How to get athletic horses with injuries to the large, complex stifle joint on the road to recovery.
The stifle. It’s the largest and one of the most complex joints in the horse’s body. It’s also key to smooth locomotion, transferring energy seamlessly from the large hind-end muscles to the long, delicate lower-limb bones. The stifle helps propel horses across turf, over obstacles, and around tight corners. Not surprisingly, with such huge forces centered on two bones cushioned by two small cartilaginous discs, injury to the stifle generally has a profound negative impact on performance.
“Stifle injuries account for a substantial number of injuries in sport horses,” says David Frisbie, DVM, PhD, Dipl. ACVS, ACVSMR, professor of equine surgery at Colorado State University’s (CSU) Equine Orthopaedic Research Center, in Fort Collins. Although the exact prevalence of stifle injuries remains unknown, Frisbie estimates that approximately 40% of sport horse injuries can be associated with the stifle.
Chris Kawcak, DVM, PhD, Dipl. ACVS, ACVSMR, a professor of orthopedics also at CSU, agrees that stifle injuries are a risk for any athletic horse but adds that they’re more prevalent in some disciplines than others. Veterinarians typically see them more frequently in Western performance horses, for instance, than in adult Thoroughbred racehorses.
While the stifle joint’s repetitive forward and backward motion during racing, or any high-speed work, can cause injury, “it’s the lateral-to-medial rotational movements of Western performance horses that tend to put significant stress on the soft tissue and cartilage in the joints,” Kawcak says—in other words, the quick side-to-side movements of spinning, cutting, and running barrels.
Regardless of breed and discipline, joint anatomy remains the same, and how veterinarians diagnose and treat common stifle injuries is transferable. In this article we’ll review stifle anatomy briefly and describe injury diagnosis. And while developmental orthopedic disorders also can result in stifle injury, they have a different cause and are diagnosed and treated differently, so we’ll skip them in this discussion. We’ll also describe the newest methods of treating common stifle injuries.
Stifle Joint Anatomy
Three bones, two menisci, and a slew of soft tissues comprise three separate compartments that make up the stifle joint. The femur, tibia, and patella (colloquially referred to as the kneecap) form the medial femorotibial joint (at the inside of the limb), lateral femorotibial joint (at the outside of the limb), and the femoropatellar joint (located between the femur and the kneecap).
Soft tissues of interest include the patellar ligaments (medial, middle, and lateral connections between bones in a joint that help a horse’s stifle “lock” so he can stand for prolonged periods) and the cranial (toward the head) and caudal (toward the hind end) cruciate ligaments within the femorotibial joint. Unique to the stifle joint is the pair of cartilaginous discs mentioned earlier, called menisci, which are C-shaped and located between the two projections of the femur bone’s lower end (called the medial and lateral condyles) and the tibia. One of their main functions is to facilitate frictionless movement of the stifle joint. Several ligaments, including the meniscotibial ligaments, hold the menisci in place between the condyles and the tibia. Finally, as in other joints, collateral ligaments located primarily on the outside of the femorotibial joints help stabilize the stifle.
Common Stifle Injuries
When we think of sports-related injuries, many of us immediately think of the worst-case scenarios: fractures. Femur and tibia fractures are, thankfully, relatively uncommon and usually restricted to trauma cases. Patellar fractures (again, following trauma) are also rare.
In a study soon to be published in the Equine Veterinary Journal, Laurie Goodrich, DVM, PhD, Dipl. ACVS, Director of the Orthopaedic Research Center at CSU, and colleagues reviewed their hospital’s medical records to gain more information about identifying common stifle injury types and how arthroscopy and ultrasound findings correlate. In that study they identified the menisci as the culprit in most cases. The medial meniscus suffered injury more frequently than did the lateral meniscus, with 25 of the 47 medial menisci (53%) injured compared to only six out of 34 lateral menisci (17%).
Other types of stifle injuries Goodrich et al. identified included:
- Medial and lateral cranial meniscotibial ligament tears (seen in up to 30% of horses in the study);
- Defects of the articular cartilage—that which is in the joint—and bone cysts of the medial and lateral condyles. Again, the medial structures were more commonly affected; 21/27 medial condyles (78%) and 15/27 lateral condyles (56%) had evidence of abnormalities; and
- Very rarely, patellar ligament injuries.
