by Dr Simon Pearce and Dr Cathy Beck
Subchondral bone disease has presumably been around for as long as horses have. In humans it is called “bone bruising”. However it was not until 1981 that veterinarians started to recognise the condition, thanks to some work performed in America by O’Brien.
The disease in those days was diagnosed by radiographs and it was understood that the disease was very serious when radiographic changes were evident. In fact, horses never recovered once the subchondral bone changed to the extent that it was clearly evident on radiographs. At post-mortem the defects in the joint, including the articular cartilage, were extensive.
Affected horses typically presented with pain that was localised to the region of the fetlocks but with unremarkable radiographs. These horses often responded well to intra-articular injection of corticosteroids, but the pain would recur.
The length of time that the intra-articular injections were effective for became shorter and shorter and eventually radiographs did show remodelling of the subchondral bone, and the horses were no longer responsive to treatment.
Many horses were even too lame for retirement and required euthanasia. If horses were rested, they would often return sound, only to become lame again as the level of exercise intensity increased.
The advent of new imaging techniques has helped us to diagnose this condition much earlier. Nuclear scintigraphy for example is a very sensitive tool for measuring changes to the subchondral bone. The great advantage of early detection is that we can diagnose the disease prior to it being seen radiographically and more importantly prior to the “point of no return”.
Typically it occurs in the fetlocks, commonly in the hind fetlock in Standardbreds and front fetlock in Thoroughbreds, but as we learn more about the condition, we are finding it in a number of different joints. Other imaging technologies, such as magnetic resonance imaging, are being used in some Universities to document the disease very precisely.
Image right: Scintigraphic image of the hind fetlock of a standardbred racehorse. There is increased uptake of the radiopharmaceutical (hotspot) in the fetlock (arrow) indicating subchondral bone disease.
In some ways, the damage to the subchondral bone is analogous to tendon injuries, and the treatment consists of a controlled exercise programme. Monitoring the healing process can be done using scintigraphy.
One important factor to keep in mind is that the development of the injury was probably related to the training programme, or individual conformational attributes. If these conditions are replicated once the horse returns to training, it may be expected that the disease will return.
By Dr Simon Pearce and Dr Cathy Beck
University of Melbourne