Stifle injuries can be devastating for a performance horse. Learn why accurate diagnosis and proper treatment are challenging—and critical—for keeping your reining horse sound.
By Barb Crabbe, DVM
The gelding was almost 3-legged lame. When I picked up his right hind leg, he snatched it away and almost fell down—all of his joints flexed, hind end shaking, and a look of extreme discomfort on his face. At 12 years old, he was retired from a successful reining career and had spent the last few years teaching kids how to ride. He’d been one of those truly great horses. Worth his weight in gold.
“It’s probably his stifle,” his owner told me. “It’s been injected pretty regularly, and the last time they put the needle in joint, fluid shot out half way across the barn aisle.”
Radiographs of the stifle joint showed significant arthritis, and when I checked on his records, I learned the horse’s stifle had been injected at least five times in the past three years. Unfortunately, no other diagnostics had been done.
Stifle injuries are tricky. They can be complicated to diagnose and frustrating to treat. They can also cause a horse to become very lame. Here, we’ll take an inside look at the stifle joint’s anatomy and learn why making an accurate diagnosis is challenging, but so important.
It’s Complicated
Your horse’s stifle anatomy is complicated, and it corresponds to the human knee. It’s made up of three bones—the femur, tibia, and patella. The large bones of the hind leg (the femur and tibia) meet, with the “kneecap” or patella resting on the front side of the joint to help distribute forces from the muscles that bend and straighten the leg.
Although we think of it as a single joint, there are actually three joint spaces that make up the stifle: the medial (inside) and lateral (outside) femorotibial joint spaces that connect the femur to the tibia, and the femoropatellar joint space that encapsulates the end of the femur and the patella. In some (but not all) horses, the femoropatellar and medial femorotibial joint spaces communicate, which can complicate the process of making an accurate diagnosis when there’s an injury.
The stifle has a very large range of motion that requires the top of the tibia to glide a significant distance along the bottom of the femur as the joint opens and closes. Because of this, the ends of the bones are outfitted with crescent-shaped structures of cartilage called the menisci that help cushion and maintain the joint’s stability. These structures lie within the medial and lateral femorotibial joint spaces.
There are many small ligaments within and surrounding the stifle that also help provide stability. These include two cruciate ligaments that connect the femur to the tibia and lie between the medial and lateral femorotibial joint spaces. Two collateral ligaments connect the femur to the tibia on the inside and the outside of the joint and the small meniscotibial ligaments connect the menisci to the tibia and help hold them in place.
Finally, healthy function of the stifle joint depends on the patella gliding smoothly up and down along a large ridge at the end of the femur. The patella is encapsulated within the tendons at the end of the large quadriceps muscles, and its proper functioning allows these muscles to do their job. The patella is also important because it’s at the core of the horse’s ability to lock his hind leg into extension so he can “sleep on his feet,” which is essential for his survival in the wild.
Sound complicated? It is! And that’s just part of why making an accurate diagnosis of a stifle injury is difficult. There’s a lot that can go wrong in this area.
Potential Problems
Before we dive in to the diagnostic challenges of the stifle joint, let’s take a look at some of the most common stifle abnormalities that can lead to lameness.
Developmental abnormalities such as osteochondrosis can result in “mushy,” unhealthy cartilage on the surface of the joint, or even cartilage fragments that might break away. If these types of problems aren’t identified and treated when the horse is very young, they can lead to lameness down the road. The same is true for bone cysts that may form either during development or following a traumatic event. A cyst may not cause lameness initially, but will eventually lead to problems. Surgical intervention to clean up cartilage or remove fragments is often an important treatment step that will help minimize chronic inflammation and lessen long-term joint damage.
A torn meniscus is another common injury that can occur after a traumatic event. When a meniscus tears, the joint’s stability is compromised. Treatment recommendations for that kind of injury vary from joint injections to reduce inflammation to regenerative therapies that can help promote healing. In some cases, surgery can clean up the tear. In all cases where a torn meniscus is diagnosed, a period of rest is critical to allow healing. Some meniscal tears will heal well, and the horse can go back to work. Others can be career-ending.
Any of the stifle ligaments can also be torn or damaged, making the joint unstable. Like meniscal tears, ligament injuries may be treated with regenerative therapies to help stimulate healing and joint injections to reduce inflammation secondary to joint instability. Once again, an adequate period of rest will be important for a successful long-term outcome.
Diagnostic Dilemmas
A typical lameness exam starts with a clinical exam, which usually includes palpation, movement evaluation, and flexion tests. Diagnostic blocks to localize the problem and imaging such as radiographs, ultrasound, or MRI are all common next steps. In difficult cases, a bone scan (nuclear scintigraphy) might be recommended, too. When it comes to stifles, however, there are potential complicating factors at almost every step.
