What some people fail to realize is the joint that we call the stifle in four-legged animals is anatomically the same as the human knee. (it doesn't help that equestrians call a joint in the front leg the "knee"-- in reality this joint is the anatomic equivalent of the human wrist). Flexion and extension of this joint is one of the main sources of power for locomotion. Pain, or restriction in motion here can not only cause lameness, but reluctance to jump, turn, go up stairs or steep hills, or even transition from one gait to another.
The Human Knee
Here's a drawing of the right knee of a human, as seen from the front. Notice that the quadriceps (your "quads") blend into the patella (kneecap) and continue as the patellar tendon. This is the tendon a doctor thumps with a rubber hammer to check your reflexes. When the quadraceps contract, the knee extends. On the back side of the femur are what we call the biceps femoris (hamstrings), which flex the knee when contracted. The knee joint is the space between your femur (thigh bone) and tibia (shin bone) and between the patella an femur. Between the femur and tibia are cresent-shaped pads of cartilage called meniscii as well as a couple of ligaments that stabilize the joint (the cruciate ligaments--more on them in a bit). Connecting the bones on the inside of the thigh is the medial collateral ligament, on the outside the lateral collateral ligament. These are the most important structures to think about as we go forward. Find them on yourselves. Get on all fours (hands and feet, not hands and knees) and see how this joint aligns in a dog or horse. Same joint, slightly different lengths of bone and alignment for moving on all fours rather than upright.
The Canine StifleLook familiar? The same structures are all there, proportionally a bit different, but very recognizable, and the muscle and joint work together in the same fashion.
The Equine Stifle
Before I get started, this is a left stifle so things are a mirror-image. See that little bone on the right? That is the fibula--it's in the dog and human image as well--and is on the outside of the leg. This was the best image I found, so hopefully not too confusing. Anyway, again, this should look familiar, but right away there are obvious differences. The end of the femur (colored blue here) is much larger on the inside and there are three patellar ligaments rather than one. This becomes very important in one of the problems we see in the horse: locking stifles. But don't let this confuse you; all those important parts are there: the quadriceps blends into the patella and continues as the patellar ligament(s); femur on top, tibia on bottom; shock absorbing meniscii (in yellow) between. This images shows those collateral ligaments very prominently, bridging the joint on either side.
Common Problems: Canine
Here's another image of the dog stifle with the patellar ligament removed, which makes it much easier to see the cruciate ligaments. The one labeled "cranial cruciate ligament" is often abbreviated CCL. "Cranial" means towards the head, so it is the one in front. In humans we say "anterior" or anterior cruciate ligament (ACL), because it is "in front", but not towards the head since we are upright. (Don't you just love anatomists?) Actually, I tend to say ACL in dogs too, because so many of us have either had personal experience or know someone with an ACL tear it makes communicating the problem easier. Anyway, back on track. CCL tears are probably the most common stifle injury/problem in dogs and is often caused because the femur and tibia align too steeply, (a "straight-legged" conformation) which make hyperextension injury more likely. Once the ligament is torn, the stifle is unstable, and most dogs will either be chronically sore or be lame after exercise. The best solution to this problem is surgery, and the best surgery is one called the Tibial Plateau Leveling Osteotomy, or TPLO. A TPLO is not lightly undertaken as it involved cutting the tibia and realigning it at a new angle to the femur. It is an expensive procedure, and recovery is long, but I have seen many dogs go back to full athletic activity afterwards.
If the CCL is only strained, or the problem is with a meniscus or collateral ligament, other therapies may be preferable to surgery. Stem cell injections, where cells are harvested from a dog's abdominal fat, processed, and reinjected into the joint, can be very helpful. Prolotherapy, cold laser, acupuncture, and hydrotherapy can all work to decrease inflammation and speed healing. For pain management NSAIDs like Rimadyl may also be appropriate, at least in the short term.
Degeneration of the joint, either from age or previous trauma, is also very common and can cause chronic pain, especially in older dogs. Many of the same therapies mentioned above are very helpful. In chronic pain a multi-modal approach generally works best; that is, using a combination of medication (like NSAIDs, opiates like Tramadol, and/or Adequan, which protects the cartilage) , acupuncture, injection therapy, and physical therapy is the best bet for a good qualtity of life.
This problem is one most commonly found in small or toy breeds. The joint itself is not anatomically normal, so the kneecap is able to pop in and out of the groove it normally slides in. This causes discomfort and eventually, arthritis. This problem can be corrected surgically, but in my experience only the most severe cases seem to do better than if they had no surgery at all. The most beneficial approach seems to be the same as for chronic pain: use multiple modalities to manage the condition.
Common Problems: Equine
Technically called Upward Fixation of the Patella (UFP), this usually occurs in young animals, especially those who have been very fit, but are then given time off for a while and lose muscle tone. Horses have a "stay apparatus" a mechanism of interacting ligaments that allows horses to sleep standing up, and the patellar ligament is part of this mechanism. Young horse, because they are growing, typically have looser ligaments than adult horses and the inside (medial) patellar ligament can sometimes get caught over the end of the femur. This same thing can occur when young, fit horses are let down and lose strength in the quads. Look at the illustration of the horse stifle above again and you can see that the inside of the femur has a much larger "roller" and that the ligament branches right above it. Usually, this issue can be corrected with exercise; that is condition the horse so that the quadriceps become stronger and tighten up the ligaments of the kneecap. Sometimes, the problem is severe enough and occurring frequently enough that a horse is not able to be conditioned appropriately. In these cases the medial patellar ligament is "blistered", injected with an irritant that causes inflammation, scarring and tightening of the ligament which will then allow conditioning to resume. Back in the old days, when I was in vet school, we used to grab for a scalpel first thing, and the most common treatment was to cut the medial patellar ligament completely. Of course, the law of unintended consequences reared it's head, and studies have shown that upwards of 20% of horses undergoing this procedure will fracture their patella at a later date. Understandably, this procedure is now reserved for only the most serious cases.
Osteochondritis Dissicans (OCD)
OCD is another very common disease of the equine stifle. This is a developmental disease, where the cartilage of the joint malforms in a growing animal, leading to pothole-like defects on the joint surface, bone chips, swelling and pain of the joint and eventual arthritis. The best treatment usually involves arthroscopic surgery to remove any chips and smooth out defects on the joint surface. The result is not a perfect joint, but most animals do very well after surgery.
Like the dog, trauma is another significant problem in the equine stifle. Strains of the collateral ligaments and tears in the meniscus are fairly common. Fortunately CCL tears are not; the size and mechainics involved in stabilizing the horse stifle mean that a horse with a full CCL tear will likely never be fully sound again. Treatment is very similar to dogs, with rest, anti-inflammatory, and physical therapy being the best bet for recovery.
Degenerative joint disease in the horse is also very common, and like the dog can result from normal wear and tear, as well as develop after an injury or as a result of OCD. Because of their size and relative ease of the procedure, direct injection into the joint is much more common in horses than dogs. Steroids, hyaluranic acid, stem cell, and platelet rich plasma (PRP) are all commonly done, and quite effective for arthritis. Like with dogs, a multi-modal approach is often best and combining injections with systemic meds like Legend, Adequan, NSAIDs, and acupuncture can be very valuable in keeping horses performing comfortably.
The stifle is a very important joint in both humans and animals. The ability to flex and extend the joint without pain or restriction is vital to speed, power and agility. This look at the function and treatment of the stifle is by no means comprehensive, but if you do have an animal with joint problems it will hopefully give you a nice place to start.