In a retrospective study, researchers from the United Kingdom found that injuries to the palmar or plantar annular ligament (PAL)–the anatomic structure that holds that superficial and deep digital flexor tendons in place as they pass the fetlock (ankle) joint–can be readily diagnosed, are common, and have a worse prognosis if observed concurrently with injury to the deep digital flexor tendon.

According to Sue Dyson, MA, VetMB, PhD, DEO, FRCVS, Head of Clinical Orthopaedics at the Animal Health Trust in Newmarket, England, and a co-author on this study, "While PAL syndrome is well recognized, there is limited information regarding horses affected with a primary injury to the annular ligament itself."

In this study, medical records from 71 horses with a history of lameness, clinical signs of PAL injury, and the ultrasonographic diagnosis of a thickened PAL (> 2 mm) were collected from three hospitals.

Results showed that most (63%) of the affected horses were used for general riding purposes and only 23% were performance horses. PAL injury occurred more commonly in the hind limb compared to the forelimb, and bilateral PAL thickening was very common (found in 68% of horses), especially in hindlimbs