How Overuse Injuries Happen, And How Imaging Can Help

There is no avoiding the reality that fractures sustained while training and racing is the most pressing of the challenges facing Thoroughbred racing. Veterinarians, trainers, and their support workers at Santa Anita Park have made some dramatic improvements to their system of checks and balances and have significantly reduced their injury rates relative to their previous numbers. In fairness to those that have recently joined the debate about these injuries, the details of these improvements are lost with the addition of each new injury and the growing injury total.

A recent opinion piece on the effects of the products Lasix, Gastroguard, and Thyro-L on fracture formation is an example of the misunderstandings associated with fractures in racing Thoroughbreds.  Although strong evidence is missing that therapeutic medication plays a role in fracture development, these topics have become entangled due to the recent events in southern California: perhaps a discussion for another day.

The purpose of this editorial is to address some of the misunderstandings of fractures in racing Thoroughbreds, to provide some insights about why these fractures are elusive and a challenge to identify, and how early identification using advanced imaging can help the situation.

When dealing with tissue, two types of injuries can occur: an acute injury and an overuse (chronic, stress) injury. An acute injury occurs when a single high force overwhelms the tissue’s ability to deal with the force and it fails. The second type, overuse injuries, can affect muscle, tendon, ligament, and bone and are the most common reason that human athletes visit a sports medicine clinic. Overuse injuries are the result of smaller forces whose damage accumulates over time. Common examples in human medicine include tennis or golfers’ elbow, Achilles tendinitis, and shin-splints.

Speaking specifically about bone injuries, an acute fracture of bone is known as a traumatic fracture. Falling off a ladder and breaking your tibia is an example of an acute (traumatic) fracture of bone. The single high force of falling overwhelms the ability of the tibia to deal with the force and it fractures. Acute (traumatic) fractures are a rare event in racing Thoroughbreds, although the phrase taking a “bad step” is often mistakenly implicated in media reports as the cause for fracture formation.

Overuse injuries of bone represent the great majority of fracture failures in the racing Thoroughbred and are known as a bone stress injury or BSI. Racing Thoroughbred, like marathon runners and the marching military, perform a sport with a repetitive cycle to the sport. For racing Thoroughbreds, their repetitive cycle is the gallop gait. In a small number of horses, the repetitive nature of the gallop gait causes bone to gradually accumulate damage over time and ultimately fail. Fractures begin at the cellular level manifested as bone marrow edema and, when combined with insufficient rest, progress into micro-cracks, macro-cracks, and ultimately a complete fracture. Two examples of bone stress injuries in racing Thoroughbreds include humeral (upper forelimb) stress fractures or condylar fractures of the cannon bone in the fetlock joint.

Equine veterinary medicine has learned so many helpful things from California racing, Santa Anita Park, and Dr. Susan Stover. In 1992, Dr. Stover made two game-changing discoveries about fracture in racing Thoroughbreds: 1) Fractures are a chronic repetitive injury that develop over time and are a sudden ending to a chronic problem when the fracture reaches a tipping point and fails; and, 2) Imaging can help us to identify the signs of a developing fracture (pre-existing disease) before the fracture occurs so that training and racing schedules can be changed “pre-fracture.” This explains the arrival of the bone scanner (nuclear scintigraphy) at Santa Anita Park in the early 1990s.

Worldwide, bone stress injury of the fetlock joint is the most common cause of fracture failure in racing Thoroughbreds. Solving the mysteries of bone stress injury in the fetlock will address approximately 60% of the fracture failure problem in the USA and the most significant fracture failure problem in the world.  Human patients describe the progression of a developing bone stress injury as a dull ache that starts about one week after intense exercise (like a race), to occurring after routine exercise, to occurring during exercise but disappears with rest, to occurring during exercise but not disappearing with rest, to loss of performance in the affected area, to occurring with normal daily movement, to getting worse when sleeping or at night.

The subtle nature of a developing bone stress injury in the fetlock joint and the non-verbal nature of our equine patients makes identifying the at-risk horse a real challenge. Fortunately, the chronic repetitive nature of the gallop gait, like all cyclic and repetitive exercises, leaves a “footprint” in bone so that a developing fracture can be identified “pre-fracture”. Unfortunately, digital radiographs have limited value, with up to 85% of developing bone stress injuries going unidentified in the early stages of fracture development and 50% going undetected during follow-up radiographic examination (radiography).

The act of training makes bone denser by making more bone, a medical principle known as Wolff’s law. When healing a micro-crack, the crack is removed and is replaced with more bone. Although the bone scanner helps us by identifying new bone production, it cannot distinguish new bone production that is creating a more dense bone from new bone production that is healing a fracture. This creates a real problem for veterinary medicine because the cannon bone of the fetlock has the highest level of density increase from training and is also the most common location of fracture failure in racing Thoroughbreds.

Human medicine has demonstrated the value of using advanced imaging to identify bone stress injury in the at-risk athlete. Currently, human medicine uses MRI as the imaging modality of choice for detecting bone stress injuries because MRI can identify bone marrow edema.  In human medicine, MRI is preferred to the bone scanner because bone marrow edema is not present when creating a more dense bone but it is present when a fracture is developing.

With this in mind, equine veterinary medicine has looked to several newer kinds of advanced imaging to identify the developing fracture in the at-risk Thoroughbred racehorse. The standing MRI has been used successfully on Thoroughbred racehorse in Newmarket in the UK and in the USA. We are also evaluating the role of standing robotic computerized tomography (CT) as this equipment can identify changes in bone density, formed macro-fractures and complete fractures, and is the industry standard to reconstruct complicated fracture in operating rooms. PET imaging tags new bone production to identify a developing bone stress injury, like the bone scanner, similarly identifies both more dense bone and a developing fracture.

Santa Anita Park is leading the charge in our industry to identify Thoroughbred racehorses at risk of fracture failure during racing. They have funding in place to use PET imaging and are gathering funding for a standing MRI to be positioned at the racetrack. Churchill Downs is constructing an Equine Medical Center with plans for several advanced imaging modalities to be positioned at the racetrack in order to quickly and efficiently evaluate the at-risk Thoroughbred.  The most recent cycle of Grayson-Jockey Club Research Foundation grants has funded many studies to evaluate the role of advanced imaging in the early identification of bone stress injuries in horses.

By following the lead of human medicine, it is likely that advanced imaging will provide us with the same successes to identify the developing fracture in the at risk horse so that training an racing schedules can be altered pre-fracture.  This also will allow us to recommend a shorter or longer period of recovery, given the degree of progression of the developed “footprint” of the bone stress injury. It is intuitive, and has been established in human medicine, that the earliest possible diagnosis and treatment of a developing bone stress injury would facilitate shorter recovery periods, a better prognosis, an earlier return to performance, and far fewer fracture failures.

Dr. John Peloso is a diplomate with the American College of Veterinary Surgeons. He is a surgeon and partner at the Equine Medical Center of Ocala.

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