INTRODUCTION

For more than 70 years, the sacroiliac joint complex has been implicated as a significant and discreet structural source of low back pain. However, of the three major groups of spinal structures identified as contributing to chronic low back pain, the sacroiliac joint complex (SIJC) stands apart from the zygapophyseal joints and intervertebral discs as the least understood from a neuroanatomic, diagnostic, and therapeutic standpoint. The prevalence of low back pain arising from the SIJC has been estimated at 15–30%,^1,2 and is likely higher in the older population. The SIJC possesses a unique and complex three-dimensional anatomy with multisegmental sensory innervation. Unlike the zygapophyseal joints and intervertebral discs, the sensory innervation of the SIJC has been described in a detailed fashion only recently, and the definitive gold standard for successful treatment of SIJC pain has not yet been rigorously defined.

Numerous studies have demonstrated a lack of pathognomonic historical, symptom, physical examination, or imaging findings specific for the individual diagnosis of zygapophyseal joint, disc, or SIJC pain.^1,3–7 The identification of specific structural sources of low back pain must currently be made through validated interventional diagnostic means. A structure-specific diagnosis is ultimately essential for the eventual consideration of appropriate evidence-supported treatment options. A specific structural diagnosis also leads to a definable prognosis, as the long-term outcomes following treatment for zygapophyseal joint pain differ from disc pain, and from SIJC pain as well.^8–14

A rational, evidence-based diagnostic algorithm for the identification of the structural source of low back pain is thus invaluable (Fig. 116.1). The area of the SIJC is frequently involved with referred pain patterns associated with other lumbar structures,^2,3,15–17 and pain of SIJC origin frequently involves somatic referred pain to various areas of the low back, pelvis, abdomen, and lower extremities,¹⁸ with nearly one in seven patients experiencing pain radiating to the foot in a pseudoradicular pattern. Successful implementation of such a diagnostic algorithm is therefore ultimately dependent not only on the physician’s understanding of the technical aspects of diagnostic interventions, but also the overlapping distributions of referred somatic pain arising from different lumbar structures, and the defined sensitivities and specificities of each test considered.

Fig. 116.1 Sample SIJ diagnostic algorithm flowchart depicts an evidence-supported interventional diagnostic algorithm for the identification and treatment of SIJC pain.

The current gold standard for the diagnosis of SIJC pain involves controlled intra-articular anesthetic injections (IAI).^1,2,6,7,19 Pain may also arise from the deep interosseous ligament (DIOL), which is exquisitely innervated. The nociceptive potential of the DIOL is supported by anatomic and histologic findings. Although a technique for injection of the DIOL has been described, the clinical overlap in diagnostic sensitivity and specificity between controlled SIJC IAI and DIOL anesthetic injections has yet to be determined,^10,20–22 and represents an ongoing area of clinical and anatomical research. Once the diagnosis of SIJC pain has been established and the presence of other sources of lumbar spine pain excluded, definitive SIJC therapeutic intervention may be considered.

A definitive, highly successful, and durable intervention for the treatment of SIJC pain may be just around the corner. Limited reports of the efficacy of surgical fusion^23,24 and radiofrequency denervation^10,25,26 are available; however, long-term prospective, controlled or comparative studies are currently lacking or are under investigation. This chapter will focus on current evidence supporting therapeutic injection and radiofrequency SIJC denervation.

RELEVANT ANATOMY

The sacroiliac joint (SIJ) is the largest syndesmotic joint in the human body. Although possessing a cartilaginous articular surface and synovial component, the sacroiliac joint primarily functions as a stress-relieving joint, insulating the lumbar spine from transmitted shock associated with ambulation. The SIJ is not bounded by capsular tissue like other synovial joints. The joint capsule is composed of ligamentous fibers, and posteriorly, contiguous with the deep interosseous ligament. The joint as a whole is intimately associated with a multitude of surrounding ligaments, which provide structural integrity to the joint complex.

The synovial component of the SIJC is composed of two divergent joint planes, a larger lateral (or anterior) pole and the smaller medial (or posterior) pole. Percutaneous access to the medial pole traverses a minimum of ligamentous tissue, whereas access to the lateral pole must traverse the DIOL (Fig. 116.2). The total volume of the synovial component of the SIJC (1.5 mL) is small compared to its surface area.^15,27

Fig. 116.2 Medial and lateral poles, SIJ. An ipsilateral oblique image of SIJ arthrography, with needle penetrating the posterior joint capsule of the medial pole. The heavy dark line separates the medial from the lateral pole of the joint. Note capsular redundancy of the distal (caudal) medial pole (white arrow), and the small amount of dorsal capsular contrast extravasation (black arrow).

