Patellar ligament injuries in the stifle are on the rise in horses, but researchers do not yet fully understand the ligament’s normal appearance on ultrasound and what they might see when the ligament is injured. Therefore, Ellen Law, DVM, ECVDI resident, a radiologist at the Diagnostic Imaging Unit at the Swedish University of Agricultural Sciences, in Uppsala, Sweden, and a research team studied the appearance of patellar ligaments in 111 riding and trotting (harness racing) horses on B-mode and color Doppler ultrasound.

“This study actually gained inspiration from human medicine, where a disease called jumper’s knee is a common orthopedic complaint in both recreational and professional athletes,” said Law. “We wanted to investigate whether a similar syndrome with chroinic pain from the patella apparatus also exists in horses.”

Objective Equine Hind-limb Gait Analysis

The horses included in the study were in training appropriate for their age—at least five days per week for mature horses—and the riders and trainers of each horse reported no impaired performance or lameness.

The researchers performed a full clinical examination and objective gait analysis on each horse within 24 hours of conducting ultrasonographic examinations. They recorded any abnormal findings during joint, tendon, and ligament palpation, or incorrect hoof conformation. They categorized each horse as hind-limb lame or sound based on the findings during the objective gait analysis.

None of the horses had effusion (swelling), increased heat in stifle joint compartments, or abnormal hoof conformation. However, one trotter horse had scar tissue in the skin over the attachment of the lateral (outer) patellar ligament.

The researchers noticed movement asymmetry in one or both hind limbs of 26 horses during the straight-line trotting gait analysis. Of these, six horses displayed push-off lameness in the left hind limb and four in the right hind limb, while three appeared to have an impact lameness in the left hindlimb and nine in the right. Two horses exhibited both impact and push-off lameness in the right  hind limb, and one experienced both in the left hind limb.

Equine Patellar Ligament Ultrasound Findings

A certified diagnostic imaging resident or veterinary radiologist examined the full length of each horse’s patellar ligaments using static and cine-loop B-mode and color Doppler ultrasound with the horse bearing weight.

The researchers commonly noted evenly distributed striations in the upper and/or lower portion of the intermediate patellar ligament, which they considered most likely to be normal. However, they considered unevenly distributed, hypoechoic (dark), wide, single, or multiple splits in the ligament to be abnormal and noted these changes in 21 horses.

Law and her colleagues described uninjured medial patellar ligaments as having a triangular shape throughout, and in 17 horses they noted a striated appearance. The lateral patellar ligament varied in shape amongst the horses but overall had an oval or triangular shape with indistinct margins. In 15 horses they noted abnormal bone formation at the origin or insertion of the lateral patellar ligament, but only four of these horses were lame in the affected limb.

“I think the most important point is that veterinarians have to be critical when they see changes on ultrasound in the patellar ligament in horses,” said Law. “Blocking to determine the clinical significance might be necessary in some cases.”

Law and her team plan to continue this research. “What has surprised us so far is the variable appearance of the patellar ligaments in horses. Dark (hypoechoic) regions don’t necessarily mean lesions as we had previously thought.”

Take-Home Message

Law and her fellow researchers did not find a strong association between hind-limb lameness and ultrasonographic findings in the patellar ligaments of the 111 horses participating in their study. She said in practice, veterinarians should perform further diagnostics to determine the clinical relevance of ultrasonographic and gait analysis findings if needed.