Sidebone in Horses: Causes, Symptoms and Treatment

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 sidebone. ^[1]

One study comparing lameness scores between horses with and without sidebone found no association between lameness and the severity of sidebone, suggesting that sidebone itself does not cause lameness. ^[9]

Similarly, studies in Finnhorses with large sidebones showed that they continue to perform without lameness, despite significant changes on X-ray. ^[3] Another study in Swedish cold-blooded horses showed no differences in the number of starts or career earnings between horses with and without sidebone. ^[10]

Lameness from Sidebone

Most veterinarians believe that ossification of the collateral cartilages is itself not painful, unless other abnormalities are present. ^[9] Abnormalities that may contribute to pain during ossification include: ^[9]

• Toed-in conformation
• Heavy body weight
• Exercise on hard ground, such as stone

In these cases, lameness usually subsides when ossification stops. ^[9]

However, some researchers believe that active ossification may be painful by irritating the extensive sensory nerve network within the collateral cartilages. ^[11] The role of active ossification in causing lameness requires further investigation.

Complications

Although sidebone itself is unlikely to cause lameness, it can contribute to other painful conditions in the foot that do cause lameness. ^[2]

Coffin Bone Fractures

Some researchers suggest that sidebone may be associated with fractures of the coffin bone. ^[2] Coffin bone fractures can cause sudden development of moderate to severe lameness. ^[8]

In most cases, coffin bone fractures with associated sidebone occur at the base of the collateral cartilage. ^[2] This suggests that the ossification of the cartilage may produce a focal stress point while weight bearing. ^[12]

Horses that have asymmetrical ossification of the cartilages may have a higher risk of coffin bone fracture. ^[1] The asymmetry may divert the forces associated with weight bearing differently between the two sides of the hoof, resulting in increased stress. ^[1]

Additionally, affected horses usually show changes in the ligaments supporting the collateral cartilages, which may further contribute to lameness. ^[12]

Many horses with coffin bone fractures recover completely with appropriate treatment. ^[8] Usually, horses require a long period of stall rest and specialized shoeing to reduce movement of the coffin bone to ensure proper healing. ^[8]

Sidebone Fractures

Since the collateral cartilage becomes mineralized in sidebone, it can fracture similar to other bones in the body. ^[2] There are reports of sidebone fracture causing lameness. ^[13]

Studies show that these fractures are rare, occurring in only about 5% of sidebone cases. ^[14] Overall, only 0.53% of horses undergoing a lameness work-up have a fractured sidebone. ^[1]

Sidebone fractures most commonly occur at the base of the collateral cartilage, and usually affect the cartilage plate on the inside of the foot. ^[13]

Most horses with sidebone fractures recover well with conservative management, such as stall rest. ^[13]

Ligament and Bone Changes

There are reports of sidebone associated with changes to ligaments elsewhere on the limb, suggesting that sidebone may cause abnormal stress during movement. ^[15] Affected areas include: ^[15][16][17]

• Ligaments supporting the proximal interphalangeal joint (between the pastern bones)
• Ligaments supporting the distal interphalangeal joint (between the short pastern and coffin bone)
• Oblique sesamoidean ligaments (ligaments supporting the sesamoid bones in the fetlock)
• Digital annual ligaments (the broad, sheet-like ligaments protecting the back of the pastern and fetlock joint)

There are also reports of additional bone formation on the coffin bone, likely a reparative response to abnormal stress placed on the coffin bone due to sidebone. ^[15]

Ligament and bone changes were more common in horses with severe sidebone than mild sidebone in one study. ^[15] From this finding, the researchers suggested that horses with severe sidebone may be more likely to have pain and lameness than mildly affected horses, due to the secondary effects on other tissues. ^[15]

Another study evaluating horses with severe sidebone showed that all of the horses examined had damage to a least one other structure in the lower limb. ^[16] Similar to the previous study, they concluded that sidebone may contribute to a vicious cycle of lameness by overloading other structures of the limb, although sidebone itself does not appear to cause lameness. ^[16]

