Changes in the way veterinarians view stifles might lead to more frequent recognition of injury

by Kenneth L. Marcella, D.V.M.

The stifle joint in the hind limb is one of the largest and most complex joints in the horse's body, and it functions comparably to the human knee. In humans, the knee tends to be the most easily injured joint. About 11-million people visit their doctors each year because of knee injuries, and more than half those injuries will necessitate some type of surgical repair.

Equine athletes incur a large number of stifle injuries. But this joint is so large and contains so many intricate structures that clinicians often are unable to completely evaluate the stifle using existing diagnostic techniques.

Racehorses are most likely to sustain stifle injuries when they accelerate out of the gate or when negotiating turns at high speed. Footing is very important in some types of stifle injuries because the irregularity of the surface can create an imbalance within the stifle joint as the horse runs through turns.

Many veterinarians who specialize in the problems of performance horses feel stifle injuries are under-diagnosed and sometimes might contribute significantly to hind-limb lameness. Use of new curvilinear ultrasound probes, which give a wider look at the joint, and a renewed dedication to the evaluation of stifle injuries might change existing ideas concerning this important joint.

A look at the stifle

The stifle joint serves as the junction of three bones:

•The femur, the large bone in the upper leg between the hip joint and the stifle;

•The tibia, the bone between the stifle and the hock; and

•The patella, the "kneecap."

The femur communicates with the tibia via the stifle joint, which is partially protected by the patella.

A series of ligaments called the patella ligaments and named according to their location--medial (inside), middle, and lateral (outside)--attach the patella to the tibia. Short, thick collateral ligaments located on the inside and outside of the stifle joint attach the femur and the tibia. Semicircular pads of thick, resilient fibrocartilage occupy the spaces between the ends of the femur and the tibia. These pads, the medial and lateral menisci, together with the cartilage of the bones and the fluid within the joint, help cushion the weight-bearing forces in the stifle.

Two strong ligamentous bands, the anterior (forward) and posterior (hind) cruciate ligaments, crisscross within the joint and span between attachment sites on the femur and the tibia. Cruciate ligaments provide support for the joint when it is subjected to pressure that pushes it excessively forward or backward. These ligaments also provide some rotational stability.

With an understanding of all the individual parts to this complex joint, it becomes easier to see why the stifle is prone to injury and why it is difficult to completely evaluate.

Types of injuries

Even though human and equine athletes share a tendency to injure the knee or stifle, respectively, there are many differences in the type and nature of their injuries. According to Troy Trumble, D.V.M., Ph.D., a board-certified orthopedic surgeon at the University of Florida's College of Veterinary Medicine, most humans tear or rupture the cruciate ligaments. This particular injury is rare in horses, which are much more likely to tear or injure the meniscal structures in the joint.

"Part of the reason for this may lie in the difference in anatomy," said Trumble. Humans have only one patella ligament, and the cruciate ligaments are located within the joint capsule; horses have three patella ligaments, and the cruciate ligaments are located outside the joint capsule, Trumble explained. "These differences probably make the equine stifle inherently more stable," he said. Nevertheless, injuries to the equine stifle do occur and perhaps more frequently than was previously thought.

Stifle injuries are relatively common in horses that jump, jump with speed, or spin and torque on their hind end. Horses competing in three-day eventing, jumping, dressage, and western performance seem to have a higher incidence of stifle problems.

Challenges in diagnosis and repair

The definitive diagnostic modality used to evaluate knee injuries in humans is magnetic resonance imaging (MRI), but MRI evaluation requires a magnetic coil large enough to admit the entire area being studied. A coil has not been developed that can accommodate the size and weight of a horse, thereby enabling it to image a horse's stifle. Current equine MRI units accommodate only horses' heads and lower limbs.

Digital radiology allows veterinarians to produce superior-quality radiographs, and this technology is proving helpful in the evaluation of stifle injuries. Arthritis and tears of the collateral ligaments or the attachment points of the cruciate ligaments can be visualized with radiology. If the stifle trauma has caused these ligaments to tear and to pull small pieces of bone attachment away from the surface, the injury will be visible.

