SIJ pain has numerous systemic and/or local etiologic factors; therefore, a thorough history of clinical symptoms and a general medical history are paramount in all patients with suspected SIJ dysfunction. SIJ-related pathologies include trauma, infection, or inflammatory diseases such as ankylosing spondylitis, Reiter syndrome, rheumatoid arthritis, and psoriatic arthritis. Secondary conditions causing SIJ pain include spinal fusion, scoliosis, and leg-length discrepancy.[9,10]
The pain diagram is often helpful. Fortin et al generated a pain referral map in asymptomatic volunteers after SIJ injection. A specified hypesthetic area, approximately 10 cm caudally and 3 cm laterally from the posterior superior iliac spine (PSIS), was diagnosed immediately after the injection, which corresponds to the maximal pain. A typical patient with isolated SIJ dysfunction may characteristically localize pain just distal and medial to the PSIS. However, the pain referral pattern from SIJ does not seem to be limited to the lumbar or buttock region. Slipman et al found 18 different patterns of SIJ pain referral in their population. The most common symptom was buttock pain (94%) followed by lower lumbar pain (72%), and about 14% reported of pain in the groin area. Approximately 50% of patients also had pain in the lower extremity; most common area was posterior and lateral thigh pain followed by leg pain distal to the knee (28%) and pain in the foot in 14%. Interestingly, they found younger patients to be more likely to report of pain distal to the knee. This variability can be attributed to several reasons including close anatomic location of the lumbar plexus supplemented with poor capsular strength around the SIJ to the complex nerve innervation of the SIJ, sclerotomal pain referral, or secondary irritation of the adjacent structures.
The patients' history and clinical findings are often helpful to rule out other pathologies that may mimic SIJ pain such as lumbar disc disease, tumor, hip joint pathology, myofascial pain syndrome, and gastrointestinal, genitourinary, or gynecologic pathologies. Inflammatory disorders that also involve the SIJ should be investigated with appropriate radiographs and laboratory tests in patients with doubtful history or clinical findings suggestive of spondyloarthropathy.
Physical examination of a patient with suspected SIJ dysfunction starts with an evaluation of gait pattern, leg-length inequality, and lower lumbar examination to rule out any obvious deformity secondarily causing SIJ symptoms. In addition, hip joint pathology should be ruled out with a range of motion examination and Trendelenburg test. The SIJ should be palpated, and the maximum area of tenderness determined. The Fortin finger test is considered positive when the maximum point of tenderness is consistently within 2 cm inferomedial to the PSIS with one finger. The reliability of this test was further confirmed by other investigators.[6,12,13] Straight leg raise test and neurologic examination should be performed to rule out spinal pathology or nerve root stretch pain.
Several provocative tests[14–16] are described for the diagnosis of SIJ pain, but data are lacking to support any single test as being highly sensitive. Some commonly performed provocative tests for SIJ pain, their sensitivity, specificity, reliability, and positive and negative predictive values are summarized in Table 1.
Dreyfuss et al studied 12 physical diagnostic tests and found none of them to be reliable in the diagnosis of SIJ pain. van der Wurff et al investigated a combination of multiple clinical tests in the diagnosis of SIJ pain. They found that when three or more provocation tests are positive, the probability is between 65% and 93% that the pain is related to the SIJ. With fewer than three positive tests, the probability is between 72% and 99% that the SIJ is not the source of pain. When three or more tests were positive, sensitivity was 85%, specificity 79%, negative predictive value 87%, and positive predictive value 77%. With only one positive test, the specificity was only 42% and the positive predictive value was 59%. Similarly, Young et al supported that three or more provocative tests are reliable for the diagnosis of SIJ pain. In addition, they found that with SIJ pathology, patients rarely presented with midline pain or pain above the L5 spinous process. Pain exaggeration while rising from sitting position was also highly correlated with SIJ origin in their study. They concluded that SIJ etiology should be suspected in a patient in whom three or more provocative tests (out of six) reproduce the patient's pain, which warrants further investigation to confirm the diagnosis of SIJ pain.[18–21] If all six tests are negative, SIJ is excluded from the differential diagnosis of back or leg pain.
Despite the above findings, studies entirely disputing the use of provocative tests in clinical practice exist. A systematic methodological review of the literature performed by van der Wurff concluded that no evidence exists showing the reliability of the clinical provocative tests. Other authors have come to the same conclusion.
Several reasons exist as to why provocative tests are not reliable in diagnosing SIJ pain. First, the range of motion of the SIJ is so limited that it may be difficult to clinically reproduce and elicit the pain from motion. The joint is anatomically confined within a bony pelvic ring, and the intra-articular ligamentous band and surrounding muscular and ligamentous structures add notable stability. Dreyfuss et al conducted clinical provocation tests in asymptomatic patients and found false-positive results in more than 20% of volunteers and attributed this to the relative hypomobility of a specific joint under test.
