Contrast Medium Volume Needed to Reach Anterior Epidural Space via the Kambin Triangle or Subpedicular Approach for Transforaminal Epidural Injection

Background: Transforaminal (TF) lumbar injection is a commonly used minimally invasiveintervention for management of chronic low back pain. TF injection can be performed usingvarious approaches to inject the drug to the anterior epidural space (AES).Objectives: To identify the volumes of contrast medium needed to reach the AES and otherlandmarks in the Kambin triangle (KB) and subpedicular (SP) approach of TF injection in patientswith lumbosacral radicular pain.Study Design: Randomized controlled trial.Setting: Pain clinic and operating room of a tertiary care hospital.Methods: Seventy-five eligible patients were randomized to receive TF epidural injection eitherby SP (SP group; n = 38) or the KB (KB group; n = 37) approach under fluoroscopic guidance. Afterconfirming the appropriate needle position, contrast medium was injected at 0.5 mL incrementsup to 2 mL under intermittent fluoroscopy. Contrast medium volumes needed to reach specificlandmarks, that is, AES, medial to superior pedicle, medial to inferior pedicle, medial aspect of boththe superior, and neural spread, were recorded. Following this, 4 mL of the drug (0.5% lidocaine1 mL + methylprednisolone 80 mg + 1 mL normal saline solution) was injected. Patients wereevaluated for Visual Analog Scale (VAS) and modified Oswestry Disability Questionnaire (MODQ)scores after epidural injections at 2 weeks, 1 month, and 2 months.Results: Average volume of contrast medium needed to reach AES was 1.10 ± 0.46 mL in the KBapproach and 1.10 ± 0.38 mL in the SP approach. Contrast medium volume needed to reach otherlandmarks showed comparable results in both groups. AES was seen in 27.02% (10/37) patientsin the KB group and 23.6% (9/38) patients in the SP group with 0.5 mL of contrast medium. Thisincreased to 56.76% (21/37) and 77.7% (28/38) with 1 mL of contrast medium (P = 0.03, chi-squaretest). No anterior spread was seen even after 2 mL of contrast medium in 4 patients in the KB groupand 2 patients in the SP group. Neural spread was seen in 100% of patients in the KB group after0.5 mL of contrast medium, but in 34 (89.4%) patients in the SP group (P = 0.03, chi-square test).We did not note any contralateral spread. Short-term effectiveness in pain relief in terms of VAS forback pain and functional improvement in terms of MODQ score over time showed similar results inboth groups. Intravascular needle puncture and needle paresthesia was comparable in both groups.Limitations: Small follow-up duration is one the limitations of this study. Future studies will beneeded to assess any long-term differences in outcome between approach methods. Also, use ofintermittent fluoroscopy might have limited detection of intravascular injections of the contrastmedium in comparison to the continuous fluoroscopy.Conclusions: To conclude, our study revealed that average volume of contrast medium neededto reach AES and other landmarks were comparable with both approaches of TF injection.Key words: Transforaminal injection, subpedicular approach, Kambin triangle approach, contrastmedium spread, anterior epidural spread