Muscles, tendons and ligaments run along the long tibia and smaller fibula to the hock joint. The hock joint is another hinge like joint about half way down the horse's back leg. The small bone that forms the point of the hock is actually similar to the human heal bone. Six small bones make up this joint, and it is often the site of strain and wear.
There is no muscle below the hock joint which is why the canon bone and splint bone feel hard, with little cushioning. It's also why this area is susceptible to injury, with only ligaments running down to the pastern joint.
Stringhalt is the over flexing of one or both back legs. It looks as if the horse is taking very high steps with the back legs. The horse will snap the hoof upwards and then stomp down. The flexing can be more subtle however, appearing occasionally and may be more obvious when the horse is asked to step backward or turning sharply. There are different types of stringhalt. Australian stringhalt is thought to be caused by a weed. False stringhalt may be caused by injury in the lower leg. The cause of unilateral and bilateral stringhalt is not known. Treatment sometimes includes removing a small section of tendon. Sometimes stringhalt improves on its own, sometimes it will get worse, and some horses will improve and then relapse. Many horses improve after being worked lightly. Many will be sound enough to be ridden.
2. Bog Spavins
Bog Spavins are caused by wear on the hock joint or an impact to the joint like a kick. Conformation problems and mineral deficiencies may also cause bog spavins. Bog spavins feel like a spongy swelling around the hock. If they are caused by an impact injury, only one hock will be affected. If both are involved, the bog spavin is probably caused by a conformation fault that places strain on the hock joint. Bog spavins are unsightly, but many horses preform well despite them. If the strain causing the spavin causes undue wear on the joint, pain and lameness can occur. The excess fluid that causes the spavin can be drained, but may reoccur.
3. Bone Spavins
Bone Spavins are caused by joint strain, concussion on hard surfaces, confirmation problems, and mineral deficiencies. Horses like jumping and reiners are susceptible to bone spavins. The hock joint is made of several bones and one or more of these bones can become infected. Hock flexibility is decreased, the horse may drag its hind toes and the stride shortened. This causes the horse pain and can result in arthritis. Most treatments are of minimal effectiveness. Eventually the bones will fuse. Horses will be sound but corrective shoeing may be required.
4. Jack Spavins
Jack Spavins are bone spavins that appear on the inside of the hock joint. It appears as a bony growth. The growth pushes against a tendon, which causes pain and makes the horse step short, dragging the toe of its back hoof.
Thoroughpins are caused when the tendon in the upper rear hock area becomes inflamed. The swelling is visible as a bulge on either side of the top of the hock joint. (A bog spavin sits slightly lower.) Treatments are aimed at decreasing the swelling. Rest, cold hosing, topical sweats, draining and radiation are some of the treatments that may help.
6. Capped Hocks
Capped Hocks are large fluid filled swellings on the points of the hock. They can be caused by impacts and from lying on hard floor. Thicker bedding may help alleviate the problem. Similar swellings can occur on the horse's elbows. The horse is unlikely to be lame. The swelling can become quite pronounced. Treatments include cold hosing, steroids and drainage. Most I've seen resolve themselves to some extent without treatment.
Curbs are inflammations of the upper hind area of the rear cannon bone. They are caused by an impact, such as a kick, that strains the Plantar Tarsal Ligament. Slight lameness may accompany the swelling. Rest is the only treatment, and although some fluid may remain.