Second, tests that presumably load the SIJ will also load and stress the structures surrounding the joint, making it difficult to differentiate the origin of pain. The unique anatomy of the SIJ, with weak anterior capsule and strong posterior ligaments, makes it difficult to distinguish between intra-articular and extra-articular pathologies. Several extra-articular structures exist around the SIJ that can be pain generators, such as facet joint pain, iliolumbar syndrome, and superior cluneal nerve entrapment. Eskander et al found examination under fluoroscopy to be a more specific (80%) and with higher positive predictive value (93%) than any other clinical provocative test. This fluoroscopically guided test was performed in the prone position to localize the point of maximum tenderness to help rule out some of these obvious surrounding structures. However, it gets more dubious when the pain generator is the posterior SIJ ligaments rather than intra-articular pathology. Borowsky and Fagen directed the corticosteroid at the posterior interosseous ligaments and S1-3 lateral branches in addition to the SIJ and found 47% increase in patients' response to the anesthetic block. Murakami et al found similar results; the improvement rate after periarticular injection was 96%, which was significantly higher than that after the intra-articular injection, which was 62%. These results suggest that the prevalence of SIJ pain, when reported only by SIJ intra-articular injection response, may be underestimated. Furthermore, the clinical provocative tests may have a high false-positive result because of the difficulty in distinguishing intra-articular versus extra-articular pain by clinical examination.
Finally, the degree and duration of force applied by the examiner performing the provocative tests may vary and is influenced by patients' body habitus. At times, myofascial pain and dermatomal stretch also aggravate the low back pain giving false-positive results. Table 1 shows the variability and range of reliability data of various tests as reported by various investigators. Wide variation exists in the sensitivity and specificity of each test, and generally, the tests have low positive predictive value and relatively higher negative predictive value. Therefore, these tests are more helpful in ruling out SIJ dysfunction than in diagnosing this condition.
No imaging studies have been found to be accurate in diagnosing SIJ pain. Inflammatory sacroilitis can present from mild erosion (grade 1) to ankylosis (grade 4) on plain radiographs according to the modified New York criteria. So, starting with a plain AP pelvis is reasonable to only rule out any other obvious reasons for pain. However, the sigmoid shape and oblique orientation of the SIJ creates challenges in visualization by conventional radiographs. Cross-sectional studies such as CT, MRI, and single photon emission CT carry distinct advantages because of their ability to create multiplanar visualization of the joint. CT scan shows erosive joint changes and subchondral sclerosis. Elgafy et al reported that CT was only 57.5% sensitive and 69% specific in the diagnosis of SIJ pain. The authors concluded that with clinical suspicion of a sacroiliac origin of pain, intra-articular injection is the only means to confirm the diagnosis. MRI can detect early inflammation and soft-tissue pathology of the SIJ in patients with spondyloarthropathy. Edema and early erosive changes across the SIJ identified on MRI are sometimes useful in the diagnosis or staging of spondyloarthropathy. MRI can also rule out other sources of pain.
The specificity of nuclear bone scans is 90%, with the sensitivity of only 12% to 46%. The positive predictive value and the negative predictive value of bone scans are 86% and 72%, respectively. Because of low sensitivity, bone scan should not be included in the routine diagnostic algorithm of SIJ pain. However, at times compared with MRI, we found bone scan to be more advantageous because increased uptake often helps to identify stress fractures, inflammatory changes, and infection or primary and metastatic tumors as well. Bone scan is also helpful when a diagnostic block cannot be performed.[31,32]
In general, limited evidence exists for the diagnostic accuracy of any imaging modality in diagnosing SIJ pain. The choice of radiographic investigation should depend on the clinical presentation and history of the individual patient. We do not routinely obtain advanced imaging studies for the diagnosis of SIJ dysfunction. These tests are more helpful in ruling out other sources of pain.
Because of the lack of reliable clinical and radiologic studies to diagnose SIJ pain, the use of SIJ contrast enhanced injections under fluoroscopic guidance has become more common as a diagnostic and therapeutic option. To minimize unnecessary diagnostic injections, several investigators have suggested that only patients with three or more positive provocative clinical tests or patients with isolated localized SIJ pain and positive Fortin Finger tenderness be candidates for a diagnostic SIJ injection.[17,18,33]
Good evidence exists to support single or dual blocks for the diagnosis of SIJ as a pain generator when pain relief cutoff is set between 75% and 100%.[16,26] Evidence is fair when the cutoff is set between 50% and 74%.[17,33] Depending on the pain response cutoff value and inclusion criteria designed in a particular study, the prevalence of SIJ pain is reported anywhere from 10% to as high as 62%.[16,18,19,26,34,35] A prevalence of greater than 50% was reported in a well-selected population.[6,16,26] Most studies suggest a point prevalence of approximately 25%. Comparative local anesthetic blocks are also useful when the diagnosis is still in doubt and/or the first block response was false-positive. In this scenario, repeated blocks are given at different occasions with different anesthetic agents (with varying duration of action) to ascertain the efficacy of the block.
Radiographic localization of the SIJ by arthrography is important before injecting the anesthetic agent, as diagnostic specificity of the injection will be otherwise compromised (Figures 1 and 2). Rosenberg et al showed that only 22% of patients received an intra-articular injection when the injection was performed by clinical palpation only. They performed immediate postinjection CT scan and found that most injected material was within 1 cm of the joint in the posterior area medial to the iliac bone and the remaining material had leaked into the sacral neural foramen and epidural spaces.
Arthrogram showing AP (A) and lateral SIJ (B) under fluoroscopic guidance before injecting anesthetic agent. SIJ = sacroiliac joint
Flowchart showing the algorithm useful in a patient with suspected SIJ pain. LBP = low back pain, PSIS = posterior superior iliac spine, PT = physical therapy, ROM = range of motion, SIJ = sacroiliac joint
The average SIJ volume in symptomatic patients is 1.08 mL (range, 0.8 to 2.5). Therefore, approximately 2 mL of injected volume should be adequate. Excessive amount may leak through the anterior capsule to the neural structures and may give a false-positive test.
J Am Acad Orthop Surg. 2019;27(3):85-93. © 2019 American Academy of Orthopaedic Surgeons