37 Sacroiliac Joint Pain
Procedures for Diagnosis and Treatment
Over the last 2 decades, the sacroiliac joint (SIJ) has increasingly been recognized as an anatomic source of pain that figures in the differential diagnosis of a patient presenting with low back pain (LBP) and/or buttock pain with or without more distant referred pain.1–7 Because the SIJ is innervated, it can be a source of pain.2,8–13
Because SIJ pain refers into the buttock and iliac crest near the posterior-superior iliac spine, and also into the groin, abdomen, and leg including the foot,14,15 it can be confused with referred pain from other sources, particularly from the lumbar intervertebral disc, lumbar zygapophysial joint (ZJ), and the hip joint. Additionally, because of its propensity for referral into the leg, SIJ pain can be confused with radicular pain. Thus, it is essential that the clinician recognizes these potentially confounding features and takes adequate steps to differentiate between SIJ pain and other pain presentations.
In the early 1900s, the SIJ was thought to be the principal source of LBP,16 and an important cause of “sciatica.”17 Subsequently, and particularly after the discovery of the disc prolapse, it was considered that the lumbar spine—and in particular the lumbar intervertebral disc—was responsible for most back problems.17
Acknowledgment of the SIJ as a source of pain in the ensuing decades commenced in rheumatologic literature, but this largely related to seronegative arthropathies and case reports of various rare infections and tumors.18,19 Subsequently there have been substantial developments in the basic sciences relating to the SIJ, initially and primarily in the osteopathic, physiotherapy, and chiropractic literature,18,19 and later in biomechanical and radiologicliterature.20–26 The SIJ was overlooked as a possible pain generator outside of the inflammatory population perhaps because it was deep and inaccessible to specific accurate injection prior to the development of C-arm fluoroscopy, and because clinical examination tests had poor sensitivity and specificity. The advent of imaging-controlled diagnostic interventions has allowed for a more rational approach to diagnosis, and as a result, there is again an increasing awareness that the SIJ is an important cause of LBP and referred pain into the pelvis and leg. It is now estimated that the SIJ may be the cause of between 15% and 38% of all cases of LBP.4,27–30
The increased awareness of the potential for the SIJ to be a source of pain has not as yet translated into proven treatments. The literature concerning SIJ interventional treatment is sparse.1,31–35
Anatomy and Biomechanics
The SIJ is the articulation between the triangular sacrum and the two ilia. It is a true diarthrodial synovial joint, and is unlike any other joint in the body. Only the ventral third of the joint is a true synovial joint with a joint capsule and synovial cells;36 the remainder is composed of three ligaments,37 the ventral sacroiliac ligament, the interosseous sacroiliac ligament, and the posterior sacroiliac ligament.38 From fetal life onward, the iliac surface is fibrocartilage and the sacral surface is hyaline cartilage. Subsequent arthrosis of the joint tends to affect the fibrocartilaginous iliac side more than the sacral hyaline cartilage.39 The dorsal transition between the ligamentous and synovial components shows marked individual variability including osseous clefts, cartilage and subchondral defects, and vascular connective tissue in the bone marrow.36 Furthermore, aging is accompanied by extensive ridging in the interosseous region of the sacrum and ilium to the extent that it is present in 100% of those 55 years or older and also by ossification of the central interosseous region of the SI ligament in 60%, making it likely that by the sixth decade of life there is little or no movement in the SIJ.40 Other degenerative changes such as marginal osteophytes are not uncommon by the start of the fifth decade, at least unilaterally.41
The articular cartilage of the SIJ does not appear to degenerate in a similar manner to other synovial joints in which articular cartilage defects result in bony ankylosis. The SIJ articular cartilage is maintained even in the elderly; it seems that fibrous tissues contribute most to ankylosis so that with aging there is interposition of fibrocartilage-like tissues within the joint (complete fibrous ankylosis).42 Bony fusion seems to occur only in ankylosing spondylitis.43 Degenerative changes and intraarticular SIJ ankylosis are substantially more common in men than women.43–45
The sacrum contains four foraminal pairs on either side, S1 to S4. Each pair has a ventral and dorsal aperture. On x-ray, the ventral component of the foramen is the most obvious; the smaller dorsal foramen can be difficult to visualize. It is important to recognize this when performing imaging-guided procedures into a sacral foramen. The dorsal component of each foramen may be difficult to visualize on a static image particularly because of the superimposition of the larger ventral component and from bowel gas. However, with the use of CT scan or more preferably a C-arm image intensifier, the dorsal component can be identified. With the C-arm, this is achieved by varying the amount of obliquity and observing that the more superficial dorsal component moves to a greater extent relative to the deeper ventral component (Fig. 37-1).
