Objective. To determine the physiologic effectiveness of single site, single depth sacral lateral branch injections.
Design. Randomized, controlled, and double-blinded study.
Setting. Outpatient pain management center.
Patients. Fifteen asymptomatic volunteers.
Interventions. The dorsal sacroiliac ligament was probed and the sacroiliac joint was injected with contrast medium until capsular distension occurred. The presence or absence of pain with each maneuver was noted. Under double-blind conditions, subjects returned 1 week later for L5 dorsal ramus and S1-4 lateral branch injections; 10 subjects received 4% lidocaine (active) injections while five subjects received saline (control) injections. After 30 minutes, subjects had repeat ligamentous probing and capsular distension of the same sacroiliac joint that was previously tested. The presence or absence of pain with each maneuver was noted. In a parallel anatomic study, S1 and S2 lateral branch injections with green dye were performed on two nonembalmed cadavers. Dissection was undertaken to quantify the degree of staining of these target lateral branch nerves.
Outcome measures. Presence or absence of pain for ligamentous probing and sacroiliac joint capsular distension.
Results. Forty percent had no discomfort upon repeat ligamentous probing after active lateral branch injections while 100% retained pain upon repeat ligamentous probing with control lateral branch injections. Forty percent of the active group and 20% of the control group did not feel repeat capsular distension of the sacroiliac joint after the lateral branch injections. In the anatomic study, 11 lateral branch nerves were isolated while staining occurred in only four cases or 36%.
Conclusions. Anatomic limitations exist with single site, single depth sacral lateral branch injections rendering them physiologically ineffective on a consistent basis.
The sacroiliac joint (SIJ) is a possible source of back pain. It is innervated, and therefore, is endowed with the necessary anatomical substrate to become painful [1–8]. In normal volunteers, experimental noxious stimulation of the joint evokes pain low in the back, which can radiate into the gluteal region and upper posterior thigh . In some patients, such patterns of pain can be relieved by anaesthetizing the SIJ [10–13].
Most commonly, intra-articular (IA) blocks have been used to diagnose SIJ pain. In patients with lumbosacral fusions, single diagnostic IA blocks suggest a prevalence of SIJ pain of 32% [14,15]. In patients with chronic low back pain, IA blocks using either anatomical controls or physiological controls, established a 10–15% prevalence of SIJ pain [16,17].
IA blocks, however, are not necessarily specific or sensitive for SIJ pain. Defects can occur in the ventral capsule and local anesthetic can leak out of the joint onto nearby neural structures, which compromises the specificity of the block [16,18]. IA injections might block the SIJ proper, but they do not necessarily anaesthetize the interosseous or dorsal sacroiliac ligaments, which could be an additional or alternative source of pain in patients with sacroiliac disorders [5–7].
Alternatives to IA injections are nerve blocks, in which the nerves that supply the joint are anaesthetized. However, the innervation of the SIJ is contentious, and has not been properly resolved. Some investigators maintain that the joint is innervated both posteriorly and anteriorly [3,4]. Others maintain that the innervation is exclusively posterior, and stems from the lateral branches (LBs) of the sacral dorsal rami [1,2].
If the latter is correct, diagnostic blocks of the sacral LBs become a putative diagnostic test of SIJ pain, including pain stemming from the posterior ligaments of the joint. Some investigators have performed such blocks in the pursuit of SIJ pain . Others have sought to coagulate the LBs in the treatment of SIJ pain [19–21]. In this context, LB blocks (LBBs) could be used as a prognostic tool to determine who should be offered LB radiofrequency neurotomy. Pivotal to these practices, however, is the validity of LBBs.
In the case of low back pain stemming from the lumbar zygapophyseal joints, lumbar medial branch blocks have been shown to be anatomically valid . Blocking the lumbar medial branches protects normal volunteers from experimentally induced pain from the lumbar zygapophysial joints . The same has not been shown for blocks of the sacral LBs.
