sacroiliac joint The practice is justified primarily by studies showing that outpatient clinic.

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1 2048 ORIGINAL ARTICLE Sources of Sacroiliac Region Pain: Insights Gained From a Study Comparing Standard Intra-Articular Injection With a Technique Combining Intra- and Peri-Articular Injection Claude D. Borowsky, MD, MPhil, Glenn Fagen, PhD ABSTRACT. Borowsky CD, Fagen G. Sources of sacroiliac Conclusions: Significant extra-articular sources of sacroiliac region pain: insights gained from a study comparing standardregion pain exist. Intra-articular diagnostic blocks underestimate the prevalence of sacroiliac region pain. intra-articular injection with a technique combining intra- and peri-articular injection. Arch Phys Med Rehabil 2008;89: Key Words: Injections; Pain; Prevalence; Rehabilitation; Sacroiliac joint by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine Objectives: To present evidence supporting the existence of and extra-articular sources for sacroiliac region pain and to present Rehabilitation evidence that intra-articular anesthetic blockade may underestimate the true prevalence of sacroiliac region pain. Design: Retrospective review of 2 large case series compar- N THE FIELD OF INTERVENTIONAL spine care, it has ing patient responses to intra-articular injection versus com-bined intra-articular and peri-articular injection of anesthetic 1-7 been common practice to treat recalcitrant pain in the region of the SIJ with intra-articular sacroiliac injection. and corticosteroid. The practice is justified primarily by studies showing that Setting: Private practice chronic pain clinic set in a hospitaldiagnostic intra-articular injection can temporarily alleviate outpatient clinic back, buttock, and leg pain in 18% to 57% of subjects. Participants: Patients (N 120) sequentially enrolled from Few controlled studies have demonstrated that injection of practice billing records. Inclusion criteria included pain in thesteroids into the SIJ results in long-term clinical improvement. 12,14-16 Most of these studies have examined sacroiliac low back below L4 and in the buttock, thigh, groin, or lower leg. If disk herniation, lumbar stenosis, or facet syndrome was 10,11,14-16 pain associated with a systemic inflammatory arthritis. previously treated with appropriately chosen injections, response to treatment had to be negative. Patients failed tostates are done in patients who do not have such forms of However, most sacroiliac injections performed in the United respond to treatment with physical therapy. Exclusion criteria arthritis. included records with an incomplete database, patients increasing pain medication use greater than 15% for pain not related clinic led us to believe that the success rate for intra-articular In 2003, an informal chart review done in our chronic pain to the sacroiliac region, severe psychiatric illness, and nonspe-sacroiliacific anesthetic blockade. One hundred sixty-seven records of 15 patients complaining of chronic 12mo) ( pain, presumed injections was less than 20%. The review consisted were reviewed to obtain the 120 study subjects. to be of sacroiliac origin, who had been treated with standard Interventions: Intra-articular injection was done according intra-articular sacroiliac injections by 2 interventional physiatrists. This low response rate raised the question of whether to the standard technique described by Fortin. Peri-articular injection was done by a slight modification of the procedureintra-articular injections failed because there might be significant extra-articular sources of sacroiliac region pain. Available described by Yin. Main Outcome Measures: Percentage change in visual literature on the innervation of the posterior sacral region and analog scale (VAS) pain scores at 3 weeks and 3 months on the biomechanics of the posterior sacroiliac ligaments was postinjection; patients self reported activities of daily living reviewed This suggested that the lateral branches of the (ADLs) improvement at 3 weeks and 3 months postinjection; S1-3 spinal nerves and the posterior sacroiliac ligaments could and percentage change in VAS pain score within 1 hour of be 2 of these extra-articular pain generators. Therefore, we injection. changed the preferred method for sacroiliac injections from Results: For intra-articular injection alone, the rate of positive response at 3 months was 12.50% versus 31.25% for the (fig 1) to a method dividing the injection into 2 portions, an 27 the standard intra-articular method described by Fortin et al combined injection (P.025). Positive response was defined as intra-articular portion and an extra-articular portion directed greater than 50% drop in VAS pain score or patients describing under the posterior sacroiliac ligaments, but maintaining the ADLs as greatly improved. Anesthetic response rates were same total steroid dose. Our previous injectate was 1.5mL higher in the combined injection group (62.5% vs 42.5%; 0.5% bupivacaine and 80mg methylprednisolone injected intra- P.037). articularly (or as much of that solution as the joint could hold). The new injectate consisted of 2mL 0.5% bupivacaine and From Pioneer Spine and Sports Physicians, Springfield, MA. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated. Reprint requests to Claude D. Borowsky, MD, MPhil, Pioneer Spine and Sports Physicians, 55 St. George St, Springfield, MA 01003, kskcdb@charter.net /08/ $34.00/0 doi: /j.apmr ADLs AIAT PSIS SIJ VAS List of Abbreviations activities of daily living ADLs Improvement Assessment Tool posterior superior iliac spine sacroiliac joint visual analog scale

