Proximal suspensory disease includes injuries to the proximal suspensory ligament (PSL) itself as well as the surrounding structures. Injury to the PSL and associated structures is a common lameness with a long list of potential causes. Veterinarians have seen a high prevalence of proximal suspensory disease (PSD) in cutting horses, which has prompted the need for more information on the impact of PSL abnormalities on performance.

“The anatomy in the PSL region is complex, making it difficult to ultrasound,” said Hayley Sullivan, DVM, MS, cVMA, an associate veterinarian at Kentucky Equine Hospital who completed a residency and master’s degree through the Colorado State University’s Orthopaedic Research Laboratory. “In this study we performed weight bearing and also non-weight-bearing ultrasound exams with angle-contrast to improve the visualization of the ligament margins. The latter technique helped improve our ability to differentiate between various tissue types in the proximal suspensory region,” she added during her presentation at the 2022 American Association of Equine Practitioners Convention held Nov. 18-22 in San Antonio, Texas.

Using this ultrasound technique, Sullivan and colleagues at Colorado State University examined 110 2-year-old cutting horses prior to commencing training and again two years later. In total, they performed 267 ultrasound examinations on both forelimb and hind-limb PSLs.

Sullivan’s team aimed to establish normal cross-sectional areas (CSA) of the PSL in forelimbs and hind limbs in the non-weight-bearing position, monitor PSL changes over time with training, and determine if PSL abnormalities affect lifetime earnings using data from the National Cutting Horse Association. They hypothesizeds that cutting horses commencing training would have PSL morphology (form and structure) changes detectable via ultrasound that would affect lifetime earnings.

The following data is from this prospective clinical trial:

• Normal PSL CSAs for both fore and hind limbs in the non-weight-bearing position were established for the common metacarpal and metatarsal (front and hind cannon bones, respectively) ultrasound zones.
• The prevalence of limbs with PSL abnormalities increased from 13.4% during the initial ultrasound exam to 53.3% at the follow-up exam two years later.
• PSD occurred more frequently in forelimbs than hind limbs.
• Similar to a previous study’s finding using MRI, osseous attachment abnormalities were diagnosed more often than abnormalities of the ligament itself.
• Despite many PSLs increasing in size over time, as determined by subjective ultrasound assessment by a board-certified equine radiologist, objective assessment of size via measurements of CSA in those PSLs did not change over time.
• The presence of PSD did not affect lifetime earnings.

“This is the first study to establish ultrasonographic CSA of the PSL by zone using the angle contrast non-weight bearing position and CSA of the PSL of horses within a specific discipline,” said Sullivan. “This is important as CSA measurements of PSLs are frequently performed by practitioners, and prior to this study there were no published normal ultrasonographic CSA ranges by zone in the non-weight-bearing position. The non-weight-bearing position with angle contrast ultrasonography (a simple technique involving angling the probe slightly proximal or distal—higher or lower) has been shown in prior studies to be superior to weight-bearing exams, allowing a more accurate assessment of the PSL and surrounding structures.

“There was a high prevalence (13.4%) of PSL lesions seen in young cutting horses prior to even starting training, which increased markedly over the two-year study period,” she added. “Both a high prevalence in young horses and the increase over time with training could mean that these abnormalities represent adaptive remodeling.”

It is possible, Sullivan said, that they found more osseous attachment than ligamentous changes because the former precede ligamentous lesions in the disease process. Alternatively, osseous changes could be standalone abnormalities, or they could appear more frequently because ultrasound is more sensitive for finding them.

“The lack of CSA change over time within PSLs that became subjectively enlarged (when graded by a board-certified equine radiologist) brings into question the usefulness of taking such measurements as part of the diagnostic process, as previous groups have also questioned,” Sullivan said.

In summary, her team established normal angle contrast ultrasonographic PSL CSAs by zone of both fore- and hind limbs in cutting horses. Cutting horses have induced ultrasound suspensory ligament morphology changes with commencing training that do not impact their lifetime earnings. Forelimb PSL abnormalities were more prevalent than hind-limb PSL abnormalities, and osseous attachment abnormalities are more prevalent than ligamentous abnormalities in this group of cutting horses.