The sacroiliac joint is the joint or site of articulation between the horse’s hind leg and spine and serves as a major point of weight and force transfer between the hind leg and the vertebral column as the leg takes weight during the stance phase of the stride. The joint is made up of two major bones: the ilium (part of the pelvis) and the sacrum (part of the spinal column). The sacroiliac joints sit on the either side of the spine but as part of their weight-transfer role there is minimal actual movement at these joints during exercise in adult horses. The joint surfaces themselves are almost flat and rely on three major sets of ligaments for support (dorsal sacroiliac ligaments, ventral sacroiliac ligaments and the interosseus sacroiliac ligaments). The muscles surrounding the sacroiliac joint have also been shown to be important in the function of the sacroiliac joint and back.
Injuries to the sacroiliac joint region fall into two main categories: primary sacroiliac injury where pain is caused by a direct trauma to the area e.g. a fall that causes ligament injury or sprain or a fracture of the associated bone and secondary sacroiliac injury where the horse develops sacroiliac joint pain and disease secondary to lameness in another part of the hind leg.
Common signs of sacroiliac joint pain include shortened hind leg stride, a “bunny-hopping” gait or the horse frequently becoming disunited at canter, being reluctant to strike off or refusing to strike off with the correct canter lead leg when ridden. Riders frequently report that the horse feels flat or lacks impulsion from behind and the loss of hind limb propulsion often feels worse to the rider than it may look from the ground. Pain from the sacroiliac joint may manifest as true lameness but is more commonly associated with lower-grade loss of performance.
Injury to the sacroiliac joint region is frequently associated with muscle loss or muscle wastage over the rump and the increased prominence of the tuber sacrale which are also known as jumper’s or hunter’s bump, where long-term muscle loss makes these bony prominences more obvious.
The diagnosis of sacroiliac joint disease is often somewhat complicated. Exclusion of other causes of lameness or bunny-hopping in the lower part of the leg is important and usually involves nerve blocks. Blocking the sacroiliac joint region is possible but the proximity of the large sciatic nerve, which is responsible for maintaining weight-bearing in the hind leg, close to the back edge of the joint does mean there is a small risk of the sacroiliac joint block temporarily affecting this nerve which results in the horse being unable to bear weight on the leg until the block wears off.
Ultrasound examination of the joint (from the outside or dorsal surface and from the inside/ventral aspect by rectal examination) is useful in looking at the supporting ligaments and the joint contour at the bottom edge of the joint. The curved nature of the central portion of the joint does preclude imaging the entire joint by ultrasound however. In view of its location and the heavy surrounding musculature, x-rays of the joint cannot be obtained. However nuclear scintigraphy (“bone scan”) can provide invaluable information about bone remodelling and inflammation in the sacroiliac joint region.
Figure 2: Bone scan images of the pelvis showing the normal appearance of the sacroiliac joint region of the pelvis. Mild increased radiopharmaceutical uptake (IRU) or “hot spot” is noted on the right side of the sacrum (red arrow) – injury to the ligament attachments in this area was noted on ultrasound examination.
Figure 3: Ultrasound images of the dorsal sacroiliac ligaments showing loss of fibre pattern and general enlargement of the ligament on the right side (white arrow) compared to the normal left side
Treatment and management of sacroiliac joint disease falls into a few different categories. In cases of secondary sacroiliac joint pain, treatment of the underlying lameness in the lower limb e.g. proximal suspensory ligament pain, is key in managing the source of the sacroiliac joint problems. Where recent ligament damage and instability of the sacroiliac joint is evident, a period of rest in the order of 30-60 days may be indicated. However in the long-term developing and restoring muscular support to the area is often key in managing pain and instability in this region. This means working and developing the major muscles of the back. abdomen and pelvis like the multifidus and gluteal muscles. This can include exercises incorporating lunging aids like the Pessoa or Equi-Ami or stretch bands, to encourage development of muscles whilst maintaining a normal outline. Water treadmill exercise, use of trotting poles and leg weights or tactile devices on the pastern encourage the horse to pick up the hind feet in a more normal foot arc and therefore develop the appropriate muscles of the upper hind limb and pelvis. These exercises can be integrated with physiotherapy stretches, once pain from the region has resolved, to encourage symmetrical muscle development and to work the deep muscles of the back e.g. multifidus which are directly linked to the sacroiliac joint capsule. Steroid medication around the sacroiliac joint is also helpful in providing pain relief, particularly in horses that are struggling to track up normally and engage the hind quarters and therefore cannot work in a sufficiently normal outline to allow normal muscle development. However medication generally provides relatively short-term relief and must be followed up with strengthening exercises in order to optimise long-term benefits.