Examining the association of body mass index and the depth of epidural space, radiation dose exposure and fluoroscopic screening time during transforaminal nerve block injection: a retrospective cohort study.

OBJECTIVE: The primary objective of this study was to examine the association between body mass index (BMI) and the depth of tissue overlying the epidural space. Secondary objectives examined the association between BMI and (1) radiation dose exposure and (2) fluoroscopic screening time during transforaminal nerve block (TFNB) injections.

METHODOLOGY: This is a retrospective cohort study including patients aged ≥ 16 years who underwent unilateral single-level TFNB in a single centre over a 28-month period, by a single spinal orthopaedic surgeon. Demographic data, BMI (kg/m2 ), fluoroscopic screening time (seconds) and 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: (1) radiation dose exposure, (2) fluoroscopic screening time. Descriptive statistics for study participants' demographics are presented. Spearman's rank (r) coefficient and linear regression analysis was performed examining the association between BMI and the outcome measures.

RESULTS: A total of 362 patients met inclusion criteria; n = 45 patients were excluded due to incomplete data, final 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). Mean BMI was 26.9 kg/m2 (IQR 24.4-28.9 kg/m2 ). 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 < 0.001)). We also found a significant association between BMI and both secondary outcomes, radiation dose exposure (adjusted coefficient 1.45, (95% CI (0.84, 2.06), p < 0.001)) and fluoroscopic screening time (adjusted coefficient 0.11, (95% CI (0.02, 0.20), p = 0.02)).

CONCLUSION: This study has demonstrated a significant association between increasing BMI and increased depth of the epidural space. Furthermore, significant associations between increasing BMI, radiation dose exposure and fluoroscopy screening time have been identified. BMI may represent a modifiable risk factor with a view to decreasing patient exposure to medical ionised radiation.