2 SOURCES OF SACROILIAC REGION PAIN, Borowsky 2049 Fig 1. Sacroiliac (SI) injection: standard technique. 40mg methylprednisolone injected intra-articularly (or as much as the joint could hold) and 2mL 0.5% bupivacaine plus 40mg methylprednisolone injected under the posterior sacroiliac ligaments, bathing the S1-3 lateral branches and the posterior ligaments (fig 2). The exact technique for injecting under the sacroiliac ligaments is described more fully in the Methods section. This change in sacroiliac injection technique provided the opportunity to conduct a retrospective study on the outcomes for 2 consecutive series of patients, the first receiving standard intra-articular sacroiliac injection and the second receiving the newer combined injection. The goals of the study were (1) to challenge the assumption that the pathology causing sacroiliac region pain is predominantly intra-articular; (2) to perform the study in a private practice setting without denying any patient the presumed standard of care, intra-articular injection (all interventions performed on patients would be within the scope of standard clinical practice, would be covered by standard informed consent agreements, and would comply with the Declaration of Helsinki guidelines for proper treatment of human subjects); (3) to assess the short-term efficacy of corticosteroid injections for chronic sacroiliac region pain; and (4) to assess whether reliance on intra-articular diagnostic injections might result in underestimating the true incidence of pain generated from the sacroiliac region. METHODS Billing records for a single practitioner were reviewed. A list of all patients who had undergone sacroiliac injection from 2002 through 2006 was created. The patients charts were reviewed in chronologic order, and patients who met inclusion and exclusion criteria were enrolled sequentially into the study until the study included 40 patients who had undergone standard sacroiliac injection and 80 patients who had undergone the new modified procedure. Patients were included if they reported pain in the low back below L4 and buttock, thigh, calf, or groin pain. 28 All patients had also failed to improve with physical therapy consisting of any or all of the following: manual manipulation, myofascial release of the lumbopelvic area, pelvic stabilization exercise, and bracing. If potential subjects had a documented disk herniation or lumbar stenosis graded as worse than mild, they had to be unresponsive to a transforaminal epidural injection directed at the root corresponding to their symptom distribution. Presence of moderate stenosis was determined by the attending neuroradiologists official reports. Consensus definitions for moderate stenosis were lateral recess anteroposterior diameter measuring 3mm or less, or central canal anteroposterior diameter measuring 9mm or less. There was no consensus definition for foraminal stenosis. If the patients had received facet injections, they had to experience minimal or no pain relief. Patients were excluded for the following reasons. The patient s chart did not include a full database of the potentially confounding variables the study was examining (table 1). 5,8,11,29 Patients increased their pain medication use by more than 15% over the course of the 3-month follow-up period. Patients were instructed that medications could be increased for a period of 5 to 7 days postprocedure to address postprocedure pain, but afterward all attempts should be made to reduce medication use to prior usage levels or below. Patients were told exceptions could be made for a serious flare-up of a new pain that they felt would not spontaneously resolve. Patients with psychiatric impairment severe enough to impair their ability to interpret the outcome measurement tools were excluded. Any patient experiencing anesthesia below the buttock or leg weakness postinjection was excluded because this was presumed to be a nonspecific block. A total of 167 records were reviewed to obtain the 120 study subjects. Only 3 subjects were excluded because of increases in narcotic dosing (1 in the standard injection group, 2 in the new Fig 2. Acceptable and unacceptable contrast patterns for S1-3 lateral branch block.

