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. 2021 Mar 8;34(5):428–434. doi: 10.1177/1971400921998966

Impact of flow pattern, body mass index, and age on intraprocedural fluoroscopic time and radiation dose during sacroiliac joint injections

Christin A Tiegs-Heiden 1,✉, Naveen S Murthy 1, Jennifer R Geske 2, Felix E Diehn 1, Vance T Lehman 1, Greta B Liebo 1, Jared T Verdoorn 1, Carrie M Carr 1, Gavin A McKenzie 1
PMCID: PMC8559008  PMID: 33685248

Abstract

Purpose

Understanding all factors that may impact radiation dose and procedural time is crucial to safe and efficient image-guided interventions, such as fluoroscopically guided sacroiliac (SI) joint injections. The purpose of this study was to evaluate the effect of flow pattern (intra- vs. periarticular), patient age, and body mass index (BMI) on radiation dose and fluoroscopy time.

Methods

A total of 134 SI joint injections were reviewed. Injectate flow pattern, age, and BMI were analyzed in respect to fluoroscopy time (minutes), radiation dose (kerma area product (KAP); µGy m2), and estimated skin dose (mGy).

Results

BMI did not affect fluoroscopy time, but increased BMI resulted in significantly higher skin and fluoroscopy doses (p < 0.001). There was no association between fluoroscopy time and flow pattern. Higher skin dose was associated with intraarticular flow (p = 0.0086), and higher KAP was associated with periarticular flow (p = 0.0128). However, the odds ratios were close to 1. There was no significant difference between fluoroscopy time or dose based on patient age.

Conclusion

Increased BMI had the largest impact on procedural radiation dose and skin dose. Flow pattern also showed a statistically significant association with radiation dose and skin dose, but the clinical difference was small. Proceduralists should be aware that BMI has the greatest impact on fluoroscopy dose and skin dose during SI joint injections compared to other factors.

Keywords: Sacroiliac joint, injection, fluoroscopy, radiation dose

Introduction

The sacroiliac (SI) joint is a common pain generator, accounting for low back, buttock, and/or referred lower extremity pain, with proven benefit from corticosteroid injections in some patients.1–4 The diagnosis of SI joint dysfunction can be complex, given symptom overlap with facetogenic and discogenic pain and SI joint block false-positive rates of 20–22%. 4 This complexity makes anatomic injection accuracy crucial for appropriate patient management. Blind injections have demonstrated poor anatomic accuracy, with appropriate needle placement reportedly ranging from 12% to 22% and uptake noted within unintended locations such as the epidural or neural foraminal zones. 5 , 6

To ensure best practice and achieve the greatest accuracy, image guidance is preferred for SI joint injections at our institution, with fluoroscopy being the predominant method of image guidance. Computed tomography is more time-consuming and more costly, it and has been shown to result in higher patient radiation doses compared to fluoroscopy during lumbar injections. 7 Ultrasound is less confidently able to confirm intraarticular injectate secondary to inherent lack of visualization deep to osseous structures. However, fluoroscopy carries risks of radiation exposure to both practitioner and patient, with increased exposure in obese patients and those undergoing serial injections. 8 It is widely accepted that keeping radiation dosage as low as reasonably achievable (ALARA) is paramount. Even as improved technology aids in reducing radiation dose, the ALARA principle remains important to patients and practitioners who may be exposed to numerous radiation-generating imaging studies over the course of their lifetime. Therefore, understanding what may impact procedural radiation doses to both patient and proceduralist continues to be studied.8–15 In addition, a proceduralist should fully understand which factors may impact the intraprocedural course and ultimate outcome of each procedure.

