Inside the equine stifle
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."
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.
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."