3 2050 SOURCES OF SACROILIAC REGION PAIN, Borowsky Table 1: Sample Demographics and Symptom Distributions Variable SIJ Injection (n 40) SI S1-3 Injection (n 80) Age (y) Duration of symptom* (mo) Mean initial VAS pain scores Sex (male/female) 10/30 24/56 Unemployment Obesity Depression Diabetes Smoking Oxycodone use 120mg/d Oxycodone use 60mg/d Oxycodone use 30mg/d Lumbar fusion Stenosis Bilaterality Traumatic Pain pattern Pain pattern Pain pattern Provocative sit Provocation by standing Provocation by walking Provocation by sit to stand Provocation by stairs Provocation by side lay Positive SLR Provocation by hip AFPS PSIS tenderness NOTE. Values are mean SD, percent, or as otherwise indicated. Pain pattern 1 includes the back, buttock, and thigh; pain pattern 2 is below the knees; pain pattern 3 includes the groin area. Abbreviations: Hip AFPS, hip adduction, flexion, posterior shear; SI S1-3, combined sacroiliac and S1-3 injection; SLR, straight-leg raise. *Variable correlated with treatment outcome at 3 months for sacroiliac group. Variable correlated with treatment outcome at 3 months for combined sacroiliac and S1-3 injection group. Significant difference between the treatment groups in frequency of this variable. needle is advanced in line with the fluoroscopic beam targeting a point just medial and inferior to the PSIS. When the interosseous ligaments are engaged, the needle is rotated medially and advanced to bone on the sacrum. Radiographic contrast is injected to demonstrate spread medial to the SIJ line both cranially and caudally (see fig 2). If only cranial or caudal spread is demonstrated, then half the injectate is injected, and the needle is then directed either cranially or caudally to achieve spread in the other direction. Care is taken to avoid imbedding the tip of the needle on the ileum, which has a bulbous enlargement in this location that can serve as an alternate surface over which medication can spread (fig 3). Care is also taken to avoid a common contrast pattern in which most of the contrast flows in channels on the surface of the sacrum and directly into one of the sacral foramina (see fig 2). Patient responses to the 2 injection techniques were evaluated in the following 3 ways. First, anesthetic response was graded as positive if the patient attained a pain reduction of 75% or greater than their preinjection VAS score. All scores were recorded on a pain diary with lines to record the preinjection VAS score and 8 hourly entries for the postinjection scores. Second, at the 3-week and 3-month follow-ups, pain was assessed by percentage change from the patient s preinjection VAS pain score. The follow-up period was limited to 3 months to minimize the chance that new problems might emerge, requiring additional interventions or medication changes that would complicate the comparison between the 2 SIJ injection techniques. All but 3 patients were maintained on their previous opioid regimens, which, in most cases, consisted of a variety of extended-release opioids combined with use of a breakthrough pain medicine 2 or 3 times daily. Most patients chronic pain medication also included 1 neuromodulator, which was usually an antidepressant or anticonvulsant. Finally, at 3 weeks and 3 months, patients change in functional status was assessed with a tool created for the pain clinic, the AIAT. This measure is a global assessment based on a patient s modified injection group). All 3 increased their medications because of problems unrelated to their sacroiliac region. Two had exacerbations of neck and shoulder pain, and 1 had newonset headaches. Two were excluded for leg anesthesia, both in the modified injection group. Thirty-two subjects were excluded because of inadequate databases. Eight subjects were excluded as a result of previous pain relief by epidural injections. Two patients were excluded retrospectively because, during review of these patients cases in regularly scheduled case conferences, our attending psychologist suggested they had Axis II disorders severe enough to inhibit their ability to use our outcome measurement tools appropriately. One of these patients was in the standard injection group and 1 in the new modified injection group. Intra-articular sacroiliac injection was done according to the technique described by Fortin et al. 27 The extra-articular injection was done by modifying slightly the technique described by Yin et al. 30 The injection site is targeted by obtaining an oblique fluoroscopic view of the PSIS that aligns the medial edge of the PSIS with the medial edge of the SIJ line (about oblique). A 22-gauge, 3.5-inch to 5-inch curved spinal Fig 3. S1-3 lateral branch block: another contrast pattern to avoid.