“Other potential causes of lameness in the stifle include synovitis (inflammation of the synovial membrane, or the joint’s inner lining) and cruciate ligament disease,” says Goodrich.
Identifying Stifle Injuries
“Although the stifle is a large joint, and swelling of one or more of the compartments of the stifle joint is usually obvious to veterinarians, the proximal location often doesn’t make the outward observation of swelling obvious,” says Kawcak, referring to the close proximity of the stifle to the bulk of the horse’s body. This is in contrast to the knee or fetlock, where swellings are more obvious to many ;owners.
A physical examination, including a lameness examination, as well as diagnostic anesthesia (i.e., nerve blocks, or injecting a local anesthetic such as lidocaine into the joint to confirm the suspected source of lameness) certainly serve as diagnostic mainstays. But to pinpoint the exact cause, nature, and severity of the injury, veterinarians typically recommend one of the following diagnostics: radiography, ultrasonography, and arthroscopy.
“Compared to other joints in the limb, such as the knee and fetlocks, the bulky muscles surrounding the horse’s stifle and the close, complex anatomy often makes diagnosis of the injured structures challenging,” Kawcak says.
Radiographs are typically a go-to diagnostic tool in many cases, helping rule out fractures and other bony changes, such as osteoarthritis, subchondral (under the cartilage) bone damage, and excessive bone growth where the ligament should attach (enthesophytes).
Ultrasonography is the next noninvasive and readily available diagnostic option. It allows the veterinarian to examine the soft tissues (specifically, the patellar and collateral ligaments) around the joint as well as in the joint, such as the menisci and meniscal ligaments. He or she can also assess the joint capsules, bone margins, and articular cartilage.
Like radiographs, however, ultrasound cannot be used to diagnose all stifle injuries, says Myra Barrett, DVM, MS, Dipl. ACVR, an assistant professor of radiology at CSU.
“Ultrasound can fall short in detecting articular cartilage damage in the absence of subchondral bone injury,” she says. “Evaluation of the cruciate ligament is very limited with ultrasound because they are located deep within the joint.”
Kawcak concurs, adding, “Owners need to remember that ultrasonography only provides a window into the joint, not a complete picture, and a combination of imaging techniques is usually required to get the best characterization of the damaged tissues.”
In some cases veterinarians perform ultrasonography in conjunction with arthroscopy (inserting a slender arthroscope into a small incision to view the joint) to get a more detailed picture of the stifle joint’s overall health.
“Arthroscopy can identify some of the injuries to the menisci, avulsion injuries (fragments pulled loose from the bone), as well as cranial medial meniscotibial ligament tearing, among others,” says Goodrich. But again, it’s not useful for diagnosing all of these injuries, and she recommends using multiple imaging modalities, such as radiographs and ultrasound, to get a global assessment of the stifle.
One alternative to traditional arthroscopy is needle arthroscopy, originally described by Frisbie in 2015. With this method the practitioner uses a disposable 18-gauge needle measuring only 1.2 mm in diameter and 100 mm in length. A camera attaches to the arthroscope at one end and projects the image on a monitor, while the other end probes the stifle joint. The needle arthroscope’s small size and maneuverability allow veterinarians to see all the major structures of the stifle, especially the menisci and collateral ligaments that other approaches, such as ultrasonography, allow them to only partially visualize.
“The utility of this minimally invasive technique is becoming popular, partly because of the short downtime associated with the procedure,” says Frisbie. “Horses are back to work in as little as 48 hours, depending what is found at arthroscopy. Further, more and more veterinarians are learning how to perform the procedure.”
For the last four years Frisbie has taught a course at CSU for veterinarians to become proficient at needle arthroscopy. He has also traveled internationally to share this technique.
“Horses that have pain localized to the stifle, but other diagnostics such as X ray or ultrasound don’t provide answers to why there is pain, turn out to be the perfect indication for this technique,” says Frisbie. “When I am in my private practice I am performing this procedure weekly.”
