount of data used to formulate the model, Tim Parkin suggested that if we had to choose between two horses starting in a race, this model would only correctly identify the horse about to sustain a fracture 65% of the time. Furthermore, the low prevalence of catastrophic injury means it will always be difficult to predict, regardless of which diagnostic procedure is employed. 

Where do the solutions lie?

A radiograph showing a racing thoroughbred’s fetlock joint. The arrow points to a linear radiolucency in the parasagittal groove of the lower cannon bone—a finding that is frequently detectable before progression to serious injury.

A radiograph showing a racing thoroughbred’s fetlock joint. The arrow points to a linear radiolucency in the parasagittal groove of the lower cannon bone—a finding that is frequently detectable before progression to serious injury.

One possible strategy to overcome the inherent challenge of predicting a rare event involves serial testing. Essentially with this approach, a sequence of tests is carried out to refine sub-populations of interest and thus improve the predictive ability of the specific tests applied. An additional consideration in the design of any such practical profiling system would have to be the ability to speedily come to a decision. For example, starting with a model based on racing and training metrics such as number of starts and length of lay-off periods, as well as information about the risk associated with any particular track or racing jurisdiction, entries could be screened to separate those that are not considered to be at increased risk of injury from a smaller sub-group of horses that warrant further evaluation and will progress to Phase 2. The second phase of screening would be something relatively simple. Although not yet available, there is hope that blood tests for bone biomarkers or genetic profiles could be used to further distil horses into a second sub-group. This second sub-group might then be subjected to more detailed veterinary examination, and from that a third sub-group, involving a very small and manageable number of horses flagged as potentially at increased risk, would undergo advanced imaging. The results of such diagnostic imaging would then allow vets to make evidence-based decisions on whether or not there is sufficient concern to prompt withdrawal of an individual from a specific race from a health and welfare perspective. Of course there are other considerations which limit the feasibility of such a system, including availability of diagnostic equipment and whether or not imaging can be quickly and safely performed without use of sedation or other drugs, which are prohibited near to a race start. 

Diagnostic techniques for fetlock injury risk profiling

Currently there is no clear consensus on the interpretation of images from all diagnostic imaging modalities, and important areas of uncertainty exist. Although a range of imaging modalities are available, each has its own strengths and weaknesses, and advances in technology currently outstrip our accumulation of published evidence on which to base interpretation of the images obtained.  

Interpretation is easy when the imaging modality shows an unequivocal fracture such as a short fissure in a cannon bone. Here the decision is simple: the horse has a fracture and must stop exercising. Many cases, however, demonstrate less clearly defined changes that may be associated with bone fatigue injury. 

Currently radiography remains the most important imaging modality in fetlock bone risk assessment. With wide availability and the knowledge gained by more advanced imaging techniques refining the most appropriate projections to use; radiography represents a relatively untapped resource that through education of primary care vets could immediately have a profound impact on injury mitigation. The most suitable projection with which to detect prodromal condylar fracture pathology in the equine distal limb is the flexed dorsopalmar (forelimb) or plantarodorsal (hindlimb) projection. On this projection, focal radiolucency in the parasagittal groove, whether well or poorly defined, with or without increased radio-opacity in the surrounding bone, should be considered representative of fracture pathology unless evidence from other diagnostic imaging modalities demonstrates otherwise. 

Computed Tomography (CT) excels at identification of structural changes and is better than radiography at showing very small fissures in the bone. However, additional research is needed to determine specific criteria for interpretation of the significance of small lesions in the parasagittal groove with respect to imminent risk of serious injury. There are good indications that fissure lesion size and proximal sesamoid bone volumetric measurements have the potential to be useful criteria for prediction of condylar and proximal sesamoid bone fractures respectively. With technological advancement, it is likely that CT will be more widely used in quantitative risk analysis in the future. 

