Sesamoiditis in horses refers to pain and injury around the sesamoid bones, pairs of small triangular bones found at the back of the fetlock. Inflammation in this area can cause pain and changes to the bony structure of the sesamoid bones, resulting in lameness.

Sesamoiditis is most common in hunter/jumper horses, young racehorses, and barrel racing horses. Treatment typically involves a prolonged period of rest, anti-inflammatory medications, and a slow return to exercise.

Not all horses that develop signs of sesamoiditis on X-rays are lame. However, these horses may be predisposed to developing lameness in the future, as well as damage to their suspensory ligaments.

Horses that develop lameness due to sesamoiditis have a poor prognosis for future soundness and performance.

Sesamoids in Horses

The fetlock joint is a major weight-bearing structure in the horse’s lower limb. At the back of the fetlock joint are two roughly triangular bones, the sesamoid bones, which help absorb concussive forces as the horse moves. [1]

The sesamoid bones have attachments to many of the major ligaments at the back of the horse’s limb, serving to cushion and disperse the force from impact. Attachment points include: [1]

  • Suspensory ligament, the major weight-bearing ligament of the limb
  • Palmar ligament, a stabilizing ligament the supports many of the fetlock’s structures
  • Distal sesamoidean ligaments, which run between the sesamoid bones and the pastern bones

When the horse stands, these ligaments and the sesamoid bones help support the horse’s body weight. Veterinarians call this group of structures the suspensory apparatus of the fetlock, due to its function in “suspending” weight.

During motion, these structures prevent overextension of the fetlock joint, preventing it from dropping too close to the ground. [1]

These structures are under the highest strain during galloping or landing off a jump, where all of the forces of the horse’s weight and movement are placed on the front limbs. [1] The tight turns performed by barrel racing horses may also cause overextension of the fetlock, particularly on the outside of the limb. [2]

Sesamoid Bones in Horses | Mad Barn CanadaIllustration:

Causes

The basic cause of sesamoiditis is excessive strain in the fetlock, resulting in damage to the attachment point of the suspensory ligament to the sesamoid bones. [1][3]

Veterinarians presume that inflammation where the suspensory ligament inserts at the sesamoid bones is what produces the pain and lameness associated with sesamoiditis. [1] The sesamoid bones have a large network of sensory nerves, which may become irritated by inflammation. [1]

Risk Factors

These injuries are most common in young racehorses, hunters, and jumpers. [4] Studies show that around 65% of Thoroughbred yearlings show changes associated with sesamoiditis on X-rays. [5]

Barrel racing horses also have a high incidence of sesamoiditis-related changes on X-rays, according to one study. [2] This study showed that 70% of the unsound barrel horses examined had severe sesamoiditis changes that likely contributed to their lameness. [2]

Symptoms

The most common symptom of sesamoiditis is lameness. Most horses show more significant lameness when they first start exercising, or when exercising on hard ground. [1]

Other symptoms include: [1]

  • Increased heat over the sesamoid bones at the back of the fetlock
  • Swelling of the fetlock
  • Pain when squeezing the sesamoid bones
  • Pain when squeezing the suspensory ligament

Horses with sesamoiditis often present to their veterinarians with poor performance. This condition is also commonly associated with injuries to the suspensory apparatus, including the enlargement of suspensory structures around the palmar fetlock. [12]

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