The present study was undertaken to assess the validity of sacral LBBs. The hypothesis tested was that if the LBs are the sole or major source of innervation of the SIJ, then anaesthetizing these nerves should protect volunteers from experimentally induced IA SIJ pain. Furthermore, as there is no reported innervation to the dorsal SIJ ligamentous complex other than the L5 dorsal ramus and the LBs of the S1-3(4) dorsal ramus [5–8], then anesthetizing these nerves should protect volunteers from experimentally induced dorsal ligamentous pain.
The study was approved by the Institutional Review Board for Human Subject Research at East Texas Medical Center in Tyler, TX. The study was performed in March–May 2000. Fifteen healthy asymptomatic volunteers were recruited and provided informed consent. All volunteers were screened with an interview, a questionnaire, and a physical examination. In order to be eligible, volunteers had to have no neurological or musculoskeletal abnormalities; have normal skin sensitivity from the iliac crest to greater trochanter and inferior gluteal folds; have no history of spine surgery or prior SIJ injections; and have no history of back pain lasting longer than 2 days during the previous 12 months. Subjects were compensated $200 if they completed the entire study or $100 if they completed only the first injection session of the study.
Procedures were performed in the fluoroscopy suite without intravenous or oral sedation. All procedures were performed by the principal investigator. An insertion point over the inferior aspect of the SIJ was selected, and the overlying skin was anaesthetized with an intradermal injection of 1.0% lidocaine. A 22-gauge 3.5-inch Quinke point needle was passed through the insertion point and used to probe the inferior regions of the dorsal sacroiliac ligament. Subjects were asked if they felt discomfort upon probing this ligament. Subsequently, an IA block of the sacroiliac joint was performed, as described by International Spine Intervention Society (ISIS) guidelines . The needle was inserted in the inferior pole of the joint. A minimal amount (0.2–0.3 cc) of contrast medium (Isovue M-300) was injected to confirm IA injection. If venous uptake or dorsal ligamentous flow was noted, the needle was redirected, usually in an anterior direction and contrast medium was once again injected. If contrast remained within the joint, a further aliquot was injected until either the subject reported discomfort upon capsular distension, or a firm endpoint was reached without discomfort, or a maximum of 2.5 cc was injected. Subjects were asked if they felt discomfort upon capsular distension. The volume of injectate that caused discomfort upon capsular distension was recorded. AP, lateral, ipsilateral, and contralateral oblique digitally formatted images were saved (Figures 1–3).
If subjects had ventral capsular tears (as detected on lateral imaging), they were excluded. Additionally, if there was unavoidable venous uptake, substantial dorsal extravasation, extravasation to regional neural elements, lack of pain upon capsular distension, or the inability to inject contrast into the SIJ these joints were excluded.
Between 4–7 days later, subjects returned to undergo L5 dorsal ramus and LB injections (LBIs) with a 25-gauge Quinke point needle.
Using computer randomization, subjects received 4% lidocaine (active) or saline (control) injections. The randomization schedule was available only to the assistant preparing the injection solutions. The operator (P.D.), the investigator obtaining data (K.P.), and the subjects were all blinded as to the agent injected. Ten subjects received active injections and five subjects received control injections on the side of the previously tested SIJ.
The L5 dorsal ramus injection was performed using a previously described and validated technique [22,23]. Single site, single depth LBIs were performed 5–10 mm lateral to the lateral aspect of the dorsal sacral foramen at the 3 o'clock position on the right and the 9 o'clock position on the left on the dorsal osseous portion of the sacrum (Figure 4). For the LBIs, 0.2 cc of contrast medium was initially injected under AP imaging (Figure 5). If epidural or venous flow was noted, the needle was redirected and contrast was re-injected. If neither epidural nor venous flow was noted, then lateral imaging was obtained and 0.25 cc of either saline or 4% preservative free lidocaine was injected per the computer randomization schedule. A stronger concentration of lidocaine was used than may be conventional in an attempt to limit potential false negative blocks. If repeat imaging revealed epidural or venous flow, then the needle was again redirected and contrast re-injected until appropriate flow was obtained. Once epidural or venous flow was avoided, then an additional quantity of either 4% lidocaine or saline was injected such that the total volume of appropriately placed injectate over the target nerves was 0.5 cc at each level. 0.5 cc total was injected at each level (S1-4), regardless if initial venous uptake or epidural flow was noted. Subjects then rested prone for 30 minutes following the LBIs.