Magnetic Resonance Imaging (MRI) has the ability to detect alterations in the fluid content of bones, which allows assessment of acute, active changes. Indeed standing, low-field MRI has been shown to be capable of detecting bone abnormalities not readily identifiable on radiography and has been successfully used for injury mitigation in racehorse practice for some time. However, when used for evaluation of cartilage and subchondral bone lesions, there is a relatively high likelihood of false positive results.  

PET is the most recent advance in diagnostic imaging. It is being developed in California and, when combined with CT, provides information on bone activity and structure. In these three images of the same fetlock, from different aspects, the orange spots indicate increased activity in the proximal sesamoid bone, which is a potential precursor to more serious injury.Image courtesy of Dr M. Spriet, University of California, Davis.

PET is the most recent advance in diagnostic imaging. It is being developed in California and, when combined with CT, provides information on bone activity and structure. In these three images of the same fetlock, from different aspects, the orange spots indicate increased activity in the proximal sesamoid bone, which is a potential precursor to more serious injury.

Image courtesy of Dr M. Spriet, University of California, Davis.

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PET scanning - reduces catastrophic fractures - latest advance in equine imaging - designed to image horse legs

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By Mathieu Spriet, Associate Professor, University of California, Davis

PET: the latest advance in equine imagingMathieu Spriet, Associate Professor, University of California, Davis<< EVJ new logo near here>>Santa Anita Park, the iconic Southern California racetrack, currently under public and political pressure due to a high number of horse fatalities during the 2019 season, announced in December 2019 the installation of a PET scanner specifically designed to image horse legs. It is hoped that this one-of-a-kind scanner will provide information about bone changes in racehorses to help prevent catastrophic breakdowns.What is PET?PET stands for positron emission tomography. Although this advanced form of imaging only recently became available for horses, the principles behind PET imaging have been commonly used at racetracks for many years. PET is a nuclear medicine imaging technique, similar to scintigraphy, which is more commonly known as “bone scan”. For nuclear imaging techniques, a small dose of radioactive tracer is injected to the horse, and the location of the tracer is identified with a camera in order to create an image. The tracers used for racehorse imaging are molecules that will attach to sites on high bone turnover, which typically occurs in areas of bone subject to high stress. Both scintigraphic and PET scans detect “hot spots” that indicate—although a conventional X-ray might not show anything abnormal in a bone—there are microscopic changes that may develop into more severe injuries.Development of PET in CaliforniaThe big innovation with the PET scan is that it provides 3D information, whereas the traditional bone scan only acquires 2D images. The PET scan also has a higher spatial resolution, which means it is able to detect smaller changes and provide a better localisation of the abnormal sites. PET’s technological challenge is that to acquire the 3D data in horses, it is necessary to use a ring of detectors that fully encircles the leg.The first ever equine PET scan was performed at the School of Veterinary Medicine at the University of California in 2015. At the time, a scanner designed to image the human brain was used (PiPET, Brain-Biosciences, Inc.). This scanner consists of a horizontal cylinder with an opening of 22cm in diameter. Although the dimensions are convenient to image the horse leg, the configuration required the horse be anesthetised in order to fit the equipment around the limb.<< Fig 1 near here>>The initial studies performed on anesthetised horses with the original scanner demonstrated the value of the technique. A first study, published in Equine Veterinary Journal, demonstrated that PET showed damage in the equine navicular bone when all other imaging techniques, including bone scan, MRI and CT did not recognise any abnormality.<< Figure 2>> near hereA pilot study looking at the racehorse fetlock, also published in Equine Veterinary Journal, showed that PET detects hot spots in areas known to be involved in catastrophic fractures. This confirmed the value of PET for racehorse imaging, but the requirement for anesthesia remained a major barrier to introducing the technology at the racetrack. To overcome this, LONGMILE Veterinary Imaging, a division of Brain-Biosciences Inc, in collaboration with the University of California Davis, designed a scanner which could image standing horses. To do this, the technology had to be adapted so that the ring of detectors could be opened and positioned around the limb.With the support from the Grayson Jockey Club Research Foundation, the Southern California Equine Foundation and the Stronach Group, this unique scanner became a reality and, after the completion of an initial validation study in Davis, the scanner was installed at Santa Anita Park in December 2019.PET at the racetrackThe new PET scanner has been used to image the equine limb from the foot to the knee. The current main focus at the racetrack is fetlock imaging, as the majority of catastrophic breakdown in racehorses affects this area. The UC Davis pilot study highlighted the value of PET for detecting abnormalities in the proximal sesamoid bones—the two small bones at the back of the cannon bone—that are commonly involved in catastrophic fractures. Previous necropsy research on horses which suffered breakdowns has shown that changes can be present in the bones prior to the development of major injuries. The goal of the Californian PET project is to detect these warning signs in order to avoid training and racing horses at high risk for catastrophic breakdown.