Veterinarians routinely perform suspensory ligament branch ultrasounds during prepurchase examinations of young Thoroughbred racehorses. However, they do not have much clear data on what constitutes a normal suspensory ligament branch. Frances Peat, BVSc, PhD, Dipl. ACVSMR, of the Orthopedic Research Center, C. Wayne McIlwraith Translational Medicine Institute at Colorado State University, in Fort Collins, Colorado, collected data from 969 ultrasound exams to establish reference ranges for forelimb suspensory ligament branches. She presented the results at the 2024 American Association of Equine Practitioners Convention, held Dec. 7-11, in Orlando, Florida.

Studying Thoroughbred Suspensory Ligament Branches

Peat said her goal was to define the normal appearance of suspensory ligament branches, identify common pathology, and determine any association between specific lesions and racing performance as well as association with radiographic changes in the sesamoid bones. This data could help veterinarians more accurately determine which horses are at risk of future injury or poor performance.

In total, she and her colleagues performed ultrasound exams on 593 sales yearlings and 376 sales 2-year-olds. During the exams, they assessed cross-sectional area of the medial and lateral (inside and outside) suspensory ligaments, width, periligamentous (around the ligament) tissue thickness, fibrillar pattern grade (0 to 3), hyperechoic foci (dark abnormality) grade, and sesamoid bone surface grade (yes or no).

They defined Grade 0 fibrillar pattern as a ligament with homogenous echogenicity (brightness) and uninterrupted fibrillar pattern. Grade 1 showed small areas of subtle irregularity, appearing as indistinct hypoechoic (darker) regions. Grades 2 and 3 showed moderate to severe hypoechogenicity and/or disrupted fibrillar pattern.

The researchers analyzed associations between ultrasonographic changes and racing performance from 2 to 4 years of age. Key findings relative to the fibrillar pattern were:

• The cross-sectional area of the medial branch of the suspensory ligament was consistently larger than that of the lateral branch;
• Cross-sectional area increased with fibrillar pattern grade, with Grade 3 lesions having the most enlarged branches;
• The prevalence of Grade 1 (mild) fibrillar pattern was 21% in yearlings and 29% in 2-year-olds;
• Grade 2 fibrillar pattern was seen in 8% of yearlings and 10% of 2-year-olds, and Grade 3 fibrillar pattern was seen in 1% of yearlings and 4% of 2-year-olds.

In some yearlings ultrasound data was available when they re-presented as 2-year-olds. Most of those horses (93%) either remained fibrillar pattern Grade 1 or improved to Grade 0 by the 2-year-old sale. However, one-third of yearlings with Grade 2 fibrillar pattern change progressed to a Grade 3 by the 2-year-old sale.

The Effects of Suspensory Changes on Horse Performance

Peat and colleagues also evaluated the effect of fibrillar pattern on racing performance, including eight measures of performance such as starting at least one race by 5 years of age, total number of starts, earnings per start, etc.

They only considered Grade 3 fibrillar pattern clinically important in reference to racing because those horses had a 0.8 probability of racing (i.e., an 80% chance they’d race), compared to 0.9-0.92 (a 90-90% chance) for horses with Grades 0-2 fibrillar pattern. They associated the presence of Grade 1 fibrillar pattern with significantly higher earnings per start compared to those with a higher grade.

The researchers concluded Grade 1 fibrillar pattern does not seem detrimental to racing performance—a key finding given its high prevalence in yearlings (20%) and 2-year-olds (30%). However, Grade 3 fibrillar pattern has a clinically important detrimental effect on racing.

Take-Home Message

Veterinarians frequently ultrasound suspensory ligament branches during Thoroughbred prepurchase exams but, until now, they’ve lacked clear agreement on what qualifies as normal. Researchers found Grade 1 fibrillar changes common and not linked to poor outcomes—in fact, they correlated with higher earnings per start. However, horses with severe changes had a significantly lower probability of racing. This research offers context for interpreting suspensory ultrasounds, helping veterinarians identify which horses are likely to stay sound—and which might be at risk for injury or poor performance.