Abstract

Objective. It has been postulated that local anesthetic administration may be the most painful part of interventional spine procedures. Despite this, there is a lack of evidence supporting the commonly used traditional technique of anesthetic delivery as part of these procedures. This study tested three hypotheses: 1) alternative method of local anesthesia injection is superior to the traditional method; 2) using the traditional method of injection is not superior to using no local anesthetic; and 3) treatment needle size, anesthetic injection technique, and sedation are associated with pain experienced during procedures.

Design. Prospective, multicenter clinical trial of 127 participants who underwent elective bilateral symmetric interventional spine procedures in outpatient spine clinics.

Methods. Primary outcomes were pain scores during and after procedures to examine the influence of anesthetic injection method and treatment needle gauge on pain experience using linear mixed model regression analysis. A post-hoc comparison of estimated marginal mean pain scores was completed on both anesthetic injection method and treatment needle gauge.

Conclusions. The alternative method was superior (P < 0.05) to the traditional method on post procedural pain scores. Injecting local anesthetic with the traditional or alternative method was not superior to using no local anesthetic. Treatment needle size, pain at one inch of treatment needle insertion, and baseline pain were all significantly (P < 0.05) associated with overall procedural pain. Further studies are needed optimize and justify local anesthetic injections for these procedures.

Introduction

Interventional spine procedures are an increasingly popular means of diagnosing and treating spinal diseases. Conventionally, local anesthetics are injected cutaneously at the beginning of these procedures with the goal of minimizing the overall pain experience. However, the infiltration of the local anesthetic itself is painful, and, anecdotally, is often described as the most painful part of these procedures. This local anesthetic injection has been studied as a possible predictor of sensitivity to pain in general [1]. Higher pain scores with this injection are associated with pre-procedural anxiety and increased body movement during injection [1]. There is also evidence to suggest that higher pain scores during cutaneous anesthetic injections prior to epidural steroid injections may impact outcomes, such as attenuated pain relief at follow up visits [2].

There is limited research on lidocaine injections for pain reduction during spinal procedures. One prospective observational study examined the hypothesis that pain scores in response to cutaneous lidocaine injections could predict treatment outcomes to the subsequent epidural steroid injection, but the actual technique of lidocaine injection administration was not detailed [2]. Investigation into needle gauge, quantity of anesthetic, and whether or not local anesthetic should be administered prior to spine procedures was investigated concurrently in just one study, with mixed results [3].

Another method that was investigated in the literature to reduce pain during spine injection procedures was the use of topical lidocaine application, which showed some promise in mitigating procedural pain [4]. In studies of cutaneous injections of lidocaine not limited to spine procedures, other factors have been investigated including: bevel position [5], angle of needle insertion[6], buffering the local anesthetic with sodium bicarbonate [7], temperature of anesthetic solutions [8], application of mechano-anesthesia adjacent to the injection site [9], injection rate [10,11], and variations in needle size [12,13]. To address the gaps in the literature regarding effectiveness of cutaneous local anesthetic injections prior to spine procedures, we conducted a clinical trial. We hypothesized that an alternative method of local anesthetic injection would be superior to the traditional technique for reducing pain experience during interventional spine procedures. We also hypothesized that administering local anesthetic with the commonly used traditional technique is not superior to using no cutaneous anesthetic injection at all in reducing post-procedural pain scores. Furthermore, we hypothesized that factors including treatment needle size, anesthetic injection technique, and the presence or absence of sedation used for procedure, would contribute significantly to the pain experienced at one inch of treatment needle insertion and overall procedural pain.

Methods

Trial Design

This was a multicenter, prospective, single-blinded trial. There were no changes to the protocol after the trial commenced. Eligible participants were over the age of 18 and were referred by their treating-physician for elective, bilateral, symmetric, single-level, interventional spine injections under fluoroscopic guidance for pain management. These procedures included transforaminal epidural steroid injections, facet joint injections, sacroiliac joint injections, and pars defect injections. Injections were in the cervical, thoracic, and lumbosacral spine. Exclusion criteria were pregnancy, systemic infection, allergy to contrast dye, cognitive impairment, and imprisonment. The study took place at two academic medical centers, the University of Michigan and Stanford University.

