A split pastern, where the first pastern bone cracks down the middle, is a common training and racing injury in thoroughbreds.
At the top of the first pastern bone is a central groove. A central ridge at the bottom of the cannon bone fits into this groove, inside the fetlock joint. The crack usually starts at the bottom of the groove and continues down the length of the pastern bone.
In severe cases, the whole bone can fall apart, requiring surgical repair with screws. Milder cases, in which the crack reaches part way before petering out, can heal with rest.
2 highly respected equine surgeons, writing in the Equine Veterinary Journal (EVJ), reported on 110 cases of split pastern. They speculated whether it might be possible to predict incipient fractures by measuring various features of the bone.
Most notably, they showed that the plate of bone beneath the joint cartilage seems to get thicker before it cracks. They compared the thickness of this bone plate with the total length of the bone, so that variations in the size of the horses were not confusing the measurements.
In pastern bones that had sustained fractures, the bone plate thickness was on average more than 4% of the total bone length. In the other non-fractured leg, it represented on average less than 4%.
The reason for this increased bone thickness before cracking is not certain. The most likely explanation is that, although the bone may be adapting to the stresses of training and racing, the bone plate does not develop sufficient strength in time to prevent a split occurring.