Obvious swelling of the carpus (erroneously called "knee") of the front limb is common. There are multiple structures in this area that can become inflamed, thickened or fluid-filled.

The carpus is a common site for direct trauma, most commonly a kick from another horse. When this area sustains direct trauma, it tends to swell severely. There are several tendon sheaths which are located just under the skin here and they tend to swell when injured.

A moderate to severe swelling above or over the carpus, with modest or no lameness but difficulty advancing the limb can indicate inflammation of the extensor tendon sheaths. It is important for a vet to differentiate this from joint swelling itself. Swelling of a joint or tendon sheath often means that there is irritation or inflammation present that could worsen without proper treatment.

Swelling along with a wound to the area could indicate that the wound has caused infection of one of these important structures. Wounds to this area can involve these sheaths or the joint itself, and can be life-threatening.

Swelling here is often seen in older horses with chronic arthritis of the carpus. These horses usually have limited range of motion, and some have difficulty and pain associated with flexing the affected limb for the farrier.

Racehorses injure the carpus frequently and become lame with swelling of this area. In this case, there is usually a bone chip or other fracture of the small bones of the carpus. Fractures must be identified and treated promptly. Even tiny fractures involving joints quickly cause arthritis within the joint, which damages the joint permanently and reduces the likelihood for a good outcome.