The recruitment of subjects took place between January 2006 and January 2011. Potential participants were identified on the daily fluoroscopic procedural unit schedule and were approached by the study team for further eligibility determination and informed consent if they were scheduled to receive bilateral symmetric single-level injections. This clinical trial was approved by the University of Michigan’s and Stanford University’s Institutional Review Boards.

Treatment Allocation

Three different injection techniques were investigated, hereafter referred to as the traditional, alternative, and no anesthetic method or technique. All subjects enrolled in this study received symmetric bilateral interventional spine injections, always beginning with the right side. Each subject received a combination of two of the three anesthetic injection techniques. A fixed sequence of the six possible technique combinations was generated and consecutive subjects were assigned as they were enrolled. Subject allocation into one of the six possible combinations of anesthetic groups is displayed in Table 1.

Table 1

Demographics and clinical characteristics

Characteristic N = 127 
Age (mean, SD) 56.46 (16.7) 
Male (N, %) 63 (49.6) 
Race (N, %)  
 White 111 (88.1) 
 Black (non-Hispanic) 7 (5.5) 
 Asian/Pacific Islander 4 (3.1) 
 Asian 1 (0.8) 
 Other 3 (2.4) 
 Baseline pain rating (0–10; mean, SD) 3.73 (2.5) 
Type of injection procedure (N, %)  
 Lumbosacral transforaminal epidural 128 (50.4) 
 Sacroiliac joint 60 (23.6) 
 Lumbosacral facet 56 (22) 
 Cervical facet 6 (2.4) 
 Thoracic transforaminal epidural 2 (0.8) 
 Pars defect 2 (0.8) 
Sedation given (yes; N, %) 19 (15.0) 
Anesthesia type (right, left administration; N, %) 
 Traditional, none 18 (14.2) 
 Traditional, alternative 26 (20.5) 
 Alternative, none 21 (16.5) 
 None, alternative 22 (17.3) 
 Alternative, traditional 22 (17.3) 
 None, traditional 18 (14.2) 
Anesthesia needle size  
 25G1.5” 10 (3.9) 
 27G1.25” 164 (64.6) 
Characteristic N = 127 
Age (mean, SD) 56.46 (16.7) 
Male (N, %) 63 (49.6) 
Race (N, %)  
 White 111 (88.1) 
 Black (non-Hispanic) 7 (5.5) 
 Asian/Pacific Islander 4 (3.1) 
 Asian 1 (0.8) 
 Other 3 (2.4) 
 Baseline pain rating (0–10; mean, SD) 3.73 (2.5) 
Type of injection procedure (N, %)  
 Lumbosacral transforaminal epidural 128 (50.4) 
 Sacroiliac joint 60 (23.6) 
 Lumbosacral facet 56 (22) 
 Cervical facet 6 (2.4) 
 Thoracic transforaminal epidural 2 (0.8) 
 Pars defect 2 (0.8) 
Sedation given (yes; N, %) 19 (15.0) 
Anesthesia type (right, left administration; N, %) 
 Traditional, none 18 (14.2) 
 Traditional, alternative 26 (20.5) 
 Alternative, none 21 (16.5) 
 None, alternative 22 (17.3) 
 Alternative, traditional 22 (17.3) 
 None, traditional 18 (14.2) 
Anesthesia needle size  
 25G1.5” 10 (3.9) 
 27G1.25” 164 (64.6) 

Intervention

In the traditional method, local anesthetic was injected with a 25-gauge 1.5” or 27-gauge 1.25” needle placed just under the superficial skin followed by formation of a subcutaneous wheal using 0.25cc 1% lidocaine. The needle was then redirected and 0.75cc 1% Lidocaine was injected along the planned treatment needle trajectory to the hub depth of the 25- or 27-gauge anesthetic needle. In the alternative method, the local anesthetic needle was inserted immediately in the planned treatment needle trajectory to the hub depth. Then, 1cc of 1% lidocaine was injected slowly as the needle was withdrawn to the skin creating a small point wheal at the skin prior to removal of the needle. For the no anesthetic method, the procedure was started using the treatment needle with no prior local anesthetic needle insertion or injection. Following completion of the assigned anesthetic method, the treatment needle was inserted to a depth of 1 inch.

