Written by Sue Dyson MA, Vet MB, PhD, DEO, FRCVS
The anatomy of the pelvic region in the horse is fairly complex, and because of the huge overlying muscles comprising the hindquarters, most of the bony elements cannot be seen or felt, and are inaccessible to routine X-ray techniques.
To understand what can go wrong, the anatomy of the area requires explanation. The three pelvic bones – the ilium, ischium and pubis – are all fused and form a ring on the left and right sides, joined in the middle by an immovable joint called a symphysis.
The tops of the left and right ilial wings are called the tubera sacrale [see box, right], and form the most prominent region of the hindquarters, the so-called jumper’s bump. The ilia are curved bones that, at their widest point, form the so-called hipbones. This is a misnomer, because the real hip joint is positioned much further back, and their correct name is the tubera coxae.
These bony prominences are susceptible to direct trauma if a horse falls over, or crashes against a stable door or gatepost, and this may result in a fracture, a so-called “knocked down hip”.
Part of each ilial wing forms a joint with the sacrum, which is part of the horse’s backbone. This is the sacroiliac joint. This joint is different to typical joints because there is negligible movement and the cartilage between the bones is unusual.
There is very little or no joint fluid. The joint is supported by huge, very strong ligaments – the ventral sacroiliac ligaments. These are quite separate from the dorsal sacroiliac ligaments, which, confusingly, have nothing to do with the sacroiliac joints. The dorsal sacroiliac ligaments are much smaller structures that run lengthways along the horse’s back from the jumper’s bump region backwards to the top parts of the sacrum.
In a fit, lean horse the jumper’s bump region may appear unusually prominent, but this is just normal anatomy not masked by excessive fat. It doesn’t mean that the horse has a problem.
Sometimes the tubera sacrale appear asymmetrical, and this can also be seen in clinically normal horses. Historically, it was attributed to misalignment of the sacroiliac joint, but there is very little evidence to suggest that this can happen.
So what can cause this apparent asymmetry? The most likely reason is difference in thickness of the overlying soft tissues, possibly the result of previous injury.
Alternatively, it could be due to a previous fracture of the ilium, so that one tuber sacrale slipped downwards. It can be a cause for concern when a horse is examined for purchase if the discrepancy in height is large.
Fractures of the ilial wing usually occur in young racehorses and are stress-related injuries. Repetitive overstress of the bone through exercise results in a series of microfractures, which may not affect performance. Ultimately, there may be a more complete large fracture, causing severe lameness and pain. The horse may suffer a lot of pain if pressure is applied over the jumper’s bump region, and one side may look lower than the other.
In some horses, a fracture can be confirmed using an ultrasound scan, looking through the hindquarter muscle mass to the upper surface of the ilium. If the fracture is incomplete or only affects the lower surface of the ilium, a bone scan may be required for definitive diagnosis. Although many of these fractures heal without difficulty, some are very close to the sacroiliac joint, and may partially destabilise it and cause long-term problems.
Diagnosis of injury to the ventral sacroiliac ligaments or the sacroiliac joints themselves is much more challenging. The hindquarter muscle mass completely surrounds the top parts of the sacroiliac joints, so it is impossible to feel the joints. An extremely limited part of the lower aspect of the joints can be felt through the rectal wall, but because of the normal irregular bony contours in this area, identifying any abnormality is extraordinarily difficult.
Unlike lower limb joints, which can be flexed to see if extreme motion causes pain, the sacroiliac joints cannot be directly manipulated. Increased stress can be placed on the joints by certain manoeuvres.
Holding one hind limb up fully flexed places an abnormal load on the opposite joint and may cause pain. Some horses with sacroiliac joint pain become difficult to shoe, because they object to standing on one hind limb for prolonged periods.
The problem with diagnosing sacroiliac pain is the inaccessibility of the joints. Joint pain is normally diagnosed either by identification of obvious pain and swelling, and intensification of lameness by flexion of the joint, or by injecting local anaesthetic solution into the joint and then seeing improvement in lameness. The joint can then be X-rayed to see if any bony abnormalities can be identified.
The deep location of the sacroiliac joint and its tight structure mean that we cannot feel the joint, nor can we inject local anaesthetic into it.
We cannot assess heat in the joint manually, or by using thermography, because this technique only measures surface temperatures. The joint cannot be X-rayed in a standing horse, and only limited views of the joint can be obtained if the horse is anaesthetised.
Bone scans can be helpful in identifying abnormal bone activity in the region, but interpretation is not easy because there is a lot of variability in normal horses, partly reflecting the shape of their pelvis, the thickness of the overlying muscle and also age effects.
Although local anaesthetic cannot be injected into the sacroiliac joints, it is possible, using very long needles, to infiltrate it around the joints. Improvement in action may reflect joint pain, but the local anaesthetic could also influence nearby structures, such as the ventral sacroiliac ligaments, or local nerve roots, or spread to affect the lumbar joints themselves.
It is therefore extremely difficult to be sure of a diagnosis of sacroiliac joint pain, although we know from post mortem studies that arthritis of these joints does occur. It remains a huge diagnostic challenge, aided by methodical elimination of the many possible causes of loss of hind limb action, and using a jigsaw of information gained from examinations.
As diagnosis is difficult, treatment recommendations are usually symptomatic, and include corticosteroids, chiropractic manipulation, acupuncture for pain relief, and modification of the work programme. Unfortunately, long-term follow-up suggests that the prognosis for horses with sacroiliac injury is poor for return to the previous level of activity. As such, it is frustrating for horse owners and vets alike.
A recent study by the author and Rachel Murray MRCVS of 74 cases of sacroiliac joint disease showed:
- Dressage and showjumping horses appeared to be at particular risk
- There was no correlation between conformation and the presence of sacroiliac joint pain
- In all horses, restricted hind limb impulsion was the predominant feature; this was most obvious when the horse was ridden. Stiffness, unwillingness to work on the bit and poor-quality canter were common
- 99% of the horses had abnormalities of the sacroiliac joint region identified using nuclear scintigraphy (bone scanning).