Hardworking HOCKS

Wear and tear over time will affect all horses' hocks, leading to osteoarthritis, or bone spavin, although the extent varies in individuals.

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ONE SPAVIN is a term used to describe osteoarthritis (degenerative joint disease) of the hock, specifically affecting the lower hock joints.

The disease process involves joint inflammation and associated adaptive modelling (changes in contour and density) of the lower hock joints, leading to cartilage erosion, bony growths and eventual fusion (ankylosis) of the joints. This joint inflammation invariably leads to pain, the condition being a leading cause of hindlimb lameness in the horse.

The hock (tarsus) of the horse is made up of 10 separate bones. These bones contribute to make up four separate joints. The upper joint, known as the tarsocrural or tibiotarsal joint, is the largest joint compartment and a high-motion joint, allowing the horse to flex and extend the hock during movement.

The remaining three joints are low-motion joints with limited movement and, from top to bottom, are known as the talocalcaneal-centroquatral, centrodistal and tarsometatarsal joints.

This sounds complicated, but the joints are in fact named in sequence after the bones that articulate (connect) to make up each joint.

The four hock joints are not separate individual synovial structures. Rather, the upper two joints communicate directly and share joint fluid, but are completely separated from the two lower joints, which also communicate directly in most horses. Bone spavin usually refers to osteoarthritis of the two lower joints, but in some cases can also involve the talocalcaneal-centroquatral joint.

"Certain breeds seem to be predisposed to bone spavin'

The low-motion joints of the hock are lined by cartilage and contain a small volume of synovial fluid, which facilitates continual low-grade sliding and avoids friction between the bones. The bones themselves are held in place with strong ligamentous and fascial attachments.