In human medicine, magnetic resonance imaging (MRI) and computed tomography (CT) are invaluable for identifying similar injuries to the knee. Horses, however, are another story: “Most (MRI and CT) units are simply not large enough to accommodate an equine stifle,” Kawcak says. “That said, large-bore CT and MRI devices are being introduced into the human market and used by some equine practices. In addition, the use of contrast agents in the joint have helped to better characterize damaged tissues in some referral centers.”
Treating Stifle Injuries
How caretakers manage a stifle injury depends on the exact nature of the lesion(s). Femur and tibia fractures, for instance, are frequently fatal, whereas a fractured patella can usually be repaired because the femorotibial joint is non-weight-bearing.
Veterinarians typically treat damage to the articular cartilage lining the femoral condyles arthroscopically by removing the damaged cartilage (called debridement). The problem with removing areas of articular cartilage from the condyles measuring greater than 5 mm is that it exposes the bony tissue underneath. As a result the joint provides less cushion during movement, causing pain and inflammation.
Over the years, veterinary researchers have attempted to remedy this problem using various “resurfacing” techniques as well as microfracture (punching small holes in the subchondral bone beneath the cartilage defect to stimulate cartilage growth). To date, none has proven overwhelmingly successful; however, various research teams refuse to be thwarted.
Most recently, surgeons from the University of Missouri’s College of Veterinary Medicine evaluated the use of a permanent synthetic implant to help repair cartilage defects located on the medial lateral condyles. They hypothesized that the prosthetic implant would serve as a cartilage substitute, decreasing pain the horse experiences following surgical debridement of articular cartilage. In this study, published in the April 2016 Veterinary Surgery, the authors found that biocompatible, synthetic cartilage implants were safe and might be effective pending additional research and technical tweaking.
Meniscal tears all too commonly result in significant injury and disappointing post-surgical performance, especially at higher levels of competition. The menisci heal poorly following injury. Even after surgery (typically arthroscopy)—which often includes removing or debriding the injured area, then rest and physical therapy—lameness persists. Recently, several research groups have studied the use of stem cell therapy for treating meniscal injuries.
Goodrich, Kawcak, and Frisbie, together with lead author Dora J. Ferris, DVM, (also at CSU) and colleagues, recently described their research on stem cell therapy for improving outcomes after surgical correction of various stifle injuries to the meniscus, cartilage, and/or ligaments.
“A single injection of cultured bone-marrow-derived stem cells, including 15-20 million stem cells, safely increased the number of horses that were able to return to work following surgery compared to previous reports,” says Goodrich.
In other words, stem cell therapy in conjunction with surgery appears to be more beneficial than surgery alone.
“After injury to such a substantial joint, which frequently involves more than one important structure, owners need to be cognizant that persistent inflammation of the joint can initiate a series of events that culminate in osteoarthritis,” says Goodrich.
Osteoarthritis, the painful degradation of articular cartilage, can develop in any age horse following even mild trauma. Evidence supports the preventive use of oral joint health supplements containing glucosamine and chondroitin sulfate (with or without additional ingredients such as methylsulfonylmethane, hyaluronic acid, and avocado-soybean unsaponifiables). Once a stifle injury occurs, veterinarians recommend a multimodal treatment approach, meaning they use several different methods to promote healing. Based on the available research, this includes:
- Appropriate surgical correction of the underlying traumatic injury;
- Non-steroidal anti-inflammatory drugs;
- Intra-articular medications (joint injections), including corticosteroids, hyaluronate sodium, and polysulfated glycosaminoglycan;
- Interleukin-1 receptor antagonist protein (IRAP);
- Weight management;
- Physical therapy;
- Extracorporeal shock wave therapy; and
- Oral joint health and omega-3 fatty acid supplements.
“Stem cell therapy may also be a useful method of supporting horses with OA,” says Goodrich. “Additional research will reveal the types of cases that will benefit the most from this therapy.”
No injury is a good injury, especially in competition horses. Stifle injuries are among the most undesirable, because the joint is very large, diagnostic and treatment options are limited, affected animals have difficulty returning to performance, and many develop long-term side effects, including osteoarthritis. Researchers say continued advances in diagnosis, surgical technique, and supplementary approaches, such as stem cell therapy, are likely to improve outcomes following injury.