Diagnosis

A veterinarian can easily diagnose sidebone through taking radiographs (X-rays) of the horse’s foot, which show mineralization of the collateral cartilages. ^[8]

There are several methods for grading sidebone lesions, however most literature uses the grading system proposed by Ruohoniemi et al. in 1993. The grades are as follows: ^[18]

• Grade 0: No ossification
• Grade 1: Minimal ossification at the base of the collateral cartilage
• Grade 2: Mild ossification at the base of the cartilage that may extend upwards
• Grade 3: Moderate ossification extending upwards to the level of the navicular bone
• Grade 4: Advanced ossification extending above the navicular bone but not beyond halfway up the short pastern
• Grade 5: Extensive ossification extending beyond the halfway point of the short pastern

Around 70% of horses have grade 0-2 sidebone on X-rays. ^[18] The grade is not correlated with the degree of lameness, unless other changes in the limb are present.

Confirming Lameness

In cases where the horse is lame, and has sidebone, confirming that the sidebone is the cause of lameness is difficult. ^[8] Signs that may suggest sidebone as the cause of lameness include: ^[1][8]

• Asymmetry of the pastern swelling
• Pain when palpating the collateral cartilage
• Lameness that worsens when turning or on a circle
• Marked improvement in lameness with a palmar/plantar digital nerve block on the more affected side of the limb

Most horses have only mild to moderate lameness from sidebone-related injuries. ^[1] However, the lameness may be more severe at the time of injury. ^[1]

Using specialized diagnostic modalities such as nuclear scintigraphy or MRI can help determine the contribution of sidebone to lameness. ^[2][13]

These modalities can distinguish between active and static bone lesions, which can allow the veterinarian to determine if ossification is currently happening. ^[11] Since there is some speculation that active ossification may be painful, identifying an active lesion may indicate a cause of lameness if no other changes are identified.

Treatment

Cases of lameness related to sidebone usually respond well to conservative management. Treatments may include: ^[2][8]

• Stall rest
• Topical non-steroidal anti-inflammatory (NSAID) creams, such as diclofenac
• Oral NSAIDs, such as phenylbutazone
• Cold water therapy or icing the area

In general, horses require at least three months of stall rest when treating sidebone fractures. ^[1]

In rare cases, horses may require a neurectomy (surgical cutting of a nerve) of the palmar/plantar digital nerve in order to be comfortable. ^[2]

Prognosis

Since most cases of sidebone do not cause lameness, the prognosis for sidebone is excellent. ^[2] Despite this prognosis, lameness experts recommend that any evidence of sidebone identified on a pre-purchase examination should be documented and discussed with the purchaser. ^[1]

This is particularly significant for sport horse breeds, which generally have a lower prevalence of sidebone. The presence of sidebone in these breeds may be a risk factor for future lameness. ^[1]

Horses that develop symptoms of lameness associated with sidebone also have a good prognosis with appropriate treatment, even in the case of sidebone or coffin bone fracture. ^[2] One study on sidebone fractures showed 71% of horses returned to full performance. ^[1]

Prevention

Since the precise cause of sidebone is unknown, there are no prevention measures currently available.

Some breed organizations require X-ray examination of breeding stallions for sidebone, and do not allow affected stallions to breed due to suspicious of genetic inheritance. ^[5] However, eliminating affected stallions from the breeding population has not proven to be effective in reducing the incidence of sidebone. ^[5]

Since sidebone appears unlikely to cause lameness in most cases, researchers currently do not recommend removing affected animals from the breeding population. ^[5]

Summary

• Sidebone in horses is the ossification of the collateral cartilages on either side of the hoof
• The exact cause of sidebone is unknown, but may be related to injuries or genetic factors
• Most horses with sidebone show no lameness and continue to have successful performance careers
• Sidebone may predispose horses to injuries in other locations of the limb, such as coffin bone fractures