Sesamoid injuries are a common and often serious injury in horses. The small sesamoid bones play a major role in reducing tension on the muscles and tendons of the body. The proximal sesamoid bones (PSBs), located in the fetlock joint, are particularly key for the movement and stability of the horse’s limbs. [1][2]

The PSBs are susceptible to catastrophic injury in performance horses, especially in high-intensity and high-speed sports, such as racing. These injuries can range from inflammation to severe fractures, often resulting in significant lameness and, in the worst cases, career-ending or life-threatening conditions.

Early diagnosis and appropriate treatment are critical to managing sesamoid injuries effectively, highlighting the importance of advanced diagnostic techniques and targeted therapeutic interventions in equine veterinary practice.

Sesamoid Bones

The fetlock joint is crucial for movement. It is located between the horse’s cannon bone and the long pastern bone in the leg, adjacent to the foot, similar to the human ankle. [2]

The fetlock joints in both the front and back legs are supported by triangular bones in the back of the joint called proximal sesamoid bones (PSBs). They connect to the tendons of the lower leg, relieving tension, and are stabilized by branches of the suspensory and sesamoidean ligaments. [3]

Sesamoid Bones in Horses | Mad Barn CanadaIllustration:

The PSBs act as mechanical levers, enhancing flexor tendon efficiency and distributing forces during movement. Together with ligaments and tendons, they: [4]

  • Absorb shock
  • Reduce friction
  • Aid in weight bearing

While these bones play a critical function in weight bearing and locomotion, their location and function make them highly vulnerable to injury. They endure substantial stress and pressure during high-impact activities like galloping and jumping.

Symptoms

Clinical signs of sesamoid injury in horses vary depending on the severity and type of injury but may include: [1][5][6]

  • Acute Lameness: one of the most common signs, varying from mild to severe depending on the extent of the injury
  • Pain and Swelling: visible swelling and tenderness around the fetlock joint, especially upon palpation
  • Heat: the fetlock and surrounding area may be hot to the touch
  • Altered Gait: changes in gait such as shortening of stride, asymmetrical movement, or favoring of one limb
  • Decreased Performance: reduced athletic performance or reluctance to perform certain movements
  • Changes in Behavior: irritability or changes in temperament, possibly due to discomfort or pain

Causes

The horse’s musculoskeletal system adapts incredibly well to increased exercise intensity with gradual conditioning and training. This process strengthens bones and supportive structures, enhancing performance and helping prevent injuries. [7]

Sudden or repetitive activity without proper conditioning can lead to injuries like stress fractures, as tissues and bones can become fatigued and sustain damage without gradual adaptation. Several factors can increase a horse’s likelihood of developing sesamoid injuries: [5][8][9][10]

  • Fatigue: during exercise, the horse’s muscles can fatigue, reducing their ability to absorb shock and support the fetlock joint effectively. Fatigue-related alterations in movement patterns can increase the risk of traumatic sesamoid injuries, particularly during or following high-speed or strenuous activities.
  • High-Speed Movement: high speed movement can subject the PSBs to significant impact forces, repetitive strain, and overextension, leading to microfractures, ligament damage, and potential bone weakening, thereby increasing the risk of PSB injuries.
  • Conformation: horses with poor conformation, such as straight fetlocks, may be predisposed to PSB fractures and injuries as they can lack the natural shock-absorbing ability of an angled joint. This increases the stress on the PSBs during movement.
  • Shoeing: incorporating a trailer, a longer outside branch on the hind horseshoes, is believed to provide added support and traction for Standardbred racehorses when completing high-speed turns on varied track surfaces. Shoes with trailers may increase the risk of fracturing the sesamoid bones in the hindlimb due to excessive torque forces on the fetlock.

Sesamoid Fractures

Proximal sesamoid bone (PSB) fractures are the leading cause of severe suspensory apparatus failure in Thoroughbred racehorses. The suspensory apparatus, which includes the suspensory ligament and other connective tissues, helps stabilize the fetlock joint and support the horse’s weight during high-intensity activity. [2][11]

When a PSB fractures, it can cause the suspensory ligament to fail, disrupting this supportive system and resulting in extreme lameness. This can potentially end the horse’s racing career. In horses, PSB fractures are categorized based on their location on the bone: apical, mid-body, basilar, and abaxial fractures. [5]

Articular vs. Non-articular Fractures

Sesamoid fractures may be characterized as articular or non-articular based on their involvement with the joint surface of the sesamoid bone. [9]

  • Articular Fractures: involve the joint surface of the sesamoid bone. This means the fracture extends into th