The ‘capsule’ of the sacroiliac joint is frequently incompetent, even in asymptomatic patients. In one series, 61% of intra-articular injections demonstrated extracapsular extravasation,²² and 25% of asymptomatic volunteers undergoing intra-articular injection demonstrated ventral capsular insufficiencies,²⁷ with contrast extravasating in the region of the traversing lumbosacral plexus. Detailed dissection of the ventral joint capsule in cadavers suggests that these ventral capsular ‘defects’ often appear as small foramina rather than traumatic capsular rents.²⁸

Neuroanatomy of the sacroiliac joint complex

Early studies of the sacroiliac joint suggested a combination of ventral and dorsal innervation,^29,30 but recent investigation has demonstrated a predominant dorsal innervation in humans^21,22 arising from the lateral branches of the L5 dorsal ramus and S1–4 dorsal rami, and composed of a wide range of sensory fiber types.^21,30 The lateral branch nerves arise from a lateral dorsal sacral plexus, and divide into multiple smaller branches, some of which enter the DIOL whereas others do not. The segmental location and number of lateral branch nerves innervating the SIJC is extremely variable. On the posterior aspect of the sacrum, these branches are often closely related (Fig. 116.3).²⁰ Unlike the lumbar medial branch nerves of the dorsal rami supplying innervation to the zygapophyseal joints, the complex three-dimensional anatomy of the lateral branch nerves innervating the SIJC combined with their close proximity to the dorsal sacral foramina renders consideration of selective anesthetic blockade futile (Fig. 116.4).²⁷

Fig. 116.3 Dorsal sacral plexus and lateral branch nerves. A representative illustration of the lateral branch nerves supplying sensation to the dorsal sacral joint complex (arrows) from L5 through S3. Note the variability in lateral branch topography between right and left. LDSIL, long dorsal sacroiliac ligament; PSIS, posterior superior iliac spine; ST, sacral tubercle; L5–S1, L5–S1 zygapophyseal joint (superior articular process).

(Courtesy of Frank Willard, PhD.)

Fig. 116.4 Incomplete lateral branch staining. Dissection in nonembalmed cadaver of lateral branch nerves, left dorsal sacral plexus following injection of 0.5 mL of colored dye lateral to S1 and S2 dorsal sacral foramina. Note incomplete staining of lateral branch nerves at S2 (black arrows). Unlike similar diagnostic injection techniques to identify painful zygapophyseal joints, it is not possible to selectively anesthetize the lateral branch nerves supplying the sacroiliac joint.

(Courtesy of Paul Dreyfuss, M.D. and Frank Willard, PhD, used with permission.)

The DIOL is richly innervated with nociceptive mechanoreceptors. Sensory fibers must traverse the DIOL to reach the posterior SIJ capsule, and understanding the topographic neuroanatomy of the SIJC becomes invaluable in consideration of the design and performance of diagnostic injection techniques and therapeutic interventions.

DIAGNOSTIC INJECTION TECHNIQUES

With the exception of imaging findings on magnetic resonance imaging (MRI) characteristic of sacroiliitis (and in the absence of tumor, fracture, or infection), there are no pathognomonic imaging findings specific for the diagnosis of SIJC pain. As previously discussed, there are also no physical examination findings that accurately identify patients with SIJC pain, although a combination of pain below L5 and localized just medial to the posterior superior iliac spine (PSIS) is suggestive. The diagnosis of SIJC pain therefore rests on a combination of interventional tests to exclude other lumbar sources of pain, and objectively verify the presence of SIJC pain.

