The sacro iliac joints are two large joints that join the sacrum to the pelvis. One on either side. To our ancestors who walked on all 4 limbs these joints were important as they slung and secured the spine to the pelvis. The equivalent for the front two limbs (or upper limb) is the scapula and the shoulder girdle. However, as we evolved to walk with an upright stance these joints essentially have no function. They, at best, have about 2 degrees of flexion and extension and arthritis in these joints is a very common cause of lower back pain.
These joints are true synovial joints however with cartilage surfaces just like hips and knees but unlike hips and knees which have very large and complex movements and are beautifully engineered to achieve maximum function, the sacro iliac joints (SIJ) are best envisaged as two cheese graters whose rough surfaces have been joined and ground together. They are rough, craggy, pitted and asymmetrical and so they eventually grind to a halt. For some people this is painful and for others it is not.
The diagnosis of SIJ pain is made clinically and there are a number of provocation manoeuvres that are used to reproduce the pain. Sometimes a nuclear medicine bone scan shows increased activity in these joints if they are inflamed but not always. The gold standard test is to have the joints injected with a long acting local anaesthetic and then try to have a relatively normal day. The test confirms the diagnosis if the pain practically disappears for 4-6 hours then rapidly returns when the anaesthetic has worn off.
I get referred dozens of patients who have had previous back surgery with still the same pain they started with. Usually they have had many second opinions, and the commonest thing I find is that they have SIJ pain which was not diagnosed to begin with.
The incidence of symptomatic SIJ degeneration in the general public has been estimated as between 15% (Cohen et al in Analgesia and Anaesthesia 2005) and almost 40% (Ha et al in Spine 2008).
The incidence of symptomatic SIJ degeneration following lumbar fusion is much higher, around the 70% mark. Almost all patients in this group however settle with some injections of cortisone (an anti-inflammatory steroid), which may need to be repeated a few times before they settle completely.
Patients whose SIJ become painful following lumbar surgery fall into three groups in terms of the cause of the SIJ pain. The first group are those that actually never had pain from their lumbar spine to begin with and have been mis-diagnosed - generally what we see is that the SIJ pain gets worse in this group. The second group are those that have had prior lumbar surgery at L5/S1 or L4/L5 and many years later get adjacent segment degeneration not cranially (higher up) but caudally (lower down). These patients have basically transferred some of the movement they had at their fusion level lower down to the SIJ and forced them to move more than the 2 degrees they are designed for. Then over time this wears out the cartilage and the joint become arthritic and unstable. The third group are those that have for years suffered with a flat-back deformity (loss of sagittal balance) and have been compensating for that by maximally using the 2 degrees of movement in the SIJ to rotate their pelvis so that they can stand up straight. If these patients have a corrective procedure - usually at L5/S1 - to restore lordosis then the SIJ suddenly get to relax. The current theory is that by the time the corrective procedure is performed the joint has already become unstable and so causes pain when back in a normal-ish state. These patients get their SIJ pain usually quite quickly following their surgery.
For those patients who do not settle with time and injections of cortisone their are two options. One is to have the joint stripped of it's nerve supply using radio-frequency probes so that no pain impulses are transmitted to the spinal cord. The other is to have the SIJ fused.
The CT scan to the right shows all of the hallmarks of degeneration in the sacroiliac joint with gas in the joint itself (the black streak running through the joint), spurs of bone on both sides (osteophytes) and dense white sclerosis on the sacral side. When you see gas inside a joint it must be grossly unstable.