Treatment needles available for use were: 26-gauge 3.5” Quinke, 25-gauge 3.5” Quinke, and 22-gauge 3.5” Quinke. The needle size was determined by the treating physician at the time of the procedure based on clinical judgment. The same treatment needle type and size was always used bilaterally. Sodium Bicarbonate was not mixed with lidocaine during any of the injections in this study. After informed consent and enrollment, subjects were transported to the fluoroscopy suite and positioned face down on the procedural table where they were prepped and draped as per procedural protocol. The treating physician was blinded to technique allocation until just prior to beginning the procedure.

Pain Ratings and Data Collection

A baseline pain score was recorded in the pre-operative area by the treating physician during the procedure consent process, prior to transport into the fluoroscopy suite. Participants were asked to rate their current pain level on an 11-point numerical rating scale of 0 to 10, with 0 being “no pain” and 10 being “the worst pain of your life.” After the baseline assessment, the subject was informed that this same pain scale would be used several times during the injection procedure to rate the pain experienced during the procedure. Once in the procedure room, regardless of the anesthetic injection technique allocation, the starting point on the skin was identified and the subject was reminded to take note of any subsequent discomfort or pain. The designated anesthetic injection technique was then completed and the treatment needle was inserted to a depth of 1 inch. Then subjects were asked to rate their pain thus far during the procedure on the same scale of 0 to 10. This determined the pain at 1 inch score. The injection procedure was then completed on the right side and the treatment needle removed. Then the subject was asked to give a final pain rating for the right-sided injection procedure from start to finish, using the same 0 to 10 scale, determining the overall procedural pain score. The procedure was then repeated on the left side with the allocated anesthetic injection technique following the same steps described above to determine both pain at 1 inch and overall procedural pain scores.

Statistical Methods

Summary statistics of clinical characteristics and pain ratings are presented for the entire sample. An intent-to-treat analysis was used to examine primary end points of pain at 1 inch and overall procedural pain scores. We used linear mixed models with random intercepts and slopes to test the following model: anesthesia type [traditional, alternative, or none] + order of administration [first or second] + gender + treatment needle size [22, 25 or 26 gauge] + sedation administration [yes or no] + baseline pain score. For overall procedural pain, the pain scores at 1 inch were added to the model. Anesthetic needle size was not included in the model because only a small number of 25 gauge needles (n = 5) were used compared to the 27 gauge needle (n = 122). Estimated marginal means were used to conduct post-hoc pairwise comparisons of anesthetic method and treatment needle size. SPSS® 22 was used to conduct all analyses.

Results

Participant Flow

One hundred and twenty seven individuals were systematically allocated to one of six study arms. Of the total 254 procedures (two procedures per subject) included in the analysis, the traditional method was administered 84 times (33.1%), the alternative method 91 times (35.8%), and 79 procedures were completed using no anesthetic method (31.1%). Baseline pain ratings were somewhat bi-modally distributed with peaks at ratings of 0 and 4; for ratings of 1 or more, the distribution was normal, as shown in Figure 1. Baseline clinical characteristics and study arm allocation are given in Table 1.

Figure 1

 Pain rating distribution at time of pain assessments, aggregated across allocations.

Figure 1

 Pain rating distribution at time of pain assessments, aggregated across allocations.

