SI Joint Dysfunction

Low back pain is one of the most common health problems in the world, and for a substantial number of people the source is not the spine itself but a pair of joints at the base of it: the sacroiliac joints. Research using reliable confirmatory testing has found that somewhere between 15% and 30% of people with chronic low back pain actually have pain coming from the sacroiliac joint. Despite this, the joint is frequently missed, and many patients spend months or years being investigated and treated for other conditions before it is even considered. The impact on daily life is real and significant - studies have shown that the burden of sacroiliac joint pain on physical function and quality of life is comparable to that of hip or knee arthritis. This page explains, in plain language, what the sacroiliac joint is, how it causes pain, why it is so often overlooked, how we diagnose it properly, and what can be done about it.

Your spine sits on top of your pelvis, and the sacroiliac joints are where the two meet. At the base of the spine is a triangular bone called the sacrum. On either side of it are the large wing-shaped bones of the pelvis, and where each one meets the sacrum there is a joint - your left and right sacroiliac joint. Together they form the structural link between your upper body and your legs, and they act as the hub through which the weight of your body is transferred down into your hips and lower limbs every time you stand, walk, climb or lift.

These joints are unusual in two important ways. First, they move only a very small amount - in the region of two to four degrees of rotation and a couple of millimetres of glide. They are built for stability and load transfer rather than for movement. Second, there are no muscles that act on the sacroiliac joint on their own; its stability depends entirely on the surrounding ligaments and on the muscles of the lower back, abdomen, pelvis and hips working together. The joint, its capsule and its ligaments are also richly supplied with pain-sensing nerve fibres, which is why, when the joint becomes a source of trouble, it can be genuinely disabling.

To understand why the sacroiliac joint sometimes becomes painful, it helps to understand how it normally stays stable. Specialists describe this using two ideas that work together, which you can think of as the architecture of the joint and the engine that powers it.

The first is FORM CLOSURE - the architecture. The sacrum is shaped a little like a keystone in an archway: it is wider at the top than the bottom and wider at the front than the back, so it wedges firmly between the two pelvic bones. The joint surfaces themselves are not smooth but covered in matching ridges and grooves that interlock, gripping against one another to resist sliding. This shape and texture give the joint its basic, built-in stability.

The second is FORCE CLOSURE - the engine. Because a joint that relied on its shape alone would be almost impossible to move, the body actively compresses the joint using the tension of ligaments and the pull of muscles such as the gluteals, the deep back muscles and the abdominal wall. These act like guy ropes and braces, squeezing the joint surfaces together at the precise moment load is applied. A key part of this is a small forward tilt of the sacrum called NUTATION, which tightens the main ligaments, presses the pelvic bones together and locks the joint for weight-bearing - this is what happens automatically when you stand or rise from a chair. The opposite, gentler backward tilt, called COUNTERNUTATION, occurs in relaxed, unloaded positions such as lying down, and is a naturally less stable state.

Healthy function depends on getting this balance right: enough compression to keep the joint stable, but not so much that it becomes stiff and irritated. When this balance is disturbed - too little support, or asymmetrical strain - the joint can become a source of pain.

One of the clearest examples of this balance being disturbed is in and after pregnancy, and it explains why sacroiliac and pelvic pain are so common in women. During pregnancy, hormonal changes - principally the hormone relaxin - deliberately loosen the ligaments of the pelvis to allow the pelvic ring to open for childbirth. This is a normal and necessary adaptation, but the resulting increase in joint laxity and mobility places more strain on the ligaments and can produce pelvic and lower back pain.

In some women the loosening is uneven, with one side becoming more lax than the other. This asymmetry places uneven stress across the pelvis and is associated with a markedly higher risk of developing moderate to severe pregnancy-related pelvic girdle pain that can persist after the baby is born. A particular ligament at the back of the joint, the long dorsal ligament, is well placed to register this uneven strain, and women with pregnancy-related pain very often feel their familiar symptoms precisely over this area, just below the bony bump at the top of the buttock. For many of these patients, restoring stability - for example with a supportive pelvic belt that gently compresses the joint and reduces excessive movement - can meaningfully reduce pain, which is one reason belts are commonly recommended in this group.