At our institution, a technique similar to that described by Nacey et al. is employed for SI injections. The primary goal of the proceduralist is intraarticular access, with the operator usually making additional attempts prior to accepting periarticular injectate. 16 As patients have shown good outcomes with intraarticular, periarticular, and mixed injections, however, each of these flow patterns are considered acceptable for diagnostic and therapeutic benefit, and remain at the discretion of the proceduralist.16–18

The primary objective of this study was to determine whether the injectate flow pattern achieved, age, and body mass index (BMI) have an effect on fluoroscopy time and radiation dose during fluoroscopically guided SI joint injections. Specifically, we aimed to determine whether the small intraprocedural nuances that are different among proceduralists with respect to the ultimate outcome of obtaining intraarticular versus periarticular flow during SI joint injections affect fluoroscopy time and radiation dose. The association between intraprocedural radiation dose and injectate flow pattern has not been previously studied to our knowledge. We hypothesized that periarticular flow patterns may be associated with longer time and higher radiation dose, as the operator may have made multiple attempts to access the joint before accepting a periarticular injection. Age has not previously been studied in relation to these factors to our knowledge. We hypothesized that increasing age may be associated with higher fluoroscopy time and dose because of a higher likelihood of hypertrophic osteophytes that may obscure the joint. Finally, we also hypothesized that BMI would be associated with increased fluoroscopy time due to a longer needle path and difficulty visualizing the joint space.

Methods

Institutional Review Board approval was obtained prior to conducting this retrospective study, and the requirement for informed patient consent was waived due to minimal risk (IRB number 18-004250).

A search of the medical record between January 1, 2010, and December 31, 2013, identified 148 patients who underwent fluoroscopically guided SI joint injections at our academic spine injection practice. From our institutional database, fluoroscopy time (minutes), radiation dose (measured by kerma area product (KAP); µGy m2), estimated skin dose (mGy), age, and BMI were noted for each patient. KAP is a product of average air kerma (Gy) multiplied by the x-ray beam cross-sectional area (cm2), and it is automatically calculated by the fluoroscopy unit at our institution. For bilateral injections, radiation doses and times were halved in order to estimate those for a single SI joint injection. Cases where the achieved flow pattern was different between the two sides (n = 14) were excluded from the study. A total of 134 injections in 134 patients were included.

The primary proceduralist for each SI joint injection was one of 14 board-certified radiologists who were fellowship trained in neuroradiology or musculoskeletal radiology with additional training in spine interventions. Procedures were performed on a fixed C-arm fluoroscope (Multi Diagnostic Eleva; Phillips Healthcare, Andover, MA) whose reflected dose values were accurate within 5% after validation with an external dosimeter. The typical frames per second (fps) was 15 and operators maximized collimation to the extent possible. Most SI joint injections consisted of two to three fluoroscopic images: pre-contrast, post-contrast, and in some cases a washout image with injectate in the anteroposterior (AP) projection, in accordance with the Spine Intervention Society guidelines for documentation and image acquisition. 19 Examinations without a washout picture were included in the study. Lateral images were not obtained, as this is not part of our standard practice.

Procedural technique

All SI joint injections were performed using the following technique. The patient was placed in a prone position, prepped using sterile technique with povidone-iodine or chlorohexidine-based scrub, and draped in usual sterile fashion. The target was identified with a fluoroscopic C-arm, and the skin and proposed trajectory in the soft tissues was anesthetized with 1% lidocaine/xylocaine without epinephrine. A 25G 3.5- or 4.75-inch spinal needle was advanced under fluoroscopic guidance to the posteroinferior aspect of the SI joint in the AP projection. Adjustments were made to the needle trajectory and C-arm position as necessary to achieve intraarticular access based on prior review of any available relevant cross-sectional imaging. A test dose of iodine or gadolinium-based contrast was used to assess the flow pattern once the proceduralist achieved the target needle position. If intraarticular and extra-vascular flow was achieved, no further needle adjustments were made. If intraarticular flow was not achieved, adjustments to the needle position were made at the discretion of the operator until the contrast flow pattern was deemed acceptable (either intraarticular or persistent periarticular). The therapeutic injectate was then administered, consisting of 2–3 mL of a 1:3 mixture of betamethasone (6 mg/mL) and 0.5% ropivacaine. In all cases, the needle was restyletted and removed.

