Bone bruising is a common condition often seen in young racehorses, such as Thoroughbreds or Standardbreds. It typically arises from repetitive stress, trauma, or microfractures affecting the subchondral bone, which is the layer of bone located beneath the joint’s cartilage.

Horses engaging in high-impact exercise or activities that involve sudden changes in direction or speed have an increased risk of developing this condition.

The main characteristics of bone bruising include inflammation and localized swelling, primarily in the limbs and joints. These symptoms often result in lameness and have a significant impact on the horse’s performance and overall comfort.

Horses affected by bone bruising require careful management and appropriate treatment to promote healing and minimize potential long-term consequences. Failure to address bone bruises may lead to complications such as osteoarthritis and stress fractures. [1]

If you suspect that your horse is suffering from bone bruising, it is important to seek guidance from a veterinarian. They can provide an accurate diagnosis and develop an appropriate treatment plan tailored to your horse’s specific needs.

Bone Bruising in Horses

Bone bruising in horses is also referred to as subchondral bone disease, nonadaptive bone remodeling, bone oedema, and bone contusion. [2][3][4]

This condition refers to a specific type of injury that affects the subchondral bone, which is the layer of bone just beneath the cartilage in a joint.

Intense training, racing, and other performance equine disciplines often lead to bone bruising. The repetitive forces placed on the horse’s joints in these activities can cause microfractures or subchondral bone damage.

In horses with bone bruising, the bone does not heal or repair itself in the typical manner. Instead of undergoing the normal remodeling process, where damaged bone is resorbed and replaced with new bone, the injured area shows signs of pathologic remodeling, such as increased bone density and changes in bone structure.

The term bone contusion is often used interchangeably with bone bruising and refers to the contusion-like appearance of the subchondral bone in imaging studies.

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Subchondral Bone

Subchondral bone, the layer of bone beneath the articular cartilage in joints, plays an important role in maintaining joint integrity and absorbing forces during locomotion. [4][5]

The rich blood supply to subchondral bone plays a crucial role in supporting the repair and remodeling processes of both the bone and cartilage.

During activities such as racing and training, repetitive stress placed on the joints can lead to damage and bleeding within the subchondral bone. In some cases, fluid can accumulate within the bone, which is known as bone oedema. This fluid buildup causes swelling and can contribute to pain and lameness in the affected horse.

The microdamage triggers the body’s natural remodeling response, initiating a process to restore the normal structure and function of the bone.

Bone Remodeling

Bone remodeling is a natural physiological process where damaged or old bone is replaced in response to stresses on the horse’s skeletal system. [6][7]

Osteoclasts (cells that breakdown and resorb bone) and osteoblasts (cells that form new bone) work together in the remodeling process to maintain the integrity of the bone tissue. This adaptive process enables the bone to withstand mechanical forces and retain its strength.

Osteoblasts deposit tissue in damaged areas to rebuild and prevent further damage. Following injury, it is common for weight-bearing bones, such as the third carpal bone of the leg, to become thicker and increase in density.

The process of new bone formation is relatively slower compared to bone resorption. While bone resorption can happen relatively quickly (within a few days to a few weeks), the formation of new bone takes several months.

When the rate of bone resorption exceeds the capacity of osteoblasts to keep up, an imbalance occurs, leading to inefficient remodeling. This condition is commonly known as maladaptive bone remodeling, resulting in bone damage and bruising. [7][8]

Bones that are undergoing active repair are also more vulnerable to injury due to decreased structural integrity. This makes them more susceptible to further damage.

Risk Factors

Bone bruising in horses is caused by various factors related to repetitive stress, trauma, and excessive loading on the skeletal system.

Some common causes of bone bruises include: [4]

  • High-impact and intense training
  • Extreme and/or repetitive concussive forces
  • Uneven ground surfaces
  • Trauma (i.e. collision, falls, injuries)

Racehorses are known to have a high prevalence of bone bruises, due to their rigorous training schedules and repetitive stresses placed on their limbs.

In the initial stages of racehorse training, the focus is typically on building stamina, conditioning the cardiovascular system, and developing the muscles through long and slow exercise bouts.

However, after a few months, there is often a sudden introduction of short periods of high-speed training and prolonged periods of stalling, which can be stressful for the young horse’s skeleton. This abrupt change from long, slow exercise to intense training can make them more susceptible to bone damage.

Clinical Signs

The clinical signs of bone bruising in horses can vary depending on the location and severity of the injury. Commonly affected areas include the fetlock and tarsus (hock) joints.

Thoroughbreds commonly develop bone bruising in the fore fetlock, while Standardbreds tend to experience it in the hind fetlocks. [1]

Typical signs of bone bruising include: [2][3][4][8][9]

  • Mild to severe acute lameness, including reluctance to bear weight on the affected limb
  • Joint effusion (fluid accumulation within the joint)
  • Localized soft tissue edema (swelling), pain, and heat
  • Decreased range of motion
  • Poor performance

Diagnosis

Diagnosing subchondral bone disease in horses typically involves a combination of clinical evaluation and imaging techniques. Prompt diagnosis is crucial to ensure a favorable prognosis.

To identify the specific area of bone bruising, palpation, lameness examination, and diagnostic analgesia are commonly used.

A flexion test may be conducted during a lameness examination to assess the horse’s reaction to stress applied to a particular part of the limb. [4] This involves temporarily stressing the joint or region of interest by flexing it and holding it in that position. The horse’s gait is then observed for signs of discomfort, which can indicate the presence of bruising or other issues.

Diagnostic analgesia- nerve or joint blocks- is a procedure that involves injecting a local anesthetic, such as lidocaine or carbocaine, near the nerve or nerves that supply sensation to a particular area of the horse’s body. By blocking these nerves, the horse’s ability to feel pain in the targeted region is temporarily eliminated or reduced, allowing the veterinarian to assess the horse’s gait without the influence of pain. This helps the vet pinpoint the exact location of lameness or diagnose the underlying issue more accurately. [10]

Bone edema does not typically show up on conventional radiographs (X-rays) in the early stages of its development. As bone edema progresses and becomes more severe, it may eventually lead to more noticeable changes in bone density or bony remodeling that can be visualized on X-rays.

In situations where bone edema is suspected but not visible on X-rays, other imaging techniques may be used to detect it. Magnetic Resonance Imaging (MRI) and Bone Scans (using radioactive tracers) are more sensitive modalities for visualizing the early and microscopic changes associated with bone edema.

Magnetic Resonance Imaging (MRI)

MRI, or Magnetic Resonance Imaging, is a medical imaging technique that uses a powerful magnetic field and radio waves to create detailed images of the internal structures of the body, including soft tissues, organs, muscles and bones. [11]

This imaging technique can be used to identify and observe contusions and microfractures in the subchondrial bone, which are typically referred to as “bone marrow lesions” (BMLs). These lesions appear different in colour from surrounding, healthy bone marrow.

BMLs are thought to be early indicators of joint deterioration and structural changes in the bones and cartilage. High-resolution MRI can provide detailed photographic information to guide diagnosis, treatment, and overall management of bone bruising in horses. [1]

Bone Scintigraphy (Bone Scan)

This non-invasive diagnostic imaging technique is used to evaluate bone health and identify any abnormalities or disease. It provides information about blood flow and can quantify changes in bo