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 USAIllustration:

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 the part of the bone that articulates with other bones in the joint. Articular fractures are often more complex and require careful management to preserve joint function and prevent long-term complications.
  • Nonarticular Fractures: do not extend into the joint surface of the sesamoid bone. Instead, they occur in other areas of the bone, away from the joint surface. Non-articular fractures may still cause significant pain and lameness but are generally less complicated to manage compared to articular fractures.
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Types of Fractures

Sesamoid fractures in horses can vary significantly in terms of location, cause, and severity. By familiarizing themselves with the different types of fractures, horse owners can better understand their veterinarian’s recommendations during treatment. It’s also helpful to understand the mechanics of how different types of fractures occur to aid in avoiding injury related to training and management.

Apical fracture (Articular)

Apical fractures, the most common type of fracture in Thoroughbreds and Standardbreds, happen in the upper third of the sesamoid bone. They typically affect less than a quarter of the area where the suspensory ligament attaches to the outer edge of the bone. [1][9][12]

Apical fractures are usually associated with overextension or acute trauma during high-impact activities, leading to sudden lameness, swelling in the fetlock area, and pain upon palpation. [1]

Mid-body fracture

Mid-body fractures, occurring in the middle part of the sesamoid bone, often result from significant trauma or stress, which can lead to splitting the PSB in half. These fractures are the most difficult to treat due to their location within the weight-bearing portion of the bone. [9]

Basilar fracture

Basilar fractures occur at the base of the sesamoid bone, typically in the lower third, and can extend into the attachments of the suspensory apparatus. These fractures often result from high-impact stress or twisting injuries. [1]

Abaxial fracture

Abaxial fractures, occurring on the outer surface of the sesamoid bone where the suspensory ligament attaches, are less common and typically result from lateral stress or trauma. They are often associated with uneven ground or awkward movements. [1]

Sesamoiditis

Sesamoiditis is a condition characterized by bony changes (growth or degeneration) in the PSBs, leading to inflammation, pain, and lameness. Affected horses can have symptoms ranging from mild discomfort to significant performance limitations. [6][13][14]

Sesamoiditis is commonly seen in racehorses, particularly Thoroughbreds and Standardbreds, due to the significant stress placed on their fetlocks during training. Horses affected by sesamoiditis may face an increased risk of developing suspensory ligament injuries once training begins.

Sesamoiditis presents in two primary forms: [1][5][13]

  • Periostitis: involves inflammation of the periosteum, the fibrous membrane around the PSBs, commonly caused by damage to the associated ligament connections. This can result in the growth of bony protrusions and changes in the shape of the sesamoid bones.
  • Osteitis: refers to inflammation within the bones themselves, which could be linked to issues in the arteries supplying blood to the bones or when parts of the bone tissue start to die. X-rays may reveal mineral loss along blood vessels or the development of bony spurs.

Axial Osteitis/Osteomyelitis of the PSBs

In rare cases, horses can develop osteitis, which is inflammation within the proximal sesamoid bones, or osteomyelitis, a bone infection, on the axial (inner) side of the PSBs. An infection results in further inflammation and the deterioration of bone tissue. [15][16]

The exact cause of these conditions is still unknown, but they’re believed to arise from vascular problems, infections, or traumatic injuries. Specifically, they are associated with the ligament that connects the sesamoid bones, called the inter-sesamoidean ligament. [17]

Osteomyelitis in the PSBs can develop as a secondary issue in horses that already have infections elsewhere in their body. For example, this condition has developed as a secondary issue in horses with infections in the digital sheath or fetlock joint. [15]

When osteomyelitis is suspected, treatment typically involves long-term systemic antibiotics and/or surgical removal of the lesion.

Diagnosis of Sesamoid Injuries

Diagnosing sesamoid injuries in horses involves a combination of client history, symptoms, and various diagnostic tests to accurately assess the condition of the proximal sesamoid bones (PSBs) and surrounding structures. [1]

  • Horse History: understanding the horse’s recent activity is the first step in diagnosis, especially if there has been strenuous exercise or acute breakdown in the past 24 hours
  • Physical Examination: a thorough physical examination of the horse while it is at rest can reveal swelling, joint effusion, and tenderness in the fetlock area
  • Gait Evaluation: observing the horse’s gait can help identify lameness and abnormal movement patterns that may indicate sesamoid injuries
  • Manipulative Tests: performing specific manipulative tests (i.e. flexion tests) can further localize pain and discomfort to the sesamoid region

Nerve Blocks

Perineural analgesia, commonly referred to as a nerve block, is a diagnostic technique used to localize the source of lameness in horses. This involves injecting a local anesthetic near specific nerves to temporarily block sensation and pain in targeted areas of the limb. [1][18]

By methodically numbing specific areas of the limb, veterinarians can pinpoint the location of pain more precisely. If a nerve block alleviates the lameness, it indicates that the source of pain is within the area served by the blocked nerve.

Nerve blocks are particularly useful for diagnosing PSB injuries, which can be challenging to locate due to their deep location in the horse’s leg. By isolating the injury site, nerve blocks enhance diagnostic accuracy and assist in planning effective treatment strategies.

Treatment

Depending on severity, horses with sesamoid injuries may receive conservative or surgical treatment. In some cases, treatment is not possible and humane euthanasia is the only option.

Conservative treatment

Conservative, or non-surgical, treatment is often the first approach for managing most sesamoid injuries, including fractures. The goal of conservative treatment is to allow the bone and associated soft tissues to heal naturally, while also managing pain and preventing further injury.

