The tarsus (hock) of a horse is the rear leg bone that connects the tibia to the metatarsal (cannon) bone. When viewed from the side, the point of the hock and cannon bone should be parallel to the point of the hip.
If a horse has a sickle hock, the hock point is parallel to the hip, but the cannon bone is slanted forward, pushing the hoof forward. In other words, the hind limb is slightly curled in the opposite direction from the front leg, similar to a sickle.
While real sickle hocks result from the hock bones being aligned at an acute angle, a long, sloped pastern or long toes on the rear hoof can generate a similar disease. Sickle hocks can also be caused by a forward-pointing hip bone. Diet may also play a role: a protein-mineral imbalance can result in bone abnormalities.
Excessive bending, whatever the source, puts a strain on the hock joint, tendons, and ligaments. The most common complication is curb, which is an inflammation of the tendons near the back of the hock. Bog spavin, a swelling right beneath the Achilles tendon, is another concern.
Inflammation of the hock joint can cause acute swelling and pain, as well as long-term bone abnormalities, or arthritis. Bone spavin is a type of arthritis that results in a bony mass on the inside of the leg.
The hock’s sharp angle can cause issues in the stifle (the joint between the femur and the tibia) higher up the leg, as well as the fetlock and hoof bones lower down. Dressage and reining horse riders occasionally prefer a somewhat sickle-hocked horse because the center of gravity is pushed slightly backwards, making it easier to collect into frame.
When training a horse, you can compensate for sickle hocks by not overworking it on hock-stressing maneuvers like reining, flying changes, and piaffe in dressage. There should be two stages to the training program. To begin, build strength and get the horse fit by doing trotting exercises (‘long trot’ in Western, ‘rising trot’ in dressage). As much as possible, eschew collections and turns.
Working on activities that aggressively engage the hocks for a short period of time is the second step. Exercises for flexing connect the two phases. It is critical to have a good farrier. The hind foot angles in a sickle-hocked horse should be kept high (45° or higher), and wedges between the hoof and shoe may be necessary.
To ease break-over and bring the hooves further backwards, the toes can be reduced by wearing a blunt-toed shoe with a rolled toe or a flat shoe with a rockered toe.
Neglect can cause a well-conformed horse to appear sickle-hocked and have the same long-term repercussions on the joints. Sickle hocks are heritable, so horses with this conformation should not be used for breeding. Training and farriery can help to keep the horse from becoming unsound.