Hunter’s bump, that obvious asymmetry to the horse’s pelvis when viewed from behind, is caused by a tearing and upward dislocation of the sacroiliac joint (see March 2002).
However, a study by Dyson and Murray of the Centre for Equine studies in the United Kingdom confirms that pain can originate in this region without a visible hunter’s bump and dislocation.
The sacroiliac joint is the connection between the horse’s pelvis and the sacral portion of the spine. It’s one of the few areas of the horse’s spine that has a lot of potential for movement when the hind end is strongly engaged.
The main stabilization for the joint is the surrounding musculature and ligaments of the joint. If a severe stress is applied and the ligaments of the joint actually tear, you’ll get an obvious hunter’s bump. However, pain and stress in this area doesn’t always cause a complete tear.
Over a five-year period, the study looked at 74 horses with a clinical exam suspicious of sacroiliac-area pain, as determined by how the horse moved and pain on palpation in some cases. The suspicion was confirmed by bone-scan findings and/or pain relief with anesthesia to the sacroiliac area.
Not surprisingly, dressage horses and jumpers were at increased risk. Other horses that also strongly engage the rear include cutting horses, barrel horses and racing Standardbreds.
The horses in this study were slightly older than most horses this clinic usually sees, probably a combination of the cumulative effect of strain over time and the fact that ligaments tend to become less flexible with age.
The horses were also taller and heavier on the average, but this finding may have been related to the large number of warmbloods that were in the study.
Other common findings were poor development of the back muscles in general, and asymmetrical muscular development in the hindquarters, with the sore side less well-muscled.
On further examination, 20% of the horses were reluctant to hold a hind leg up for any extended time. You may see this behavior in your own horse when picking his feet or during farrier visits.
Most of the horses traveled straight when viewed from behind. They didn’t bring the leg toward the midline, as you see with hock problems, or swing it wide as is common with stifle pain. In all the cases, the problem was most obvious when the horse was ridden, and it was most obvious to the rider. The most frequent rider complaints were stiffness, unwillingness to work on the bit and a poor canter.