Pain Outcomes

Results of the linear mixed model supported the primary hypothesis that the alternative method of cutaneous anesthesia is superior to the traditional method for reducing overall procedural pain scores, but not pain at 1 inch of treatment needle insertion. Linear mixed model regression also supported the hypothesis that the traditional technique was not superior to using no anesthetic injection, as shown in Tables 2 and 3. Treatment needle size had a statistically significant association with overall procedural pain scores for both 26- and 25-gauge needles compared to 22-gauge needles. There were no statistically significant differences found between 26- and 25-gauge treatment needles. Higher baseline pain was significantly associated to greater pain reported for both pain at 1 inch, and overall procedural pain.

Table 2

Linear mixed model results for pain at 1 inch

Parameter Est. SE Sig. 95% Confidence interval
 
    Lower bound Upper bound 
Intercept 3.61 0.66 < 0.001 2.30 4.92 
Right (vs. left) 0.07 0.19 0.71 −0.30 0.44 
Traditional (vs. none)*, 0.07 0.26 0.77 −0.44 0.59 
Alternative (vs. none) *, 0.06 0.25 0.80 −0.44 0.56 
Baseline pain rating 0.19 0.07 0.01 0.06 0.32 
Male (vs. women) −0.39 0.36 0.28 −1.09 0.32 
No sedation (vs. sedation) −0.75 0.49 0.13 −1.72 0.21 
Treatment needle gauge, 26 (vs. 22),§ −0.84 0.60 0.17 −2.03 0.35 
Treatment needle gauge, 25 (vs. 22),§ −0.59 0.46 0.20 −1.49 0.31 
Parameter Est. SE Sig. 95% Confidence interval
 
    Lower bound Upper bound 
Intercept 3.61 0.66 < 0.001 2.30 4.92 
Right (vs. left) 0.07 0.19 0.71 −0.30 0.44 
Traditional (vs. none)*, 0.07 0.26 0.77 −0.44 0.59 
Alternative (vs. none) *, 0.06 0.25 0.80 −0.44 0.56 
Baseline pain rating 0.19 0.07 0.01 0.06 0.32 
Male (vs. women) −0.39 0.36 0.28 −1.09 0.32 
No sedation (vs. sedation) −0.75 0.49 0.13 −1.72 0.21 
Treatment needle gauge, 26 (vs. 22),§ −0.84 0.60 0.17 −2.03 0.35 
Treatment needle gauge, 25 (vs. 22),§ −0.59 0.46 0.20 −1.49 0.31 

*Post-hoc comparison of traditional vs. alternative was non-significant (P =  0.976).

Analysis included 82 cases of traditional anesthesia, 88 cases of alternative anesthesia cases and 76 cases of no anesthesia.

Analysis included 34 cases of the 26-gauge needle, 162 cases of the 25-gauge needle, and 50 cases of the 22-gauge needle.

§

Post hoc comparison of 26- vs. 25-gauge needle was not statically significant (P =  0.633).

Table 3

Linear mixed model results for overall procedural pain

Parameter Est. SE Sig. 95% Confidence interval
 
    Lower bound Upper bound 
Intercept 3.04 0.66 <0.001 1.73 4.35 
Right (vs. left) −0.18 0.20 0.36 −0.57 0.21 
Traditional (vs. none)*, 0.24 0.27 0.38 −0.29 0.77 
Alternative (vs. none)*, −0.29 0.26 0.27 −0.81 0.23 
Baseline pain rating 0.20 0.06 <0.001 0.07 0.33 
Male (vs. women) −0.51 0.33 0.13 −1.17 0.15 
No sedation (vs. sedation) 0.44 0.46 0.34 −0.48 1.36 
Treatment need gauge, 26 (vs. 22),§ 1.46 0.56 0.01 2.57 0.34 
Treatment needle gauge, 25 (vs. 22),§ 0.97 0.42 0.03 1.81 0.13 
Pain at 1 inch 0.44 0.06 <0.001 0.31 0.56 
Parameter Est. SE Sig. 95% Confidence interval
 