4 SOURCES OF SACROILIAC REGION PAIN, Borowsky 2051 self-report of improvement in a basket of ADLs including housework, mobility, leisure activities, shopping, childcare, and work (if appropriate). The patient is asked to describe the overall change in these ADLs as worse, no change, slightly improved, greatly improved, or totally restored. The tool is scored like the patient-specified quality of life tool, a validated tool for assessing response to interventions in patients with spinal pain. 31 A score of 0 (worse), 1 (no change), 2 (slight improvement), 3 (great improvement), and 4 (restored to normal). The AIAT was not previously validated in a chronic pain population, so data were collected to determine the degree of correlation with the VAS score data. Prior studies of VAS pain-reporting tools had shown that 15% to 20% is the minimum change in VAS score usually regarded as clinically significant For our purposes, the more stringent criterion of 50% or more change in VAS score was regarded as the threshold for treatment success. Because the AIAT specified 5 scores, it seemed reasonable to scale the AIAT for compatibility with the VAS by looking at the following correlations: (1) a negative VAS change with an AIAT score of 0 (worse), (2) VAS change between 0% and 19.99% with an AIAT score of 1 (no change), (3) VAS change between 20% and 49.99% with an AIAT score of 2 (slightly improved), and (4) VAS change of 50% or more with an AIAT score of 3 (greatly improved). An AIAT score of 4 was not analyzed because it occurred so rarely. The AIAT and VAS correlations were assessed by determining the sensitivity, specificity, and positive-predictive value with which each AIAT score predicted a patient s VAS score changes. The following statistical analyses were done to compare responses to the 2 injection techniques. Percentage changes in the VAS pain scores were compared by using an independent Student t test. The 2 groups AIAT scores were compared using the Mann-Whitney rank-sum test. In addition, a Pearson Chisquare statistic was performed to check whether (1) the rates of response to treatment differed between the 2 procedures at 3 weeks and 3 months, and (2) the rates of initial anesthetic response differed between the 2 groups. Positive response to treatment was defined as a 50% or greater improvement in VAS pain rating or a score of 3 (greatly improved) or greater on the AIAT. Analysis for potential confounding variables was done in the following way. The frequencies for all the independent variables summarized in table 1 were assessed to determine whether there were any differences between the 2 treatment groups. Pearson Chi-square statistics were calculated to determine whether any differences between the 2 groups were significant. In addition, 2 2 contingency tables were constructed to evaluate whether any of the variables in table 1 predicted a positive response to either sacroiliac intra-articular injection or the new modified procedure. Because the patients in the study were on a variety of opioid regimens, dosage comparisons were done by converting everyone s total daily dose to its oxycodone equivalent. To test for correlations with high-dose opiate use, patients were stratified into 3 groups in terms of opiate use: (1) use greater than or less than 120mg oxycodone equivalent a day, (2) use greater than or less than 60mg oxycodone equivalent a day, or (3) use greater than or less than 30mg oxycodone equivalent a day. Correlations were tested using the Pearson Chi-square statistic and the coefficient of association. When 2 2 contingency tables contained entries with very low frequencies (such as the number of diabetics in the study or the number of patients who denied symptom provocation by standing or walking), the Fisher exact probability test was performed. An independent Student t test was used to check whether there was any difference between the treatment groups ages and initial pain scores. The Student t test could not be used to do the same cross-check analysis on symptom durations because these values were not normally distributed (see SD data, table 1). A Mann-Whitney rank-sum test was used instead. Finally, we calculated the sensitivity and specificity of