The Sacroiliac Joint

Anatomy and Function

The sacroiliac (SI) joint is the point of articulation between the sacral vertebrae (bones forming the spine) and the inner surface of the pelvis (Fig 1).

This bony connection functions to transfer the propulsive forces generated by the horse’s hindlimbs to the spine. The horse has two specialised SI joints, one to the left and one to the right of the midline, both of which are relatively inaccessible due to their deep location. The SI joint is long and narrow; yet, each contains less than 1ml of joint fluid, and, unlike most synovial joints, contains two distinct types of cartilage.

Additionally, the strong sacroiliac and sacrosciatic ligaments further support each SI joint.

The SI joint is designed to undergo gliding movements and as such is subjected to large shearing forces rather than direct compressive forces as experienced by other synovial joints.

Post-mortem studies have demonstrated abnormalities of the SI joint in up to 100% of horses that have been euthanised for unrelated purposes. However, despite this high incidence of abnormality, only about 15% of performance horses experience an aberration in their degree of athleticism due to sacroiliac dysfunction. The main changes recorded are those attributable to osteoarthritis.

These include cartilage damage and the formation of new bone around the edges of the SI joint, similar to those seen in other joints such as the fetlocks. Other findings noted were damage to the supporting ligaments and complete or impending fracture of the pelvis or sacral vertebrae.

Ultimately, these changes result in inflammation and therefore pain which is manifested as a decrease in the horse’s performance.

Sacroiliac Dysfunction

Sacroiliac dysfunction may be classified as either acute or chronic. Horses with acute SI dysfunction display sudden lameness of either one or both hind limbs. This is usually attributable to slipping, falling or some other form of trauma in the recent past. A moderate to severe pain response can be easily elicited on manipulation of the bony or soft tissue pelvic structures.

Chronic SI dysfunction, on the other hand, tends to have a more insidious onset and signs may range from mild to severe. This is by far and away the most common presentation of SI dysfunction. In many circumstances unresolved acute SI dysfunction may become chronic, and as such chronic SI dysfunction is simply a continuum of the acute presentation. Performance horses, in particular those competing at a high level such as show jumpers, dressage horses and thoroughbred racehorses are the most frequently affected. Trainers/riders commonly report an overall decrease in the horse’s level of performance. More specifically some complain of:

Diagnosis

Historically, definitive diagnosis of SI dysfunction has been difficult. This has improved in recent years due to a number of cadaver studies combined with the increasing availability of advanced diagnostic imaging techniques and more importantly, improvements in veterinarian expertise.

Diagnosis is based on obtaining a consistent history from the trainer/rider, coupled with a detailed physical examination of the horse. Specific manipulations, which stress the SI joints, may elicit variable degrees of discomfort and resentment by the horse.

Frequently some degree of pelvic asymmetry is observed which may be due to muscle wastage on the more affected side or swelling of damaged sacroiliac ligaments. Muscle wastage results from improper use of the affected limb (Fig 2); however, this may be due to other causes of lameness in the lower portion of the limb, which should first be excluded.

Commonly, horses with SI dysfunction also have pain in their hock joints. Moreover, many horses with SI pain also have evidence of co-existing back pain further forward along the spine.

In a large number of cases, this develops secondary to the abnormal gait which results from SI or hock pain and may also need to be addressed.

Further diagnostics such as the use of ultrasound or scintigraphy may be particularly useful in identifying SI abnormalities. Unfortunately, radiography is of limited or no use due to the sheer size of the horse’s hind end. Likewise, injecting local anesthesia into the joint is also rarely performed, as inadvertent injection of the surrounding nerves may cause the horse to acutely collapse.

Treatment and Outcome

Acute cases of SI dysfunction are treated with painkillers, anti-inflammatories and strict box rest for four-six weeks after which time horses are re-evaluated. Chronic cases typically do not respond well to extended periods of rest as this may result in progression of any pre-existing pelvic muscle wastage. Short courses of systemic anti-inflammatories may be warranted following diagnosis, however therapeutic shoeing and modifications of the training regimen have proven to be more beneficial in the long-term.

Swimming may be particularly beneficial in maintaining and building muscle mass without straining the injured structures. If these changes are unsuccessful at restoring the horse to its previous level of performance, the SI joints may need to be injected with anti-inflammatories and pain-relieving drugs.

This is an advanced technical procedure that involves passing a spinal needle up to 25 cms in length into the SI joint either blindly or under ultrasound guidance. Many horses will require this procedure once or twice a year.

Unfortunately, not all horses competing at a high level that develop SI dysfunction are able to return to their previous level of performance, even after appropriate treatment. This being said many are able to return to an acceptable, albeit lower, level of competition.

Turlough McNally MVB Dip ACVS Dip ECVS MRCVS is a member of the Veterinary Ireland Equine Group and is a specialist in equine surgery at Anglesey Lodge Equine Hospital, The Curragh, Co Kildare.

Email: hq@vetireland.ie

Telephone: 01-4577976