Fluoroscopy image analysis

Two board-certified fellowship-trained musculoskeletal radiologists (four and five years’ experience; G.A.M. and C.A.T.) with subspecialty practices in interventional pain management performed an independent retrospective review of the intraprocedural fluoroscopic images. Reviewers were blinded to patient charts, procedural dictations, and patient-reported outcomes. Reviewers classified intraarticular flow as linear contrast uptake flowing away from the spinal needle, outlining the articular surface in a non-vascular flow pattern. Distention of the inferior capsular recess was not necessary but considered consistent with intraarticular flow. In the absence of the aforementioned characteristics, flow patterns were considered periarticular. Any flow pattern with a discrepancy was reviewed later by consensus, blinded to the initial interpretations. Example flow patterns are shown in Figure 1.

Figure 1.

Figure 1.

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(a) Example of intraarticular contrast flow with linear contrast outlining the sacroiliac joint and distending the inferior capsular recess. (b) Example of periarticular contrast flow, with non-linear contrast pooling near the needle tip.

Statistical methods

Multivariable regression models were used to examine age, BMI, fluoroscopy time, KAP, and skin dose effects after adjusting for the other, using logistic regression for flow pattern and linear regression for fluoroscopy time, KAP, and skin dose. All pairwise and three-way interaction terms were considered in multivariable models. Significant interaction terms (p < 0.05) were included in the models. For figure purposes, fluoroscopy time, KAP, and skin dose were summarized with descriptive statistics, with distributions formally tested across two-level categories of flow pattern. Wilcoxon rank sum tests were used for the two-level category of flow pattern. Spearman correlations were used to test for an association of age and BMI with fluoroscopy time, KAP, and dose. Analyses were conducted using SAS v9.4 (SAS Institute, Cary, NC), and plots utilized RStudio v3.6.2 (The R Foundation for Statistical Computing, Vienna, Austria). A p-value of  < 0.05 was considered statistically significant.

Results

This retrospective review included 134 SI joint injections in 134 patients. There were 97 (72%) female and 37 (28%) male patients. Upon review of the captured images, 54/134 (40.3%) injections were intraarticular, and 80 (59.7%) were periarticular. Twenty-three (17.2%) injections were bilateral injections that had the same flow pattern achieved on both sides. The mean patient age was 66.79 years (range 19–90 years). The mean BMI was 29.72 kg/m2 (range 16.96–43.43 kg/m2).

Results of the logistic regression model are shown in Table 1. This demonstrated a significant relationship between skin dose and flow pattern (odds ratio (OR) = 0.963, p = 0.0086), with a higher skin dose being more likely to be associated with intraarticular flow. There was also a significant relationship between KAP and flow pattern (OR = 1.004, p = 0.0128), with a higher fluoroscopy dose being more likely to be associated with periarticular flow. Fluoroscopy time, age, and BMI were not independent predictors of flow pattern. The model concordance statistic of 0.719 indicated moderate discrimination between flow patterns.

Table 1.

Logistic regression model showing adjusted effects of fluoroscopy time, skin dose, KAP, age, and BMI.

Outcome Predictor OR 95% CI p-Value Model C-statistic
Flow pattern Fluoro time 0.696 0.37–1.31 0.2614 0.719
Skin dose 0.963 0.94–0.99 0.0086
KAP 1.004 1.00–1.01 0.0128
Age 1.018 0.99–1.05 0.2223
BMI 1.030 0.95–1.12 0.5014
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KAP: kerma area product; BMI: body mass index; OR: odds ratio; CI: confidence interval.

Results of the linear regression model are shown in Table 2. Fluoroscopy time and BMI were significant predictors of both skin dose and KAP (p < 0.001). BMI was not a significant predictor of fluoroscopy time. Age was not a significant predictor for any outcome. For the skin dose outcome, fluoroscopy time and BMI demonstrated a significant interaction (estimate = 1.30, SE = 0.38, p = 0.0009), with increased fluoroscopy time demonstrating a greater effect on skin dose as BMI increased (Figure 2). All other interactions were assessed and were not statistically significant.

