The suspensory ligament is a fibrous structure that runs from behind the knee or hock down the limb to insert on the sesamoid bones just above the fetlock joint. It consists of an origin at the top, a body in the middle, and, a little farther down, a bifurcation, where it splits into two structures: a medial (inner) and lateral (outer) suspensory branch. Each branch attaches to the related medial or lateral sesamoid bone. The ligament and its branches form an integral component of the suspensory apparatus, suspending the fetlock during maximum weightbearing when, without these structures, the joint would collapse to the ground.
Injury Significance & Prevalence
Thankfully, a suspensory branch injury itself is not typically catastrophic. However, inflammation in the area can cause pain that can result in potentially performance-limiting lameness. In some cases branch injury can also result in effusion (fluid accumulation) and arthritis in the fetlock joint. Even in horses that show no clinical signs of branch injury, veterinarians might detect one following flexion tests or ultrasound during a prepurchase examination, which can result in failure to sell or a reduced selling price.
Until recently few researchers had studied the significance of suspensory branch injuries. We now know, however, that in juvenile Thoroughbred racehorses in training, moderate-level injuries to at least one branch that result in swelling, pain, or lameness confer a decreased chance of starting a race as a 2- or 3-year old. Additionally, affected horses make fewer career starts through their 4-year old seasons, have decreased earnings per start compared to matched siblings, and are at increased risk of reinjury. Horses with milder cases might race unaffected.
In show horses evaluated for lameness arising from suspensory branch injury, researchers reported that 43% returned to normal levels of work, although fibrosis (scarring) around the ligament and increased fetlock “drop” were negative predictors of future soundness. It is fair to say that in some cases this injury can be a significant detriment to the athlete.
Researchers on the above retrospective study looked at clinical cases—in other words, horses with lameness or pain attributable to branch injury. It becomes more difficult when we try to predict the significance of branch injury in currently sound horses. A group of researchers (Ramzan et al.) from Newmarket, England, evaluated the forelimb suspensory branches of 60 sound horses in full race training. They observed that 6.7% had moderate or greater injury as assessed by ultrasound yet never showed lameness for decreased performance. They found a higher number had mild ultrasound changes. Therefore, veterinarians should use caution when ultrasound shows an abnormal branch in a clinically normal horse. It seems the prevalence of an abnormal scan does not directly translate to a similar prevalence of clinical disease.
In a study of 896 juvenile Thoroughbreds in training, our research team (Plevin/McLellan) determined the prevalence of branch injury to be around 9.5%. However, we also observed that juvenile Thoroughbreds with significant ultrasonographic changes in their branches as yearlings were up to five times more likely to sustain a clinical injury to the branch than yearlings with ultrasonographically normal branches. This risk increased to 11 times if both the branch and the adjacent sesamoid were abnormal as a yearling. It seems appropriate to conclude that minor abnormalities on ultrasound might be forgivable in young Thoroughbreds, but more significant changes, even in a currently sound horse, should be viewed as potential weak spots in future athletic endeavors.
Clinical signs such as heat, thickening, and pain on palpation or flexion are useful indicators of branch injury. Lameness is not always a consistent finding. Ultrasound has been the mainstay for diagnosing and monitoring injuries to this region, and increases in ligament size, scarring, and fiber disruption are useful findings when making prognostic decisions. Recently, researchers have reported Doppler ultrasound, ultrasonographic tissue characterization, elastography, and MRI as providing potentially useful diagnostic and prognostic information. With some of these modern imaging techniques, it might be possible to image the injury before it becomes clinically apparent. This would be ideal from a welfare and rehabilitation viewpoint, because the prognosis for recovery is superior when we detect injury before clinical signs develop.