The synovial part of the joint is more prominent caudally. At the level of the S1 foramen, the ventral 25% of the joint is synovial; at S2, the ventral 50% to 75% is synovial and at S3, 100% of the SIJ is synovial.46
The pelvis is made up of three bones, with the sacrum positioned as the keystone in an arch from femur to femur (Fig,37-2). The stability of the sacrum within the pelvis is dependant on (1) the shape and orientation of the sacrum and its articulations with the ilia, (2) the integrity of the ligamentous structure around the joint, and (3) the extent of muscular compression across the joint. Variations in the shape and orientation of the sacrum and its articulations are rarely a problem except when these aberrations are quite extreme; for example, after pelvic fractures and in some congenital conditions. The ligamentous function of the SIJ is dependant on intact, stable ligaments and the orientation of the sacrum. The sacrum pivots in a sagittal plane around its true articular joint surface by between 6 and 11 degrees. Tilting of the superior sacrum in an anterior direction is called sacral nutation. This is its normal position, encouraged by the lumbar lordosis. Excess nutation can occur. Superiorly, such excess nutation is limited directly by the deep interosseous and long dorsal ligaments and indirectly by the iliolumbar ligament. Inferiorly, it is limited by the sacrococcygeal and sacrotuberous ligaments. If all of these ligaments are intact, nutation of the sacrum has the benefit of helping to pull the pelvic ring closed, thereby compressing the SIJs. This, the so-called “locked” position of the pelvis, has great biomechanical strength with force transfer occurring primarily through a well supported and mechanically advantaged joint surface and it ligaments. This passive locking mechanism, depending on the shape of the keystone and the integrity of the articular capsule and surrounding ligaments, is called form closure of the SIJ.
The ligaments and capsule of the SIJ cannot, however, provide adequate compression across the joint surface on their own. The primary compressors of the SIJs are muscular; their actions are termed force closure of the SIJ. Three main muscle groups have been identified: (1) muscles of the pelvic floor, (2) transverse abdominis, and (3) a posterior sling consisting of latissimus dorsi through the thoracolumbar fascia to the contralateral gluteals.20–22,47–56
Innervation
Sacroiliac joint innervation is important because the only method for making a diagnosis of SIJ origin pain is anesthetic block of the joint or its nerve supply, and one possible 53–56method of treatment is radiofrequency (RF) treatment directed at target nerves. The SIJ is definitely innervated and it can be a source of pain. The periarticular tissues of the SIJ contain mechanoreceptors and nociceptors.2 Nerve fibers varying from 0.2 micron to 2.5 microns in diameter end in five morphologically different terminals and these terminals are present in the SIJ capsule and adjacent ligaments.9 Substance P and calcitonin gene-related peptide (CGRP) immunoreactive nerve fibers have been found in the anterior SIJ capsule and interosseous ligament,13 the superficial layer of sacral and iliac cartilage, and the surrounding ligamentous structures.12 Nerves supplying the SIJ are distributed not only to the superficial and deep dorsal sacroiliac ligaments, but also to the sacrotuberous and sacrospinous ligaments; the dorsal rami continue their course laterally, sandwiched between superficial and deep portions of sacroiliac ligaments, and pierce the origin of the gluteus maximus muscle.57
It is considered that the synovial component of the SIJ has a different innervation to the posterior ligamentous component. The synovial joint is likely to be innervated mainly by ventral sources;8,9 its upper ventral portion is mainly innervated by the L5 ventral ramus and the lower ventral portion by the S2 ventral ramus or by branches from the sacral plexus.9 The synovial component has minimal innervation by the sacral dorsal rami.58
The dorsal sacroiliac ligaments are innervated by at least the L5 dorsal ramus and lateral branches of the S1-S3 dorsal rami. The L4 medial branch may be involved. The upper dorsal ligamentous structures are innervated by the L5 dorsal ramus; the lower dorsal ligaments by nerves arising from a plexus composed of lateral branches of the dorsal rami of the sacral nerves.9 These nerves range from 0.292 mm to 0.997 mm in diameter, and the nerves supplying both the synovial and ligamentous components of the SIJ complex have similar diameters.9 The lateral branches of the sacral dorsal rami emerge from the sacral foramina in a varied array, radiating cephalad, transverse, or caudad.58 When they emerge, they do not run in a constant plane,11,58,59 but run across the dorsal sacrum either through, superficial to, or deep to the dorsal sacroiliac ligament at a variable depth of up to 1 cm superficial to bone (Fig. 37-3).58,60