    Lower bound Upper bound 
Intercept 3.04 0.66 <0.001 1.73 4.35 
Right (vs. left) −0.18 0.20 0.36 −0.57 0.21 
Traditional (vs. none)*, 0.24 0.27 0.38 −0.29 0.77 
Alternative (vs. none)*, −0.29 0.26 0.27 −0.81 0.23 
Baseline pain rating 0.20 0.06 <0.001 0.07 0.33 
Male (vs. women) −0.51 0.33 0.13 −1.17 0.15 
No sedation (vs. sedation) 0.44 0.46 0.34 −0.48 1.36 
Treatment need gauge, 26 (vs. 22),§ 1.46 0.56 0.01 2.57 0.34 
Treatment needle gauge, 25 (vs. 22),§ 0.97 0.42 0.03 1.81 0.13 
Pain at 1 inch 0.44 0.06 <0.001 0.31 0.56 

*Post-hoc comparison of traditional vs. alternative was significant (P =  0.041).

Analysis included 82 cases of traditional anesthesia, 88 cases of alternative anesthesia cases, and 76 cases of no anesthesia.

Analysis included 34 cases of the 26-gauge needle, 162 cases of the 25-gauge needle, and 50 cases of the 22-gauge needle.

§

Post-hoc comparison of 26- vs. 25-gauge needle was not statically significant (P =  0.319).

Discussion

The findings of this study suggest that baseline pain scores are significantly predictive of pain at 1 inch of treatment needle insertion and overall procedural pain, regardless of anesthetic method. Treatment needle size was also significantly associated with overall procedural pain scores as expected, with greater pain reported using 22-gauge needles relative to 25- or 26-gauge needles. Treatment needle gauge, however, was not a significant predictor of pain experienced at 1 inch of insertion. With respect to anesthetic injection technique, the alternative method was associated with significantly lower overall procedural pain scores than the traditional method. None of the three anesthetic methods had a significant association with pain at 1 inch of treatment needle insertion. Findings indicated that both traditional and alternative anesthetic injection techniques were not statistically superior to using no local anesthetic in reducing pain at 1 inch. Lastly, we found that the pain at 1 inch was significantly predictive of overall procedural pain scores. Use of conscious sedation, gender, and order or side of injection had no significant association with pain scores at either time point.

The association of higher baseline pain scores and overall pain experience may be explained by several factors. First, hyperalgesia is common among people with chronic back pain [14]. Greater pain prior to initiation of the procedure may influence the hyperalgesia associated with the procedure. A limitation of this trial may have been not examining the acuity or chronicity of each subject’s painful condition, and not taking into account their past and or current medication regimen. Assessing these factors may have helped to further characterize our sample. Acute inflammatory pain, even in the chronic pain population, may lead to secondary hyperalgesia, characterized by further increased pain with application of mechanical stimulus to the affected region [15]. Controlling for additional causes of hyperalgesia, such as opiate use, anxiety, and sleep deprivation were not assessed in this study, but may be helpful in strengthening future studies. It is possible that participants with higher baseline pain scores may have lower pain tolerance overall, leading to inflated procedural pain scores. However, more studies are needed to confirm this supposition. Another possible reason for the observed influence of baseline pain scores may be related to greater pre-procedural anxiety in this population, as pre-procedural anxiety is strongly predictive of greater pain with cutaneous lidocaine injections in spine patients [1]. Subjects enrolled in this study were not screened regarding pre-procedural anxiety and/or familiarity with these injection procedures. The degree to which the bedside nurse played a role in decreasing anxiety through comfort and reassurance was also not standardized or measured and may be an area of interest for future studies. Investigation into other modifiable factors that play into the baseline pain scores should be a target for future studies on optimizing pain experience during interventional spine procedures.

Our findings indicate that pain experienced after cutaneous anesthesia up to the point of a 1 inch depth treatment needle insertion was significantly associated with overall procedural pain scores. This finding supports the goal of minimizing the initial painful stimuli during these procedures, in an effort to set the stage for a more tolerable procedure overall. Proposed mechanisms for this association may include a heightened sense of pain awareness with a more painful initial injection, pre-existing hyperalgesia, and/or a lower pain tolerance. Here again, more research is warranted. It is possible that greater pain during anesthetic injection may cause further anxiety and/or excessive body movement during the remainder of the procedure. Excessive body movement during cutaneous lidocaine injections is associated with a more painful experience [1], although the cause and effect of this relationship is not yet determined.

