SI joint fusion stiffens the joint between the sacrum and the ilium — the sacroiliac joint, SI joint for short. Through a short skin incision at the side of the buttock, screws or triangular titanium implants are introduced through the ilium across the joint into the sacrum. The aim is to end the residual movement that remains, so that the joint fuses as bone and drops out as a source of pain. The procedure is rare and tied to a strict indication: it only comes into consideration once SI joint syndrome has genuinely been established as the cause and treatment without an operation has not been enough over months. We perform this procedure.
In this operation it is the diagnosis that decides the outcome, not the technique. No image proves that the pain comes from the sacroiliac joint. Only several examination manoeuvres that agree with one another, together with a targeted anaesthetic block of the joint that gives clear relief, make the diagnosis robust — and only then is a fusion a subject at all.
Who is SI joint fusion suitable for?
For very few people with low back pain. The sacroiliac joint is the source in only a proportion of those affected, and the great majority manage without an operation. We therefore require several conditions to be met at the same time — not one of them, but all.
- The symptoms fit — one-sided pain low in the back, below the belt line, radiating into the buttock, the groin or the back of the thigh, worse when getting up from sitting, when climbing stairs and when standing on one leg.
- Several provocation tests are positive — examination manoeuvres that put the joint under tension in a targeted way. A single test says little; only when several of them trigger the familiar pain does the accuracy rise markedly.
- A targeted infiltration has clearly relieved the pain — under X-ray or CT guidance a local anaesthetic is injected into the joint. If the pain clearly eases for as long as the anaesthetic acts, that speaks for the joint. Because this test too can deceive, it is repeated before an operation.
- Other causes have been ruled out — the disc, the spinal canal, the facet joints, the hip, a stress fracture of the sacrum in osteoporosis and an inflammatory disease that is treated by a rheumatologist.
- Treatment without an operation has been carried through consistently — over about six months, with training, with the correction of loading factors and, where appropriate, with infiltrations, without sufficient improvement.
- Everyday life is considerably restricted — work, sleep, walking, driving. A pain one lives well with is no reason for a fusion.
Another, rarer reason is a true instability of the joint after an injury to the pelvis — there it is not a matter of wear pain, but of load-bearing capacity.
Telling it apart from the lumbar spine
Low back pain from the sacroiliac joint and low back pain from the lower lumbar spine feel similar, and both can exist side by side. That is the most important reason why this operation can go wrong: if the pain in truth comes from a worn facet joint, from a narrowing of the spinal canal or from a slipped vertebra, fusing the pelvis changes nothing about it.
A pain that runs as a band down into the lower leg or the foot, with tingling, numbness or weakness, speaks for an irritated nerve root — that is, for sciatica; symptoms that increase on walking and recover on sitting speak for spinal stenosis. In SI joint syndrome the neurological examination is normal; more on this on the page back pain. We look particularly carefully where the lower lumbar spine has already been fused: after a spinal fusion, the sacroiliac joint takes over more movement and becomes painful more often. The few situations in which low back pain must not be waited out are on the page Warning signs.
What happens exactly?
The joint is not opened. The implants are pushed across through the bone, so that they bridge the joint and shut down the residual movement.
- PreparationExamination, review of all the images and of the reports on earlier injections, a conversation about the aim and the limits of the procedure. Added to that an anaesthetic consultation and blood values. Your list of medication is important, especially blood thinners; whether and when they are paused is settled beforehand.
- The procedureUnder general anaesthetic, lying on your front or on your side. Under X-ray guidance, where appropriate with navigation, a channel is created through a short skin incision at the side of the buttock through the ilium, across the joint space, into the sacrum. Two to three screws or triangular titanium implants are anchored in it. The position is checked in several X-ray planes, because the nerve roots run immediately beside the sacrum. The wound is then closed and the skin sutured.
- AfterwardsGetting up on the day of the operation or the day after. For a few weeks the operated leg is only partly loaded, usually with forearm crutches; after that comes the building up of load with physiotherapy. Fusing as bone takes months and is checked with X-ray or CT images.
Open procedures, in which the joint is exposed and packed with bone, are larger operations and today the exception. The mobility of the back hardly changes as a result of the fusion, because the joint is almost immobile in any case — unlike a fusion of vertebrae.
What the fusion can achieve — and what it cannot
Two randomised trials have compared the procedure with treatment without an operation. In the European trial, 103 participants at nine centres were assigned; after two years the operated group reported markedly less pain, less restriction in everyday life and a better quality of life. The proportion of those taking opioids fell in the operated group from 56 out of 100 to 33 out of 100. The American trial with 148 participants came to a similar result.