Pathophysiology
As noted previously, the SIJ is innervated and has the potential to be a source of pain. It can become painful as a result of both intrinsic and extrinsic factors. Intrinsic mechanisms include definitive biomedical processes such as sacroiliitis and tumors. These constitute red-flag conditions and are not covered in this chapter. The other intrinsic mechanism considered to be a risk factor in SIJ origin pain relates to aberration of biomechanical function. The technical terms used to describe these biomechanical features are form closure and force closure. Poor form closure of the SIJ is caused by inefficient bony structure/alignment, absent or stretched SIJ ligaments, or sacral counter nutation. Poor force closure is considered to arise through pain inhibition and poor firing of the compressive muscles (Fig. 37-4).61–67 It is likely that long-term lack of force closure across the SIJ can lead to increased strain on the ligamentous structures, which, over time may lengthen and cause further loss of pressure across the joint. Such a joint may then be resistant to muscular retraining.
Extrinsic mechanisms causing loss of form or force closure include trauma (macro or repetitive microtrauma), infection, and pregnancy.20,47,63,68–73
Pain in the region of the SIJ is not uncommon after posterior iliac graft harvesting.74 It occurs in 6% to 39% in patients who have had iliac grafting for spinal fusion.75,76 One possible cause is sacroiliac joint instability, caused by ligamentous disruption or violation of the synovial part of the SIJ during surgery. In patients who have had posterior iliac grafting and have persistent SIJ region pain with CT scan evidence of synovial disruption, SIJ degeneration is more prevalent on CT.77
SIJ pain is also not uncommon after spinal fusion. The prevalence of SIJ pain, diagnosed on the basis of 75% or more pain relief with local anesthetic block, in one group of patients with significant and chronic LBP postlumbar fusion surgery was found to be 35%,78 a figure that seems to be similar to that in other reports.79 Bone scan does not assist in the diagnosis of postfusion SIJ origin pain using local anesthetic as the diagnostic tool; however, clinical features that are somewhat predictive include pain that is different to the prefusion pain, particularly if it commences some time after the fusion has been performed.78 SIJ pain postfusion occurs as fusion increases the computed sacral angular motions and the average stress on the SIJ articular surface motion particularly in combined L4-5 and L5-S1 fusion.79 This can lead not only to an increased chance of pain, but also to premature SIJ degeneration; the prevalence of CT scan determined SIJ degeneration over a 5-year period postlumbar fusion was found to be 75% in one study that found a prevalence of only 38% in a matched control group.80 Consistent with the finding that L5-S1 fusion had the more significant biomechanical effect, the prevalence of SIJ degeneration was higher if the L5-S1 segment was included in the fusion.80
Sacroiliitis is characterized by chronic inflammation within the joint and the deep interosseous ligament, which forms the posterior capsule and is the largest syndesmosis in the body. Both hyperparathyroidism and repetitive shear stress injuries in athletes can mimic the presentation of sacroiliitis.81
In a study of long-term outcomes from SIJ radiofrequency neurotomy (RFN), SIJ pain was considered to be idiopathic in 30% of cases, and to derive from motor vehicle accidents in 9%, from a fall or slip in 24%, from overload or a work injury in 26%, and from other incidents in 11% of cases.82
Clinical Presentation and Diagnosis
The diagnosis of SIJ pain is predicated on diagnostic anesthetic injection because there are no other clinical or radiologic correlations that have been found to be reasonable surrogates. The most significant feature on clinical assessment of a patient considered to have SIJ origin pain is the site of pain; if the patient points to the posterior superior iliac spine (PSIS), then the pain is more likely to derive from the SIJ.28 Clinical examination has possible utility when examination findings are considered collectively. Radiologic findings are of no particular use other than in the exclusion of some red-flag conditions.