The statistically significant difference in overall pain scores between a 26-gauge needle and a 22-gauge needle was, on average, 1.8 pain points, approaching the generally accepted clinically significant change in pain score of 2 points on an 11-point numerical rating scale [16]. The statistically significant change in pain scores when comparing the 25- and 22-gauge needles was about 1 pain point may be less clinically significant, although no statistically significant differences were noted between 25- and 26-gauge treatment needles. The finding of treatment needle gauge as a predictor of overall procedural pain scores was not surprising. A study conducted on needle diameter and pain was completed using automated injection delivery device controlling for velocity, angle, and depth of needle insertion with different needle diameters [12]. The larger diameter needle consistently and significantly predicted more painful injections [12]. These findings are also consistent with a Japanese study [3], which found that administration of 2 ml 1% lidocaine with a 24-gauge needle was more painful than 0.5 ml 1% lidocaine with a 27-gauge needle. In this same study, both anesthetic injections increased the overall pain experienced during the spine procedure when compared to inserting a 25-gauge treatment needle without cutaneous anesthesia. The researchers observed that both anesthetic techniques reduced the pain upon insertion of the 25-gauge treatment needle, but anesthetic injections actually increased the overall pain of the procedure when compared to using no cutaneous anesthetic. This is in contrast to our current study, which took into account both the lidocaine injection and treatment needle insertion when participants rated their pain at 1 inch depth.

Proposed mechanisms for significant increases in overall procedural pain with larger treatment needles may be explained by more tissue damage with manipulation of a larger needle, especially in the deeper non-anesthetized tissues. As described in other studies, there is a deep phase pain component to these procedures [4]. The alternative method significantly decreased overall procedural pain compared to the traditional technique. It may be hypothesized that there was a more accurate trajectory and deeper spread of the local anesthetic using the alternative method of injection due to the initial insertion of the anesthetic needle in the trajectory of the treatment needle down to hub depth. This evidence may indicate that the deep phase component of pain may account for a larger portion of overall procedural pain scores than once thought. These methods likely allowed for similar superficial cutaneous anesthesia, whereas the traditional method may not have achieved optimal deeper anesthesia along the treatment needle trajectory.

In the current study, the traditional method was initially thought be less effective than the alternative method of injection for several reasons. The initial needle insertion through the skin, rich in nerve endings, with the traditional injection is often incorrectly performed at an angle relatively parallel to the skin due to increased ease of obtaining an intradermal skin wheal. This can result in increased time spent superficially, which ultimately disturbs and distends nerve fibers during the formation of the superficial skin wheal through mechanical and chemical stimulation. This was thought to be more painful than a direct needle insertion in a more perpendicular anesthetic needle trajectory combined with slow infiltration of anesthetic sub-dermally while withdrawing the needle and coming to a pin-point intradermal wheal. There is evidence against both rapid distension of superficial nerve endings, and angle of insertion parallel to the skin in the literature [6,10].

It is unclear why both of these local anesthetic injections did not show a significant difference in pain experience compared to using no cutaneous anesthetic in our study. Interventional spine procedures, including the use of lidocaine injections for cutaneous anesthesia, remain a popular means of pain control for those with back pain. Several factors need to be addressed, and specifically, in combination, to optimize anesthetic delivery and increase procedural tolerance. Future studies are recommended to optimize local anesthetic injections prior to interventional spine procedures. We suggest a combination of the bevel up needle position [5], perpendicular angle of needle insertion [6], buffering the local anesthetic with sodium bicarbonate [7], warming anesthetic solutions [8], application of mechano-anesthesia adjacent to the injection site [9], slower injection rate [10,11], non-continuous injection pattern [10], distraction of subject [10], and smaller needle circumference [12,13].

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