the anesthetic block for predicting positive response to treatment at 3 months. RESULTS When comparing sacroiliac injection to the newer combined procedure (combined sacroiliac injection and S1 3 lateral branch block at the posterior sacroiliac ligaments), there were statistically significant differences between the changes in VAS pain scores at the 3-week and 3-month follow-ups (table 2). Similarly, the AIAT scores differed at 3 weeks and 3 months, with the differences more statistically significant at the 3-month follow-up than at the 3-week follow-up (Mann-Whitney, P.006, P.050, respectively). The actual distribution of AIAT scores is shown in figure 4. Furthermore, treatment response rates (defined as a 50% or better improvement in VAS score or a score of 3 or higher on the AIAT) differed significantly at 3 weeks and 3 months with the difference more significant at 3 months than at 3 weeks (table 3). In all cases, the combined procedure was superior to the standard procedure. In addition, the 2 procedures differed with respect to the reported rates of positive response to anesthetic block (table 4). Despite the fact that the combined procedure showed a higher anesthetic block rate, the report of a positive response to anesthetic block predicted treatment response at 3 months with poor sensitivity and specificity. Conducting this analysis on the whole study population yielded a sensitivity of 88%, a specificity of 49%, and a positive-predictive value of only 44%. If we restricted that analysis to a group that was more likely to respond to treatment (eg, nonsmokers with symptom duration 24mo), the recalculated sensitivity was 90%, the specificity was 70%, and positive-predictive value was 75%. Comparing the 2 treatment groups, they were similar with respect to all variables studied except symptom provocation by sit-to-stand transfers. The frequency of this variable was sig- Table 2: Percentage Change in Pain VAS Scores at 3 Weeks and 3 Months Injection Measurement Interval Mean Initial VAS Initial 95% CI Mean VAS 95% CI (%) P Sacroiliac injection 3wk % 8.9 NA SI S1-3 3wk % 7.0 NA Sacroiliac vs SI S Sacroiliac injection 3mo % 6.4 NA SI S1-3 3mo % 6.3 NA Sacroiliac vs SI S Abbreviations: CI, confidence interval; NA, not applicable; SI S1-3, combined sacroiliac and S1-3 injection.

5 2052 SOURCES OF SACROILIAC REGION PAIN, Borowsky Fig 4. AIAT score distributions at (A) 3-week follow-up and (B) 3-month follow-up. Gray bars indicate SI joint injection; black bars indicate SI S1-S3 injection. Mann-Whitney P values at 3-week follow-up, P.05; and at 3-month follow-up, P.006. nificantly higher in the sacroiliac injection group (77.5% vs 42.5%; P.002), but this variable did not correlate with treatment outcome. Furthermore, Student t testing showed no significant differences between the 2 treatment groups ages or initial pain scores. Mann-Whitney analysis of the duration of symptoms reported by the 2 groups also showed no significant difference between the 2 populations. The only epidemiologic variable that showed a significant positive correlation (P.05) with response to treatment by the combined sacroiliac and S1-3 injection was symptom duration 24 months or less. The following variables were associated with a decreased chance of a positive outcome: symptom duration of more than 36 months, smoking, symptom provocation by walking, symptom provocation by stair climbing, and symptom provocation by standing. The strengths of these correlations are summarized in table 5. When the same analysis was done to assess variables affecting response to SIJ injection alone, the following results were obtained. Age and duration of symptoms had a weak effect on response to treatment at 3 months. The prevalence of both of these variables was the same in our 2 treatment groups. Finally, the AIAT and VAS correlations were based on a total of 240 data pairs, 3-week and 3-month data on 40 sacroiliac injections and 80 combined sacroiliac and S1-3 injections. The ability of the individual AIAT scores to predict the VAS changes with which they were paired is summarized in the sensitivity and specificity and positive predictive value data of table 6. With 2 exceptions, the correlations were quite strong.

