The Ins and Outs of (Popped) Splints in Horses

Updated December 15, 2023 | Reviewed by: Joan Maree Hinken, DVM, CVA, CVSMT

Anatomy of Splint Bones in Horses

The splint bones are the small bones on the inside (medial) and outside (lateral) aspect of the cannon bone.

Splint Bone Terminology

• Second metacarpal (MCII) – the medial splint bone in the front legs.
• Fourth metacarpal (MCIV) – the lateral splint bone in the front legs.
• Second metatarsal (MTII) – the medial splint bone in the hind legs.
• Fourth metatarsal (MTIV) – the lateral splint bone in the hind legs.

The cannon bone is the third metacarpal (MCIII) in the front legs and third metatarsal (MTIII) in the hind legs.

Location and Purpose of Splint Bones in Horses

The splint bones begin just below the carpus (knee joint) and gradually taper to a small button about 2/3 of the way down the cannon bone. The splint bones are believed to be remnants of second and fourth toes before horses evolved to have a single toe per leg (their hooves).

Splints serve a supportive function to the carpal and tarsal bones with the medial splint bones (MCII and MTII) bearing a greater load then the lateral splint bones (MCIV and MTIV). As more of the horse’s weight is carried on the front end of the horse, the forelimbs have more developed splint bones and more of a supportive role than those of the hindlimbs.

There is a small ligament between the splint bones and cannon bones called the interosseous ligament which helps keep the splint bones attached to the cannon bones.

What Does it Mean When a Horse “Pops a Splint?”

When a horse “pops” or has a splint, it means there is a localized swelling on the lateral or medial aspect of the metacarpals or metatarsals. Splints can range from a small, soft swelling to a firm, hard bump. Some are the size of a pea, while others can become very pronounced growths about the size of your thumb.

There can be a single lesion or multiple lesions affecting the same bone. Some splints can only be palpated when the limb is raised off the ground and fingers are run along the shaft of the splint bone. Splints occur most commonly on MCII but can occur on all splint bones.

Causes of Splints in Horses

One of the most common causes of splints is direct trauma to the area, such as a horse knocking his own legs together. This results in inflammation of the connective tissue surrounding the underlying bone (periostitis) causing swelling and localized internal bleeding in the traumatized area.

Poor conformation can predispose a horse to splints. Offset knees or toeing in or out will put strain on the interosseous ligament and splint bones. Repetitive circular exercises can also cause strain of the interosseous ligament, resulting in its tearing and an associated periostitis of the underlying bones.

Young horse’s splint bones are more mobile than the cannon bone. It’s more common for splints to be found on the inside of the front legs because the metacarpals have more articulating, or touching, surfaces with the carpal bones than the metatarsals have with the tarsal bones (hocks). A horse with a severely inflamed suspensory ligament is more at risk of developing a splint or potentially fracturing the splint bone.

Diagnosing Splint Injuries

Veterinarians diagnose splints by the presence of swelling on the medial or lateral aspect of the metacarpal or metatarsal bones. When initially forming, there is often pain on palpation, along with heat in the area.

Splints and Lameness

In mild splint cases, horses are often not lame. However, the more tearing of the interosseous ligament and periostitis of the surrounding area that occurs, there will be more significant lameness. This may range from the horse being sound at the walk with slight lameness at the trot, to lameness when walking and trotting.

Lameness will be exacerbated when traveling on a circle with the affected area being to the inside of the circle. If a horse has developed splints on both front and hind legs there may not be an appreciable nod or hip hike when moving, but the horse will move in a stilted, choppy type gait.

A splint that causes significant lameness, is very large, or has an abnormal amount of swelling associated with it may indicate a more serious problem such as a fractured splint bone or involvement of the suspensory ligament. In these cases, further diagnostics like radiographs to assess the splint bone(s) and ultrasonography to assess the suspensory ligament are recommended.

Can you ride or exercise a horse with splints if they are not lame?

Because every situation is different, the veterinarian that is working with you to diagnose and treat the condition is the best person to answer this question about continuing to work your horse through a splint. Sometimes the answer is yes, sometimes the answer is no, and your veterinarian can provide the best advice for your individual horse.

Treatment for Splints in Horses

The goal in treating splints is to reduce or eliminate the cause of the splint and reduce the inflammation to minimize the amount of boney growth.

• Exercise should be limited in the early stages. In the majority of cases, the horse is restricted to stall rest with hand walking until their lameness and sensitivity to palpation has subsided.
• Depending on the case, anti-inflammatories such as phenylbutazone, flunixin meglumine, or firocoxib may be prescribed to be given for a few days or up to a few weeks.
• Cold therapy, such as cold hosing or icing, can be beneficial in the early stages.
• Laser therapy, therapeutic ultrasound, magnetic boots and PEMF (Pulse Electromagnetic Pulse Therapy) have been used successfully.
• DMSO applied topically may have some benefits.
• Supplements that support normal healing can help manage discomfort and promote joint and tissue health.

Healing Splints

When splints first occur, there is associated swelling and heat in the affected area. As the horse’s body attempts to heal a splint, it will lay down bone to stabilize the area resulting in a firm, hard, or “set” knot which usually will not cause further problems or pain to the horse. At this stage, a popped splint is considered a blemish.

How long does it take for a horse splint to heal?

With the appropriate diagnosis and no complications, horses typically recover in 2-6 weeks with proper therapies, rest, and preventative measures taken.

Can you get rid of an old splint?

If the old splint is simply a blemish, it’s best to just ignore it and leave it alone. Surgery to remove it may result in more irritation and create a more unsightly blemish. Over time, the splint may reduce in size as the body resorbs the unnecessary boney proliferation.

Surgical Treatment for Splints

In cases where the splint is not responding to these management practices, surgery may be an option. This usually occurs when there is so much bone growth that the suspensory ligament is being traumatized, there is an open wound, or there is a non-healing fracture of the splint bone.

Equine surgeons have found that if the splint “pops” in the bottom two-thirds of the bone, there may be too much movement for the splint to heal normally, that is, by laying new bone between it and the cannon bone. Instead, in some situations, the best results may be obtained by surgically removing the bottom of the splint bone (when a splint bone fracture occurs in the bottom two-thirds of the bone, it is also surgically removed).

Recovery and Preventing Splints

The prognosis for most horses that develop splints is generally good, as long as the appropriate treatments are prescribed and adhered to. Most horses will recover from a splint with no effects on their athletic performance or career. Splints will eventually become set, often as a small boney swelling in the affected area that is just a blemish.

Recurrent or chronic splints occur when the splint goes undiagnosed or not allowed enough time to heal. Splints that occur in older horses take longer to heal and are more likely to become chronic or recurrent. Follow your veterinarian’s advice for the splint your horse may have and take as much time as your horse needs for it to heal completely before going back to any work.

Horses whose legs interfere - meaning they seem to always be bumping their legs against each other - can be protected by putting on splint boots or leg wraps when riding and in turnout. Some horses may require corrective shoeing or trimming to prevent further trauma.

It’s impossible to design an exercise program that would be appropriate for each breed and discipline to prevent horses from developing splints. The best proactive measure owners can take is to monitor their horse for early signs of swelling and take appropriate action if symptoms occur.