With the news that Kentucky Derby hopeful Algorithms is off the trail due to a fractured splint bone, let's look at what it really means to "pop a splint".
The horse's cannon bone is accompanied by a thin sliver of bone on each side that begins at the knee and glides to a point a few inches above the pastern. These bones are not weight-bearing, nor are they part of a joint. They are vestiges of the toes of the horse's prehistoric ancestors.
When a horse is young, the splint bones are attached to the cannon bone by a tissue called the interosseous ligament. As the horse matures, the ligament ossifies, or becomes solid, and the splints are fused to the cannon. The horse's bones are covered by a membrane called the periosteum, which provides bloodflow to the bone and contributes to the healing process when bones are injured.
When the periosteum is irritated, or the interosseous ligament is injured, it becomes inflamed. This inflammation is what is referred to as a "popped splint." The condition is most often caused by concussion, usually from training. However, a horse can pop a splint by kicking a stall wall or a kick from another horse.
So you see, while this type of injury is an ouchie for the horse and a headache for the horseman, it is a relatively minor one and is best treated by time off.
Algorithms reportedly suffered a splint fracture. This injury is slightly more serious, but still not career- or life-threatening. It often requires surgery to remove the bone fragments. Fractured splints may be set in a cast to prevent formation of callouses.
As horses mature and the interosseus ligament calcifies, they become less likely to develop splints.
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-- by Candice Curtis