“Splints” in Horses: Metacarpal Exostosis & Interosseous Desmitis Risk Factors, Causes & Treatment

Splints are a common condition in horses characterized by bony growths or swellings along the splint bones, which run parallel to the cannon bone in the horse’s lower legs. “Splints” is a broad term used in common language to describe pain in the horse’s shins.

The root cause of splint pain may be related to metacarpal or metatarsal exostosis, new bone production around the splint bones, or interosseous desmitis, inflammation of the ligament between the splint bone and the cannon bone, or both. This condition often develops in young, active horses undergoing intense training.

While splints can cause pain, swelling, and lameness, they typically respond well to rest and conservative management. Understanding the causes, symptoms, and treatment options for splints is essential for maintaining your horse’s soundness and performance.

Read on to learn everything you need to know about identifying splints so you can manage the condition effectively and support your horse’s recovery.

Splints in Horses

Splints are a common condition in horses, marked by the development of bony growths (exostoses) along the splint bones, which run beside the central cannon bone in the lower leg. ^[1][2][3]

There are multiple terms used to describe this condition, including: ^[1]

• Metacarpal exostosis
• Metatarsal exostosis
• Popped splints

Often occurring in young horses, splints are typically caused by trauma, strain, or instability in the connections between the splint bones and the cannon bone. These factors can lead to inflammation or damage to the periosteum, the membrane covering the bone, triggering new bone growth and the formation of a visible bump. ^[1]

This condition is most often seen in young horses that are beginning rigorous training, but they can occur in horses of all ages. ^[1]

This type of splint is usually the result of trauma leading to bleeding and subsequent bone growth under the periosteum, which is referred to as subperiosteal hemorrhage. The damage to the bone stimulates new bone growth, resulting in a bony bump under the skin. ^[3]

Owners and caretakers also use the word “splint” in reference to interosseous desmitis, which presents similarly to exostotic lumps in the shin region. The interosseous ligament, which runs between the splint and cannon bones, hardens naturally as the horse ages. In some cases, inflammation of this ligament during the hardening process can cause a bony bump that appears the same as an exostotic lump under the skin in the area of the splint bone. ^[4]

Early diagnosis and rest improve the chances of a full recovery. If the condition becomes chronic, the risk of lameness increases. ^[2]

Causes

There are two splint bones in each leg of a horse, one on each side of the cannon bone (third metacarpal/metatarsal bone). Each splint bone runs parallel to the cannon bone and is attached by the interosseous ligament. The splint bones aid in stabilizing the knee in the front legs and the hock in the hind legs. ^[1]

“Splints” can arise from one or a combination of physiological changes in the shin known as:

• Interosseous Desmitis
• Metacarpal/Metatarsal Exostosis

Both types of splints present similarly in horses, with shin pain and a bony lump on the affected lower leg, and have similar risk factors and treatment options.

Interosseous Desmitis

As horses mature, typically by three to four years of age, the interosseous ligament undergoes ossification (i.e. hardening into bone), leading to partial or complete fusion of the splint bones to the cannon bone. ^[1]

This process is a normal aspect of equine development. ^[5] In cases of interosseous desmitis, fusion causes pain and may result in a bony outgrowth or splint.

Metacarpal & Metatarsal Exostosis

The splint bones, like all bones, are covered by a membrane called the periosteum that contains cells responsible for growing and repairing bone. When the periosteum is damaged, bone remodeling and ossification processes are triggered to repair the bone and make it stronger. ^[1]

While bone remodeling and ossification is a normal, healthy process, in cases where it is stimulated by excessive force or trauma, it can result in exostosis – development of a bony outgrowth – from the metacarpal or metatarsal bones. The size of the bump depends on the size and severity of damage in the affected area. The bump typically forms along the length of the splint bone and lies parallel to it. ^[1]

Damage to the periosteum can be caused by: ^[1][2]

• Tearing of the ligament that connects the splint bone to the cannon bone
• External trauma such as hitting the leg against something
• Fractures
• Interference such as the horse kicking its own legs in training

Splints can also be the result of conformation abnormalities. For example, if the carpal bones are misaligned, this can lead to uneven stress on the bones, which can cause splints. ^[7]

Additionally, imbalances in the foot can alter the forces placed on the metacarpal region which can have the same effect. ^[6][7] This underscores the importance of maintaining robust farriery care for growing horses.

Splints can also develop because of uneven or excessive loading. ^[6] This can occur when the horse’s body weight or the forces required for movement are distributed unevenly, or when the limbs are subjected to more stress than they can handle.

Risk factors for uneven or excessive loading include: ^[2]

• Conformation issues
• Improper hoof care
• Overexertion
• Poor/inappropriate footing
• Carrying excessive weight

Symptoms

The characteristic sign of splints is the formation of a bony growth or bump on the splint bone. ^[1][6] This swelling is typically firm and warm to the touch. ^[7] It usually appears about 3 in (7 cm) below the knee or hock where the splint bone connects to the cannon bone.

Splints are more common in the front limbs, but can occur in any leg. ^[1] The growths themselves are not usually painful but the horse may have sensitivity if the splint is palpated. ^[6][8]

Additionally, the horse may experience varying degrees of lameness, especially during work on hard ground. ^[9] In mild cases, lameness may only appear while trotting. ^[1]

Severity

There are several specific locations on the horse’s legs that are susceptible to splints: ^[8]

• Blind splints: Arise on the inner side of the splint bone and are less visually obvious compared to other locations. These splints can cause lameness if the new bone growth presses on the suspensory ligament.
• Knee splints: Also referred to as proximal exostoses. These can be painful and may increase the risk of inflammation and stiffness in the joints of the knee.
• True splints: Develop along the interosseous ligament and usually refer to interosseous desmitis rather than exostosis.

