Other Names
- Ossification of the second and fourth metacarpal bones
- Splint bone disease
- Periostitis of the splint bones
What are Splints in Horses?
Splints in horses refer to bony growths that develop along the inner or outer aspect of the cannon bone, most commonly on the forelimbs. They arise from ossification of the interosseous ligament that connects the small splint bones — the second and fourth metacarpal bones — to the large cannon bone (third metacarpal). While the term sounds alarming, many horses develop these bony enlargements without long-term consequence. However, during the active phase of formation, splints can be acutely painful and cause significant lameness.
Splints are particularly common in young horses between two and five years of age, especially those in early training. Once fully calcified and hardened, the bony growth is typically permanent, though in many cases it becomes clinically silent — meaning the horse shows no ongoing discomfort. The location and size of the splint determines whether it continues to cause problems over the horse’s career.
Horses with poor conformation — particularly those with narrow, upright feet or toed-in posture — are at higher risk, as uneven loading places additional stress on the splint bones. The Thoroughbred is among the breeds most frequently affected owing to their fine-boned structure and high athletic workload.
Causes
The primary cause of splints is trauma or excessive stress to the periosteum of the splint bones, which triggers an inflammatory response that eventually leads to new bone formation. Common causes include:
- Repetitive concussion: Hard ground creates micro-trauma at the junction between the cannon and splint bones, especially in young horses whose interosseous ligament is not yet fully ossified.
- Poor conformation: Horses with offset knees, narrow chests, or base-narrow conformation load the medial splint bone abnormally.
- Dietary imbalances: Calcium-to-phosphorus imbalances, particularly high-phosphorus diets heavy in bran, may predispose young horses to splint formation.
- Direct trauma: A kick from another horse or striking the inside of the leg during exercise can trigger localised periostitis.
- Incorrect shoeing: Uneven trimming or shoeing that causes asymmetrical loading can contribute over time.
Symptoms and Signs
During the active (hot) phase of splint formation, a horse may show the following signs:
- Lameness, typically mild to moderate, that worsens on hard ground
- Heat and swelling along the inner aspect of the cannon bone, approximately one-third of the way down from the knee
- Pain on firm palpation of the affected area
- A visible or palpable bony lump
- Reluctance to work or resistance under saddle
Once the splint matures and hardens, lameness typically resolves. A splint that forms beneath the suspensory ligament or close to the knee joint can cause ongoing mechanical problems.
Diagnosis
Your vet will perform a thorough clinical examination, including:
- Palpation: Feeling for heat, swelling, and pain along the splint bone
- Trot-up and flexion tests: To localise lameness and assess severity
- Radiography (X-rays): To confirm location, size, and maturity of the bony growth and rule out fractures
- Scintigraphy (bone scan): In complex cases, detects active bone remodelling before it is visible on X-ray
- Nerve blocks: Diagnostic anaesthesia can confirm the splint as the source of pain when the picture is unclear
Treatment
Treatment depends on whether the splint is active (inflamed) or mature (cold). Options include:
- Rest: The cornerstone of treatment during the active phase. Typically four to eight weeks of reduced work is advised.
- Cold therapy: Icing or hosing the affected area reduces inflammation and pain in the acute phase.
- NSAIDs: Phenylbutazone (“bute”) or flunixin meglumine help manage pain and inflammation under veterinary guidance.
- Topical treatments: Cooling gels or DMSO-based preparations may assist superficial inflammation.
- Corrective shoeing: An experienced farrier can redistribute load away from the affected area.
- Surgical removal: In rare cases where a splint is mechanically impinging on a joint or the suspensory ligament, periosteal scraping or removal may be recommended by a specialist.
Prevention
- Avoid overworking young horses on hard ground before their musculoskeletal system has matured
- Ensure correct dietary balance, particularly appropriate calcium-to-phosphorus ratios in growing horses
- Use brushing boots during exercise to prevent direct trauma to the splint bones
- Address poor conformation with corrective trimming and shoeing at an early age where possible
- Monitor legs regularly post-exercise — detecting heat and swelling early allows for quicker intervention
Prognosis
The prognosis for horses with splints is generally good to excellent. The majority of horses with uncomplicated splints return to full work once the bony growth has matured and the inflammation has resolved. A cold, painless splint is simply a cosmetic blemish that need not affect performance. Exceptions include splints that form adjacent to the knee joint, press against the suspensory ligament, or involve a fracture of the splint bone — in these cases the outlook for a high-performance career is more guarded. BEVA guidance on equine lameness investigation recommends thorough imaging before returning an affected horse to competitive work.
When to Call the Vet
- Your horse is noticeably lame on a forelimb with localised heat and swelling below the knee
- The lameness does not improve with a few days of rest
- A bony lump appears suddenly following trauma
- The swelling appears close to the knee joint rather than mid-cannon
- The horse is in competition preparation — prompt treatment protects the career timeline
Frequently Asked Questions
Can a horse with splints still compete?
Yes, in most cases. Once the splint has fully calcified and is cold and pain-free, it does not interfere with performance. Many competition horses carry splints identified during early training. The key is that the splint is completely mature and not located near a joint or ligament. Related conditions to be aware of during rehabilitation include tying-up, which can occur when horses are brought back into work too quickly after a rest period.
How long does it take for a splint to mature?
The active, inflammatory phase typically lasts four to eight weeks. Full calcification usually takes several months. During this period, the horse requires reduced or altered work, particularly avoiding hard surfaces and high-impact exercise.
Are splints hereditary?
There is a conformational component to splint susceptibility and conformation does have a genetic basis. However, management factors — diet, footing, and training load — play a significant role and can mitigate risk even in predisposed horses.
Do splints need treatment if the horse is not lame?
If a splint is cold, well-calcified, and the horse is entirely sound, no treatment is needed. Regular monitoring is sensible — particularly noting if heat or pain returns — but a mature, asymptomatic splint requires no intervention.