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What is the SI and why is it so important it functions well and without pain?

The SI is involved in every stride and stance of the horse. Equine vet Lucinda Ticehurst explains why pain from this area can significantly impact a horse's performance and wellbeing.
What is the SI and why is it so important it functions well and without pain?

What is the SI Joint?

The SI joint – or sacroiliac joint - is involved in every stride and stance of the horse. We can almost think of it as a bus terminal or central station that every action has to travel through. It transfers the propulsion of movement and energy from the hindlimbs to the spine and the trunk of the body, aiding with stability, shock absorption and connectivity. The ‘SI’ is where the five fused vertebra of the sacrum run underneath the ilium of the pelvis. The bones are supported by ligaments.

Because of its huge importance, the SI can be a source of primary injury or the site of strain because of a problem elsewhere. Either way, pain in this area can have a significant impact on a horse’s performance and wellbeing. However, because of its location, associated and complex structures, and the sheer size and bulk of the muscle involved, diagnosing specific issues can be difficult.

Clinical signs of SI pain can include:

  • Poor performance: less impulsion, especially on deeper ground a feeling of ‘the engine not working like it used to’
  • Reluctance to work through from behind, poor engagement
  • Intermittent lameness (uncommonly overt lameness)
  • Stilted canter, wrong canter lead, disunited canter, reluctance to canter
  • Change in action when asked to work in an outline
  • Weakness or resistance in lateral work, often most noticeable to the rider.
  • Conflict behaviour such bucking or kicking out
  • Cranky demeanour, weight shifting, issues with the farrier
  • Wide-based gait behind, abnormal foot placement on circles

 These signs can be subtle and insidious in onset and progression.

What is the job of the vet in trying to get to the bottom of SI issues?

Firstly, it is crucial that a vet gathers information. This should include a full of history plus whether the horse has had any previous injuries or accidents. The vet will also want to look at the horse’s posture at rest and conduct a physical examination.

Sometimes horses with SI issues can have what is described as ‘downhill conformation’: put simply when the back end of the horse appears higher than the front. A vet will assess whether there is a loss of hindlimb muscle, low heels and long toes as there are compounding factors that cause and can be a consequence of SI pain.

Finally, a lameness/performance work up and flexion tests are usually performed to assess the animal’s range of movement.

What are the diagnostic options available?

These are varied, and not all will be needed or affordable to some clients. It is important to note that it might take several approaches to get an accurate picture as diagnostics are often summative. It is the vet’s job to discuss all options with the owner to make an informed clinical plan relevant and suitable for the individual animal.

  • Nerve block If this is the site of pain the clinical signs should temporarily resolve when local anaesthetic is used to numb the area.
  • Bone scan (nuclear scintigraphy) This looks at active skeletal remodelling and is not sensitive to minor ligament damage but gives a great deal of information about the whole body to help with localising primary and secondary issues. It is expensive and can be stressful to the horse.
  • Transrectal ultrasonography A good ultrasound scanner and experience are needed for this procedure, with ideally the horse in stocks accompanied with the administration of a smooth muscle relaxant. It allows the vet to see irregular joint margins and ligament damage (not all can be visualised though).
  • Treatment trial with medication of the area with long-acting corticosteroids This should be strictly based on a comprehensive physical exam and the clinical signs, with all information about the horse taken into account. A disadvantage of this approach can be that it could slow down ligament healing and mask discomfort. There is also a risk of side effects of the medication so it is not suitable for all cases but is a calculated and minimal risk in many.
  • Thermography This is a safe diagnostic that suits horses that are easily stressed. It is repeatable so vets can assess the physiological response to treatment. It is also useful in the detection of nerve dysfunction too.

What are the treatment options available?

There are many practical options to help our horses not just perform better but also feel better. It is important to treat not just the primary issue but any related issues. Options include treatment with long-acting corticosteroids but also non-medical interventions.

Physiotherapy, rehabilitation and fitness can play an important role. Strengthening hindquarter muscles provides great support to the SI. Never underestimate the significance of this, including non-ridden exercises such as lunging long and low but with the horse remaining active behind, plus pole work. Also include cross-training techniques and avoid repetition. In addition, it is important that your horse has turnout. A small paddock with good, even terrain and footing is ideal.

The SI can be a really tricky area to diagnose, treat and manage. However, when done successfully it can mean the difference between simply crossing big country or really travelling over it.


West Cork Flash was a good racehorse in his prime. He was then a great schoolmaster point-to-point horse, jumping me round my first 14 point-to-points, always performing well.

After retiring from racing at 13 years old for no physical reason, other than that he had done his time, he went to a friend who kept him in a lot more and so he lost a lot of muscle bulk with the change of discipline and reduction in intensity of exercise and training.

Flash started to bucked a few times, which he had never done before and he struggled to strike off cleanly into canter and maintain canter on a circle; previously he could do tempi changes! A lameness work up found no other areas of concern and his SI was medicated after a thermography scan that highlighted activity in that area.

He responded very well to treatment and now at the age of 18 years old will happily carry a side saddle all day and jump decent hedges.

About the Author

BSc BVMed Sci Hons BVM BVS MRCVS. Lucinda is an equine vet, born with a passion for horses and helping horse owners.

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