After 30 minutes the subjects returned to the fluoroscopy suite. The subjects were again asked if they felt discomfort upon probing the inferior dorsal sacroiliac ligament, prior to placing the needle into the joint cavity. The previously injected SIJ was re-injected with contrast medium using the same angles, and needle entry site used for the first SIJ injection. Subjects were asked if they felt discomfort upon capsular distension. Identical volume was used as for the first injection, if possible, to reproduce capsular distension without capsular extravasation. The volume of injectate that caused discomfort during capsular distension was recorded.
After completion of all injections, subjects were examined with pinprick testing to determine if they developed any areas of cutaneous numbness in the back, buttock, or upper thigh after the LBIs. If a subject had numbness, its distribution was noted.
A secondary anatomic study was undertaken. Two fresh frozen cadaveric torsos were brought to 37°C. Single site LBIs were performed by the principal investigator using green dye at S1 and S2 on the left on both specimens, and at S2 on the right of one specimen. Careful dissection was undertaken by two coinvestigators (F.W. and J.C.) to isolate and quantify green dye staining of the targeted LB nerves.
The right or left SIJ was screened in 12 subjects and bilateral SIJs were screened in the remaining 12 subjects in order to find one SIJ that met the anatomic inclusion criteria. Nine of 36 joints (25%) screened had a ventral capsular defect, and these joints were excluded. Twelve additional SIJs were excluded for other reasons including dorsal extravasation (four), inability to inject adequate contrast volume (one), inadvertent venous uptake (four), or partial flow to the S1 or S2 dorsal sacral foramen (three). Fifteen subjects (15 SIJs) were eligible for the study; 10 females and five males. The median age of the subjects in the study was 32 (range 19–52).
All 15 subjects felt pain when the dorsal sacroiliac ligament was probed. The average volume that produced capsular distention and associated discomfort was 1.5 cc (range 1.0–2.5 cc). Similar volumes were observed in all subjects upon repeat IA SIJ injections without evidence of new capsular defects.
Four out of 10 subjects (40%) in the active group and no subjects in the control group required redirecting of the needle due to epidural spread from one level of their LBIs.
Forty percent (4/10) of the active group had no discomfort upon repeat ligamentous probing after the LBIs. All subjects in the control group felt repeat ligamentous probing (Table 1).
|Agent used||Ligament Pain||Joint Pain||Buttock|
|Agent used||Ligament Pain||Joint Pain||Buttock|
Four out of 10 (40%) of the active group and 1/5 (20%) of the control group did not feel repeat capsular distension of the SIJ after the LBIs.
Five out of 10 (50%) of the active group and none of the control group experienced partial buttock numbness after the LBIs (Table 1). This numbness was appreciated in the upper outer region of the buttock and was no greater than 13 cm in diameter.
In the anatomic study, 11 LB nerves were isolated by dissection. Of these, only four (36%) demonstrated green dye staining. On the left side of specimen A, one out of two isolated nerves was stained at S1, and zero out of two isolated nerves were stained at S2. On the right at S2, one out of three isolated nerves was stained. On the left side of specimen B, two out of two isolated nerves were stained at S1, and zero out of two isolated nerves was stained at S2.
The experimental paradigm adopted in this study has previously been successful when testing the physiological effectiveness of lumbar medial branch blocks . The active group was shown to be protected from the experimental pain stimulus while the control group was not protected from the experimental pain stimulus .
This did not occur in the present study. LBBs did not consistently protect volunteers from experimentally induced pain, from either the dorsal sacroiliac ligament or the sacroiliac joint.