<<Figure 3 near here>>Alternative imaging techniquesOther imaging techniques are available for examining equine bone. Scintigraphic bone scans are doing an excellent job at detecting stress fractures of the humerus or tibia, and this has helped markedly decrease catastrophic injuries in these areas. Bone scan is also used for fetlocks; but “hot fetlocks” are common on bone scan, and the lower resolution 2D images often do not allow to truly determine whether horses are at high risk of fractures or have normal bone adaptation to training.MRI is used for fetlock imaging too, and MRI scanners designed for imaging standing horses have been available for over 15 years. Several large racing centers are equipped with such scanners, and MRI excels in particular at detecting changes in the cannon bone that precede condylar fractures. MRI can detect areas of bone densification, or even accumulation of fluid in the bone, typically indicative of microtrauma that can weaken the bone.Computed tomography (CT) has also recently been used for standing imaging of the fetlock. At the moment, there are a few centers equipped with a CT scanner allowing standing fetlock imaging, but they are only available at, for example, New Bolton Center, Pennsylvania - USA, and the University of Melbourne, Australia. CT uses X-rays to create 3D images. Similar to MRI, CT can detect areas of bone densification or areas of bone loss.PET’s advantagesThe big advantage of PET is what is called “sensitivity”—the ability to detect early and subtle findings. This is because PET detects changes at the molecular level before structural changes have occurred. MRI and CT rely on changes in the density and shape of the structures they are imaging; i.e., structural change must have occurred before these techniques can identify that the bone is abnormal. MRI and CT might miss early information that a PET scan can detect; but they provide complementary information, and these techniques will be important to further characterise abnormalities found on PET. For these reasons, PET and MRI or CT can be combined: a PET image is “fused” on an MRI or a CT, combining the sensitivity of PET with the anatomical detail of the other imaging tool.<< Figure 5 near here>>As PET is a newly available modality at the racetrack, there is still a lot to learn. The goal of the first year at Santa Anita is to image as many horses as possible and compare with the PET information with bone scan or MRI information. The pilot study at Davis and the initial cases at Santa Anita tend to show that it is normal to see some bone activity in specific areas of the fetlock, e.g., the palmar condyles; but the presence of hot spots in other areas, for example in the middle of the sesamoid bones, is an abnormal finding that could be an indicator of higher risk of fracture.Other roles for PETIn addition to its use in racehorses, PET has been used in over a 100 sport horses at UC Davis in the last three years. All these scans have been performed with horses under anesthesia and combined to a CT. The main reason to perform a PET scan is either when other imaging modalities do not find a reason to explain a lameness or to better understand changes seen with other modalities. PET is a “functional” technique; this means that a hot spot indicates an area where an injury is active. MRI can meet difficulties distinguishing between scar tissue and active injury, but PET is the ideal modality for this. The majority of the work done in sport horses has used the same bone tracer as in racehorses. The most common injuries found with this tracer in sport horses result in navicular disease and early arthritis (joint disease).PET is not restricted to imaging; with an alternative tracer, it can be used to look at injuries in the soft tissues. This is something that is not possible with scintigraphy, and the soft tissue tracer has been used successfully to identify tendon injuries—distinguishing between active and inactive tendon lesions. Another important area of interest where the soft tissue tracer has been used is for the assessment of laminitis. This disease is extremely complex, and PET is bringing new information about laminitis, which hopefully will help find new ways to fight this serious life-threatening disease.PET in the futureThe development of equine PET is the biggest step forward in horse imaging since the introduction of equine MRI over 20 years ago. The development of the standing system has considerably facilitated the use of the technique. PET is currently at the forefront of the solutions proposed to improve racehorse safety, but PET will also help with other important health issues in horses.

