The stifle joint is a large complex joint that requires careful consideration when evaluating hind end lameness. Due to its action and anatomical structures there are a number of instances when it could impact a horseâ€™s soundness. The joint itself is most analogous to our knee as it is made up of the same bones, including a kneecap or patella, plus the same internal cruciate ligaments and similar external ligaments. We often call the horseâ€™s knee the joint in the front leg above the fetlock and below the elbow and although it moves like a knee, in reality it is most similar to our wrist joint.
Stifle lameness may begin as a primary problem or may be secondary to another joint or region. I often find that they are involved with hock lamenesses. Also, there are plenty of times when what we consider a typical hock problem turns out to involve the stifle instead. They can develop early in life and manifest themselves as an acute lameness with swelling in foals or yearlings or they can develop with work. Depending on actual joint or soft tissue damage, they may respond quickly to treatment or require more invisible treatments including surgery.
Anatomically the stifle is made up of the end of the femur bone and the beginning of the tibia, both surfaces are covered with cartilage. Riding on the anterior surface of the femur is the patella, commonly know as the kneecap. There are two internal ligaments called cruciate ligaments that stabilize the joint. Menisci or cartilage disks lie between the cartilage surfaces and help prevent concussion injuries to the joint surfaces. Outside of the joint are the external ligaments that are on the inside and outside of the leg called the collateral ligaments and the front of the stifle has three large patellar ligaments that extend from the quadriceps muscle just above the patella and insert on the upper tibia or shin region. The reason its worth knowing these structures is that any of them either individually or in combination can be injured and cause a similar lameness.
Starting with the earliest onset of problems, a mild to severe lameness can develop in foals to yearlings with osteochondral disease (OCD). OCD develops when there is damage or weakness at the bone to cartilage junction such that a flap develops in the normally smooth cartilage or a hole forms that is termed a cyst. The cyst is actually abnormal tissue in the bone. Typically when OCD is seen it has a fairly acute onset, there may be reluctance to trot and often joint swelling. Radiographs are used to find the lesion and surgery is usually elected especially in these young animals. When OCD is found early in life surgery may have the most favorable outcome, when OCD develops into a problem later, especially with cysts, then surgery may be chosen after more conservative treatment such as systemic or intra-joint injections. Unfortunately the prognosis with cysts is guarded. Some surgeons suggest a soundness rate of 50-65% post surgery and this seems consistent with results I have seen in this practice over the past 20 years. Fortunately the horses with the more simple cartilage flaps, erosions or chips respond better to surgery or in some instances may never require any treatment other than conservative management.
Another common stifle disorder that develops in young animals, generally between 1-4 years of age is termed upward fixation of the patella. This refers to the patella actually getting fixed or locked in place along the femur such that it prevents the stifle from flexing. It may happen completely or there may be intermittent catches that gives the joint a "popping" appearance and sound. Often it develops due to a straight legged conformation, weakness in the thigh muscles and relaxation of the patellar ligaments and/or actual stretching of these ligaments. We see it frequently in miniature horses probably due to conformation and therefore genetics can be a factor. Horses coming out of prolonged periods of rest may also develop this. The horse with the leg actually locked in extension is the most alarming to horse owners as the leg will not flex and the horse drags its toe. Sometimes backing the horse or pivoting on the leg may allow it to pop free or it may need to be physically replaced by a veterinarian. This is a simple manipulation we do in the stall. Following this a steady exercise regimen may be used to prevent the problem. Exercise is by far the most successful treatment. The principle behind this is that strengthening the thigh or quadriceps muscles will help to put more tension on these ligaments thus reducing the relaxed. Similarly when a horse develops this after a period of prolong rest exercise is used first. At times this condition may be less obvious then the visible catching of the ligament. I find there are gait abnormalities when the role of these ligaments should be considered. For instance, horses that do not maintain a good canter, that seems to collapse in the hind end for a stride and/or maybe do not trot well in tight corners may have this condition. History, palpation of the ligaments and observance of the horse at work helps to locate the problem. Treatment options are many. I usually start with a 4-6 week course of working the horse up hills and over trotting poles to strengthen the muscle. Usually we avoid working on the lunge in tight circles. If improvement is limited we may recommend or start with exercise, a course of treatment with an estrogen related drug that actually can be quite effective when given in the muscle for these mildly affected horses. Injecting of an irritant directly into the ligaments is used frequently as a way to "tighten the ligament" by causing a limited scarring reaction at the injection site. This procedure is often very successful and well tolerated by the horse. The medications, typically used are those with an oil plus 2% iodine. Exercise is still important to maintain the improvement and conditioning should never re overlooked. The final treatment that is rarely used is a surgery where one of the ligaments is actually cut. The removal of this ligament prevents the catching and does not compromise the stability of the joint. The breed that generally needs surgery the most are the minis.
Joint inflammation is another common reason for stifle lameness. This could involve the soft tissues of the joint â€“ joint capsule, internal ligaments and/or menisci or it could involve the actual cartilage. Diagnosis is frequently made by "blocking the joint", injecting local anesthetic into the joint space, and observing the horse after to see if lameness resolves. If the lameness improves radiographs are taken to evaluate for any signs of degeneration or arthritis. Unfortunately one cannot see actual soft tissue injuries but the attachments of these areas to the bone may yield some information. As some of you know, soft tissue injuries with people are usually found with a MRI, which is not currently not available in this part of the limb. Generally we start by trying to determine if these are radiographic signs of degenerative joint disease (DJD) or arthritis. If this is found then we may be looking at a chronic condition. The initial treatment may be the same but management, likelihood of recurrence and modification of work may be worth considering. When DJD is not apparent that soft tissue inflammation is suspected the problem with this is actually making this diagnosis. Again we are limited diagnostically by which structures we can fully visualize.
As the above images indicate ultrasound can be a very accurate diagnostic method to use along with radiography. There are times when the findings during arthroscopic surgery may be worse than is what is seen during an ultrasound exam, but the presence of soft tissue lesions should usually be considered potentially significant. Trauma to the internal ligaments and/or menisci may preclude a quick recovery while milder joint inflammation may be more manageable. Inflammation of the joint capsule may cause fluid accumulation in the joint and lameness, but responds better to therapy. Initially the treatment may involve intra-joint injections of steroids plus hyaluronic acid (HA). The steroids are the most potent anti-inflammatories and are quite effective when used in this manner. The HA helps to reduce inflammation, to lubricate the joint surfaces and to offset any negative effects of the steroids. Frequently we will utilize a period of rest, anti-inflammatories (i.e. bute), and systemic joint treatments. These systemic treatments may be the use of intramuscular Adequan, intravenous Legend or oral supplements (i.e. glucosamines, chondroitins) often these later treatments are included for prophylactic purposes.
If a problem has been localized to the stifle and the treatments have not been effective then referral to a surgical facility may be recommended. There may be times when they would be able to better visualize a problem with their ultrasound machine or by doing an arthroscopic procedure. The advantage of arthroscopy is to allow placement of a scope into the joint and to permit visualization of the joint surfaces and structures. If a problem is located then cleaning up and removal of damaged tissue can take place. We have had a few instances where this procedure ultimately allowed for a cure of cartilage lesions that were not seen on radiographs and were not responsive to treatment.
Obviously there are a multiple of reasons why a problem with the stifle may develop and success will be determined by the length of time before a diagnosis is made, the nature of the injury and the anatomical structures involved.