Non-articular fractures, which do not involve the joint surface, are generally less complicated and can often be effectively managed with conservative treatment. Although basilar fractures typically have a poorer prognosis, some cases can still be managed conservatively, especially if the fracture is stable and not severely displaced. [1]

While under conservative treatment, the horse is typically put on strict stall rest to minimize movement and stress on the affected limb, lasting from several weeks to several months, depending on the fracture’s severity and the horse’s response to treatment. Non-steroidal anti-inflammatory medication (NSAIDs) may be prescribed by a veterinarian to minimize pain.

After the rest period and at the discretion of a veterinarian, controlled exercise is gradually introduced to strengthen the ligaments and surrounding structures. The intensity and duration of exercise are slowly increased over time, building strength and resilience without overloading the affected area.

Surgery

Surgery may be considered for certain types of sesamoid injuries in horses, particularly those that do not respond to conservative treatment or cases where there is a significant structural damage that requires surgical intervention (apical, midbody and some basilar fractures). [1]

One common aspect of these surgeries is removing bone fragments to aid healing. Depending on the specific fracture, the veterinarian might opt for arthroscopic removal, using a small camera to guide extraction through tiny incisions, or fixing the fragments in place with lag screws. [1][2]

Complex fractures may require more extensive surgical techniques, such as arthrodesis (fusing the bones of the joint) or external fixation, to maintain joint stability and function.

Prognosis and Prevention

The prognosis for sesamoid injuries in horses depends on factors like injury severity, treatment efficacy, and the horse’s response to treatment. Less severe injuries may recover with non-surgical methods like rest and medication, allowing for a return to previous activity levels. [1][14]

More serious fractures that require surgical intervention have variable outcomes influenced by surgical technique and post-operative care. Complications like infection or delayed healing can impact prognosis, as can the potential for long-term lameness or limitations.

In severe cases, retirement from high-intensity activity may be required for the horse’s welfare, or euthanasia may be considered.

While it is impossible to completely eliminate the risk of sesamoid injuries in horses, regular monitoring of the horse’s fetlocks is crucial for identifying potential issues. Early detection of minor problems provides the best chance of prevention progression to a more severe and potentially debilitating injury.

Summary

Sesamoid injuries impact the proximal sesamoid bones (PSBs) located in the horse’s fetlock joint, which are vital for maintaining limb stability.

  • Sesamoid injuries are prevalent among racehorses, particularly Thoroughbreds and Standardbreds
  • These injuries can range from inflammation to severe fractures, often resulting in pain, swelling, and considerable lameness
  • Diagnosis involves a comprehensive approach including client history review, physical examinations, gait assessment, diagnostic imaging (radiography, ultrasound), and nerve blocks
  • Treatment options range from conservative measures to surgical intervention, depending on the injury type and severity

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References

  1. Adams, S. et al., Proximal sesamoid: fracture in Horses (Equis).Vetlexicon.
  2. Brokken, M.T., Disorders of the Fetlock and Pastern in Horses.Merck. 2019.
  3. Pearce, D.J. et al., Biomechanical and Microstructural Properties of Subchondral Bone From Three Metacarpophalangeal Joint Sites in Thoroughbred Racehorses. Front Vet Sci. 2022. View Summary
  4. Yeung, A.Y. et al., Anatomy, Sesamoid Bones. In: StatPearls. StatPearls Publishing. 2024.
  5. Brokken, M.T., Fractures of the Proximal Sesamoid Bones in Horses. Merck. 2015.
  6. Brokken, M.T., Sesamoiditis in Horses. Merck. 2015.
  7. Rivero, J.-L.L., A Scientific Background for Skeletal Muscle Conditioning in Equine Practice. J Vet Med A Physiol Pathol Clin Med. 2007. View Summary
  8. Anthenill, L.A. et al., Risk Factors for Proximal Sesamoid Bone Fractures Associated with Exercise History and Horseshoe Characteristics in Thoroughbred Racehorses. American Journal of Veterinary Research. 2007. View Summary
  9. Schnabel, L.V. and Redding, W.R., Diagnosis and Management of Proximal Sesamoid Bone Fractures in the Horse. Equine Veterinary Education. 2018.
  10. Beccati, F. et al., Morphologic Radiographic Study of the Proximal Sesamoid Bones of the Forelimb in Thoroughbred Racehorses in Training. Anat Histol Embryol. 2014.
  11. Spargo, K.E. et al., Catastrophic Musculoskeletal Injuries in Thoroughbred Racehorses on Racetracks in Gauteng, South Africa. J S Afr Vet Assoc. 2019. View Summary
  12. Woodie, J.B. et al., Apical Fracture of the Proximal Sesamoid Bone in Standardbred Horses: 43 Cases (1990-1996). J AVMA. 1999. View Summary
  13. Schramme, M.C.A. and Labens, R., Chapter 16 – Orthopaedics 2. Diseases of the foot and distal limbs. In: Equine Medicine, Surgery and Reproduction (Second Edition). W.B. Saunders. 2012.
  14. Munroe, G., Proximal sesamoid: sesamoiditis in Horses (Equis). Vetlexicon.
  15. Wisner, E.R. et al., Osteomyelitis of the Axial Border of the Proximal Sesamoid Bones in Seven Horses. Equine Veterinary Journal. 1991.
  16. le Roux, C. and Carstens, A., Axial Sesamoiditis in the Horse: A Review. J S Afr Vet Assoc. 2018. View Summary
  17. Baxter, G.M., Adams and Stashak’s Lameness in Horses, 7th Edition. Wiley. 2020.
  18. Schumacher, J. and Schumacher, J., Regional Anesthesia in Equine Lameness. Merck. 2015.