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

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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Diagnosis

Diagnosis of sesamoiditis first focuses on identifying the general location of the lameness through a lameness examination, followed by further diagnostic tests to confirm the sesamoids as the source of lameness.

Lameness Examination

In most cases, thorough palpation of the limbs by a veterinarian is sufficient to identify the sesamoid bones as the likely source of lameness.

Horses with sesamoiditis typically show a pain response when the veterinarian squeezes the sesamoids or the suspensory ligament. [1] Many horses also have swelling and heat around the sesamoid bones, providing further clues that the sesamoid bones are affected. [1]

Flexion tests typically show increased lameness with a distal limb (lower limb) flexion. [1]

Some veterinarians may choose to perform nerve blocks or joint blocks as part of their diagnostic work-up. Horses with sesamoiditis typically show reduced lameness after a low four-point nerve block, which blocks sensation in the entire fetlock and hoof. [1]

X-rays

X-rays of the fetlock are the most common method for diagnosing sesamoiditis. Changes associated with sesamoiditis include:[1], [3]

  • Changes in the size of the sesamoid bones
  • Increased number of vessel channels through the bones
  • Increased irregularity of the vessel channels
  • Coarse or mottled appearance to the bone

Some horses that develop sudden lameness from sesamoiditis may show no X-ray changes at the time of examination. [1] Repeat X-rays around 3 weeks after the horse initially develops lameness are usually required to confirm the diagnosis. [1]

Horses with sesamoiditis may also show signs of calcification of the suspensory ligament or soft tissues surrounding the sesamoid bones. [1] This additional bone production is likely due to inflammation and weakening of the ligaments, resulting in tearing. [1] In response, the body produces mineral and bone in an attempt to stabilize the area. [1]

Nuclear Scintigraphy

Nuclear scintigraphy (bone scan) is another diagnostic method that can identify sesamoiditis. Some large referral practices may offer this diagnostic method.

To perform nuclear scintigraphy, the veterinarian administers a radioactive medication to the horse. Over the course of 2-4 hours, the radioactive material absorbs into the horse’s bones, with higher absorption rates in areas of inflammation or increased blood flow. [1]

The veterinarian then uses a specialized camera that highlights areas of radioactivity to identify areas where there is more radioactivity than expected. [1] These areas are the most likely source of lameness.

Ultrasound

Ultrasound is not able to identify the bony changes associated with sesamoiditis, however it can identify changes in the ligaments and other tissues surrounding the sesamoid bones. [1] Damage to these tissues can contribute to lameness and is worth investigating in sesamoiditis cases. [1]

Clinical Significance

Evidence of sesamoiditis is not always associated with lameness. [1] In young racehorses, changes on X-rays associated with sesamoiditis are common, even in clinically sound horses. [1]

These findings likely reflect the significant remodeling, or bony change, that the sesamoid bones undergo when a horse begins intensive training. [1] The increased exercise associated with starting training causes mild inflammation of the sesamoid bones in response to stress. This is a normal response in all species increasing their exercise levels.

Sesamoiditis associated with lameness occurs if there is overexertion of the sesamoid bones during this remodeling phase. This results in stress placed on the sesamoid bones that exceeds their current strength. [1] Damage to the suspensory ligament may also occur due to overexertion, further contributing to lameness. [1]

Some horses showing evidence of sesamoiditis on X-rays may also be sound if their body has had time to heal the injury. [1] These horses may be performing successfully at low levels that do not irritate their previous injury.

Evidence of sesamoiditis on X-rays must be taken in context with the overall picture of the horse’s health, including the findings of a lameness examination.

Treatment

Treatment of sesamoiditis focuses on reducing inflammation, followed by a slow return to work to prevent re-injury of the sesamoid bones. Most horses require rest from exercise until they show complete soundness at the trot. [1][4]

Anti-inflammatories

Anti-inflammatory medications are the main treatment for sesamoiditis. [1] Both topical anti-inflammatories applied to the site of injury and systemic anti-inflammatories, such as oral or injectable non-steroidal anti-inflammatory drugs (NSAIDs), can be used. [1]

Topical anti-inflammatories used for sesamoiditis include: [1]

  • Diclofenac cream
  • Ibuprofen cream
  • Ketoprofen gel

Common systemic NSAIDs used in horses include: [1]

Return to Work

Many horses require 6 – 8 months of complete rest from exercise. [1] When reintroducing exercise, the exercise plan must be progressive and avoid overexertion of the fetlocks. [1]

The veterinarian managing the case will provide general guidelines on how to increase workload slowly to prevent re-injury of the sesamoid bones. [1]

Other Treatments

Shockwave Therapy

Some veterinarians recommend the use of extra-corporeal shockwave therapy (ECSWT) in treating cases of sesamoid injury. [1] However, there is limited scientific evidence to support the use of shockwave therapy in sesamoiditis.

One study involving 10 horses reported that 60% of the horses treated with ECSWT showed improvement in soundness at 3 and 6 months after original injury. [6] Horses were treated 2-4 times every 3-4 weeks in this study, and no adverse effects were reported. [6]

However, there was no comparison to alternative treatment modalities or horses treated with stall rest alone, so the true efficacy of ECSWT for treating sesamoiditis is unknown. [6]

Platelet-Rich Plasma

Platelet-rich plasma (PRP) is a common treatment for soft tissue injuries and other causes of lameness. Some veterinarians may use PRP as part of their treatment protocol for sesamoiditis.

In this procedure, the veterinarian collects whole blood from the horse and removes the plasma, the component of blood containing white blood cells and healing factors. [1] The veterinarian then injects the plasma into an area of the body requiring additional healing.

