Transforaminal Insertion of a Thermocouple on the Posterior Vertebral Wall Combined with Hydrodissection during Lumbar Spinal Radiofrequency Ablation

DISCUSSION

As demonstrated in this case series, the transforaminal insertion of a small thermosensor into the anterior epidural space at the midlevel of a vertebral body is technically feasible, can be performed with stand-alone fluoroscopy, and adds little time to the procedure. Double-oblique access is mandatory to access the midlevel of the vertebral body wall because it allows lining up of the thermosensor with the ideal trajectory. Because the thermocouple is very thin, a more direct approach (similar to a discal puncture) cannot achieve such positioning because the thermometer will not “take the curve” along the posterior part of the vertebral body. The craniocaudal approach should always be favored to avoid transgressing the danger zone located at the anterior and superior parts of the foramina, which contains the radicular nerve and adjacent vessels.⁶ The double-oblique approach allows the safest passage (posterior and inferior) through the foramen.^7,8 In the aging spine, the anatomy of the foramen can, however, be modified by a bulging disc and/or osteophytes. Careful examination of the planning CT is therefore recommended before thermosensor insertion.

Although more challenging than a posterior epidural or a straight transforaminal approach, the technique is the only method to access the anterior epidural space at exactly the level of the RFA probes where fluctuations in local temperature are believed to be the most critical because of the shape and extent of the expected ablation zone. Moreover, it can easily be combined with hydrodissection, which allows separation of the dural sac from the ablation area if the tumor does not extend into the anterior epidural fat. The major challenge of the technique is to precisely navigate through the foramen under fluoroscopy with a double-oblique approach. Any deviation from the ideal trajectory might lead to either a suboptimal position or the impossibility of advancing the thermosensor because of bony interposition.

In this series, we had a single case in which active fluid injection was required to maintain the local temperature at a maximum threshold of 45°. This excellent safety profile of ablation might be explained by the preferential use of dedicated bone bipolar RFA devices that are known not to diffuse through an intact osseous cortex and form smaller, precise, and more predictable ablation zones than other ablation techniques (cryoablation, microwave, monopolar RFA).^2,9 Nevertheless, local thermal monitoring gives extra confidence and reassurance to the operating physician who can follow the real-time evolution of local temperatures, thereby increasing the safety profile of the procedure, even though the present study is not powered enough to statistically prove it. This technique is particularly useful if the procedure is being performed with the patient under general anesthesia without access to intraprocedural neurophysiologic testing to monitor neural conduction.¹⁰ It can theoretically be used in combination with all heat- or cold-based ablation modalities (RFA, microwave, laser, cryoablation) to avoid breaching above 45° or below 0°, which are known to be the neurotoxic threshold temperatures.¹¹ Although favorable and with many potential uses, this technique is unfortunately not transferable to the dorsal spine because of the interposition of the ribs, which preclude a required steeper sagittal approach. The present study is limited by the small cohort of patients, which does not allow definitive conclusions regarding efficacy and safety. Moreover, all the procedures were performed by interventional radiologists trained in the spinal procedure, likely representing a bias in terms of reproducibility.