The stifle is made up of the femorotibial and femoropatellar joints. It corresponds to the knee joint in humans. Disorders of the stifle include fractures, gonitis, dislocation, and bone cysts.

Fracture of the Stifle

Severe fractures of the stifle involving either the femur or the tibia are uncommon. Treatment is difficult or impossible, owing to the damage to the joint, ligaments, and cartilage, and the soft-tissue swelling that occurs. Fractures of the kneecap usually cause much less severe lameness and swelling. X-rays are necessary to confirm the diagnosis. The condition may respond to conservative treatment or, if a large bone fragment is involved, require surgical repair.

Gonitis

Gonitis is inflammation of the stifle. Because of the joint's complexity, there are many possible causes, including osteochondritis dissecans (see Bone, Joint, and Muscle Disorders in Horses: Osteochondritis Dissecans), “locking” of the kneecap in an extended position, injuries to the ligaments or cartilage, erosions of the joint cartilage, or infection, whether as a result of a puncture wound or a bloodborne bacteria (for example, pyosepticemia).

Signs may vary, depending on the condition's cause and how advanced it is. The joint membrane may swell just below the kneecap. Lameness may be visible in the swinging leg as a shortening of the forward phase. At rest, the horse will keep the fetlock bent with only the toe touching the ground. In moderately severe cases, lameness may be seen in both the supporting and the swinging legs. In severe cases, the horse may carry the leg in a bent position. If the cartilage or ligaments of the joint have ruptured, a grating or crackling sound may be audible. X-rays help confirm bone or cartilage involvement, whereas ultrasonography allows your veterinarian to view ligaments, cartilage, and soft tissue.

Treatment requires prolonged rest. Repeated injections of steroids or hyaluronic acid into the joint may be useful. Anti-inflammatory drugs and whole-body steroids may relieve the lameness in less severe cases. Cases of gonitis that are due to rupture of ligaments or damage to the joint cartilage rarely respond satisfactorily and rapidly progress to secondary arthritis. The outlook for recovery is poor if the condition is longterm or if severe injuries to the joint surface, ligaments, or the joint cartilage have occurred.

Dislocation of the Kneecap (Patella)

True dislocation of the kneecap is uncommon in horses. When it does occur, it is usually a serious injury and the dislocation is very obvious. Some breeds are prone to a congenital form of dislocation.

The most frequent problem involving the kneecap is upward fixation or locking of the patellar ligament. Some pony breeds inherit a tendency to develop this condition. Dislocation may also be seen in immature animals with poorly developed thigh muscles. It may occur in one or both hindlimbs. Signs include an intermittent locking of the limb in extension followed by a sudden jerk or overbending of the joint when the patellar ligament unlocks. These signs are more common after the horse has been standing still for any period (for example, overnight in the stable, or after traveling in a trailer). However, the signs are often much less dramatic, which makes diagnosis difficult. There may simply be a lack of hindlimb forward motion associated with a rather jerky patellar action.

In many cases, a general improvement in fitness and muscle tone of the hindquarters can eliminate the problem. In the more severe and persistent cases, surgery to realign the kneecap may be recommended. Following surgery, rest must be sufficient to permit complete healing (4 to 6 weeks) before resuming any training.

Bone Cyst beneath Cartilage

Large, cyst-like structures may occur in various sites in a horse's body, particularly in the stifle. Their cause and development are not completely understood, but they may arise after trauma to the joint cartilage or as a result of a bone or cartilage disorder (for example, osteochondritis dissecans).

In the stifle, cysts are most common in young Thoroughbreds (aged 1 to 2 years). Lameness is first noticed when training begins or while the horse is being broken in. The condition commonly produces swelling of the knee joint, although the cysts can cause severe lameness with no swelling or pain. X-rays can confirm the diagnosis. Some horses respond well to rest for 4 to 6 months and treatment with anti-inflammatory drugs. If this conservative treatment fails, particularly in more mature animals, surgery is recommended. A variety of surgical techniques are in use, and because of the favorable results surgery tends to produce, it may be considered before more conservative treatment.

Last full review/revision July 2011 by Russel R. Hanson, DVM, DACVS, DACVECC; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS; Andrew P. Bathe, MA, VetMB, DACVS, DEO, MRCVS; Leo B. Jeffcott, MA, BVM, PhD, FRCVS, DVSc, VD; Svend E. Kold, DMV, MRCVS, RCVS Specialist in Equine Surgery (Orthopedics); C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS; Dale A. Moore, MS, DVM, MPVM, PhD; Sheldon Padgett, DVM, MS, DACVS; Tracy A. Turner, DVM, MS, DACVS, DABT; Stephanie J. Valberg, DVM, PhD, DACVIM; John F. Van Vleet, DVM, PhD