long pastern bone damage in sport horses

With the rise of magnetic resonance imaging (MRI) in equine medicine, veterinarians have picked up on a variety of lameness conditions that were harder to identify and/or evaluate in the past. One that appears to occur most frequently in sport horses such as eventers, show jumpers, and dressage horses is osseous trauma (bone damage) of the sagittal groove of the long pastern bone (proximal phalanx), which lies beneath the ridge in the bottom of the cannon bone at the base of the fetlock.

Ellen Singer, BA, DVM, DVSc, Dipl. ACVS, ECVS, MRCVS, formerly a senior lecturer in equine orthopedics at the University of Liverpool and now a private consultant, described the pathophysiology behind this finding and how to diagnose and treat it at the 2018 British Equine Veterinary Association Congress, held Sept. 12-15, in Birmingham, U.K.

What Is It?

Osseous trauma to this region can include subchondral bone (below the joint cartilage) damage, articular cartilage damage, and fracture. Singer said it’s either a repetitive stress injury or a one-off incidence of acute overload with excessive twisting.

To better understand how this injury can occur, she described the biomechanics behind the joint involved: the metacarpophalangeal joint, or fetlock.

The bottom of the cannon bone fits quite nicely into the sagittal groove of the pastern bone, Singer explained; it pushes down into the pastern bone with each step. While the joint naturally rotates some as the foot hits the ground, soft tissue limits significant joint movement outside of this sagittal plane. The degree of twisting in this groove, however, varies between horses, disciplines, conformation, shoeing, footing, and much more. This force can ultimately contribute to osseous trauma.

Diagnosis

The prevalence of this condition is low, Singer said—less than 0.1%. Associated lameness ranges from mild to severe but is typically acute and occurs more frequently in the forelimbs than the hind. Veterinarians usually use diagnostic analgesia (nerve blocks) to pinpoint the problem, but radiographs often don’t pick up on any abnormalities.

“MRI is the gold standard diagnostic test,” Singer said.

Treatment

Veterinarians can take a conservative or surgical approach to treating osseous trauma. Singer said the conservative route consists of six to 12 months of restricted and controlled exercise and non-steroidal anti-inflammatories for pain management. Surgery involves placing one or two screws across the abnormal area.

Prognosis is only fair, she said, and signs of damage often persist on follow-up MRI exams. Various studies have shown return-to-soundness rates ranging from 32% to 75% with conservative treatment. Overall, however, she said “surgical versus conservative treatments appear to have similar results: 50% (return to soundness).”

Wrapping up, said Singer, “Osseous trauma and fractures of the proximal phalanx may have similar underlying causes related to the biomechanics of the fetlock joint; however, the results of treatment appear worse for osseous trauma.”