Table 2.

Linear regression models for skin dose, fluoroscopy time, and KAP showing adjusted effects of age, BMI, and fluoroscopy time.

Outcome Predictor Estimate SE p-Value Model R2
Skin dose Fluoro time 17.48 2.05 <0.001 0.48
Age –0.06 0.15 0.6934
BMI 1.80 0.35 <0.001
Fluoro time Age 0.0003 0.007 0.9688 <0.01
BMI 0.0007 0.015 0.9601
KAP Fluoro time 149.11 17.96 <0.0001 0.46
Age 0.77 1.33 0.5613
BMI 20.22 3.05 <0.0001
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Figure 2.

Figure 2.

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Body mass index (BMI) and fluoroscopy time demonstrated a significant interaction for the outcome of skin dose, with greater increase in skin dose as fluoroscopy time increased in patients in the overweight and obese categories.

For intraarticular injections, the mean KAP was 317.9 µGy m2 (SD = 255.2 µGy m2), and the median KAP was 231.5 µGy m2 (range 28.4–915.0 µGy m2). For periarticular injections, the mean KAP was 354.6 µGy m2 (SD = 294.6), and the median KAP was 252.3 µGy m2 (range 38.7–1420.0 µGy m2). The p-value for KAP was 0.4209. These values are displayed in Figure 3.

Figure 3.

Figure 3.

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Analysis of variance (ANOVA) of kerma area product (KAP), a measure of fluoroscopy dose, illustrating the distribution of skin dose in the intraarticular and periarticular groups.

For intraarticular injections, the mean skin dose was 34.1 mGy (SD = 35.3 mGy), and the median skin dose was 24.0 mGy (range 3.0–186.0 mGy). For periarticular injections, the mean skin dose was 26.3 mGy (SD = 24.0 mGy), and the median skin dose was 19.0 mGy (range 0.6–156.0 mGy). The p-value for skin dose was 0.4181. These values are displayed in Figure 4.

Figure 4.

Figure 4.

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ANOVA of skin dose by injectate flow pattern, illustrating the distribution of skin dose in the intraarticular and periarticular groups.

For intraarticular injections, the mean fluoroscopy time was 1.56 minutes (SD = 0.85 minutes), and the median fluoroscopy time was 1.39 minutes (range 0.33–4.00 minutes). For periarticular injections, the mean fluoroscopy time was 1.37 minutes (SD = 0.89 minutes), and the median fluoroscopy time was 1.37 minutes (range 0.35–5.70 minutes). The p-value for fluoroscopy time was 0.1260. These values are displayed in Figure 5.

Figure 5.

Figure 5.

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ANOVA for fluoroscopy time by injectate flow pattern showing fluoroscopy time did not differ significantly between intraarticular or periarticular flow patterns.

In the intraarticular group, the mean BMI was 28.7 kg/m2 (SD = 5.3 kg/m2), and the median BMI was 28.5 kg/m2 (range 19.8–38.9 kg/m2). In the periarticular group, the mean BMI was 30.0 kg/m2 (SD = 5.9 kg/m2), and the median BMI was 29.3 kg/m2 (range 17.0–43.4 kg/m2). The p-value for BMI and flow pattern was 0.2133. Pearson’s correlation between BMI and fluoroscopy time was 0.022 (p = 0.795; Figure 6). Pearson’s correlation between BMI and KAP was 0.435 (p < 0.0001). Pearson’s correlation between BMI and fluoroscopy dose was 0.386 (p < 0.0001). Pearson’s correlation between age and fluoroscopy time was 0.034 (p = 0.696). Pearson’s correlation between age and KAP was –0.006 (p = 0.947). Spearman’s correlation between age and fluoroscopy dose was –0.045 (p = 0.599).

Figure 6.

Figure 6.