Sacroiliac joint pain is frequently overlooked, and there are clear reasons for this. The joint produces pain that closely resembles pain from other parts of the spine, with no single symptom, movement or position that is unique to it. Pain is usually felt in the lower back and buttock, but it can also spread into the groin and down the leg - sometimes as far as the calf or foot - which means it is commonly mistaken for sciatica, a disc problem or a trapped nerve.

Standard imaging adds to the difficulty. X-ray, CT and MRI scans usually do not show sacroiliac joint pain. Apart from specific inflammatory conditions, scans are mostly useful for ruling out other problems rather than confirming this one, so patients with genuine sacroiliac pain are often told their scans are essentially normal while the cause of their pain remains unexplained. For many years the joint was also simply not considered as a cause, partly because there were no reliable surgical treatments for it, so clinicians were not actively looking for it. The result is a familiar pattern of painkillers, physiotherapy, spinal injections and investigations for disc or nerve problems, with only brief or partial relief, before the joint is finally considered. The encouraging news is that this has changed: there are now well-validated ways of examining the joint, a reliable confirmatory test, and treatments - including a minimally invasive operation - supported by high-quality research.

Although there is no single signature symptom, certain patterns are common and can be a useful clue. In patients whose sacroiliac pain was confirmed by injection, the buttock was by far the most common site of pain, affecting around 94%, followed by the lower back in roughly 72%. Importantly, the pain does not stay confined to the back and buttock. Groin pain was reported by about 14% of patients and has been specifically linked to the sacroiliac joint. Pain travelling into the leg is also common, reaching the thigh in nearly half of patients and, in a meaningful proportion, extending below the knee into the calf and even the foot. Interestingly, younger patients appear more likely to feel pain travelling further down the leg, which is one reason sacroiliac pain in younger people is so easily mistaken for a nerve problem.

Patients also frequently describe pain that is worse when sitting, standing for long periods, rising from a chair, climbing or descending stairs, walking, or turning over in bed. Many can point quite precisely to the painful spot, which is itself a recognised diagnostic clue described below.

If you have already had spinal surgery and your pain has continued, returned, or changed, the sacroiliac joint deserves particular attention. When the lower spine is fused, the joints next to it are forced to absorb more load, and the sacroiliac joints sit directly in that line of stress. Radiographic signs of sacroiliac degeneration have been found in up to three-quarters of patients in the years following lumbar fusion, and in patients with persistent pain after lumbar fusion the sacroiliac joint has been identified as the actual source of pain in around 43% of cases. In other words, a significant share of so-called "failed back surgery" pain is not coming from where the original surgery was performed at all - it is coming from the sacroiliac joint, which had not been identified or treated. For these patients, recognising the true source of pain can open up an effective treatment that earlier surgery was never going to address.

Because no single test is perfect on its own, a confident diagnosis comes from combining several lines of evidence - your history, the pattern of your pain, a focused physical examination, appropriate imaging to exclude other causes, and finally a confirmatory injection. This step-by-step approach is exactly what the best research recommends.

A simple and useful starting point is the FORTIN FINGER TEST. During the examination you are asked to point, with a single finger, to the exact spot where your pain is. The test is considered positive when you repeatedly identify the same area and that area lies within about a centimetre of the bony prominence at the top of the buttock, typically just below and toward the midline of it. A positive result is a strong early indicator that the joint may be involved, and notably this test was a required criterion for diagnosing sacroiliac pain before patients entered the major surgical trials.

The cornerstone of the physical examination is a group of PAIN PROVOCATION TESTS. Each one applies a controlled, firm stress across the joint in a slightly different direction, and we watch to see whether your familiar pain - the very pain that brought you in - is reproduced, as opposed to ordinary discomfort that any firm pressure might cause. The main tests are distraction, thigh thrust, compression, sacral thrust, Gaenslen's and the FABER (Patrick's) test. The research is clear that no single test is enough, but a cluster is powerful: when three or more of these tests reproduce your familiar pain, sacroiliac involvement becomes highly likely. Equally useful is the reverse finding - in patients with confirmed sacroiliac pain at least one test is almost always positive, so if every provocation test is negative the joint can usually be confidently ruled out, sparing you unnecessary treatment.

The final and most definitive step is a DIAGNOSTIC INJECTION, which is the accepted reference standard worldwide. Under X-ray or CT guidance, and using a small amount of contrast dye to confirm the needle is genuinely inside the joint, a local anaesthetic is injected. We measure your pain before and after, and a substantial reduction - generally in the order of 75% or more - confirms that this joint is the source of your pain. The principle is simple: if numbing the joint switches off the pain, the joint was causing the pain. Because a single block can occasionally give a false-positive result, the most rigorous approach uses two confirmatory injections before a firm diagnosis is made.

