Hello everybody, Whinny here! This topic comes to you by popular request. I will be discussing an essential anatomical and functional component of the equine skeletal system – the sacroiliac joint. We’ll explore the structure, function, and clinical implications of these important joints.

First, a definition: the sacroiliac joints (SI joints) are the articulations between the sacrum and the ilium. But what are those? The equine sacrum and ilium are two important components of the horse’s skeletal system, specifically the pelvis. The sacrum is a large, triangular bone located at the base of the spine in horses and other mammals. It is formed by the fusion of several vertebrae, typically five in horses. The sacrum serves as a vital connection between the vertebral column and the pelvic girdle, providing structural support and facilitating the transfer of forces during movement.

The Sacrum

In horses, the sacrum is positioned posteriorly (towards the rear) and forms the back part of the pelvis. Its shape is uniquely adapted to withstand the forces generated during locomotion. The sacrum consists of a body and a broad, wing-like portion on each side known as the alae. The dorsal (upper) surface of the sacrum is slightly concave, while the ventral (lower) surface is mostly flat.

The sacrum plays a crucial role in distributing weight and forces from the spine and hind limbs to the pelvis and vice versa. It provides stability and support for the horse’s body, particularly during activities that involve weight-bearing, such as running, jumping, carrying a rider, or trying desperately to avoid stepping on a mouse who happens to be in the area.

The Ilium

The ilium is one of the three bones that make up the pelvis, alongside the ischium and pubis. In horses, as well as in humans, mice, and many other mammals, the ilium is the largest and most prominent of the three pelvic bones. It is located on the lateral (outer) side of the pelvis, forming the upper part of the hip bone.

The equine ilium has a distinctive shape, characterized by a broad, flat wing-like structure known as the iliac blade. The ilium extends dorsally and cranially (towards the back and front, respectively) from the sacrum, curving downward to meet the ischium and pubis bones at the acetabulum, the socket where the femur (thigh bone) articulates.

The ilium serves as an attachment site for various muscles involved in locomotion, such as those responsible for hip extension and lateral movement. It provides a strong foundation for muscle attachment, allowing efficient force generation and control during movement.

How It All Works

Together, the equine sacrum and ilium form an integral part of the horse’s pelvis, contributing to overall stability, load-bearing capacity, and locomotor function. Their anatomical characteristics and proper alignment are crucial for the horse’s performance and soundness.

The sacroiliac joint is relatively immobile in adult horses. This immobility and stability is reliant on muscular support. Increased mobility causes pain from instability and subsequent osteoarthritis as the bones move against one another. This instability can result in damage to the sacroiliac ligaments which support the joint.

Resistance to engaging the SI joint causes atrophy of the epaxial musculature and low back pain. Signs of sacroiliac pain can be insidious and hard to identify. Some examples would include back soreness, shifting hind limb lameness, or just poor performance. In certain cases, horses may drag their toes—gotta watch out for those when I’m frolicking in the field with them! Keep in mind that these symptoms can mean a wide variety of things, so don’t assume it’s the SI joint without a good diagnosis by one of my docs.

Diagnosing the Problem

Diagnosis often involves imaging and watching the horse go in a variety of ways. The vet may even want to see video of your horse doing what they do best—so jumping or running barrels, especially if the lameness or discomfort is more subtle and not apparent on a simple straight jog. It’s mostly impossible to radiograph the bones of the horse’s lower back because of their size, so ultrasound is one of the most commonly used tools to visualize the joints and bones.

Treatment

Treatment for SI joint pain is often multimodal. Just like when you humans injure yourself, keeping recovery simple is often the best step. My docs will often recommend things like chiropractic, acupuncture, FES, and at-home exercises.

One of the best exercises for your horse’s back involves using cavaletti poles which you probably already have at home. You only need 2-4 but can use more if you’d like. All you have to do is elevate one side of each pole by a small amount, just enough space for me to limbo under. Then, simple as can be, you walk your horse over the poles, making sure they stay in the center so that one side of their body has to lift over the elevated pole and the other side stays mostly flat. This gives them a nice oblique workout and helps strengthen their core, which leads to a healthier back.

Dr. Yorke and Dr. Carter just did a seminar on acupuncture, which you can watch on my YouTube Channel, and my mentor Tony has written several blogs on that and chiropractic. FES, or functional electrical stimulation, is another great tool in our arsenal for back pain. Tony has a full blog on that HERE, but the gist is that this treatment does a fantastic job of helping decrease muscle spasms and relieve pain. It creates equal amounts of contraction and relaxation of the muscles which helps balance things out. A common sequela of sacroiliac pain is lumbar muscle spasm, pain, and atrophy. FES helps a ton with it!

Of course, as with many joint issues in horses, an option that we sometimes have to go to is joint injection. The sacroiliac joint is one that responds well to steroids to decrease inflammation, so that is often our first go-to (whereas in a lot of other joints we have moved away from steroids as the first approach). This injection is ultrasound guided, and most often is done at the clinic so our doctors can use our stock system to contain the horse. Because there is so much muscle covering the bones of the pelvis, the doctors use deeply penetrating ultrasound beams to visualize the space they need to get the medication. As with any joint injection or use of steroids, there are risks involved, so they’ll often focus on strengthening exercises and other modalities before jumping into injections.

Whinny Wisdom: Every time steroids are injected into a joint, the total lifespan of that joint gets shorter.

The next time our doctors are out for your horse’s wellness exam, talk to them about exercises you can do to strengthen and stretch your horse’s lower back! Prevention is always better than treatment, just like mice are always better than cats 😉

I’ll get BACK with ya later!

~Whinny

P.S. Word around the Clinic has it that Justin just finished writing Adventures of the Horse Doctor’s Husband 3, and it will be released on September 15th! Keep an eye on the Books Page of my website for links to purchase in the coming weeks, or get your signed copy at our annual Open House, which is coming up on September 30th!