Abstract
BACKGROUND AND PURPOSE: Despite ongoing clinical suspicion regarding the relationship between sacroiliac joint (SIJ) dysfunction and lower extremity symptoms, there is a paucity of scientific literature addressing this topic. The purpose of this study was to describe patterns of contrast extravasation during SIJ arthrography and postarthrography CT in patients with lower back pain and to determine whether there are pathways of communication between the SIJ and nearby neural structures.
METHODS: Fluoroscopically guided SIJ arthrography was performed on 76 SIJs. After the injection of contrast medium, anteroposterior, lateral, and oblique radiographs as well as 5-mm contiguous axial and direct coronal CT images were obtained. Contrast extravasation patterns were recorded for each joint. These observations included a search for contrast extravasation from the SIJ that contacted nearby lumbosacral nerve roots or structures of the plexus.
RESULTS: Sixty-one percent of all joints studied revealed one of five contrast extravasation patterns. Three of these observed patterns show a pathway of communication between the SIJ and nearby neural structures. These included posterior extravasation into the dorsal sacral foramina, superior recess extravasation at the sacral alar level to the fifth lumbar epiradicular sheath, and ventral extravasation to the lumbosacral plexus.
CONCLUSION: Three pathways between the SIJ and neural structures exist.
At the turn of the century, strain of the sacroiliac joint (SIJ) was regarded as the primary etiologic factor leading to symptoms of sciatica (1). Later, in 1936, Pitkin and Pheasant (2) described lower extremity pain as originating in the sacroiliac and lumbosacral joints and their accessory ligaments and coined the term sacroarthrogenic telalgia. For decades, SIJ fusion was a common surgical procedure used to alleviate symptoms of sciatica (3). After Mixter and Barr's (4) initial description of the herniated nucleus pulposus, focus on the SIJ as a mediator of lower extremity symptoms fell from favor.
Since that time, however, there has been a growing understanding that morphologic changes are not necessarily predictive of the symptom complex of any individual with lower back pain (5−7). For instance, structural changes of the intervertebral disk, including some disk herniations causing neural compression, are seen in asymptomatic individuals (5, 7). Despite ongoing clinical suspicion regarding the relationship of the SIJ to lower extremity symptoms, there is a paucity of scientific literature addressing this topic.
Recently, Fortin et al (8, 9) documented that provocation (direct capsular stimulation at the time of injection) of the SIJ produces a referred pain pattern over the medial gluteal region in asymptomatic volunteers. There are also numerous reports of patients presumed to have idiopathic SIJ pain who also have an array of lower extremity symptoms, including radicular pain (1, 10, 11). Capsular irritation of the SIJ is thought to be the underlying mechanism leading to lower extremity symptoms, yet there are main nerve trunks adjacent to the SIJ, the role of which in the mediation of lower extremity symptoms remains unexplored. Moreover, recent studies identifying nociceptive and neuropathic inflammatory activation within spinal structures, including facet joints and intervertebral disks (12−14), emphasize the need to investigate pathways of communication between spinal structures and nearby neural elements. The purpose of this study was to report patterns of SIJ contrast extravasation during SIJ arthrography and observe the relationship between the SIJ and nearby neural structures.
Methods
Forty-three consecutive patients with lower back pain (mean age, 33 years; age range, 24−48 years), were enrolled in the study after receiving Internal Review Board approval. The participants met the following criteria: back pain present for longer than 2 months; lower back pain:lower extremity pain ratio of greater than or equal to 75%:25%; no evidence of motor or sensory changes revealed by examination to suggest radiculopathy; no allergy to contrast media or iodine; Beta human chorionic gonadotropin-negative; and willingness to participate in the study. Investigators chose to study the SIJs on the symptomatic sides of the participants. Thirty-one of the 43 participants had bilateral complaints, which allowed for examination of 76 SIJs.
SIJ arthrography was performed as described in a previous study by Fortin and Tolchin (16). Complete arthrograms were obtained in all cases, and no complications resulting from the procedure were observed. No complications were anticipated because previous studies suggested that arthrography is a safe procedure in carefully selected participants (8, 9, 16). Nonetheless, patients were monitored for allergy to contrast medium, postprocedural infection, and bleeding. These problems were minimized by excluding those with histories of allergy to iodinated materials or contrast medium, using a sterile technique and 25-gauge spinal needles. Moreover, there are no major vessels along the needle pathway when using the technique previously described by Fortin et al (8, 9). After the injection of contrast medium, anteroposterior, lateral, and oblique radiographs were obtained for each SIJ. Postarthrography CT was performed on the same joints. Five-millimeter contiguous transaxial and direct coronal images were acquired. A −20° gantry tilt was used for the transaxial scans, and a +20° gantry angle was used for the coronal scans. Window-level settings as well as bone-detail algorithms were adjusted for optimal visualization of soft tissue and osseous structures. Two observers (J.F., F.F.), both of whom are experienced interpreters of SIJ arthrograms, graded the arthrograms and postarthrography CT findings. The joints were divided into anterior, posterior, superior, and inferior components for regional recording of contrast patterns from both techniques. Choices of contrast patterns included anterior, posterior, superior, and inferior extravasation. Other findings, such as capsular attenuation (ie, capsular bulge without associated extravasation) or diverticula, were noted by the observers but were not the focus of this study. In addition to recording the region of contrast extravasation from the joints, the observers were asked to search for extravasation of contrast medium from the SIJ that contacted nearby lumbosacral nerve roots or structures of the sacral plexus.