Treatment follows a sensible ladder, and almost everyone begins with non-surgical care. Conservative management centres on physiotherapy aimed specifically at improving the stability of the lower back and pelvis - strengthening the muscles that provide the joint's "force closure" - together with anti-inflammatory and other medication where appropriate, and a supportive pelvic belt, which is particularly helpful for women with pregnancy-related pain. For many people these measures provide enough relief to get on with life.

For a significant number, however, conservative care offers only modest or temporary improvement. This was demonstrated strikingly in the major clinical trials, where a large proportion of patients who had not benefited from six months of conservative care chose to cross over to surgery - and once they did, their pain and function improved just as much as the patients who had been treated surgically from the start.

Between simple conservative care and surgery sits a group of minimally invasive procedures that target the joint or the nerves that carry pain from it. These are often the next step when physiotherapy, medication and a support belt have not given enough relief, and they can be both diagnostic and therapeutic.

Steroid Injections into the Joint

The same image-guided injection used to confirm the diagnosis can also be used to treat the joint. After the local anaesthetic has confirmed the joint as the source of pain, a corticosteroid - a powerful anti-inflammatory medication - is added. The aim is to calm the inflammation within and around the joint and provide a longer period of relief than the anaesthetic alone. For some patients this relief is substantial and lasting; for others it is partial or temporary, and the injection may need to be repeated periodically. Steroid injections tend to be most useful where there is an inflammatory component to the pain, and they are generally low-risk when performed under X-ray or CT guidance. They are best thought of as a way of controlling symptoms and buying time rather than as a permanent cure, and the response to them also gives us useful information about how the joint is likely to behave with other treatments.

Radiofrequency Ablation (Lateral Branch Neurotomy)

Radiofrequency ablation, sometimes called rhizotomy or lateral branch neurotomy, takes a different approach. Rather than treating the joint itself, it targets the small sensory nerves - the lateral branches of the sacral nerve roots - that carry pain signals away from the back of the joint. Using a needle-like probe placed under X-ray guidance, a controlled amount of heat is applied to these nerves to interrupt the pain signals they transmit. Many patients gain meaningful relief, and because the procedure is minimally invasive it can be repeated if needed.

It is important, however, to understand what radiofrequency ablation does and does not achieve. The joint is not truly switched off; rather, the nerves are desensitised. Because nerves can regenerate over time, the pain commonly returns within around a year in many patients, at which point the procedure may be repeated. The evidence for radiofrequency treatment is also genuinely mixed - some studies show clear short-term benefit while others show little - so we are honest about the fact that results vary from person to person. It tends to be most appropriate for patients who get good but temporary relief from injections and who are not yet candidates for, or who wish to avoid or defer, fusion surgery.

Endoscopic Rhizolysis

Endoscopic rhizolysis is a more recent, minimally invasive refinement of the principle behind radiofrequency ablation. Instead of working blindly with a heated probe, the surgeon uses a thin endoscope - a small camera passed through a tiny incision - to see the sensory nerve branches supplying the back of the joint directly, before dividing or ablating them under direct vision. The intention is to treat the nerves more completely and precisely than a standard radiofrequency probe can, in the hope of achieving longer-lasting relief while still avoiding open surgery. Because the nerves are visualised rather than targeted indirectly, more of the relevant branches can potentially be addressed in a single procedure.

Endoscopic rhizolysis is performed through a very small incision, typically as a day-case or short-stay procedure, with a quick recovery. As with radiofrequency ablation, it aims to quieten the pain signals from the joint rather than to fuse or stabilise the joint itself, so it is most suited to patients whose pain is confirmed as sacroiliac and who have responded to diagnostic blocks but want a less invasive option than fusion, or who are not candidates for fusion. It is a newer technique and the body of long-term evidence behind it is still developing, so we will discuss with you honestly how it compares with the more established options and whether it is appropriate for your particular situation.