The International Association for the Study of Pain (IASP) has proposed a three-part criteria for the diagnosis of SIJ pain.83 In particular, diagnosis requires the following:
Site of Pain
In asymptomatic subjects, noxious stimulation of the SIJ evokes pain in the low back, buttock, and upper posterior thigh.4 In patients with SIJ pain established by fluoroscopically guided SIJ injection, 94% described buttock pain, 72% described lower lumbar pain, 14% described groin pain, 50% described associated lower-extremity pain, 28% described leg pain distal to the knee, and 14% reported foot pain.14 However, as noted, the most prominent site of pain presentation for SIJ pain is pain over the PSIS.28 SIJ pain rarely extends above L5.28 SIJ origin pain has a similar referral pattern to pain derived from the lumbar spine and from the hip joint,14,28 and thus, analysis of pain patterns is not in itself a reliable diagnostic factor.84
Type of Pain
SIJ pain can present with local PSIS pain and/or somatic referred pain. Local SIJ region pain can be deep and aching, but it can also be sharp and activated by movement. Somatic referred pain is generally described as diffuse, aching, and poorly localized. It is different from typical lumbar radicular pain, which is generally described as long, thin, sharp lancinating pain that can concentrate distally.85 Because SIJ-referred pain can extend into the leg, it can be confused with radicular pain, potentially leading to unnecessary spinal treatment.17,86 If the predominant pain is radicular, it is most likely to arise from the lumbar spine owing to disc prolapse or canal stenosis. However, it is not impossible for lumbosacral radicular pain to be caused by SIJ pathology. In one series, ventral capsule disruption was present in 70% of patients diagnosed with SIJ pain by intraarticular block.30 Joint injury, if associated with inflammation, can theoretically be associated with extrasacral perineural inflammation and pain.
It is also important to distinguish neuropathic pain from somatic referred pain. Neuropathic pain typically presents with descriptors such as burning, buzzing, and tingling, and has clinical features including allodynia. It can occur in association with somatic referred pain.85,86 Although uncommon, lumbosacral plexopathy, which is defined as neurologic deficit derived from the lumbosacral plexus, is more common in patients after sacral fractures than it is among the entire population of patients with pelvic and acetabular fractures.87 It stands to reason that SIJ trauma and pain can be associated with local neural damage, and hence, neuropathic pain. Fortin described five patterns of contrast medium extravasation after fluoroscopically guided SIJ injections by viewing postarthrography CT in 76 patients with LBP, of whom 61% displayed extravasation.3 It seems that extravasation does occur dorsally into the dorsal S1 foramen, superiorly at the sacral alar level to the L5 epiradicular sheath, and ventrally to the lumbosacral plexus, making it possible for local neural structures to be insulted by SIJ inflammation.