Clinical Exam and Flexion Tests
What’s involved: Your vet looks at your horse; palpates his tendons, ligaments, and joints; and watches him move to identify the lame leg. She performs flexion (holding the leg in a position designed to stress a specific structure) and watches your horse trot away to see if it makes the lameness worse.
Stifle signs: A horse with a stifle injury will often be quite lame and hyperflex his leg when you try to pick it up. Effusion, or fluid, in the joint may be easily palpable. In some cases, a flexion test of the upper portion of the leg can make the lameness worse.
Dilemmas: Some stifle injuries will have no palpable effusion or fluid accumulation. Although specific techniques can be used to try to isolate the stifle joint, the joints of the horse’s hind legs all flex and extend together, making it difficult (if not impossible) to completely separate the stifle from the hock, or even the fetlock joint, when performing flexion tests.
Diagnostic Blocks
What’s involved: Your vet injects a numbing substance into either the nerves that supply a specific area or directly into a joint. For a suspected stifle lameness, that means injecting all three joints of the stifle. Your vet might choose to block the stifle joints one at a time or all together.
Stifle signs: If your horse’s lameness improves dramatically or resolves within 30 minutes of the joint(s) being blocked, he’s likely suffering from a stifle injury.
Dilemmas: Some stifle injuries won’t respond to blocks, meaning the lameness won’t resolve even when the joint has been injected with a numbing medication. Recent studies have indicated that a stifle block can resolve a foot lameness after enough time has passed. Some horses simply won’t tolerate a needle in their joint (especially in the hind leg) without sedation, and sedation makes it difficult to accurately evaluate a change in soundness.
Radiographs
What’s involved: Your vet takes a picture of the skeletal structures that make up the stifle joint.
Stifle signs: Disruptions of the joint surface, bone chips, fractures, or osteophytes (bone spurs) on joint margins can all be easily identified on radiographs.
Dilemmas: Soft-tissue injuries of the stifle are common, but those won’t show up on radiographs. Radiographic changes that indicate arthritis often appear only years after a soft-tissue injury has caused chronic instability of the joint. By that time, it may be too late.
Ultrasound
What’s involved: Sound waves pass through tissues to create an image of the soft-tissue structures and bone surfaces within the stifle.
Stifle signs: Ultrasound is one of the most helpful diagnostic tools for identifying stifle injuries. Fluid and inflammatory debris within the joint spaces, ligament injuries, and torn menisci can all be accurately identified. Even cartilage disruptions on the joint surface might be seen with an ultrasound.
Dilemmas: Some injuries aren’t visible with ultrasound because of the complicated structure of the joint. For example, a meniscal tear in certain locations can’t be visualized using this diagnostic method. The only way to properly diagnose them is with arthroscopic surgery. On the flip side, some meniscal tears seen on ultrasound can’t be located during arthroscopy. Accurate ultrasound examination of the stifle requires a high level of skill. Injuries can easily be missed.
Nuclear Scintigraphy (Bone Scan)
What’s involved: Your horse is injected with a radioactive substance that circulates through his bloodstream. A specialized camera detects areas of increased blood flow, indicting increased inflammation.
Stifle signs: A “hot spot” visible in or near the stifle area that’s detected on a bone scan would indicate a stifle injury.
Dilemmas: Scintigraphy is best used for identifying injuries that occur to the bone or in places where soft tissues connect to bone. Many stifle injures, even when they’re severe, don’t show up on bone scans.
Lessons Learned
The stifle is a complicated joint with many crucial structures that can be injured. This typically leads to instability of the joint and subsequent inflammation, pain, and lameness. So, why is it a mistake to simply inject an inflamed stifle joint with corticosteroids to reduce that inflammation if it makes your horse feel better? Employing this strategy doesn’t address the underlying cause of instability—and that’s where things can go wrong. If your horse keeps working on his injured joint until the damage becomes even more severe, he’ll end up out of the show pen for good.
Simply put, relying on injections to treat the joint without a diagnosis is a short-term fix with potential long-term consequences.
Although it may be complicated to make a specific diagnosis of a stifle injury, don’t give up! Your vet may need to perform multiple diagnostic tests before giving you a solid answer, but it’s worth the effort. Pursuit of targeted treatments such as surgical removal of cartilage fragments, regenerative therapy for injured structures, and (sometimes most important) adequate rest to allow for healing can mean the difference between early retirement and a long career.