Kent Allen, D.V.M., owner of Virginia Equine Imaging in Middleburg, feels the specific qualities of digital radiology also allow for an evaluation of the collateral ligaments and meniscal surfaces.

"If you adjust the way the films are viewed, you can get a better look at these structures," Allen said. "But it is still bits and pieces."

Most clinicians agree that trauma to the stifle that results in instability eventually will cause cartilage changes that will be visible via radiographs. The major challenge is to diagnose stifle injuries as early as possible to prevent more trauma from occurring in a horse that continues to perform, and then begin a treatment and rehabilitation program as soon as possible.

"Imagery gets better every year," Allen said. "The machines get better, and we get better," he added. "So we see more structures and different views all the time."

Advanced ultrasound

Another imaging technique that has been improved so it now is more useful in diagnosing stifle injuries is ultrasound. The use of a curvilinear ultrasound probe is allowing veterinarians to view more of the stifle joint than ever before. Superior to the standard ultrasound probe that sends out a straight-line signal and requires contact along the entire length of the structure being scanned, a curvilinear probe sends out a spray-like signal so a small probe can visualize a larger area. This feature allows ultrasonographers to evaluate collateral ligaments, meniscal surfaces, and even some sections of the cruciate ligaments within the equine stifle joint.

The difference between standard and curvilinear ultrasound can be compared to that of a photo taken with a regular lens in contrast to one taken with a wide-angle lens. This approach is fast replacing even arthroscopic surgery as the best means of diagnosing stifle injuries.

Using arthroscopy to diagnose and repair a joint injury, a surgeon inserts a small scope or camera through an incision into the joint, which enables him or her to view and manipulate surgical instruments inserted through another small hole in the joint. But the size of the equine stifle and the curved bones and deep recesses make some areas unreachable even with this specialized technique.

Alison Morton, D.V.M., M.S., assistant professor and board-certified veterinary surgeon at the University of Florida, is one of the veterinarians making use of these new imaging techniques.

"The curvilinear probe has made it far easier to see some meniscal injuries and to follow subtle shape and surface changes that can give an indication of injury or of repair and healing," Morton said. She added that the curvilinear probe is especially useful in lameness cases where the stifle injury might not cause much swelling and the horse's performance problems may be minimal. These horses typically improve with localized anesthesia of the stifle joint, but they have no radiographic changes in the joint. In these cases, the clinician might suspect a stifle injury but cannot locate a specific problem. Often these horses are rested for a short while and then returned to work only to aggravate their injury or cause more extensive damage.

Morton suggested that once the diagnosis of a meniscal tear or other ligamentous injury to the stifle is made, use of the curvilinear ultrasound could allow the clinician to treat and monitor a stifle injury in the same manner as other types of ligament injuries using standard ultrasound.

Returning a horse with a suspensory ligament tear to work too soon can be disastrous; the same applies to damage of the soft tissues of the stifle joint.

"Meniscal injuries heal even slower than regular tendon or ligament injuries because the fibrocartilage has very little blood supply," Morton cautioned. She suggested a diligent rehabilitation program of rest, anti-inflammatory therapy, hand walking, and medication to protect the joint from further injury. Periodic follow-up ultrasound evaluations are crucial.

As more veterinarians become proficient at ultrasound of the stifle joint, horsemen can expect more subtle meniscal and ligamentous injuries to be diagnosed. Proficiency in imaging the stifle is not an easily acquired skill, however, because it requires advanced ultrasonography skills, a good knowledge of the relevant anatomy, and practice.

Morton and Allen encourage all equine practitioners to scan the stifles if there is a reason to suspect their involvement in hind-limb lameness. Finding and treating these problems at an early stage may prevent more serious problems and breakdowns in horses that otherwise might have been put back to work too soon.

"It is now commonly thought that we have been under-diagnosing meniscal and other stifle injuries," Morton said. "Over the past five years, the increased use of ultrasound has found more of these horses, but there are still lesions that get missed because some veterinarians are not looking for them."

Kenneth L. Marcella, D.V.M., is a practicing veterinarian in Canton, Georgia.