Once other structures of low back pain have been excluded, consideration of SIJC-specific diagnostic interventions may be entertained. The current gold standard involves blinded, controlled intra-articular local anesthetic injections of the SIJC, or saline placebo-controlled blocks. Because the synovial component of the SIJC is rarely accessed successfully without image guidance, injection of the joint must be performed under fluoroscopy, or possibly computed tomography (CT) guidance.^31,32 However, it must be remembered that the sacroiliac joint capsule is frequently incompetent, and extravasation of local anesthetic into surrounding structures, especially from the ventral joint capsule in the region of the traversing lumbosacral plexus, will render such an injection non-specific, and therefore nondiagnostic. Additionally, since the false-positive rate of single anesthetic injections may approach 40%,^33–36 no therapeutic decisions regarding ablation or surgery should be made based on a single response to an isolated analgesic test. Patient responses to controlled, comparative or placebo-controlled anesthetic testing must be rigorously assessed and documented. As with any diagnostic procedure, analgesic tests generate negative, indeterminate, and positive results, and have associated false-negative and false-positive rates. Understanding what constitutes a negative, indeterminate, and positive result is essential. The methodology of comparative and placebo-controlled analgesic testing has been extensively validated elsewhere, and should be considered mandatory reading for all interventional spine diagnosticians.^{34,35,37–39} It is further recommended that any analgesic test be performed without sedation. Patient responses in the immediate postprocedural period should be rigorously documented, preferably with a q15 minute or q30 minute pain log. Sedation is an inadequate excuse for poor injection technique.

Access techniques for the medial^40–44 and lateral poles^10,45 of the synovial component of the SIJC have been described, as has a technique for isolated injection of the DIOL.^10,45 Any image-guided injection of the SIJC must include the use of radiopaque contrast to obviate vascular uptake, and verify contained distribution of injected local anesthetic. Fluoroscopy offers several intrinsic advantages over CT, including the capability for real-time imaging (and detection of vascular uptake of contrast), although the superior contrast and spatial resolution of CT may offer potential advantages in documenting extracapsular or extraligamentous extravasation patterns. In patients with spinal or pelvic hardware, advanced fluoroscopic imaging technologies, such as digital subtraction algorithm (DSA) imaging permits imaging free of the metallic imaging artifacts common to CT.

THERAPEUTIC INTERVENTIONS

Injections of the SIJC have not been demonstrated to have therapeutic value except in the isolated instance of sacroiliitis associated with seronegative spondyloarthropathies.^46–50 No structure-specific diagnostic information can be gleaned from steroid injection; if a patient experiences relief beyond the duration of action of local anesthetic, all that can be ascertained is that a steroid response may be present. A prolonged steroid response implies that a component of inflammation may be present, but is not structure-specific; local anesthetic injection in and of itself may also provide prolonged analgesia, well beyond the expected duration of conduction block.³⁷ Once the diagnosis of SIJC pain has been established, therapeutic intervention may be considered.

Therapeutic options for SIJC pain may be categorized into surgical interventions, other interventions, and denervation procedures. Surgical intervention for SIJC pain is covered elsewhere in this text. Some religiously advocate other therapeutic options, such as manipulation and proliferative therapies. Regrettably, the efficacy of manual medicine techniques and proliferative treatments remain anecdotal.

Several denervation procedures for SIJC pain have described targeting the lateral branch nerves of the dorsal rami at various points between the dorsal sacral foramina and the posterior SIJ capsule. Among the first techniques is that described by Kline, involving the generation of a series of bipolar radiofrequency (RF) lesions along the posterior joint line (Fig. 116.5).⁵¹ Recognizing the multisegmental innervation of the SIJC, Kline reasoned that the lateral branch nerves must converge along the posterior joint capsule, and serial lesions along the posterior joint line should effectively denervate the joint. However, a study by Ferrante et al., utilizing uncontrolled single anesthetic diagnostic blocks, found that only 36% of patients experienced more than 6 months of >60% relief, and no patients experienced complete relief.²⁶ A more rigorous evaluation with controlled anesthetic blocks and rigorous postblock assessment may have yielded better results, as initial false-positive block responders would have been eliminated from consideration.

Fig. 116.5 Bipolar SIJ RF lesioning technique. Cone down anteroposterior radiograph of bipolar electrode placement over caudal dorsal sacroiliac joint capsule.

(Courtesy of Matthew T. Kline, M.D.)

Another approach toward denervation of lateral branch nerves at L5 and S1 has been described under CT guidance by Gevargez et al.²⁵ Like the Ferrante study, a single anesthetic block was used for patient selection. Thirty-five of 38 patients were followed for 3 months, with 34.2% reporting complete relief, and an additional 32% reporting ‘substantial relief.’