In the clinical study it was possible to protect subjects from dorsal ligamentous pain and SIJ pain in four out of 10 cases. In these cases, the subjects felt pain from the dorsal ligament and from the joint before the LBs were blocked, but not after they were blocked. The other six subjects were not protected from the blocks. This raises two possible interpretations.
Distending the joint capsule may be a capricious stimulus. It cannot be guaranteed, necessarily to evoke pain. It may fail to be an adequate stimulus. This appears to have happened in the one volunteer who felt no pain despite having had blocks with normal saline. However, the fact that the other four subjects, who underwent control blocks, did feel pain indicates that capsular distension is, nevertheless, a reasonable test of nociceptive capacity. Accordingly, it would appear unlikely that all six subjects, who underwent active blocks, but nevertheless felt pain, did so because of an inadequate test stimulus. This promotes the second interpretation of why they still felt pain.
Subjects could be unprotected from pain because the target nerves were inadequately anaesthetized. The anatomical data of the present study corroborate this interpretation. In only four of 11 injections (36%) did the dye cover the target nerve. This yield (36%) is remarkably like that of the success rate of the active blocks in the volunteer subjects (40%). In the remaining cases in the cadavers, the injections failed to saturate the target nerves. The anatomy of the nerves underlies this failure.
The LBs of the sacral dorsal rami do not run in a single or a constant plane [7,8]. Upon emerging from the dorsal sacral foramina, LBs sometimes initially run across bone, on the dorsum of the sacrum; only subsequently do they enter the dorsal sacroiliac ligament. At other times, LBs enter the ligament immediately upon leaving the dorsal foramina. Once LBs enter the ligament, they do not assume a constant course. The ligament is laminated, with spaces between laminae through which nerves can run, but LBs do not run in particular spaces. Some enter spaces between deeper laminae; others enter spaces that are more superficial in the ligament. These variations govern the depth of the nerves and the access of needles to them.
If a nerve happens to lie relatively superficially within the ligament, a needle placed at the depth of bone may fail to infiltrate the nerve. Injectate spreads across bone but not between the laminae that lodge the nerve. This, indeed, is what the cadaver specimens revealed. Injections into the dorsal sacroiliac ligament disperse across the planes of least resistance, at the depth of injection, but they do not track to more superficial depths. In other words, injectate will disperse at the depth of the bevel of the needle, but not more superficially. Consequently, if a needle is placed at the depth of bone, it risks not infiltrating LBs that happen to run more superifically, by one bevel length or more.
This becomes the major liability of LBBs when performed in the manner adopted in the present study. Placing the needle at a single depth risks missing nerves that run at a different depth. This can explain the variation found between skin anesthesia, ligamentous protection, and joint anesthesia in the active group. This single depth injection produces an inadequate rate of physiologically effective LBBs. This compromises the sensitivity of LBBs.
These deductions do not compromise the specificity of LBBs. Anatomical features do not invalidate positive responses when they do occur. LBBs can cause partial anesthesia in the portion of buttock they innervate. Moreover, the present data show that successful blockade of LBs may protect volunteers from pain stemming from the dorsal sacroiliac ligament, as well as the sacroiliac joint itself. Prima facie, therefore, LBBs would appear to be a valid test for dorsal sacroiliac ligament pain. That validity, however, would need to be confirmed by performing controlled blocks in order to guard against false-positive responses.
What operators need to be mindful of is that LBBs, with placement of the needle at a single depth are compromised by a high false-negative rate. Such blocks, therefore, may fail to identify patients with pain from the sacroiliac joint or from the dorsal sacroiliac ligament. In order to overcome this liability, a different technique is required. The anatomical data of the present study suggest that injections should be made at varying depths at each target point, in order to capture nerves that run at different depths. This modification is currently under study.
This study showed that single site, single depth LBIs do not adequately anesthetize the inferior dorsal SIJ ligament or IA portion of the SIJ. Single site, single depth LB injections should not be used clinically for diagnostic or prognostic purposes.
This project was possible due to the generosity of the ETMC Neurological Institute and Dr. Kevin Pauza and Dr. Douglas Kennedy.