The Sacroiliac Joint in the Horse: The Key to Movement

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The Sacroiliac Joint in the Horse: The Key to Movement

Knowing early signs of problems in the sacroiliac joint can help with a diagnosis and set a pathway back to health for your horse.

The sacroiliac joint is one of the most important parts of the horse’s body—and perhaps one of the least understood. This joint, and its related structures, is the connection between the horse’s pelvis and the spine. It functions to allow the transfer of energy from the horse’s powerful hind legs to the pelvis and then to the spinal column. This results in the horse’s all-important forward motion.

All the motions that are required by performance horses need a functioning SI joint. These include running at speed, quick stops and tight turns, jumping, lead changes and gymnastic movements. ©Amy K. Dragoo

However, the SI joint is different from other joints, such as the fetlocks or the hocks, in that it has relatively little actual motion itself, has no specific joint capsule, has very little associated joint fluid and is composed of two different types of cartilage. Stability of the SI joint is achieved by a complex ligamentous structure and associated muscles.

Unfortunately, this important and intricate joint is very difficult to evaluate. Due to its location deep in the pelvis and its protection by thick muscles and fat, the equine SI joint is not possible to see or easily palpate (physically touch or manipulate). The size and density of that part of the horse’s body make it hard to obtain conventional radiographs, and ultrasound imaging can also be challenging. The clinical complaints and signs of potential SI problems can also be caused by any number of other issues and conditions in the horse. Consequently, identifying and accurately diagnosing equine SI problems is a significant challenge for horse owners and their veterinarians.

But understanding how important the SI joint is to the horse’s basic motion and knowing the early signs of problems are the first steps to getting a diagnosis and establishing a pathway back to health.

Signs of Potential SI Issues

All the motions that we ask performance horses to do require a functioning SI joint. These include running at speed, quick stops and tight turns, jumping, lead changes and gymnastic movements. Problems or issues with the SI joint can result in any number of vague motion-related problems. Often riders report feeling or sensing problems that are not visible as a true “lameness” when the horse’s motion is observed. These can include:

A recent and progressive unwillingness to work with a loss of normal forward motion or impulsion.
Resistance to collection or rounding through the back.
Tightness and stiffness, especially in transitions.

Additionally, affected horses may begin to have trouble with lead changes, refuse jumps or even begin to buck and kick out.

Other observations include:

A mild dragging of one or both hind limbs.
Asymmetry of the hind end or uneven pelvic motion and muscle development in more long-standing cases.
Abnormal tail carriage or unequal tracking of the hind legs especially on a circle.
Alterations of a fluid, rhythmic canter.
A “bunny-hopping” motion of the hind legs.

It also is always advisable to ask the farrier whether there have been any subtle changes in the horse’s behavior while being shod. Occasionally, very early SI changes are noted by the farrier as the horse may not move over easily when cross-tied and asked to step laterally. Or the horse may have more trouble balancing on a hind leg when the opposite hind is lifted and manipulated by the farrier.

A 2010 review of 74 horses with diagnosed SI pain showed that dressage and show-jumping horses appeared to be at greater risk for SI problems than horses used in other disciplines. Older horses, larger horses and heavier horses were also more highly represented, suggesting that “wear and tear” due to long-term use and the demands on the body from a large, heavy frame resulted in more SI problems. ©Amy K. Dragoo

Since SI issues can be subtle and hard to definitively diagnose, all these seemingly small complaints should be treated with importance. If any of these signs are noted, then a diagnosis of a potential SI problem is justified. But because these signs can also be caused by a number of other conditions, it is important that a complete clinical examination and history be performed on affected horses.

Diagnosing SI Issues

One category of SI problems can be considered acute. Acute or very recent development of SI-related clinical signs is likely to be trauma-related. Slipping on wet or uneven footing, falling while engaged in “pasture play” or in competition, kicking out or other forceful movements can all cause damage to the ligaments and muscles supporting the SI joint. If the trauma is severe enough, then the joint surface itself can be affected. A thorough history, paying attention to the condition’s timeline, will often reveal a likely underlying traumatic event.

Another category of SI problems involves a chronic, more subtle, slowly progressing version. Sue Dyson, DVM, of the Center of Equine Studies, Animal Health Trust in New Market, U.K., published a review in the Equine Veterinary Journal in 2010 of 74 horses with diagnosed SI pain. Her finding showed that dressage and show-jumping horses appeared to be at greater risk for SI problems than horses used in other disciplines. Older horses, larger horses and heavier horses were also more highly represented, suggesting that “wear-and-tear”inflammation and stress on the SI joint due to long-term use and the demands on the body from a large, heavy frame resulted in more problems. Not surprisingly, warmbloods made up a higher proportion (51%) of affected horses compared to other breeds. Another important aspect of Dyson’s study was the finding that SI joint pain was seen alone in only 47% of the horses in her study group. Many horses had other problems or issues in other locations that were believed to be contributing to their SI problems.