One study examined the effect of treating sesamoiditis in yearling Thoroughbreds with PRP. [7] Horses treated with PRP were more likely to start racing as two-year-olds compared to untreated peers. [7] However, their racing results were similar to their untreated peers. [7]

From this study, the researchers concluded that PRP does not improve future racing performance. [7] However, this study did not examine symptoms or X-ray findings of sesamoiditis before and after PRP therapy, so the efficacy of PRP as a treatment for sesamoiditis is still relatively unknown.

Prognosis

Horses that develop lameness from sesamoiditis have a poor prognosis for long-term soundness. [1] Many horses do not return to their previous level of performance, even with a slow progression in workload.

Sound Horses

Horses that are not lame, but show changes associated with sesamoiditis on X-rays, may have a worse prognosis for performance compared to normal horses. [1]

One study in yearling Thoroughbreds showed that horses with changes in the sesamoid bones had fewer race starts and reduced earnings compared to their peers. [8] From the study, the researchers concluded that X-rays of the sesamoid bones in yearlings are an important component of pre-purchase examinations, as they can provide information about the horse’s long-term soundness. [8]

Another study showed similar findings, but also observed that horses with sesamoid changes in the front limbs were less likely to begin a racing career, while horses with changes in the hind limbs were more likely to have a delayed start to their racing career. [9] This is likely due to the increased concussive forces on the front limbs making forelimb changes more significant in the horse’s overall soundness. [9]

In mature racehorses, X-ray changes of sesamoiditis were associated with fewer career placings. [10] However, the overall career earnings for affected horses were similar to their unaffected peers. [10]

Suspensory Ligament Injuries

Horses with changes indicative of sesamoiditis on X-rays may be predisposed to suspensory ligament injuries. [1]

One study found that yearling Thoroughbreds with sesamoiditis were more likely to exhibit changes affecting the integrity of the suspensory ligament branch that attaches to the sesamoid bones. [5] Changes identified in the ligaments included: [5]

  • Irregular fibre patterns
  • Areas of fibre loss or significant defects in the structure of the tendon

Another study from the same research group showed that yearlings with significant sesamoiditis changes on X-ray have a 5 times greater risk of developing suspensory ligament branch injuries compared to unaffected horses. [11] Suspensory ligament branch injuries can adversely impact a horse’s future performance. [5][11]

From these results, the researchers recommend that all horses with evidence of sesamoiditis should undergo ultrasound of the suspensory ligament branches to identify any changes that may cause future lameness. [5]

Summary

  • Sesamoiditis refers to inflammation of the sesamoid bones and surrounding structures
  • Sesamoiditis is most common in hunter/jumpers, young racehorses, and barrel racing horses
  • Not all horses with signs of sesamoiditis on X-rays are lame
  • Treatment requires prolonged rest, anti-inflammatories, and a slow return to work
  • Horses that develop lameness from sesamoiditis have a poor prognosis for future soundness

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References

  1. Baxter. G. M., Ed., Adams and Stashak’s lameness in horses, Seventh edition. Hoboken, NJ: Wiley-Blackwell, 2020.
  2. Menarim. B. C. et al., Radiographic Abnormalities in Barrel Racing Horses with Lameness Referable to the Metacarpophalangeal Joint. Journal of Equine Veterinary Science. 2012. doi: 10.1016/j.jevs.2011.09.064.
  3. Vanderperren. K. and Saunders. J. H., Diagnostic Imaging of the Equine Fetlock Region Using Radiography and Ultrasonography. Part 2: The Bony Disorders. The Veterinary Journal. 2009. doi: 10.1016/j.tvjl.2008.01.017.
  4. Baxter. G. M., Ed., Manual of Equine Lameness. Wiley Blackwell, 2011.
  5. Plevin. S. et al., Association between Sesamoiditis, Subclinical Ultrasonographic Suspensory Ligament Branch Change and Subsequent Clinical Injury in Yearling Thoroughbreds. Equine Veterinary Journal. 2016. doi: 10.1111/evj.12497.
  6. Boström. A. et al., Systematic Review of Complementary and Alternative Veterinary Medicine in Sport and Companion Animals: Extracorporeal Shockwave Therapy. Animals. 2022. doi: 10.3390/ani12223124.
  7. Garrett. K. S. et al., Injection of Platelet- and Leukocyte-Rich Plasma at the Junction of the Proximal Sesamoid Bone and the Suspensory Ligament Branch for Treatment of Yearling Thoroughbreds with Proximal Sesamoid Bone Inflammation and Associated Suspensory Ligament Branch Desmitis. Journal of the American Veterinary Medical Association. 2013. doi: 10.2460/javma.243.1.120.
  8. Spike-Pierce. D. L. and Bramlage. L. R., Correlation of Racing Performance with Radiographic Changes in the Proximal Sesamoid Bones of 487 Thoroughbred Yearlings. Equine Veterinary Journal. 2003. doi: 10.2746/042516403776014262.
  9. Peat. F. J. et al., Radiological Findings in the Proximal Sesamoid Bones of Yearling and 2-Year-Old Thoroughbred Sales Horses: Prevalence, Progression and Associations with Racing Performance. Equine Veterinary Journal doi: 10.1111/evj.14051.
  10. Lloyd. K. A. et al., Associations between the Radiographic Appearance of Vascular Channels in Proximal Sesamoid Bones, Their Microstructural Characteristics and Past Racing Performance in Thoroughbreds. Equine Veterinary Journal. 2020. doi: 10.1111/evj.13239.
  11. McLellan. J. and Plevin. S., Do Radiographic Signs of Sesamoiditis in Yearling Thoroughbreds Predispose the Development of Suspensory Ligament Branch Injury?. Equine Veterinary Journal. 2014. doi: 10.1111/evj.12154.
  12. Munroe, G. Proximal sesamoid: sesamoiditis. Vetlexicon Equis.