Inside the equine stifle
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.
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.