Sacroiliac Joint Motion and Pelvic Deformation, AAEP 2008


An equine athlete uses the pelvis and sacroiliac (SI) joint in many different fashions: A rope horse uses the pelvis to rapidly accelerate and decelerate, a Standardbred races at high speed at a constrained gait, and a dressage horse exercises with repetitive and isometric control of his haunches. Furthermore, a vaulting horse is constrained to a circle, but he must provide stability of his back, a jumping horse requires large propulsive forces to use his rear legs to launch over a jump, and a Thoroughbred racehorse at a high-speed run experiences lots of forces through his pelvis, while a cutting horse needs agility and fine-tuned proprioceptive control and balance during rapid hind limb movements. At the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif., Kevin Haussler, DVM, DC, PhD, an assistant professor at Colorado State University, discussed the relationship between motions at the SI joint to deformation of the pelvis in the exercising horse.


Sacroiliac joint diagram

Problems that occur in the sacroiliac joint region include osteoarthritis, ilial wing stress fractures, tuber sacrale asymmetry, desmitis (ligament inflammation), and muscle strain.

The SI joint is a modified synovial articulation (a joint in which the bone ends are covered by articular cartilage), and the pelvis attaches the hind legs to the body, provides stability with weight bearing, is a location for insertion of muscles and ligaments, and transfers large propulsive forces for locomotion. Low mobility movements of the SI joint cause it to glide and shear and provide pelvic stability. Actions involving larger ranges of motion in the SI joint occur along with activation of other nearby joints (the lumbosacral, sacrocaudal, and coxofemoral joints) to generate locomotion.

Complicated motions of the SI joint include flexion, extension, lateral bending, and rotation. The pelvis is not a rigid structure as previously thought, and it deforms with all these SI movements. Haussler noted that sacropelvic pathology (deviation from normal that constitutes disease) is usually related to a mechanical cause. For example, asymmetry of the tuber sacrale (part of the pelvis just to either side of the spine), a condition known as a hunter's bump, is not localized only to the visible bump--there is also pelvic deformation and bone remodeling.

Fifty-five percent of horses with a chronic SI injury also have pelvic asymmetry associated with muscular imbalances, chronic lameness, compensatory gait forces, or other musculoskeletal injuries. Height asymmetry of the tuber sacrale is associated with SI joint luxation or subluxation (dislocation or partial dislocation) or a pelvic fracture. To determine which side of the tuber sacrale is normal, it helps to look at the second sacral spinous process (the top of the second vertebrae--the five vertebrae of the sacrum that are essentially fused together as one bone); the normal side will be in line with that.

The horse is stiffest in flexion and extension movements, a bit less stiff with lateral bending, and even less so with axial rotation. Most propulsive movements require stiffness to provide stability when both hind limbs are on the ground. Yet horses often don't have both hind feet on the ground at the same time; usually there is a leading or trailing limb or only one leg contacting the ground. The result is some asymmetry created by lateral bending or axial rotation, and it is these positions that elicit the most pelvic deformation and potential for injury.

Haussler notes that the principles discussed should help those who perform manual therapy achieve the greatest therapeutic effect--lateral bending or axial rotation techniques achieve the most motion around the SI joint or the pelvis.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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