Inside the equine stifle

The veterinary community is paying close attention to new information being uncovered using Magnetic Resonance Imaging (MRI) techniques. The increased availability of MRI units and the development of the standing MRI scans have made it possible to hone evaluations, and much has been reported of late about soft-tissue injuries in the foot, especially the caudal heel. Many horses that might have been generally classified as having navicular syndrome prior to MRI use are now being diagnosed with impar ligament damage, deep digital flexor attachment injury or any number of other soft-tissue problems occurring within the hoof capsule.

Horses that jump are at increased risk for meniscal injuries as the forces on the hind legs, and the stifles in particular, are intensified when horses take off and land. Poor riding, poor footing and other factors can also contribute to possible stifle injuries.

The technical limitations of MRI technology currently prohibit veterinarians from viewing many other areas of the body that would greatly benefit from the detailed observations and information only available with MRI (see related story). Perhaps chief among these hard-to-evaluate areas and a location that experiences considerable problems in the athletic horse is the stifle joint.

"The preferred diagnostic modality for diagnosing soft-tissue injuries in the human knee is MRI," says Troy Trumble DVM, PhD, Dipl. ACVS, an orthopedic surgeon at the University of Florida College of Veterinary Medicine. "This would be the ideal modality for the horse as well. However, until we have a better way to diagnostically image the stifle, the locations and types of soft-tissue injuries will be debated."

But there is little debate about the volume of injuries that occur in this joint. In humans, more than 9.5-million people visited an orthopedic surgeon because of knee injuries in 2003, making it the most commonly injured joint. Dr. J. Lane Easter of Performance Equine Associates in Texas considers the stifle joint the No. 2 cause of hind-end lameness in the western performance horse (behind proximal suspensory desmitis). Dr. Sue Dyson a lameness referral specialist with The Animal Health Trust in Newmarket, England, says horses that jump solid fences, such as three-day eventers and steeplechase runners, are more likely to experience trauma to the cranial surface of their stifles and therefore exhibit a higher percentage of such problems. Cutting horses, jumpers, racing Thoroughbreds, dressage horses and equine athletes of all disciplines can and do experience significant stifle injuries. But because this joint and its associated structures can be so difficult to evaluate, many stifle injuries still go undiagnosed and overlooked.

This thermography shows increased heat and swelling of the lateral collateral ligament of the stifle joint. The large number of structures all closely associated with this joint makes accurate diagnosis a challenge.

The complexity of the stifle lends itself to many different problems. In a study of 86 cases of stifle lameness, Drs. L. Jeffcoat and S. Kolb found 38 percent of the horses had subchondral bone cysts. Many of these were thoroughbreds noted to be developing stifle lameness at, or before, the onset of training. This study showed 15 percent had upward fixation of the patella; 13 percent had osteochondrosis dissecans (OCD) lesions, while a small percentage of horses showed either osteoarthritis (3 percent), fractures (4 percent), epiphysitis (1 percent) or unknown causes (13 percent). Stifle ligament and meniscal damage was noted in 12 percent of the cases, and many clinicians feel that these soft-tissue injuries are the types of stifle problems that are currently most under diagnosed.

Anatomically correct

The major soft-tissue structures of the equine stifle are the medial collateral ligament (MCL), the lateral collateral ligament (LCL), the anterior and posterior cruciate ligaments (ACL and PCL) and the medial and lateral menisci. The most common injury to the human and canine knee is rupture of the anterior or cranial cruciate ligament. But this injury is very uncommon in the horse.

"Part of the reason is a difference in anatomy," Trumble explains. "In humans and dogs, there is one joint and one patella ligament with the cruciate ligaments located within the joint capsule. Horses have three joints, three patellar ligaments, and the cruciate ligaments are extra capsular making the equine stifle inherently more stable."

Consequently, horses simply can tolerate more forceful stress to their stifles. But significant trauma to the front or cranial surface of the stifle still can cause ACL rupture.

Dyson says she often sees this type of injury associated with patella fractures and cautions practitioners to obtain a "skyline" radiograph (tangentially taken of the slightly flexed stifle) to carefully evaluate for this complication. Damage to the ACL is also believed to occur from quick changes in direction, rapid deceleration (this stresses the stifle as the horse "sits back" on its hind end and slows itself) and from the pressure created as a horse lands a jump.

"Any horse that routinely jumps as part of its performance is at a slightly higher risk for meniscal injuries as well," she says.

Differentiating injuries of all these structures is the primary task facing a clinician examining a horse with a stifle problem, and the history and physical examination are still very important parts of this process. Because of the size and relatively "open" structure of the stifle, "some degree of swelling usually develops in this joint, but the absence of distinct swelling does not always preclude severe damage," Dyson says. "The degree of lameness usually reflects the severity of lameness."

Once the stifle joint has been identified as the cause of the problem, even without MRI, there are still many diagnostic modalities available to clinicians for evaluation of the equine stifle.

Image options

Digital radiography is allowing clinicians to more accurately view many areas, and the stifle is one of them. If correctly used, digital radiographs allow a good visualization of the collateral ligaments and meniscal structures, says Dr. Kent Allen of Virginia Equine Imaging in Middleburg, Va. Allen says he visualizes these areas with ultrasound, and recent advances in probe design have made this modality more diagnostic as well.

Many clinicians are using a curvilinear ultrasound probe to obtain images of the collateral ligaments, meniscal surfaces and even of some sections of the ACL and PCL that were not able to be seen previously. A standard linear ultrasound probe sends out a straight-line signal and requires contact along its length. It was difficult to view anything within the curved surface of the stifle and behind the tibial tuberosity.

A curvilinear probe, however, sends out a spray-like signal allowing for visualization of a bigger area and allowing a look around or behind some bone structures.

"The difference between standard and curvilinear ultrasound can be compared to a photo taken through a standard lens and a wide-angled lens," says Alison Morton, DVM, MS, surgeon at the University of Florida College of Veterinary Medicine. Because of the degree of information currently available with this new ultrasound technique and because it does not require general anesthesia, this modality is fast replacing arthroscopic surgery as the best means of diagnosing stifle injuries.

Stifle injuries, especially documented ligamentous problems, should be handled like tendon or ligament injuries in other areas of the horse, Morton advises. Ultrasound should be used to monitor healing and to help determine the exercise/rehabilitation program. All too often in the past, a horse was suspected of having a stifle injury and might or might not have had much swelling. A period of rest resolved the swelling, and the horse appeared to be "sound" again. However, putting that horse back into work often resulted in a re-injury that was often worse than the original problem. The curvilinear probe allows the clinician to monitor the degree and quality of the healing process, and it determines exactly how and when to reintroduce that horse to exercise.

"Meniscal injuries heal even slower than regular tendon or ligament injuries because the fibrocartilage has very little blood supply," Morton explains.

This makes follow-up examination even more important, and though every practitioner can eventually learn to effectively ultrasound the equine stifle, the technique takes skill and practice so consultation with a boarded ultrasonographer is suggested.

Severe and subtle stifle injuries are out there, and some have been slipping through the cracks. Newer technology is available to help equine clinicians better diagnose these conditions. The knowledge provided by MRI has shown veterinarians what we have been previously missing in many areas, and this observation is also important as it is applied to the equine stifle. There are undoubtedly horses currently undiagnosed that have minor meniscal tears, bruised collateral ligaments, small strains of the cruciate ligaments and other subtle injuries to the stifle. Veterinarians should keep these problems in mind and take advantage of new diagnostic tools to find them when presented with lameness problems.