Introduction
Both globally and in the United States, low back pain (LBP) has been the number-one cause of years lived with disability from 1990 to 2016. As the global population continues to grow, the prevalence of LBP follows suit, and the burden it places on the global health system continues to increase, particularly in the United States. , Within this population, a subgroup of roughly 16% to 30% suffers from nonspecific LBP, which is attributed to a sacroiliac joint (SIJ) etiology. Certain factors have been found to predispose patients to developing sacroiliac joint pain, including leg length discrepancy, gait abnormalities, pregnancy, and spinal fusion involving the sacrum, as discussed by Steven Cohen. The unique anatomy, biomechanics, and variability that exist within the SIJ further make diagnosis and treatment of SIJ pain difficult. In this chapter we will discuss the use of radiofrequency ablation (RFA) as one of the modalities for the treatment of SIJ mediated pain in patients.
Anatomy
The sacroiliac joints are the largest joints in the axial skeleton with the responsibility of transferring forces between the lower extremities and the upper body through the lumbar spine. The sacroiliac joints are found on lateral sides of the sacrum directly articulating with the medial ilium and acting as the link between both pelvic girdles to form the pelvic ring. The SIJ is a diarthroidal joint, where only the rostral third surface of the joint is considered to be a true synovial joint. The remaining surface of the joint is composed of interosseous ligamentous structures that primarily function to limit motion of the SIJ in all planes. The joint itself is anywhere from 1 to 2 mm in width and enveloped by a fibrous capsule. When comparing the sacral and iliac surfaces of the joint, the sacral surface is covered with thicker hyaline cartilage while the iliac surface is covered with a thinner fibrocartilagenous surface. Although numerous muscles traverse the SIJ, none are designed to directly act on the joint causing motion. Rather, the muscles and numerous ligamentous structures that support this SIJ are designed to help transmit, dissipate, and limit shear forces while wedging it tightly into place. This is due to the poor stability the SIJ encounters when exposed to shear forces.
Histological studies on cadavers have been performed verifying the presence of nociceptive fibers intraarticularly and also in the periarticular ligaments, , but somewhat more debated are the nerve roots that actually innervate the joint. The SIJ is believed to have anterior and posterior innervation, and the nerves supplying the posterior innervation are targeted when performing procedures such as radiofrequency ablation. Numerous studies show that the lateral branches of S1–S3 provide innervation to the posterior portion of the joint via the posterior sacral network (PSN), but some evidence also shows that L5 and S4 may contribute to the PSN and posterior innervation of the SIJ. In an analysis of 25 hemipelvises, S1 and S2 were found to have contributed to the SIJ in all specimens whereas S3, L5, and S4 had 88%, 8%, and 4% contributions, respectively. This same cadaveric study attempted to document SIJ innervation relative to bony landmarks to help identify reference points when performing injections under ultrasound or fluoroscopy. They found that S1 usually comes from the inferolateral aspect of the respective nerve root, S2 from the superolateral and inferolateral quadrant, and S3 from the superolateral quadrant. The lateral branches for L5 that were found to innervate the SIJ in two specimens emerged from the inferolateral quadrant of the L5/S1 intervertebral foramen, traveled across the superior articular process of S1/the sacrum, and merged with the lateral branch of S1. One specimen had S4 innervation to the SIJ, and the nerve root exits from the superolateral quadrant of the foramen.