Weakness, unevenness or pain in other joints, muscles, ligaments or tendons will affect how the horse moves over time. Because of its location and purpose, the SI joint will be put under increased stress when the horse attempts to compensate for issues like these.

Randy Eggleston, DVM, and clinical professor and surgeon in the Department of Large Animal Medicine, Surgery and Lameness Service of the University of Georgia College of Veterinary Medicine, agrees that many SI problems stem from other issues in the horse. “While the sacroiliac joint has begun to receive much more attention recently and has almost become a trendy ‘popular’ diagnosis, the majority of suspected SI cases seen in our hospital have problems in other areas,” Eggleston said.

SI issues can be subtle and hard to definitively diagnose. Signs can also be caused by a number of other conditions, so it’s important that a complete clinical examination and history be performed on horses suspected of having SI pain. ©Amy K. Dragoo

Careful clinical evaluation of all these potential problems must be done first, according to Eggleston, and attention to and resolution of these issues often results in the elimination of SI pain. (See sidebar, “Underlying Conditions Contributing to Sacroiliac Issues,” below.)

Diagnosing Primary SI Issues

If no other physical problems are found on clinical examination, then a potential diagnosis of a primary SI condition should be considered and other specific diagnostic tests need to be done.

Ultrasound examination of the SI joint is the most commonly used diagnostic procedure and can show evidence of degenerative disease (arthritis) of the joint or can indicate scarring and damage of the supporting soft-tissue structures. Large, heavily muscled horses can be difficult to scan due to the thickness of this area.

Transrectal ultrasound, where the ultrasound probe is inserted rectally, just as when a mare is scanned for pregnancy, may provide an additional “look” at the ventral (bottom) surface of the SI joint and can occasionally provide additional information.

Radiography of the horse’s pelvis and SI is especially problematic due to both the size and thickness of that area. Conventional X-rays of the pelvis must be taken with the horse anesthetized and lying on his back. This positioning can sometimes allow a film to be taken, which may provide some useful information, but it is not always productive due to the horse’s challenging pelvic anatomy. Many veterinarians are sometimes reluctant to try radiography on these cases because of the stress of anesthesia induction and recovery. This stress can be especially difficult on horses that may already have pelvic instability and problems with their SI joints.

Thermography, or infrared imaging, can be useful in identifying areas of inflammation or overuse in the SI region. Scanning the horse, then working the horse and rescanning, is especially helpful. This kinetic thermography can show a “before-and-after” change in blood flow and heat to a specific SI area and help confirm a diagnosis of SI injury. Dynamic diagnostic capabilities, or the modalities that can show the current physiological state of structures, are very important in confirming SI injuries because, as Eggleston explained, “We may see changes to the SI joint or surrounding structures with ultrasound and even radiography, but that doesn’t prove that those changes are significant in that particular horse or that they are even contributing to the horse’s pain and lameness.” Many horses can show old, non-active injuries or other chronic changes that are not related to that horse’s present condition.

1. This thermography scan shows a hind view of a horse with SI pain. The horse’s head is pointed away from the thermographer. The thermography scan scale is on the far right and shows a gradient of color from black (cold) to white (hot). As the colors become closer to the top of the scale, the temperature is greatest. Temperature is a product of blood flow, muscle activity, inflammation and so forth. In this scan, the tail head in the center of the image is hot (white). This is normal because of the heat from the rectum under the tail area. This horse also shows increased heat over the left SI and gluteal area. Courtesy Kenneth Marcella, DVM

2. This is a thermography scan of the SI area of the same horse in Photo 1 after exercise. The left gluteal area and SI area showed increased heat compared to the pre-exercise scan (Photo 1). The rest of the horse shows increased heat as well due to increased blood flow and muscle activity from exercise, but the SI and left gluteal area changes are more dramatic. This “kinetic” thermography (comparison of pre- and post-exercise scans) allows the clinician to have greater confidence that this horse’s problems are actively related to his sacroiliac joint area. Courtesy Kenneth Marcella, DVM

Nuclear scintigraphy is another dynamic modality that can provide evidence that an SI joint is indeed currently inflamed and irritated. A nuclear-labeled product is injected into the horse. This substance circulates throughout the horse’s body and it is picked up or concentrated only in areas where there has been damage and undergoing repair. A specialized scan of the body is done next, and the nuclear material that has been incorporated into these active areas shows up as “hot that horse’s present condition.