Additionally, the severity of splints depends on whether they lead to lameness in the affected horse. In mild cases, the bony growth is small and does not affect any of the surrounding structures. In these cases, the horse may not show any lameness, or may only have lameness at the trot. ^[1]

Larger growths, or growths that affect surrounding structures are considered more severe and may lead to significant lameness. ^[1] In some cases, scar-like tissue (adhesion) develops between the bony growth and the ligament. ^[2]

Risk Factors

Splints are a common condition in horses. ^[2] Several factors can increase a horse’s risk of developing splints.

They are most frequently seen in young horses, especially at the onset of training or during periods of high-intensity training. ^[1][6][8] Although less common in older horses, splints can still occur. ^[1]

Horses with poor nutritional status are more likely to develop splints. Mineral imbalances are also a risk factor for this condition. ^[1]

Horses that are overweight or who are exercised too much before their bones and ligaments fully mature are at higher risk. ^[1]

Horses with poor leg alignment such as offset knees or other conformation issues are more likely to develop splints. ^[1][4][8]

Other conformation faults that increase the risk include: ^[2][7][8]

• Narrow chest
• Base-narrow stance
• “Bench knees”
• Long toes
• Low heels

Several environmental and management factors may contribute to the development of splints in horses. These risk factors include: ^[1][6][8]

• Inadequate farriery
• Excessive stalling
• Footing too hard or too soft during training

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Diagnosis

The diagnosis of splints is usually based on the presence of the characteristic bony growth on the splint bone during a physical examination. ^{[1][2][6][7][8]} Additionally, a lameness evaluation may be performed. ^[8]

Other diagnostic tools may be used to confirm the diagnosis and distinguish normal bone growth from splints. These include: ^{[1][2][6][7][8]}

• Nerve blocks to localize pain
• Diagnostic imaging
• Specialized imaging, such as nuclear scintigraphy (“bone scans”)

Treatment

The main goal of treatment for splints is to reduce inflammation and bone irritation. ^[8] Normally, horses with splints can recover without intervention if given enough rest. ^{[1][2][3][7][9]}

“Rest, controlled hand-walking, and judicious use of NSAIDs are cornerstones of managing splints in horses. Adjunct therapies like shockwave can speed recovery. Collaborate with your veterinarian on a tailored rehab plan to safely return your horse to work and reduce the risk of recurrence.”

— Dr. Madison Ricard, DVM
Mad Barn Veterinary Educator

Stall rest for 30 to 45 days is recommended. Once the inflammation is showing signs of improvement, the treating veterinarian may clear the horse for hand-walking for 15 to 20 minutes twice a day. ^[1]

Depending on the specific case, veterinarians may use a combination of therapies while the horse is on stall rest, including: ^{[1][2][3][6][7]}

• Local anesthetic
• Anti-inflammatory medications
• Shockwave therapy
• Icing
• Topical anti-inflammatory creams
• Acupuncture
• Massage
• Pressure wraps
• Corticosteroids
• Hyaluronic acid injections
• Corrective shoeing
• Physical therapy

Horses with chronic splints or excessive bone growth may require surgery. ^[1][6] If the splint is caused by interference (i.e. hitting the opposite leg while moving), using splint boots or fixing incorrect trimming or shoeing can prevent further injury. ^[1]

Prognosis

The prognosis for horses with splints is good. Most horses make a full recovery if given enough time to recover. ^[1][9] Horses with splints have a very high rate of return to athletic performance once recovered. ^[7][8] However, insufficient rest and recovery may lead to chronic splints and lameness. ^[1]

The prognosis is slightly worse in cases where the exostosis is large and presses on the ligaments or joints, or if arthritis develops in the lower knee or hock joints. ^[1][7][8]

Horses that have surgery to remove the bony outgrowth tend to recover well although there is some chance of recurrence. ^[8] Surgery may also help speed up recovery so the horse can resume their athletic activities sooner. ^[1]

In cases where the growths are caused by problems with the horse’s conformation, the underlying issues may continue to put stress on the bones and surrounding tissues. This can mean further exostoses may develop, or scar tissue may form. This can lead to cosmetic deficiencies in the legs that may impact some types of showing. ^[7][8]

Prevention

Preventive measures for splints relate to the risk factors of developing the condition. Strategies include: ^[4]

• Maintaining robust farriery
• Training to support conformation
• Minimizing injuries
• Preventing over-exertion
• Providing sufficient rest and recovery between workouts
• Maintaining ideal body weight and condition
• Using appropriate footing for the type of training

In addition, ensuring that young horses in training receive a well-balanced diet that meets without exceeding their energy demands is critical for proper development. This approach also helps support bone growth and minimizes the risk of both chronic and acute injuries, such as splints.

If you are unsure whether your new equine athlete is getting all the nutrition they need, consider working with a qualified equine nutritionist to create a tailored plan that accounts for their age, breed and level of training.

Frequently Asked Questions

Here are some frequently asked questions about splints in horses:

Summary

"Splints" are common in young horses starting athletic training, and refer to two types of painful shin bumps: metatarsal or metacarpal exostosis and interosseous desmitis.

• Metacarpal or metatarsal exostosis refers to bony outgrowth of the shin bones, usually in response to strain or injury to the periosteum, the membrane that covers bone
• Interosseous desmitis refers to pain and irritation that sometimes develops into a hardened bump on the horse's shin related to the normal hardening process of the interosseous ligament
• Risk factors for splints include over-exertion, excessive or uneven loading, and inappropriate footing
• The main treatments are pain relief and stall rest followed by a gradual return to exercise
• Prognosis for a full return to performance is good as long as affected horses have time to make a full recovery and are reintroduced to activity gradually