They’re the most common cause of rear-limb lameness in sport horses, the third-most common of the same in show jumpers, fifth-most in event horses, and eighth-most in endurance horses. But despite their propensity for sidelining athletes, injuries to the top of the rear cannon bone—called proximal suspensory disease, or PSD—are some of the most diagnostically elusive. An equine surgeon recently addressed a group of veterinarians at the 2013 American Association of Equine Practitioners Convention, describing how even careful regional nerve blocks and advanced imaging techniques can be fruitless in diagnosing these cases.

In a forum on sport horse lameness at the convention , which was held Dec. 7-11 in Nashville, Tenn., Michael Schramme, DrMedVet, CertEO, PhD, Dipl. ECVS, ACVS, from France’s Campus Veterinaire de l’Universite de Lyon, described how he gets to the bottom of these injuries, which occur where the interosseous medius muscle’s (suspensory ligament) attaches to the top 1.5 inches of the rear cannon bone. Because the canal through which the suspensory ligament runs at the top of the cannon bone doesn’t distend, the suspensory ligament’s constraints can cause potential pain and injury.

Schramme notes that veterinarians attribute the pain in 63% of these cases to the proximal suspensory and/or its bone attachment. Riders of affected horses typically complain of bilateral stiffness, decreased hind-limb impulsion, difficulty making transitions, resistance to lateral exercise, evasive behavior, and decreased power over jumps.

Predisposing factors to developing PSD include riding discipline and the horse’s conformation. For example, the dressage horses Schramme sees with this injury are “typically big-moving horses in advanced work exerting plenty of repetitious exercises.” Also, straight rear-limb conformation, low-sloping pasterns, and low fetlocks seem highly correlated to PSD