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1st May 2012

Swelling Around the Hock –  By Dr Kevin Squire,  Garrards Horse & Hound Autumn 2012

In our opinion the hock is one of the most complex joints in a horse.

There are 10 and sometimes 11 bones involved in what we call the hock joint, and these are arranged in four layers of joints; this complex joint structure is held together with numerous ligaments.

There are large tendons that run over its surface to transfer the movement from the muscles to propel the hind leg. This complexity is further compounded by the massive forces that pass through the hind limbs of a 500kg animal at full gallop. Added even to these factors is the awesome biomechanical function of the hind limb of an animal at speed. The hind leg of a horse at speed acts as a shock absorber as the horse lands its huge weight on it. It also acts as a spring, as the large weight compresses the structures of the hind limb, while the hind leg also acts as a massive catapult which drives the horse forward as the huge muscles from the rump and the hind limb contract.

The range of motion of the hind limb in a galloping horse is much greater than the range of motion in the forelimb. In other words, the joints of the hind limb extend through a larger degree of angular movement than the joints of the forelimb. Putting all this together, the complexity of the hock joint, the large range of motion of the hock in a galloping horse and the massive weights and forces passing through and over the hock make it a joint to be respected!

When things go wrong in the vicinity of the joint, they can go wrong in a big way; equine veterinarians pay a lot of attention to swellings around the hock. The Garrards Gold Coast practice recently has three interesting cases of soft swellings in the hock area, which are noted, along with a fourth cause of soft swelling in the hock.

1. Bog Spavin

Bog spavin is a soft swelling of the large main joint of the hock.

The two joints are combined as one in a large joint, the tarso-crural joint, the big soft pouch in front of the hock. When this joint gets inflamed with arthritis it swells with joint fluid. Some cases of bog spavin are painful and cause lameness, while other cases are non- painful. The painful cases are nearly always active arthritis. The non-painful cases may be due to wrenching the joint and after a brief period of pain from torn ligaments the horse often is non- painful and not lame but has a large soft swelling of the tarso-crural joint, often for the rest of its life. We always recommend taking X-rays of the horses with bog spavin to find out if an active arthritis or OCD condition is the cause of the problem.

2. Capped Hock

A capped hock is a soft ball of fluid on the top of the point of the hock on the back of the leg.

There is a soft fluid filled pouch at this location to cushion the tendons running over the point of the hock. If this pouch suffers trauma it often swells with extra fluid; this is what we see as a capped hock. Once the initial cause of the trauma subsides, most uncomplicated capped hocks cause no pain or lameness. Garrard also recommends an X-ray to confirm that there has been no damage to the underlying bone, and a tendon scan to see if any damage to tendons has occurred. If there is damage to the major tendon running over the point of the hock then significant pain and lameness can occur. Such lameness is not due to the capped hock, but would be due to the tendon or bone damage if this has happened. We are often asked if we can drain capped hocks, but our advice is to leave them – uncomplicated capped hocks are rarely painful. Draining them can cause pain, can introduce infection and will not solve the problem as the fluid always refills the drained sack. Leave them alone unless there is a major lameness problem and intervention is necessary.

3. Thoroughpin

Thoroughpin is an uncommon soft swelling of the tendon sheath that surrounds the deep flexor tendon.

All tendons that run over joints are surrounded by an oil-filled sheath to facilitate their movement over the joint as it bends. The deep flexor tendon is at the back of the hock. If the tendon or the sheath is wrenched then extra oil will be produced which distends the tendon sheath. The soft swollen tendon sheath can then be seen on one or both sides (inside or outside) of the hock towards the back at the level of the point of the hock. The swellings can extend from above the point of the hock to below the level of the point of the hock. Thoroughpin is usually not painful, and does not cause lameness. The condition usually occurs in younger horses in training. Garrards vets are often asked to drain these, but our advice is to leave them alone. Draining them can cause pain, may introduce infection and the drained sheath will always swell back up with fluid. Why risk causing lameness for a futile attempt to remove the fluid?

4. Cellulitis

Cellulitis is an infection under the skin. Hocks can be a terrible location for cellulitis.

These infections usually have a rapid onset often within a few hours; the horse can be perfectly normal and thein in a short time can have a massively swollen hock with severe pain. The cause is sometimes from a small injury or laceration near the hock. But sometimes there can be no initiating injury at all and bacteria that are normally circulating in the animals body just localise at the hock. The hock swells rapidly and becomes very painful, usually so much so that the horse does not want to put any weight on the leg. The skin over the hock becomes distended and feels hot to touch. The cause of the cellulitis is bacterial infection under the skin. The treatment consists of large doses of antibiotics and anti-inflammatory agents. Garrards vets support the swollen and painful limb with a thick wrap and bandage. These infections are so severe that if they are not treated aggressively or if the bacteria are not susceptible to the first choice of antibiotics, then the infection can progress and kill the horse.

–          Dr Kevin Squire