Examining the Association of Body Mass Index With the Depth of Epidural Space, Radiation Dose Exposure and Fluoroscopic Screening Time During Transforaminal Nerve Block Injection: A Retrospective Cohort Study

2019-11-22T18:09:32Z (GMT) by Adam Galbraith


The Transforaminal Nerve Block (TFNB) represents an effective treatment modality utilised in the setting of lumbosacral radicular pain. Fluoroscopic imaging guidance aids the accurate placement of injectate resulting in ionised radiation exposure to both patients and physicians. With increasing utilisation of radiologically assisted procedures it is conceivable that a patients’ cumulative lifetime ionised radiation exposure profile will increase and with that, the potential for development of associated adverse effects.

The primary objective of this study was to examine the association between Body Mass Index (BMI) and depth of epidural space (distance of tissue from skin to epidural space). Secondary objectives were to examine the association between BMI and the following outcomes: i) radiation dose exposure; and, ii) fluoroscopic screening time.


Study design and study population: Retrospective cohort study including patients aged ≥16 years who underwent unilateral single level TFNB in a single hospital centre over a 28 months period. Study procedure: All injections were performed by a single experienced spinal orthopaedic surgeon. Demographic data, BMI (kg/m2), fluoroscopic screening time (seconds) and unit area radiation dose exposure (centi-Gray per square centimetre squared(cGy-cm2)) were recorded. Exposure of interest: BMI, Primary outcome: Depth of epidural space, Secondary outcomes: i.) Patient radiation dose exposure, ii.) Fluoroscopic screening time. Statistical analysis: Descriptive statistics for study participants’ demographics are presented. Spearman’s rank (r) coefficient was utilised to determine correlations between BMI, and primary and secondary outcomes. Linear regression analysis was performed examining the association between BMI, and primary and secondary outcomes. Unadjusted and adjusted regression coefficients (RC) with 95% confidence intervals (CIs) and p-values are presented.


A total of 362 patients met inclusion criteria; n=45 patients were excluded due to unavailable MRI images, final statistical analysis included 317 patients. Mean age was 62.6 years (IQR 53-74). Male: Female ratio was 37.9% (n=120): 62.1% (n=197). The mean participant BMI was 26.9 kg/m2 (IQR 24.4 – 28.9 kg/m2). The majority (93.1%, n=295) of injections were carried out at the L4/5 and L5/S1 (lumbosacral junction) region. Following adjustment for age, gender and spinal comorbidities there is a statistically significant association between BMI and the depth of tissue overlying the epidural space (adjusted coefficient 2.41 (95% CI 2.14, 2.68, p


This study demonstrates that there is an association between increasing BMI and increased depth of the epidural space. Furthermore, our study has indicated that increasing BMI is associated with increased radiation dose exposure and fluoroscopy screening time during transforaminal nerve block injections. It is generally accepted that there is no amount of radiation that is completely without risk and ultimately care should be taken to maximise safety for patients and physicians alike.