“The stifle region represents one of the most commonly injured and most frequently requested ultrasound examinations in the horse,” said Betsy Vaughan, DVM, Dipl. ACVSMR, at the 2022 Veterinary Meeting and Expo (VMX), held in Orlando, Florida. Vaughan is a clinical professor of large animal ultrasound at the University of California, Davis, School of Veterinary Medicine.

Regardless of the suspected origin of pain, hind-limb lameness warrants a thorough physical and lameness exam before reaching for the ultrasound probe, said Vaughan. “Upper hind-limb lameness is difficult to localize, especially if characterized by an inconsistent response to flexion tests and to nerve blocks or when intermittent in nature,” she explained. “Any evidence of trauma—such as wounds or swelling—helps point us in the right direction in terms of localizing the source of lameness.” If the horse fails to respond to more distal (lower) nerve blocks, veterinarians can also perform an intra-articular block of the stifle joint itself to try to pinpoint the area of pain.

“Ultrasound of the stifle region is very rewarding because it can lead to conclusive findings that shape the treatment and rehabilitation protocols of a particular horse,” Vaughan said. “Using primarily our high-frequency linear transducer and mid-frequency microconvex transducer, we’re able to get good visualization of many of the structures associated with the stifle region.” Indeed, the stifle region includes much more than the femorotibial joint. Vaughan described surrounding tissues that also carry the potential for injury:

• Patellar ligaments (PL). The middle PL is the most commonly injured of the three but is rarely the primary injury or cause of lameness. Injuries involving the medial (toward the horse’s midline) PL are usually small and incidental, and those affecting the lateral (away from the midline) PL are most often associated with wounds due to the ligament’s location on the horse’s limb.
• Lateral and medial femorotibial joints. The medial femorotibial joint can often have increased fluid swelling (effusion) with or without other abnormalities, but the lateral femorotibial joint rarely has visible effusion without an underlying injury.
• Lateral and medial menisci, with the latter being more commonly injured than the former. If the meniscal injury is significant, veterinarians often see concurrent effusion, synovitis (inflammation of the joint lining), and irregular bone margins (which indicate osteoarthritis) on ultrasound and radiographs.
• Lateral and medial collateral ligaments are uncommonly injured but should be evaluated to rule out rare cases. Vaughan noted they’re more easily evaluated with the limb in a non-weight-bearing position.

“With all of that in mind, it is not uncommon for a horse referred to us for a stifle injury to actually be suffering from a lameness that’s stemming from pain elsewhere in the body,” she said.

“Elsewhere” can be either proximal or distal to (above or below) the stifle region. Horses initially presenting for stifle evaluation can end up with a multitude of diagnoses—most commonly pelvic fractures, but other findings include injury to the tarsus, metatarsus, or pastern.

It’s not all black or white—stifle or not stifle—either. Vaughan has completed ultrasound exams on stifles that revealed some abnormalities, but not enough to explain the degree or duration of the lameness. “It’s important not to presume that subtle or mild findings on stifle ultrasound or radiographic examinations constitute the cause for severe, persistent lameness,” she noted. “Don’t hesitate to go back to the drawing board when the diagnostic imaging findings don’t fit the clinical picture.

“Whether the injury is in the stifle or elsewhere in the hind limb, at the end of the day, pinpointing its precise origin is really important for not only an accurate diagnosis but also for proper treatment and a successful outcome,” Vaughan said.