There was complete agreement between both reviewers regarding all but one case in which extension of contrast medium from the superior aspect of the joint had occurred. Both reviewers observed the leakage of contrast medium, yet one failed to document the extension of contrast medium into the adjacent L5 root canal, which was undoubtedly present on postarthrography CT scans (Fig 5). Complete concordance of findings between plain films and postarthrography CT was achieved in terms of the region of extravasation. CT was necessary to visualize the neural elements and to determine whether contrast medium contacted them.
Results
Thirty-nine percent of all SIJs studied showed no contrast medium extravasation. A normal SIJ arthrogram is shown in Figure 1. Sixty-one percent of all SIJs studied revealed extravasation of contrast medium. Observed contrast medium extravasation patterns from the ventral, dorsal, superior, and inferior aspects of the 76 SIJ capsules are displayed in Table 1.
Ventral extravasation near the lumbosacral plexus was observed in 12 (16%) of the SIJs studied (Fig 2). Other ventral capsular findings, not included as frank contrast medium extravasation, deserve special mention. Six arthrograms revealed ventral capsular attenuation without extravasation, whereas nine others exhibited a wispy pattern of extravasation through a small ventral schism (Fig 3).
Dorsal leakage of contrast medium was seen in 24 (32%) of all SIJs. In six of these, contrast medium was observed to enter the first dorsal sacral foramen, which overlies the first sacral nerve root (Fig 4). In the others, contrast medium layered between the dorsal sacroiliac ligament and the sacrum.
In addition, two (3%) arthrograms exhibited direct flow of contrast medium from the superior capsular recess along the sacral ala to the fifth lumbar epiradicular sheath (Fig 5).
Discussion
During the past several decades, there have been numerous reports of patients suspected of having idiopathic SIJ pain who also have associated lower extremity symptoms, including radicular pain (2, 10, 11). The mechanism by which lower extremity pain occurs in patients with SIJ dysfunction remains unknown. Unfortunately, spinal morphologic studies often fail to be adequately predictive of the true pain generator in any given patient. Studies that seek to explain morphologic and physiologic changes and how they relate to the evolution of spinal pain syndromes are much needed.
In this study, five principal patterns of extracapsular contrast extravasation from the SIJ were observed using plain-film arthrography and postarthrography CT. Three of these patterns reveal a potential pathway of communication between the SIJ and nearby neural structures. These include posterior extension into the dorsal sacral foramina, superior recess extravasation at the sacral alar level to the fifth lumbar epiradicular sheath, and ventral leakage to the lumbosacral plexus.
Frank extravasation of contrast medium is easily visible on both plain-film arthrograms and postarthrography CT scans. The added value of postarthrography CT lies in its greater sensitivity for detection of subtle capsular changes and extravasation of small amounts of contrast medium. This is especially true for the ventral capsule in which changes of normal capsular structure are seen. The range of ventral capsular findings may represent points on a continuum of the same process. The patterns of ventral capsular attenuation and schism later might become frank ventral tears.
The large percentage of observed dorsal extravasation is not surprising considering that the dorsal capsule is discontinuous compared with the ventral capsule, which is a continuous sheet of connective tissue (10). The dorsal capsule allows contrast medium to track along the posterior aspect of the sacrum and enter the dorsal sacral foramen relatively unimpeded.
Our findings raised some intriguing questions that have provided the substrate for investigations that are currently underway. Is it plausible that in the setting of capsular disruption, intra-articular contents, including inflammatory chemical mediators in symptomatic patients, could leak from the SIJ in a manner similar to the extracapsular contrast extravasation patterns observed in this study? If so, irritation of adjacent neural structures could manifest as lower extremity symptoms.
The role of contrast-enhanced anatomic studies as progenitors in revealing spine pain mechanisms bears historical significance. For example, Lindblom (18) provided the foundation for the understanding of discogenic pain when he injected red dye into the nucleus pulposus of cadaveric disks and observed it leaking through annular rents. Similarly, we think that patterns of communication from the SIJ to nearby neural structures may have significance. There is evidence of a substantial role for biochemical or inflammatory mediators in discogenic and facet joint pain (12−15). Accordingly, the pathophysiologic mechanism by which SIJ dysfunction leads to lower extremity pain may include biochemical factors that affect regional nerves via direct communication from the SIJ.
Conclusion
Contrast extravasation after SIJ arthrography can be visualized in patients with lower back pain. Five extravasation patterns were observed, including three pathways of communication between the SIJ and nearby neural structures.
Footnotes
↵1 Address reprint requests to Joseph D. Fortin, DO, Division of Spine Technology and Rehabilitation, 7230 Engle Road, Suite 210 Fort Wayne, IN 46804.
- Received June 13, 1997.
- Accepted after revision March 9, 1999.
- Copyright © American Society of Neuroradiology