Choosing Between These Options

In practice, these treatments often form a logical sequence. A diagnostic block confirms the joint; a therapeutic steroid injection may follow; if relief is good but short-lived, radiofrequency ablation or endoscopic rhizolysis can be considered to target the nerves for longer-lasting effect; and if the joint itself remains the persistent source of disabling pain despite these measures, fusion becomes the option that addresses the underlying mechanical problem directly. Which of these is right for you depends on your diagnosis, how you have responded to previous treatments, your general health and your own preferences, all of which we will work through together.

For patients in whom the joint has been reliably confirmed as the source of pain, and where the treatments above have not given lasting relief, MINIMALLY INVASIVE SACROILIAC JOINT FUSION is the modern surgical option. The aim is to stabilise the painful joint and ultimately allow bone to grow across it, stopping the small, painful movements that drive the pain. The procedure is performed through a small incision in the side of the buttock area, under X-ray guidance, and the most thoroughly studied approach uses TRIANGULAR TITANIUM IMPLANTS placed across the joint, usually three per side. These implants are designed to provide immediate stability, while their specially textured porous surface encourages bone to grow onto and across them over the following months. The operation is short, blood loss is minimal, and most patients go home quickly. Where both joints are painful, we generally treat one side at a time, and it is common for the second side to improve once the first has healed, sometimes avoiding the need for a second operation altogether.

This contrasts with traditional open fusion, which has existed since the 1920s. Open surgery can relieve pain but involves larger incisions, more muscle damage, more bleeding and a longer recovery, and is today largely reserved for specific situations such as pelvic fractures after major trauma rather than for routine chronic sacroiliac pain.

We believe patients should be able to see the real evidence behind any operation they are offered, and minimally invasive sacroiliac fusion is in fact one of the better-studied procedures in spinal surgery. In high-quality randomised controlled trials - the most rigorous type of study - surgery was clearly superior to continued conservative care. At one year, low back pain improved by an average of around 42 points on a 100-point scale after surgery, compared with about 14 points with ongoing conservative treatment, and disability improved roughly three times more in the surgical group. By six months, around four in five surgical patients had achieved a clinically meaningful improvement in pain, compared with only about one in five of those continuing conservative care. Two separate randomised trials, one in Europe and one in the United States, reached almost identical conclusions, which gives real confidence that the benefit is genuine and reproducible.

The results also hold up over the long term. Five-year follow-up of patients treated with triangular titanium implants showed that the improvements in pain, disability and quality of life were durable, that satisfaction remained high with the great majority of patients saying they would choose the procedure again, and that opioid painkiller use fell substantially - from around 77% of patients before surgery to about 41% five years later. Independent analysis of CT scans confirmed that bone had bonded to the implants in around 98% of cases, with the joint itself fusing in around 88%. The need for further surgery was low, in the region of 3% or less, and the procedure's overall safety profile was reassuring, with serious device-related problems being uncommon.

While every patient and every surgeon's practice differs slightly, the published experience gives a realistic picture of what to expect. The operation itself is short, typically under an hour for one joint, and is carried out through a small incision under X-ray guidance with very little blood loss. Most patients stay in hospital only briefly, often going home the same day or within a day or two. To protect the joint while it begins to heal, you will usually be asked to limit how much weight you put through the operated side for the first few weeks before progressing to full walking as comfort allows. Many patients notice improvement early, often within the first month, and this tends to be maintained over the following months and years as the joint gradually fuses. As pain settles, many are able to reduce their reliance on painkillers over time. The specific guidance for your recovery, including activity and return to work, will be tailored to you, and follow-up appointments allow us to track your progress and the healing of the joint.

If you have struggled for a long time with lower back, buttock or leg pain that has never been fully explained - or if your pain has persisted after spinal surgery - the sacroiliac joint is a common, genuine and treatable possibility that deserves to be properly assessed. With a careful history, the Fortin finger test, a cluster of well-validated provocation tests, appropriate imaging to exclude other causes, and a confirmatory image-guided injection, we can usually give you a clear answer, often for the first time. Most patients are then helped by conservative treatment and minimally invasive procedures, and for those who are not, a well-studied operation offers a strong chance of meaningful, lasting relief. The single most important step is simply having the joint considered in the first place.

------------------------------------------------------------------------------- This information is provided for general education and does not replace individual medical advice. Please discuss your specific circumstances with your surgeon or healthcare professional before making any treatment decisions.

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• —NICE — Low Back Pain and Sciatica in Over 16s (NG59)
• —NHS — Spinal Conditions
• —British Association of Spine Surgeons (BASS)

This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.