Looking at the Equine Lumbo-Sacral Joint

While I was looking for information on the horse’s back online I found that the number of sites discussing the sacro-iliac joint overwhelming outnumbers sites discussing the lumbo-sacral joint. In this photo, from Rod Nikkel’s website, you can see why. The lumbo-sacral joint is well-protected, nested deep within the triangle formed by the five solidly fused vertebrae of the sacrum and the wings or ilium of the pelvis.

This photo shows an unusual variation in the form of the lumbar vertebrae as well as illustrating the lumbo-sacral and sacro-iliac joints. There are facets between the transverse processes of two lumbar vertebra. These facets are only found in horses and rhinoceroses, the only two large grazing mammals that can accelerate from a standstill to speeds up to 55 MPH in a few strides. Speed is essential to both species survival, although they direct it differently. The rhinoceros uses their speed to charge at threats. The horse uses their speed to flee. Both species take the design of vertebrae that rotate and then lock together to most efficiently redirect the vertical forces of impact into forward momentum to the extreme.

The muscles that attach to the ilium and the processes of the lumbar vertebra have considerable leverage and leeway to rock-n-roll the lumbo-sacral joint. The wide flat inner surfaces of the ilium are tightly bound to the equally flat sides of the sacrum with thick fibrous connective tissue. The sacro-iliac joint is a vital part of the biomechanical structure redirects the vertical forces of impact into forward momentum at each stride.

You can also see the ischium or the horizontal bone pointing to the rear. The end of the ischial process forms the point of the buttock. The boney protuberances at the hip and buttock develop from the stresses of major muscle attachment. The slope of a line drawn from the point pf the hip to the greater tuberosity of the hip joint on the living horse does indicate the bio-mechanics of their pelvis.

Because the sacro-iliac joint is stabilized by the massive muscles of the horse’s haunches, arthritis can develop from overwork when the hind legs are working in opposition to each other, usually at the trot or pace . Endurance horses asked to trot for extended periods of time when fatigued and harness racers asked to perform at the limits of their abilities are the most likely to develop arthritis of the sacro-iliac joint.

The ‘hunter’s bump’ is named for an injury that occurs when one hind-leg slips, usually as a horse is taking off or landing over a jump, tearing the thick fibrous connective tissue. Given enough time (at least nine months) the injury can heal. The horse is usually sound enough to ride. But the two sacro-iliac joints may be uneven.

If you think of the sacrum as a flat plane like a table top, you can imagine how the angle of the pelvis supports it. Think of how a folding table collapses when the legs are left at an angle instead of being locked into their upright position.

If the stifle lies behind the sacro-iliac joint as it does in most modern horses, it demands more muscular effort to stabilize the hind leg. In the Square Horse, not only is the angle of the pelvis steeper, supporting the sacrum with less muscular effort, the stifle of supporting hind leg is directly under the sacro-iliac joint. The right angles at the hip and stifle flex like springs so there is much less stress on the sacro-iliac joint.

And a final note, I have appreciated Gillian HIggins’ anatomically painted horses for over a decade, but I have put off actually reading her books. To my very great disappointment, in How Your Horse Moves, she describes the lumbo-sacral joint as a ‘hinge joint’ which is just plain flat out wrong. All 50,000 vertebrate species on the planet have vertebrae that are composed of irregular shaped bones. The vertebrae protect the spinal cord from compression and kinks so not a single vertebrate has a hinge joint in their spine.

I looked up the definition of a ‘hinge joint’ and it turns out that the only joints in the human body that can be genuinely labeled ‘hinge joints’ are our fingers and toes. Hinge joints are composed of two and only two bones designed to open and close in one and only one direction. So appreciate Higgins images, but always double check the stories being told about them…

and remember that the stories we humans like to tell ourselves and what is actually happening may be very different things!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.