Clinical Examination
The validity of physical examination tests is reduced because they tend to stress adjacent soft tissue structures as well as the lumbar spine and hips.68
Singular examination tests such as palpation and movement tests are generally considered to be unhelpful in the diagnosis of SIJ pain.27,37,68,88,89 However, combined tests may be useful. Synovial SIJ pain can be predicted when three of five pain provocation tests are positive with a sensitivity and specificity of 91% and 78%, respectively, and a consequent likelihood ratio of 4.1.90–95 Specificity improves with the absence of centralization of pain. The tests used are:
It is unknown if these tests can predict ligamentous sources of SIJ pain; clinical examination has not been assessed using ligament injection as the criterion standard. These tests individually can be positive in up to 20% of the asymptomatic population.96
The SI joint is more likely to be the source of pain if the following occur:
Tests for SIJ instability have been proved reliable particularly in the postpartum population, but there is no data supporting their efficacy in the management of SIJ pain, except in the postpartum population.51,52,73,97–99
Multiple authors raise the concept of SIJ dysfunction, where the self-locking mechanism of the SIJ complex fails due to a loss of form and/or force closure.20–22,47–56,61–64,66,69–73,97–101 The tests used to assess SIJ dysfunction are reliable and valid.49–52 The presence of SIJ dysfunction is proposed by these authors as a putative cause of SIJ pain. However, although outcome studies on treating this SIJ dysfunction show significant improvements in disability, changes in pain are less impressive.61,64,100,102
Imaging
The diagnosis of SIJ origin pain is difficult because there are no valid or reliable correlations between imaging changes and SIJ pain. Imaging cannot be used as a criterion standard for diagnosis or as a basis on which to assess the validity of treatment. In one study, diagnostic CT-guided intrasynovial SIJ injections had a sensitivity of 57.5 %, a specificity of 69%, and a consequent poor likelihood ratio of 1.9,75 thus negating the use of CT in a presentation of putative SIJ origin pain except to rule out red-flag conditions. Bone scan has a very low sensitivity but a high specificity for SIJ pain diagnosed with diagnostic blocks, and is not worth performing.103 Similarly, plain radiography and MRI cannot reliably detect non–red-flag SIJ origin pain.28
Changes are often noted on imaging but they are not clinically significant. For example, the CT appearance of the SIJ is closely related to patient’s age, gender, BMI, and in women, parity.104 The widths of the SIJ space and of the subchondral sclerosis on the iliac and sacral sides narrow over time; they were measured to be 2.3±0.4mm, 2.5±1.6mm and 1.4±0.5 mm, respectively, in patients younger than 40 years of age and 1.9±0.2 mm, 3.6±2.1 mm and 2.3±1.1 mm, respectively, in patients older than 40 years of age.104 SIJ changes include increased joint space narrowing and loss of joint space uniformity. Subchondral sclerosis appears to be wider and less uniform in the elderly.104 Osteophytes are present even in younger patients and their prevalence increases with advancing age.104 CT has identified six anatomic variants termed accessory joints (19.1% of assessed SIJs), iliosacral complex (5.8%), bipartite iliac bony plate (4.1%), crescent-like iliac bony plate (3.7%), semicircular defects at the sacral or iliac side (3%), and ossification centers (0.6%).105
Diagnostic Injections
Properly conducted SIJ injection is considered the criterion standard diagnostic technique.27 Intricacies and subtleties in this diagnostic approach to SIJ pain need to be understood. Diagnostic injections can be performed using various forms of imaging guidance such as C-arm fluoroscopy, ultrasound, and CT. More recently, image fusion, in which a software technology matching real-time ultrasonography and a previously obtained CT, has been tested and found to be accurate, but it is slow, taking on average about 20 minutes.106 MR-guided sacroiliac and other spinal injections can also be performed in open high-field MRI using fast TSE sequence designs.107 However, although there was a reported accuracy of drug delivery of 100% for nerve root injections, the accuracy for ZJ and SIJ delivery was only 87%, and the average time taken was 29 minutes (range 19 to 67 minutes).107
Site of Injection
There are two components to the SIJ and it appears that SIJ pain can be established as the likely source of pain only if both components are assessed. The need to assess both components, however, should be predicated on the treatment that might ensue from such a diagnostic approach. As discussed earlier, the ligamentous component of the joint is innervated by at least the dorsal ramus of L5, and certainly by the lateral branches of the dorsal rami that emerge from the S1, S2, and S3 foramina. If the treatment is to be RFN of these nerves, then it stands to reason that the diagnostic injection should be directed at these nerves. As the synovial SIJ is innervated ventrally, nerve blocks cannot be used as a diagnostic test for synovial SIJ pain; lateral branch blocks do not anesthetize the synovial SIJ.58 The only method that can be used to diagnose synovial SIJ pain is intraarticular injection. This might be used as a test if steroid or other material is to be injected into the synovial component, or if a surgical procedure such as SIJ is considered relevant.
Because the sacral lateral branches run at a variable depth and have a variable course over the sacrum, multisite, multidepth, sacral lateral branch blocks are recommended; this method renders the interosseous and dorsal sacral ligaments insensate in 70% of subjects.58,60
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