Santa Anita Park, the iconic Southern California racetrack, currently under public and political pressure due to a high number of horse fatalities during the 2019 season, announced in December 2019 the installation of a PET scanner specifically designed to image horse legs. It is hoped that this one-of-a-kind scanner will provide information about bone changes in racehorses to help prevent catastrophic breakdowns.

What is PET?

Figure 1: The first equine PET was performed in 2015 at the University of California Davis on a research horse laid down with anesthesia. The scanner used was a PET prototype designed for the human brain (piPET, Brain- Biosciences Inc., Rockville, MD).

Figure 1: The first equine PET was performed in 2015 at the University of California Davis on a research horse laid down with anesthesia. The scanner used was a PET prototype designed for the human brain (piPET, Brain- Biosciences Inc., Rockville, MD).

PET stands for positron emission tomography. Although this advanced form of imaging only recently became available for horses, the principles behind PET imaging have been commonly used at racetracks for many years. PET is a nuclear medicine imaging technique, similar to scintigraphy, which is more commonly known as “bone scan”. For nuclear imaging techniques, a small dose of radioactive tracer is injected to the horse, and the location of the tracer is identified with a camera in order to create an image. The tracers used for racehorse imaging are molecules that will attach to sites on high bone turnover, which typically occurs in areas of bone subject to high stress. Both scintigraphic and PET scans detect “hot spots” that indicate—although a conventional X-ray might not show anything abnormal in a bone—there are microscopic changes that may develop into more severe injuries.

Development of PET in California

Santa Anita_ 6N2A9803.jpg

The big innovation with the PET scan is that it provides 3D information, whereas the traditional bone scan only acquires 2D images. The PET scan also has a higher spatial resolution, which means it is able to detect smaller changes and provide a better localisation of the abnormal sites. PET’s technological challenge is that to acquire the 3D data in horses, it is necessary to use a ring of detectors that fully encircles the leg. 

The first ever equine PET scan was performed at the School of Veterinary Medicine at the University of California in 2015. At the time, a scanner designed to image the human brain was used (PiPET, Brain-Biosciences, Inc.). This scanner consists of a horizontal cylinder with an opening of 22cm in diameter. Although the dimensions are convenient to image the horse leg, the configuration required the horse be anesthetised in order to fit the equipment around the limb. 

The initial studies performed on anesthetised horses with the original scanner demonstrated the value of the technique. A first study, published in Equine Veterinary Journal, demonstrated that PET showed damage in the equine navicular bone when all other imaging techniques, including bone scan, MRI and CT did not recognise any abnormality.

A pilot study looking at the racehorse fetlock, also published in Equine Veterinary Journal,  showed that PET detects hot spots in areas known to be involved in catastrophic fractures.

Figure 2: These are images from the first horse image with PET. From left to right, PET, CT, MRI, and bone scan. The top row shows the left front foot that has a severe navicular bone injury. This is shown by the yellow area on the PET image and abnormalities are also seen with CT, MRI and bone scan. The bottom row is the right front foot from the same horse, the PET shows a small yellow area that indicates that the navicular bone is also abnormal. The other imaging techniques however did not recognize any abnormalities.

