There are two different types of windgalls, which are a soft swelling located to the rear of and slightly above the fetlock joint on a horse’s leg. This swelling is either an enlargement of the fetlock joint capsule (an articular windgall) or more commonly a swelling of the protective digital flexor tendon sheath (a non-articular windgall).
Windgalls are often dismissed as an inevitable side effect of an active life. Many apparently normal horses in full work have slight windgalls due to digital flexor tendon sheath swelling, particularly in the hindlimbs. The amount of swelling may vary according to the environmental temperature, being less obvious in cold weather and larger in hot weather.
The swelling may also be influenced by exercise. Work often results in some reduction in size, whereas stable rest may result in accumulation of fluid and greater swelling. However, repeated work on hard ground can lead to an increase in the size of the windgall.
Usually these swellings are similar in size between pairs of limbs, and symmetrical swellings are normally no cause for concern, provided they are cool to touch and not painful, and most importantly readily compressible with the fluid being moveable between different outpouchings of the tendon sheath. For example, if the top part of the tendon sheath is compressed by finger pressure, then increased bulging will be seen on the back of the pastern.
It helps to understand the anatomy: The tendon sheath encloses the superficial and deep digital flexor tendons, together with their attachments to the sheath wall. The membranous synovial lining covers the internal surface of the sheath, producing synovial fluid that contributes to both the health and lubrication of the enclosed structures.
If the sheath is damaged, it becomes inflamed and, synovial fluid production increases. This accumulated fluid can protrude from the leg at points where the tendon sheath is not constrained by other structures, causing a pronounced bulge towards the back of the fetlock.