These results are to be read with caution, and we shall tell you why. Neither trial was blinded — those treated and those examining them knew who had been operated on, and precisely where pain is the outcome measure that has a strong effect. Both were funded by the manufacturer of the implants. In the European trial, 21 of those treated without an operation switched to the operation after six months after all, which makes the comparison more difficult. A trial against a sham procedure is lacking, and data over many years are scarce.
What the fusion does not achieve: it treats only the one joint, changes nothing about symptoms from the lumbar spine or the hip, and cannot be undone — a fused joint does not become mobile again. In a proportion of those operated on, symptoms remain even when everything has succeeded technically; usually because the joint was not the sole source after all.
Risks and side effects
As with every operation: infection, post-operative bleeding, thrombosis and the risks of the anaesthetic. Specific to the procedure: a malposition of the implants with irritation or injury of a nerve root beside the sacrum — with altered sensation, pain or weakness in the leg; a fracture of the ilium; a later loosening; a failure to fuse as bone; persistent pain despite correct positioning. A proportion of these problems lead to a second operation.
After the fusion, the neighbours are loaded more heavily: the opposite side, the hip and the lower lumbar spine. New symptoms there are a reason to come back and be seen. Because the work is carried out under X-ray guidance, a dose of radiation is part of it; it serves to protect the nerve roots. Which risks are to the fore in your situation is something we discuss before the procedure.
Before and after
Before the procedure: the decision is not taken in the first consultation. Bring all images on a storage medium, together with the reports on earlier injections and your own observation of how long and how strongly an anaesthetic block of the joint helped. Arrange who will support you at home in the first few weeks — crutches change everyday life more than one expects. Anyone who smokes should stop before a fusion: smoking makes fusing as bone more difficult.
After the procedure: walking with partial weight-bearing as instructed, no heavy lifting, no twisting under load. Showering is usually possible soon after the wound is closed; bathing and swimming wait until the wound has healed. Physiotherapy first builds up the buttock and trunk muscles, later stamina. How long you are unfit for work depends on your occupation and on the course of recovery. Drive only once you can sit freely and brake firmly if needed. Follow-up checks are part of it until the fusion can be judged — more on the page Aftercare and rehabilitation.
Alternatives
The most important alternative is to continue treatment without an operation consistently — often that is precisely the real difference. It includes targeted training of the buttock, abdominal and deep back muscles, stretching of shortened hip muscles, the correction of loading factors such as a difference in leg length or a disease of the hip, and in pregnancy and the period after birth a pelvic belt.
- Infiltration of the joint — an anti-inflammatory medication under image guidance can relieve the pain for weeks to months and bridge the time needed for building up muscle; a lasting effect is not established. It comes into consideration when the diagnostic anaesthetic block has made the joint likely.
- Denervation of the joint nerves — the small nerve branches at the back of the joint are switched off with a probe using heat. That can come into consideration when an anaesthetic block has clearly helped; the evidence is inconsistent and the effect limited in time.
- Pain-medicine treatment — where symptoms last for months, a concept made up of movement, pain medicine and psychological methods; more on the page Special pain therapy.
- Treatment of the actual cause — if the examination shows the lumbar spine to be the source after all, other routes apply, from an injection at the nerve root to a decompression.
Frequently asked questions
Will my back be stiff afterwards?
No. The joint allows only a few degrees of movement in any case; if it is shut down, the mobility of the back hardly changes. Stiffness in the first few weeks comes from taking things easy, not from the implants.
How do I know that the joint really is to blame?
With complete certainty, never. That is why, before an operation, we put several building blocks together: matching symptoms, provocation tests that agree with one another, a repeated image-guided anaesthetic block with clear relief, and the exclusion of other causes. If one of these is missing, we advise against it.
Do I have to use crutches?
As a rule yes, for the first few weeks, so that the implants can grow in before the full body weight rests on them. How long and with how much weight we determine individually and adjust at the follow-up check.
Do the screws have to be removed again?
No, they stay. They are removed or corrected only if they cause trouble — in the case of a malposition with nerve irritation, of a loosening, or if the fusion fails to occur. That is then a procedure of its own.
Can the other side cause symptoms later as well?
It is possible, because the opposite side and the hip take on more load after a fusion. Whether that turns into a problem requiring treatment cannot be predicted. Regular training of the buttock and trunk muscles is the best thing you can do about it.
Why do you advise against this operation so often?
Because the diagnosis is difficult and the procedure is final. If the pain comes from another source, even a technically flawless fusion does not help. We would rather say once too often that the preconditions are not met.
If a fusion of the sacroiliac joint has been recommended to you elsewhere, you can obtain a second opinion from us. We look at your images, examine you, go through the preconditions point by point and tell you frankly whether we share the recommendation — and what happens if you wait for the time being. What you should bring is on the page Your first appointment.