Research is also currently underway to develop the capabilities to produce a full-body CAT scan in the standing, sedated horse. This technology would drastically improve the way that SI problems are diagnosed and would allow clinicians to actually image the SI joint, with minimal risk to the horse, and to be able to evaluate its soft-tissue connections as well. Though not currently available, these units have been produced and are actively being tested and refined in clinical settings.

Treating SI Issues

Once a diagnosis of an SI problem has been made, there are a number of treatment options available for your horse. The SI joint can be injected with anti-inflammatory medication just like any other joint. This medication is intended to reduce the inflammation and pain in that area. Injections can be coupled with muscle-relaxing drugs and other systemic anti-inflammatory medications designed to allow the horse to move more easily and more correctly. While most horses are rested from hard work and competition while they recover from SI issues, controlled physiotherapy is still also very important. Horses are exercised on a very specific program designed to promote mobility of the supporting structures of the SI area and to help strengthen the muscles that stabilize the joint. Acupuncture, pulsed wave electromagnetic energy, therapeutic laser and ultrasound are all also used to help reduce tightness and spasm through the SI area during recovery.

Once a diagnosis of an SI problem has been made, there are a number of treatment options available for your horse. Acupuncture, therapeutic laser and ultrasound are used to help reduce tightness and spasm through the SI area during recovery. *©Amy K. Dragoo*

Research suggests that horses with primary SI injuries take longer to heal and often don’t return to full athletic function. However, horses with a strained SI joint caused by other problems can do very well once the sore joint, strained ligament, imbalanced hoof or inappropriate riding stress is resolved. Learning about the SI joint and how it influences the horse’s basic motion and knowing the early signs of problems are the important steps to getting a diagnosis and setting up a pathway back to a healthy SI joint.

Sidebar: Underlying Conditions Contributing to SI Issues

Because of its location and purpose, the SI joint will be put under increased stress when the horse attempts to compensate for other issues. A detailed history in cases of horses with suspected SI pain will help identify known risk factors. Additionally, a complete physical examination will help rule out joint inflammation, foot balance issues, back conditions and other possible contributing factors.

This is a lateral radiograph of a normal hind foot showing the normal pedal bone, or PIII, angle. PIII is the third phalanx and the main bone within the hoof. The green arrows show the angle that this bone should have. The tip is slightly lower than the heel. This provides a good balanced stance and the ability to drive off the toe during forward motion. *Courtesy Kenneth Marcella, DVM*

This is a lateral radiograph of a horse’s hind foot with a negative palmar angle. The tip of PIII is higher than the back of the bone. This is shown by the differing height of the red arrows. Rather than the two- to seven-degree normal angle, this configuration creates a negative angle. This horse is not balanced and has more weight on his heels. That posture translates into increased stress and strain on numerous structures, from the hind suspensory ligaments to the hamstrings to the SI and lower back. This condition is often seen in warmbloods, and those breeds have a higher incidence of SI issues as well. *Courtesy Kenneth Marcella, DVM*

Horses with suspected SI problems should be carefully evaluated for the following:

Hock arthritis can make horses reluctant to switch gaits and round through their backs and may inhibit forward motion.
High suspensory ligament desmitis (inflammation of the ligament fibers near the top of the cannon bone that can be from mild to severe), especially in the hind legs, can produce similar signs as hock arthritis.
Overriding dorsal spinous processes, or kissing spines, can make horses back sore, inhibit forward motion and may manifest as behavioral responses ranging from a subtle reluctance to work, all the way to flat out refusal to go forward under saddle and kicking out.
Negative palmar foot angle is relatively common in warmbloods. The pedal bone, or PIII, is the bone inside a horse’s hoof. It is generally thought that the bottom surface of this bone should be at two to seven degrees relative to the ground surface. This creates a slightly toe downward angle and provides the horse with optimal balance and push off the feet, especially the hind legs. Horses with flat to negative palmar foot angles have a PIII bone that is elevated at the cranial edge (toe) and lower at the caudal edge (heel). These horses are essentially standing and moving from a “rocked-back” position, which places additional stress on the hind leg (high suspensory ligaments, hamstrings and gluteal muscles) and on the SI joint area.

About Kenneth L. Marcella, DVM

*Kenneth Marcella, DVM | Courtesy Kenneth Marcella, DVM*

Kenneth L. Marcella, DVM, is a graduate of the New York State College of Veterinary Medicine at Cornell University. For more than 30 years, he has treated sporthorses of all disciplines and levels, including international competitors. Marcella has served as a veterinary official at many events around the world, including national championships, world championships and Olympic competitions. He is board-certified in thermal imaging and is currently a member of the board of directors of the American Academy of Thermology. With an undergraduate degree in English from Dartmouth College, Marcella has also written articles for numerous publications.

This article originally appeared in the Summer 2024 issue of Practical Horseman.