6 SOURCES OF SACROILIAC REGION PAIN, Borowsky 2053 Table 3: Positive Treatment Response Rate at 3 Weeks and 3 Months Injection Measurement Interval Positive Response (%) Sacroiliac injection 3wk 27.5 NA SI S1-3 3wk 42.5 NA Sacroiliac vs SI S Sacroiliac injection 3mo 12.5 NA SI S1-3 3mo NA Sacroiliac vs SI S P Table 5: Variables Associated With a Response Combined Sacroiliac and S1-3 Injection Variable Odds Ratio Coefficient P Symptom 24mo Symptom 36mo Smoking Pain provoked by standing Pain provoked by walking Pain provoked by stair climbing Abbreviations: NA, not applicable; SI S1-3, combined sacroiliac and S1-3 injection. The sensitivity of the AIAT score of 0 (clinically worse) is low, and the positive predictive value of an AIAT score of 2 (slightly improved) is only fair. DISCUSSION This study suggests that directing the corticosteroid dose not only into the SIJ but also at the posterior interosseous ligament and S1-3 lateral branches improves clinical outcome for patients with chronic sacroiliac region pain. A survey of the epidemiologic data shows the 2 treatment groups to be well matched (see table 1) despite this being a retrospective study. It should be stressed that the treatment response rates and variables associated with treatment response cited in this study may be very specific to this particular patient population, patients at a tertiary referral center for the treatment of chronic pain. Some of this study s findings have been replicated in previous studies. For instance, the finding that treatment response correlates inversely with symptom duration has been observed in many studies on spinal pain Similarly, numerous studies have demonstrated a negative effect of smoking on the outcomes of spinal procedures, although the absence of an effect has also been observed Some previous studies have also observed the absence of a correlation between chronic opioid use and the outcomes of spinal procedures, whereas others have shown that opioid use has a negative effect on prognosis. 35,38,39 This study s finding that symptom exacerbation by walking, standing, and stair climbing correlated inversely with response to treatment is statistically accurate but probably is not a relevant finding. It most likely can be explained by the low treatment response rate (31.25%) combined with the extremely high prevalence of symptom exacerbation by these 3 activities. There is little scientific literature describing the effects of lumbar stenosis or prior lumbar fusion on the outcomes of sacroiliac injections. However, we were surprised by our finding that neither of these conditions had an effect on outcome, despite the fact that they often cause substantial pathology of the lumbosacral nerve roots. The suggestion that there are important extra-articular sources of sacroiliac region pain is consistent with a number of previous studies. There are studies that demonstrate injection Table 4: Differences in Response to Anesthetic Block Injection Positive Response (%) P Sacroiliac injection 42.5 NA SI S NA Sacroiliac vs SI S Abbreviations: NA, not applicable; SI S1-3, combined sacroiliac and S1-3 injection. around the posterior interosseous ligament results in rates of improvement comparable to intra-articular injection. 40,41 Also, a recent study by Dreyfuss et al 8 showed that the Fortin sign (where patients identify the PSIS and posterior ligaments as the primary site of their pain) was extremely sensitive for predicting the results of an intra-articular anesthetic block. If the primary pathology were entirely intra-articular, it is not clear why the region of the posterior interosseous ligament would be the primary focus of most patients attention. Notably, the study by Dreyfuss 8 also showed no ability to correlate numerous physical examination tests and historical features with response to diagnostic intra-articular blocks. 8 Similar results have been reported by other authors, although these analyses have been contradicted in a few other published studies If the pathology were simply intra-articular, one would expect to be able to identify some physical tests or some feature of the patients history that demonstrated reasonably high predictive value. However, a more complex pathologic mechanism involving multiple structures, susceptible to a diverse set of provocations, might defy such clear correlations. Finally, it is a common experience in clinical practice to see sacroiliac region pain develop after painful injuries to the low back or after spinal fusions. 