The anterior portion of the SIJ can also be a source of SIJ pain, but this is not an area where RFA can be used to provide relief. Innervation of the anterior aspect of the SIJ is also rather unclear but has been studied both histologically and in cadaveric studies. It is believed that the ventral roots of L4, L5, and possibly S2 are responsible for the innervation of the ventral portion of this joint. ,
Diagnosis
In an attempt to provide clarity when diagnosing and considering treatment for SIJ pain, in 1994 the International Association for the Study of Pain (IASP) developed a set of diagnostic criteria to help guide clinicians in the diagnosis of SIJ dysfunction. These include:
- •
Pain in the region of the SIJ
- •
Exacerbating/reproducing a patient’s pain in the region of the SIJ through provocative maneuvers, or
- •
Relieving of a patient’s pain through injection of the SIJ with local anesthetic
When patients present with pain in the lower back just below the belt line, SIJ mediated pain should be considered. Some patients may even be able to point with one finger to localize the pain, otherwise known as the Fortin finger test. Nevertheless, it is crucial to rule out other pain mediators that may mask themselves as SIJ pain. This includes, but is not limited to, referred lumbar spine mediated pain, enthesopathies, seronegative HLA-B27 spondyloarthropathies, piriformis mediated pain, fractures of the sacrum, ligamentous injury, and even proximal hamstring tendinopathy, to name a few. With a thorough patient history and physical exam, as well as imaging modalities when needed, these can typically be ruled in or out when evaluating patients with SIJ pain. Certain risk factors such as leg length discrepancy, gait abnormalities, scoliosis with or without a history of spinal fusion, and pregnancy have been known to increase a patient’s risk of developing SIJ pain,
After other causes of SIJ mediated pain are ruled out, further physical exam tests to elicit the pain in patients can be performed to assist with the diagnosis. An analysis of multiple studies that looked to correlate positive physical exam findings with patients’ improvement in SIJ pain after receiving diagnostic analgesic block found that eliciting three or more positive responses on provocative tests has a high sensitivity and specificity. The following physical exam maneuvers have been found to be highly sensitive and specific, particularly when used in combination: compression test, distraction test, FABER test, Gaenslen’s maneuver, and thigh thrust test.
Compression test: The patient starts by lying in a lateral recumbent position, with the painful SIJ facing away from the table, and then proceeds to flex their knees and hips to about 90 degrees into a comfortable position. The clinician must stand behind the patient at the level of the pelvis and place one hand over the lateral belly of the gluteus maximus muscle between the anterior superior iliac spine (ASIS) and greater trochanter and anterior to the ischial tuberosity. The clinician’s free hand is then positioned over the other for added stability. A vertical force is then applied through the affected SIJ toward the table, and reproduction of the patient’s pain is deemed a positive test.
Distraction test: The patient should lay in a supine position, and the examiner should be standing next to the patient just below the level of the pelvis on the ipsilateral side of the SIJ pain. The clinician will proceed to place the palms of their hands on the patient’s ASIS and, while keeping their elbows extended, apply a posterior-lateral force. A positive test is considered reproducing the patient’s SIJ pain.
FABER test: With the patient in a supine position, the clinician should stand just below the level of the patient’s pelvis on the side of the patient’s SIJ pain. By taking the leg ipsilateral to the SIJ pain and positioning the lateral calf of that lower extremity on the anterior aspect of the contralateral thigh, the clinician will have proceeded to flex, abduct, and externally rotate the patient’s hip. The patient’s final positioning should resemble the figure “4.” The clinician should then place one hand on the ASIS opposite to the SIJ pain and their other hand on the medial aspect of the opposite knee. While applying posterior pressure through the patient’s knee, reproduction of ipsilateral posterior SIJ pain is considered to be a positive result.
Gaenslen’s maneuver: The patient should lay supine with the leg of the affected SIJ at the edge of the table. The patient then flexes and holds the contralateral hip and knee as close to their chest as possible. The clinician then carefully allows the leg on the side of the affected SIJ to extend at the hip while draping down from the edge of the table. While using one hand to brace the patient’s flexed knee, the other hand is placed over the anterior thigh of the extended leg while simultaneously applying a downward force. Reproduction of pain on the side of the affected SIJ is a positive result.
Thigh thrust test: The patient is lying in a supine position and the examiner must stand on the side of the affected SIJ. The patient’s ipsilateral hip is then flexed to 90 degrees while simultaneously allowing the patient’s knee to flex. While placing one hand over the patient’s contralateral ASIS, the other is placed over the patient’s flexed knee and a downward force is applied towards the table through the femur. Reproduction of the patient’s SIJ pain is considered a positive test result
To further assist in determining the cause of SIJ pain, imaging modalities are typically used, most often starting with anterior/posterior, oblique, and lateral view x-rays of the pelvis to help rule out other underlying pathologies such as fractures, spondylolisthesis, hip osteoarthritis, malignancies, and even infection. After ruling out these other pathologies, MRI can be considered if attempting to rule out sacroiliitis due to its ability to capture bone marrow edema and edema surrounding the SIJ. Otherwise, single photon emission computed tomography and CT (SPECT/CT) can be used to more clearly confirm or evaluate how the SIJ adapts to transferring forces or loads. Failure of the SIJ to properly transfer these forces has been found to correlate with increased scintigraphy uptake in the joint’s posterior and superior ligamentous structures. , Although history, physical exam, and imaging can help direct the clinician toward considering the SIJ as the primary pain mediator for patients, the debated but widely accepted gold standard for diagnosis is image-guided injection into the SIJ with local anesthetic, which we will further discuss.
