The equine hock is made up of four joints. The top joint accounts for 90% of the range of motion. The three lower joints are like stacked bricks with only a small total range of motion. The 2 lowest joints are the ones that are typically affected by arthritis, and are the site of the pain and lameness.

With repeated use and stress the lower hock joints become sore and inflamed and the joint. Cartilage is damaged. New none is formed around the edges of the joint, and sometimes even gradually forms a bridge of bone across the joint (joint fusion). The fusion and loss of these joints is actually desirable in the progression of this condition. Because most of the motion in the hock comes from the upper joint, the limb functions well without these joints. Once the joints are fused, there is no longer pain.

Hock pain is common in horses that require heavy loading of the hindquarters, such as dressage horses, jumpers, and cutting and reining horses. Conformation plays an important predisposing role. Horses with sickle hocks, very straight hind limb conformation and angular limbs at the hock, all are predisposed to developing this syndrome.

This condition is not always noticed as an obvious lameness. More commonly, the complaint is a reduction in performance. Affected horses often cross canter, miss lead changes, poorly engage the hindquarters. Western horses tend not to stop or roll back properly.

In racehorses, complaints may include excessive lead changes, not finishing well in a race, trying to get out on the turns or bracing on one side of the bit.

Horses that have sore hocks are often back sore as well.

The diagnosis is usually suspected at the lameness exam. Horses with the condition often "flex very positive" . ie they are lame following hock/stifle flexion. This is why that flexion is often known as the "spavin test". Joint blocking and x-ray confirm the diagnosis. But some horses with no x-ray changes still experience pain here.

The good news is that there are a variety of treatment options and horses can usually go on performing while this slow burning process continues.

"Jack Spavin" refers to a common finding in hock arthritis. It refers to a large hard swelling on the inner (medial) surface where the hock joint meets the cannon and splint bones of the lower limb.