13,45 A number of biomechanic studies suggest that tension on various parts of the posterior sacroiliac ligaments is influenced by the actions of multiple muscles, whose activity changes in response to spinal and perispinal pain With recent anatomic studies showing an intimate relationship between the posterior sacroiliac ligaments and the S1-3 lateral branch nerves, changes in tension on the ligaments may have an effect on the activity of these branches These data also suggest that the literature on sacroiliac pain may be underestimating the true prevalence of pain generated from the sacroiliac region by basing those estimates purely on intra-articular diagnostic blocks. In this study, adding the extraarticular block to the intra-articular block was associated with a 47% increase in patients reports that their presumed sacroiliac pain responded to anesthetic blockade. Study Limitations The data on anesthetic response can be criticized for a number of reasons. First, a 75% reduction in pain was used as the criterion for a positive block, whereas many authors have used the more stringent criterion of 90% reduction in VAS pain score. 8,30,42,46-48 Given this study population of patients with chronic pain, whose reported pain levels tend to be influenced by multiple factors, and who tend to resist ever reporting total absence of pain, it was felt that setting the threshold at 90% pain relief would have resulted in spuriously low rates of positive anesthetic blocks. Using the 75% criterion, the reported rate for positive sacroiliac intra-articular blocks (42.5%) was consistent with previous reports in the literature ranging from 18% to 57% Second, one could contend that the

7 2054 SOURCES OF SACROILIAC REGION PAIN, Borowsky AIAT Prediction of VAS Score Table 6: Correlations of AIAT Scores With VAS Changes VAS 0 VAS 0% 19.9% VAS 20% 49.9% VAS 50% AIAT Score 0 AIAT Score 1 AIAT Score 2 AIAT Score 3 Sensitivity Specificity Positive predictive value NOTE. Values are percentages. anatomic basis for the extra-articular block technique employed in this study is flawed. Anatomic studies of the innervation of the SIJ suggest that the L4-5 medial branches may play a significant role in pain signal transmission in conjunction with the S1-3 lateral branches. 16,23,30 Therefore, a better sacroiliac region blockade might have been achieved by combining an L4-5 medial branch block with S1-3 lateral branch block. This critique does not invalidate the contention that intra-articular blocks may underestimate the incidence of sacroiliac region pain. However, it does raise the possibility that this study also underestimates the true incidence of sacroiliac region pain by neglecting the medial branch nociceptors. This might partially explain why the anesthetic blocks in our combined sacroiliac and S1-3 injection treatment group had a positive predictive value of only 44%. Other potential reasons for this finding are (1) the low (31.25%) overall rate of treatment response at 3 months, (2) the fact that previous studies have shown false-positive rates of up to 20% when single uncontrolled intra-articular blocks are used, 29,42,46-48 (3) the generally low specificity of targeted spinal injections, 49 and (4) the fact that high volumes may have undermined the specificity of our blocks despite there being no gross physical examination evidence of anterior spinal root blockade postinjection. 50 A number of additional issues need to be addressed when assessing the validity of this study. The AIAT is a measure that was not previously validated in a chronic pain or spinal pain population. The data, however, showed strong correlations with the VAS, which is a validated measure. Such correlations between VAS pain scales and functional measures have been replicated in other studies. 51,52 The weakest correlations were with the AIAT scores of 0 (worse) and 2 (slightly improved), but one can argue that the most relevant AIAT scores in this study are those indicating no improvement (1) or great improvement (3), and these showed extremely strong correlations with their respective VAS scores. Some authors will criticize our decision to define clinical improvement as 50% or better improvement in the VAS pain score, recommending instead using higher thresholds. 