Osteoarthritis in horses: definition, causes and risk factors

Osteoarthritis is one of the most common reasons horses go lame — and one of the most misunderstood. It is not simply a disease of old age, and it does not always start with obvious pain. By the time most owners recognise the signs, the joint has usually been struggling for a while.

What is a joint, exactly?

A joint is the point where two bones meet. On the surface of each bone, a layer of cartilage acts as a shock absorber: smooth, slightly elastic, and designed to distribute load evenly. Surrounding the whole structure is the joint capsule, a fibrous sleeve lined with synovial membrane. That membrane produces synovial fluid — the joint's lubricant and nutrient supply, since cartilage has no blood vessels of its own.

Ligaments stabilise the joint from the outside, connecting bone to bone and keeping the movement controlled. Every structure depends on the others. When one part starts to fail, the rest compensates — and not always well.

Arthritis versus osteoarthritis — the distinction matters

People use these two terms interchangeably. They are not the same thing, and the difference matters more than most owners realise.

Arthritis is general inflammation of a joint. It can be temporary — a response to injury, infection, or irritation — and with the right treatment, the joint can recover completely.

Osteoarthritis (also called osteoarthrosis or degenerative joint disease) specifically means that the cartilage itself has been damaged or worn down. Once cartilage is gone, it does not grow back. That is what makes osteoarthritis a long-term management problem rather than something you simply treat and resolve.

The distinction shapes every decision you make about a horse's workload, medication, and supplementation.

The two routes to cartilage damage

In clinical practice, osteoarthritis develops through one of two mechanisms. Both end in damaged cartilage, but they start from very different places.

Abnormal pressure on normal cartilage

This is the more common route in sport horses. Healthy cartilage has limits. Push a joint beyond those limits often enough — through intensive training, repetitive strain, poor posture, or the forces that follow a fracture — and the cartilage starts to break down. The surface cracks, the matrix degrades, and inflammation follows. Over time, bone remodels in response to the altered load: spurs form, joint spaces narrow, and the horse loses range of motion.

Normal pressure on abnormal cartilage

The second route involves cartilage that was never quite right to begin with. In osteochondrosis, normal developmental processes go wrong during growth: cartilage that should mineralise into bone fails to do so, leaving areas of weak, unstable tissue. When this progresses to the point where fragments of cartilage or underlying bone become detached, it becomes osteochondrosis dissecans — OCD. Those loose fragments irritate the joint, cause inflammation, and set the stage for early-onset osteoarthritis even in young horses.

Which horses are at greatest risk?

Any horse can develop osteoarthritis, but in clinical practice certain groups present far more often.

Sport horses carry higher cumulative joint loads than leisure horses. Repetition is the issue as much as peak force — a horse that trains six days a week accumulates far more joint stress over a season than one working three.

Older horses see a gradual decline in cartilage quality as they age. The matrix becomes less resilient, synovial fluid thins slightly, and the joint's ability to absorb and distribute shock decreases.

Overweight horses place increased load on every joint at every step. This is a risk factor that owners can actually change — and one that is often underestimated.

Horses with prior joint surgery or a history of significant joint injury are at substantially higher risk of developing osteoarthritis in the affected joint, even after a full apparent recovery.

Recognising the signs

Osteoarthritis rarely announces itself cleanly. The early signs are easy to dismiss: a horse that takes a few extra minutes to loosen up, stiffness after a rest day, subtle resistance when asked to bend or flex. As the condition progresses, lameness becomes more consistent and easier to localise. The horse may refuse certain movements, show shortened stride, or start to compensate by loading other joints differently — which creates its own problems.

If you notice any of these signs, it is worth having the joint assessed by a vet. Identifying the affected joint and the degree of damage early gives you far more management options than waiting until the horse is clearly, persistently lame.