Sidebone in horses is a common condition that affects the collateral cartilages of the hoof. It occurs when the normally flexible cartilage plates in the horse’s hoof gradually become hardened and calcified into bone.

According to research, an estimated 80% of all horses have signs of sidebone. Most horses experience no symptoms due to sidebone, and many have successful performance careers.

However, sidebone may predispose horses to developing injuries or changes to ligaments and bones elsewhere in the limb. Debate is ongoing among veterinarian as to whether sidebone can contribute to pain and lameness in affected horses.

The exact cause of sidebone is unknown, but it may be related to traumatic injury or genetic factors. Some breed organizations prevent stallions with sidebone from breeding to reduce the risk of producing affected offspring.

Sidebone in Horses

Sidebone refers to ossification (development of bone) in the collateral cartilages of the hoof. The collateral cartilages are C- or L-shaped cartilage plates found on either side of the hoof, helping support the hoof wall and cushioning the heel. [1]

In some horses, these bony formations protrude visibly from the hoof while in others are hardly noticeable. [19] The condition is most frequently observed in the front feet, and in older horses. [19]

Sidebone is typically diagnosed as an incidental finding during a thorough physical examination of the horse involving radiography (x-ray) of the feet.

Collateral Cartilage Anatomy

The collateral cartilages are plates of cartilage found primarily within the hoof capsule. [2] They originate from the coffin bone and extend upwards beyond the coronary band, where they can be felt under the skin. [2]

There is one cartilage plate for each side of the hoof. [2] There are also five ligaments stabilizing each collateral cartilage plate.

The main structures that the cartilage plates connect to include: [2]

  • Long pastern bone
  • Short pastern bone
  • Coffin bone
  • Navicular bone
  • Digital cushion
  • Insertion point of the common digital extensor tendon (main tendon on the front of the limb)

As the horse moves, the flexible collateral cartilages act as shock absorbers to reduce strain on other components of the hoof. [2] Ossification may impair this shock absorption function by reducing the flexibility of the cartilage.

The cartilages also contain an extensive network of small blood vessels. [1] As the hoof contacts the ground, increased pressure within the hoof capsule forces blood to move through the small vessels, which helps further dissipate mechanical energy. [1]

Causes

The exact cause of sidebone in horses is unknown. [2] Suggested causes and risk factors include: [2][3][4][5]

  • Traumatic injuries to the cartilage
  • Large breed horses such as Warmbloods, draft horses, and Finnhorses carrying additional body weight that causes strain on the tissues
  • Genetic predisposition in Finnhorses and Norwegian coldblooded horses
  • Female horses in genetically predisposed breeds

Conformational Faults

Some literature indicates that conformationally base narrow or base wide horses may be predisposed to developing sidebone. [2] However, a recent study suggested that poor conformation is unlikely to influence the development of sidebone, in contrast with previous thinking. [2]

Similarly, research literature frequently suggests that poor shoeing or trimming may contribute to the development of sidebone. [2] However, a study in Finnhorses showed that poor hoof balance and uneven weightbearing on the hoof was not directly correlated with sidebone. [6] The effect of sidebone also did not appear to cause changes in the conformation of the hoof. [6]

Development

Sidebone most commonly occurs in the front feet of horses, and it usually affects the lateral cartilage, which is located on the outside of the hoof. [2]

Ossification of the cartilage either begins at the base of the cartilage, next to the coffin bone, or centrally within the cartilage. [2] From there, the ossification spreads outwards to affect the majority of the cartilage plate. Usually, the area of cartilage closest to the back of the leg is unaffected. [2]

Ossification appears to begin early in the horse’s life, usually before two years of age. [3][7] Generally, the progression of ossification is quicker in young horses, and slows in adults. [3][7] The rapid period of ossification may be associated with sexual maturity. [3]

Symptoms

Most cases of sidebone show no symptoms, and horses typically do not develop lameness. [2] Many sound horses show evidence of sidebone on X-ray. [2]

In horses with large sidebones, the sidebone may be visible around the horse’s pastern, appearing as a firm enlargement of the pastern. [8] Some horses may experience pain when the sidebone is palpated. [8]

Clinical Significance

As mentioned, sidebone rarely causes lameness, even in performance horses. [2] Finding evidence of sidebone in a lameness work-up does not indicate the source of lameness. [2] Studies show that around 80% of horses in general populations have evidence of