Figure 2: These are images from the first horse image with PET. From left to right, PET, CT, MRI, and bone scan. The top row shows the left front foot that has a severe navicular bone injury. This is shown by the yellow area on the PET image and abnormalities are also seen with CT, MRI and bone scan. The bottom row is the right front foot from the same horse, the PET shows a small yellow area that indicates that the navicular bone is also abnormal. The other imaging techniques however did not recognize any abnormalities.

This confirmed the value of PET for racehorse imaging, but the requirement for anesthesia remained a major barrier to introducing the technology at the racetrack. To overcome this, LONGMILE Veterinary Imaging, a division of Brain-Biosciences Inc, in collaboration with the University of California Davis, designed a scanner which could image standing horses. To do this, the technology had to be adapted so that the ring of detectors could be opened and positioned around the limb. 

With the support from the Grayson Jockey Club Research Foundation, the Southern California Equine Foundation and the Stronach Group, this unique scanner became a reality and, after the completion of an initial validation study in Davis, the scanner was installed at Santa Anita Park in December 2019.

Figure 3: The two images on the left are bone scan images from a 4-year-old Thoroughbred racehorse. The images on the right are 3D projection of PET images of the same fetlock. The bone scan revealed an abnormality at the bottom of the cannon bone. The PET scan confirmed this abnormality and helped better localize it. In addition, several other abnormalities were found on the PET scan in the sesamoid bones.

Figure 3: The two images on the left are bone scan images from a 4-year-old Thoroughbred racehorse. The images on the right are 3D projection of PET images of the same fetlock. The bone scan revealed an abnormality at the bottom of the cannon bone. The PET scan confirmed this abnormality and helped better localize it. In addition, several other abnormalities were found on the PET scan in the sesamoid bones.

PET at the racetrack

The new PET scanner has been used to image the equine limb from the foot to the knee. The current main focus at the racetrack is fetlock imaging, as the majority of catastrophic breakdown in racehorses affects this area. The UC Davis pilot study highlighted the value of PET for detecting abnormalities in the proximal sesamoid bones—the two small bones at the back of the cannon bone—that are commonly involved in catastrophic fractures. Previous necropsy research on horses which suffered breakdowns has shown that changes can be present in the bones prior to the development of major injuries. The goal of the Californian PET project is to detect these warning signs in order to avoid training and racing horses at high risk for catastrophic breakdown.

Alternative imaging techniques

Other imaging techniques are available for examining equine bone. Scintigraphic bone scans are doing an excellent job at detecting stress fractures of the humerus or tibia, and this has helped markedly decrease catastrophic injuries in these areas. Bone scan is also used for fetlocks; but “hot fetlocks” are common on bone scan, and the lower resolution 2D images often do not allow to truly determine whether horses are at high risk of fractures or have normal bone adaptation to training.

Figure 4: The MILE-PET scanner (LONGMILE Veterinary imaging, Rockville, MD) is the first PET scanner specifically designed to image standing horses. An openable ring of detectors allows easy positioning and safe scanning.

Figure 4: The MILE-PET scanner (LONGMILE Veterinary imaging, Rockville, MD) is the first PET scanner specifically designed to image standing horses. An openable ring of detectors allows easy positioning and safe scanning.

MRI is used for fetlock imaging too, and MRI scanners designed for imaging standing horses have been available for over 15 years. Several large racing centers are equipped with such scanners, and MRI excels in particular at detecting changes in the cannon bone that precede condylar fractures. MRI can detect areas of bone densification, or even accumulation of fluid in the bone, typically indicative of microtrauma that can weaken the bone.

Computed tomography (CT) has also recently been used for standing imaging of the fetlock. At the moment, there are a few centers equipped with a CT scanner allowing standing fetlock imaging, but they are only available at, for example, New Bolton Center, Pennsylvania - USA, and the University of Melbourne, Australia. CT uses X-rays to create 3D images. Similar to MRI, CT can detect areas of bone densification or areas of bone loss. …

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