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Correlations of fluoroscopy time, KAP, and skin dose with BMI, illustrating increased skin dose and KAP in subjects with higher BMIs but no association of fluoroscopy time with BMI.

Discussion

This study did not demonstrate a significant relationship between BMI and fluoroscopy time, even when controlling for patient age. This is consistent with the findings of prior studies. 9 , 10 The current study demonstrated a statistically significant increase in both fluoroscopy dose and skin dose with increasing BMI, including when controlling for other variables, again consistent with prior research. 8 , 10 Additionally, BMI and fluoroscopy time demonstrated a significant interaction upon skin dose, with increasing time leading to larger increases in skin dose as BMI increased (Figure 2). BMI was not associated with the injectate flow pattern achieved in both regression analysis and descriptive statistics. We are not aware that this association has previously been studied. Despite a presumably longer needle path to the injection target in patients with a larger BMI, this did not affect either the fluoroscopy time or the ability to obtain intraarticular flow in our patient population.

There were no significant relationships between patient age and fluoroscopy time or dose, including when controlling for other variables such as BMI. Age was not associated with the injectate flow pattern achieved when accounting for other factors. We are not aware that any of these associations with age have previously been studied. We hypothesized that older patients were more likely to have hypertrophic osteophytes related to degenerative arthritis, which may make it more challenging to access the joint. However, in our practice, the proceduralist typically reviews any available cross-sectional imaging of the joint, which assists in choosing the approach that will most likely result in intraarticular access.

There was no significant difference in fluoroscopy time between intraarticular and periarticular injections, which is consistent with a prior study. 16 To our knowledge, intraprocedural radiation dose based on injectate flow pattern has not been previously studied. Regression models demonstrated that higher skin dose is more likely to be associated with intraarticular injections. A higher KAP (estimated fluoroscopy dose) is more likely to be associated with periarticular injections. Although statistically significant, the odds ratios were close to 1, and the dose differences between the intraarticular and periarticular groups were clinically small with considerable overlap, as demonstrated in the graphical display of the descriptive statistics (Figures 3 and 4). These findings support the proceduralist’s discretion in determining the final flow pattern without concern for a clinically meaningful increase in radiation dose for either flow pattern.

Limitations of our study include it being a retrospective review with analysis of available fluoroscopic images, which precluded real-time visualization of injectate flow pattern. There was considerable variability in the fluoroscopy times, KAP, and skin dose in the study group. Some of this may be attributable to differences in the proceduralist’s preference regarding how many attempts he or she will make to reposition the needle to achieve intraarticular flow. Vascular uptake can also occur and requires repositioning. Such a flow pattern would increase procedural time and radiation dose and was not evaluated in this retrospective study. For bilateral injections, fluoroscopy time and dose were divided in half in order to estimate values for a single injection. However, it is unlikely that the two sides took perfectly equal amounts of time. Additionally, at our institution, fluoroscopy time and dose values are manually transcribed from the fluoroscopy unit into the patient’s chart. Such a manual process is subject to human error. Other factors that we did not analyze were whether a trainee participated in the procedures and the experience of the proceduralist.

It is critical for proceduralists to understand which factors may or may not increase procedural time and radiation dosage during fluoroscopically guided interventions in order to provide safe and efficient injections. This retrospective study assessed the relationship between intra- versus periarticular flow pattern, age, and BMI with fluoroscopy time, radiation dose, and skin dose at a single institution with 14 proceduralists. None of the examined factors significantly affected fluoroscopy time. A higher BMI demonstrated the greatest increases in fluoroscopy and skin doses. The current study elucidates that procedural differences between intraarticular and periarticular flow do not affect fluoroscopy time and have a relatively small impact on dose. Of the evaluated factors, BMI has the largest impact on dose during fluoroscopically guided SI joint injections.

Footnotes

Conflict of interest: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding: The author received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs: Christin A Tiegs-Heiden https://orcid.org/0000-0003-3794-6003

Jared T Verdoorn https://orcid.org/0000-0002-1592-1182

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