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 occurrence.

“The strain on the suspensory ligaments is directly related to the fetlock angle,” he said and, with age, conformation might worsen, leading to an increased risk of PSD development.

Another predisposing structural concern is related to foot balance: “A long toe-low heel hoof configuration with a negative plantar angle of the coffin bone (the angle the bottom of the coffin bone makes with the ground) may put the horse at risk for this injury,” said Schramme. He explained that an imaginary line drawn through the coronary band of either rear foot should intersect the forelimb at the carpus or below. If this line intersects higher up the limb, then the horse likely has a negative plantar hoof angle.

He said to watch for specific clinical signs of the injury, because it can be difficult to spot pathology (damage or disease) in this area even with advanced imaging techniques. Schramme urged veterinarians to carefully monitor the groove between the lateral (outer) splint bone and the lateral margin of the superficial digital flexor tendon. “If this groove flattens out and becomes convex, pay attention,” he advises.

Another useful test is what he terms the “strangulation test,” in which the veterinarian places his or her fingers across the back surface of the tendons and squeezes. A horse with a moderately advanced case of PSD will withdraw his leg away from this pressure fairly immediately. Other signs of proximal suspensory-related problems are evident as the horse moves: he will exhibit restricted hind leg action, decreased fetlock joint extension, decreased foot flight arc, and a shortened cranial (forward) phase of the stride. Most horses with proximal suspensory pain push off less with the affected rear limb to reduce discomfort that occurs during the second part of the stance phase (when the hoof is on the ground just before breakover). In some cases, lameness is not particularly obvious until the horse is ridden and the rider sits as the affected diagonal limb lands or when he or she asks for lateral work.

Schramme suggested using the Lameness Locator to help identify gait abnormalities during diagnosis, particularly in the early stages of PSD. The device involves mounting wireless inertial sensors on the horse’s head and croup for an objective quantification of lameness. “These sensors measure the vertical displacement of the torso and have a 10 times higher sensitivity than the human eye to evaluate gait asymmetry,” said Schramme.

Once the area of concern is identified and localized through a thorough clinical exam and diagnostic anesthesia, MRI is one of the best imaging techniques for assessing the proximal suspensory ligament, said Schramme.