Diagnostic blocks
Performing a diagnostic block by injecting an analgesic agent into the SIJ continues to be the agreed-upon means of confirming or excluding the intraarticular area of the joint as the source of a patient’s SIJ pain. These injections are typically done under image guidance, such as fluoroscopy or ultrasound. One particular study looking at fluoroscopy-guided versus clinically guided SIJ injections (no image guidance) found that only 22% of patients were able to successfully have the injection deposited into the intraarticular space when fluoroscopic guidance was not used. When considering which imaging modality to use, it is important to weigh the degree of accuracy against the clinician’s experience or comfort and the concern for injecting into extracapsular structures such as vasculature.
Ultrasound does provide a means for actively avoiding vascular structures surrounding the joint, and it is typically more available and feasible to obtain compared to a fluoroscopy machine. One study looked at the success rate of performing intraarticular injections using ultrasound and found that in the first 30 injections it is around 60%, but as the clinician becomes more adept in using ultrasound, it can reach almost 94% after the subsequent 30 injections. Another study that evaluated fluoroscopic-guided versus ultrasound-guided therapeutic SIJ injections found that although both imaging modalities provide similar improvement in patient’s pain relief and function, the fluoroscopic technique proved to be more accurate (98.2% vs 87.3%). When attempting to introduce the analgesic agent into the intraarticular area for a diagnostic block, this accuracy can be crucial.
After properly executing the diagnostic block, typically near-complete relief of SIJ pain is considered a positive response. A systematic review of numerous studies found that there is Level II evidence for the use of a dual analgesic agent block when patients have 70% pain relief, and Level III evidence for the use of single agent when patients have at least 75% pain reduction. , By utilizing the dual-block technique, the chance of a false positive is decreased because the use of a short- and longer-acting analgesic agent provides confirmation that during the duration of patient’s relief, both agents, rather than one, were effective in limiting pain in the intraarticular area injected. Typically, the short- and long-acting agents used are lidocaine and bupivacaine, respectively.
When performing this injection, patients lie prone on the fluoroscopy table, draped, and sterilely prepped while maintaining access to the posterior-inferior one-third region of the joint. Lumbosacral anatomy and SIJ anatomy can be variable from patient to patient, but the goal in positioning the C-arm should remain the same, namely attempting to align the posterior and anterior SIJ spaces within one plane while maintaining a caudal orientation of the C-arm. This will help guide needle placement correctly into the posterior-inferior region of the joint.
After using a local anesthetic over the primary entry site, a 3.5-inch, 22-gauge spinal needle is used to infiltrate the posterior-inferior region of the SIJ, and placement is confirmed through the use of a contrast agent. Arthrograms can then be taken in the anterior-posterior, lateral, ipsilateral, and contralateral oblique views to confirm proper placement. With the intention of keeping the diagnostic analgesic agent within the joint, it is important to keep the total injected volume below 2.5 mL to help maintain specificity of the diagnostic block. If any extravasation of analgesic agent does occur, it may obscure whether pain is being mediated by intraarticular pain receptors or extraarticular pain mediators such as the joint capsule or supporting ligaments. ,
Following these diagnostic blocks, it is important for patients to continue to do their day-to-day tasks and perform maneuvers that previously were known to exacerbate their SIJ pain. In addition, keeping a pain diary and monitoring fluctuations in pain following the injection can help the clinician better understand the efficacy of the injection.