8,30,46-48 Given the previously described characteristics of this chronic pain population, we felt the lower threshold was justifiable. However, a better justification for the 50% threshold is its high correlation with the AIAT score of 3 (greatly improved). A more serious critique of the experimental design in this study is that it does not preclude the possibility that the different outcomes in the 2 injection groups could be explained solely by differences in the 2 groups response rates to the intra-articular portion of the injection. If this were true, it would undermine the contention that the addition of the extraarticular component to the combined injection accounts for its observed superiority over the intra-articular injection alone. However, several lines of evidence argue against this interpretation of the data. In both treatment groups, we analyzed the technical success rates of the sacroiliac intra-articular injection. We defined a technically successful intra-articular injection as one in which the whole SIJ line was outlined by contrast or, at a minimum, the inferior recess and one half of 1 limb of the SIJ was outlined. It was deemed unreasonable to require that the whole joint line be outlined in every case because of the high incidence of capsular defects reported in anatomical studies of the SIJ. 4,5,16,53,54 The technical success rates for the 2 groups were not significantly different, 85% for the sacroiliac group and 88.75% for the combined sacroiliac and S1-3 injection group. Furthermore, when we looked at variables that predicted an increased chance of responding to the intra-articular injection, there was no significant difference between our 2 treatment groups. One remote possibility we could not exclude was that intra-articular injection of 40mg methylprednisolone might have superior results to intra-articular injection of 80mg. If this were true, it would clearly weaken the contention that this study presents evidence supporting the existence of extraarticular pain generators. Finally, the decision to exclude from the study 3 patients who increased their opioid doses greater than 15% needs to be discussed more fully. The primary purpose of this study was to compare the efficacy of 2 different injection techniques for the treatment of back, buttock, and leg pain presumed to be of sacroiliac origin. If these patients had increased their opioid use because of increased pain in the back, buttock, or leg, then excluding them would have been inappropriate, because it clearly would have risked biasing the study. Because all 3 patients increased their medications for symptoms other than back, buttock, or leg pain, including them in the study would have biased the study more than excluding them. The extra medication they were taking would have confounded the comparison between the 2 injection techniques. Even if one does not accept that argument, the bias caused by excluding these patients is likely to be small because they represent only 3 of 123 potential subjects, and they are distributed in equal proportions between the 2 injection groups (2/80 and 1/40). CONCLUSIONS The data presented in this study suggest that the clinical entity we describe as chronic sacroiliac mediated pain has both intra-articular and extra-articular sources. The level of evidence presented in this article is not sufficient to claim that the combined sacroiliac and S1-3 injection technique is clearly a superior treatment technique to sacroiliac injection alone. A prospective randomized study would be required to support that claim. To assess the relative contributions of intra-articular and extra-articular sources, a 3-limb study would be required comparing intra-articular injection to both extra-articular injection alone and the combined procedure. Addition of a fourth group, placebo injection, would increase the power of such a study. This study also suggests the need for new studies on the prevalence of sacroiliac region pain. Because all the best epidemiologic data we have derive the prevalence of sacroiliac mediated pain from response to intra-articular anesthetic blocks, the literature probably underestimates the true prevalence of pain generated from the sacroiliac region. Additional work will need to be done in order to assess which sacroiliac

8 SOURCES OF SACROILIAC REGION PAIN, Borowsky 2055 block techniques best estimate the prevalence of sacroiliac region pain. Finally, it should be stressed that the best treatment response rate we could achieve at 3 months with corticosteroid injections was 31.25%, a rate not markedly different from the placebo response rates quoted in many studies New, better strategies are required for the treatment of chronic sacroiliac region pain. Acknowledgments: We thank Curt Loud, PAC, who provided an objective pair of eyes with which to collate and analyze our data; Vassar College and Richard Lowry, PhD, whose web site VassarStats was used for all statistical computations in this study; and the staff of the Mercy Comprehensive Pain Management Clinic, without whose hard work this study would not have been possible. References 1. Schwarzer AC, Aprill CN, Bogduk N. The sacroiliac joint in chronic low back pain. Spine 1995;20: Paris VP. 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