Radiofrequency ablation (RFA)
When considering RFA of the SIJ, there are numerous techniques that can be used, each with a varying degree of evidence. A systematic review in 2015 performed by Simopoulos et al. showed that cooled RFA of the SIJ has Level II–III evidence versus traditional RFA of the SIJ with Level III–IV evidence. Here we will discuss these and other techniques for the treatment of SIJ-mediated pain. Regardless of which technique is utilized, the goal of performing RFA in the SIJ is to target the lateral branches exiting from the S1–S3 dorsal rami and the L5 and S4 branches that innervate the SIJ and exit through their respective dorsal rami.
This is why RFA is particularly helpful in alleviating SIJ pain stemming from the dorsal SIJ ligamentous complex, which does not receive innervation from any ventral branches but predominantly from dorsal nerve branches. Unlike the lumbar spine where blockade of the medial branches innervating the facet joints is more definitive, it is more difficult to accurately block the sacral lateral branches due to the overlying ligaments and fascia, which limits the spread of the injectate. Therefore, rather than performing a sacral lateral branch block to determine if a patient is a good candidate for SIJ RFA, injecting a diagnostic block with or without steroid into the intraarticular space of the SIJ, while ruling out other possible referred sources of pain, is a more suitable means for confirming SIJ mediated pain before considering a patient for SIJ RFA.
Conventional RFA
In two studies performed by Cohen et al. , which utilized the conventional RFA, L4 and L5 dorsal ramus lesions were created by inserting a 22-gauge needle with a 5-mm active tip. In the lumbar vertebrae the tip of the RFA needle was positioned parallel to the course of the nerve. At the L4 nerve root level they aimed to position the needle at the angle formed between the superior border of the transverse process and the superior articular processes of L5 vertebrae. At L5 nerve root level the needle was directed toward the groove of the sacral ala. To confirm placement over the target area dorsal sensory nerves, electrostimulation at 50 Hz at less than 0.5 V was used to provoke sensation. Simultaneously, confirmation of no ventral motor root involvement was done by monitoring leg contraction while providing electrostimulation at 2 Hz. Once electrode placement was confirmed, lidocaine 2% (0.5 mL) mixed with 5 mg of methylprednisolone was injected. Subsequently, the RFA probe was reintroduced and 80°C lesions were created by performing a 90 s ablation.
When performing S1–S3 ablations in these studies, 22-gauge RFA probes were placed perpendicular to the bone roughly 3 to 5 mm from the sacral foramina. At the S1 and S2 foramina the RFA probe was positioned at 1:00, 3:00, and 5:30 o’clock positions on the right and 11:00, 9:00, and 6:30 o’clock positions on the left. At the S3 level needles were placed at the 1:30 and 4:30 o’clock positions on the right and 7:30 and 10:30 o’clock positions on the left. Certain patients received RFA at the S4 level when the foramen was located at or below the level of the SIJ. In those patients, one lesion was performed at the superior aspect of the S4 foramina. Location of these various RFA lesions, as done by Cohen et al. in this study, are seen in Fig. 8.1 . After the first needle was placed at each of the S1–S3 foramina, sensory stimulation followed by injection of local anesthetic with steroid was performed similarly to the L4/L5 level. The duration and temperature of the lesioning was also similar to the lumbar region.
The advent of multilesion probes, such as the Simplicity III as depicted in Fig. 8.2 , has allowed practitioners to simultaneously target and lesion the lateral branches that innervate the SIJ with one continuous strip lesion that is created through only one inferior-lateral sacral entry site. Gilligan et al. evaluated the use of a modified Seldinger approach utilizing the curved multilesion Simplicity III RFA probe. This unique probe has three active areas that allow it to simultaneously create two bipolar lesions and three monopolar lesions and thus create a 9 mm x 52.5 mm lesion, which is showcased in Fig. 8.2 . One of the largest benefits of this novel technique involves decreased risk of bowel perforation, which otherwise may occur if the probe accidentally passes through a sacral foramen in the conventional technique. Also, greater accuracy was ensured with the use of one hollow micropuncture needle to simultaneously create a tract for precise placement of the Simplicity III probe and injection of the analgesic agent. Traditionally, an analgesic injection is followed by insertion of the RFA probe, and there is a risk that the RFA probe does not track precisely through the tract created by the analgesic needle.
