Dupuy et al. [7] measured differences in heat transmission for radiofrequency energy in cancellous and cortical bones and performed the first radiofrequency ablation on a metastatic hemangiopericytoma of L2 vertebra under CT guidance. Their early work showed that despite the use of internally cooled electrodes at maximum output, injurious elevations of temperatures in the epidural space did not occur. This work confirmed decreased heat transmission in cancellous bone and an insulative effect on cortical bone. They postulated that additional factors that may account for differences in heat distribution were the local heat sink from the rich epidural venous plexus and CSF pulsations. They also identified that a larger margin of safety could be gained by successfully preserving cancellous or cortical bone between the lesion and the spine. A vital point is that RFA that heats tissue to 45 °C is cytotoxic to the spinal cord and peripheral nerves [8]. Cortical bone insulates and protects the spinal canal, but an absent or lytic posterior vertebral body cortex or a widened neural foramen could increase the risk of thermal nerve damage.

The literature describes additional techniques for neuronal protection: Nakatsuka et al. [9] monitored spinal canal temperature in real time with thermocouples inserted in the epidural space between the tumor and the spinal cord. Buy et al. [10] introduced the practice of carbon dioxide dissection with thermocouples to protect vital structures. However, some authors have performed radiofrequency ablation of vertebral tumors with posterior wall defects followed by vertebroplasty with no significant complications [11].

Groenemeyer et al. [12] and Schaefer et al. [13] were among the first researchers to describe the combination of radiofrequency ablation and subsequent cement augmentation to achieve stability after tumor removal as a palliative treatment for metastatic spinal lesions. Both groups used commercially available radiofrequency systems employed routinely for ablation in other parts of the body, namely, the RITA system (RITA Medical Systems, Mountain View, CA), wherein the radiofrequency generator is connected to an expandable electrode catheter with multiple retractable arrays. The first group [12] reported a series of 12 patients where four of them underwent vertebroplasty few days after the ablation. The second group [13] reported a single case performed under CT guidance, where a 2 × 3 cm solitary lytic renal cell cancer metastasis of L3 was ablated using a 16-gauge LeVeen needle electrode and a radiofrequency generator (RF 3000; RadioTherapeutics, Sunnyvale, CA). A simple vertebroplasty immediately followed the ablation. The whole procedure was performed under general anesthesia.

Larger patient series were then published describing patients treated by combined radiofrequency ablation and cementoplasty. All papers reported pain relief despite borderline safety profile. Nakatsuka et al. [14] reported a series of 17 cases done under CT guidance with four major neurological complications. The authors used Cosman Coagulator-1 (Radionics, Burlington, MA). Gevargez et al. [15] reported 41 patients where ablation was performed using cool-tip electrodes (Radionics, Burlington, MA) and a RITA system (Mountain View, CA). Twenty-two patients underwent vertebroplasty after ablation, four of whom experienced radiculopathy as a side effect. All cases were also performed under CT guidance. Munk et al. [16] reported a group of 19 patients where they used biplane fluoroscopy and general anesthesia. They reported minor complications in seven patients related mainly to leakage. Lane et al. [17] reported performing the procedure in 36 patients under general anesthesia or with conscious sedation, with visualization aided by CT guidance or using a biplane fluoroscopy. Ablation was again performed using multiple RF machines not specifically designed for spine ablation but reporting only a few transient complications during the procedures.

In 2007, Georgy et al. [18] introduced the concept of performing plasma-mediated radiofrequency (pmRF) ablation before cement augmentation in metastatic spinal lesions. They presented a series of 15 patients with metastatic lesions presenting with posterior cortical disruption and/or epidural extensions. They hypothesized that creating a cavity by tissue removal, rather than displacement, before cement injection in this subset of patients would facilitate safer procedures, preventing possible leakage and tissue displacement in the spinal canal.

pmRF ablation operates by using radiofrequency energy to excite the electrolytes of a conductive medium, such as a saline solution, to create a precisely focused plasma field. The energized particles in this plasma field possess sufficient energy to break molecular bonds, excising or dissolving soft tissues at relatively low temperatures (40–70°C) [19]. The device (Cavity SpineWand; ArthroCare, Sunnyvale, CA) is a stronger version of the more commonly used nucleoplasty probe for disk material removal (Fig. 11.1). The device has a gentle curve to permit more medial directionality inside the vertebral body, but is not flexible, which limits its ability to precisely target particular lesions inside the vertebral body. It also requires continuous saline infusion during ablation.

The device was posited to be safer to use near the spinal cord and nerves due to its lower energy profile [20] compared to the standard RF probes originally designed for soft tissue ablation, which require a large safety zone. However, this resulted in weaker energy production that was unable to completely eradicate tumor masses.

The technique was marketed as a way of debulking tumors, and the same group [21] who introduced the technique later showed that decreasing the intravertebral pressure by creating a cone-shaped void through tissue removal and not displacement results in a predictable pattern of cement deposition in the anterior two thirds of the vertebral body away from the compromised posterior border (Figs. 11.2, 11.3, and 11.4).

Subsequent studies confirmed and duplicated the original work in Europe. Cianfoni et al. [22] reported a series of 48 patients (70 levels) with excellent short-term pain relief and epidural leakage in 14.2 % of cases (not clinically significant). In Germany, Dabravolski et al. [23] reported a series of 250 patients treated with the same technique over the course of 6 years. In 59 patients, posterior percutaneous instrumentation was added to the technique. After surgery, significant pain reduction, satisfaction, early mobilization, and improvement in quality of life were demonstrated in all patients. Immediate radio- and chemotherapy could be carried out. In 38 cases, cement escaped laterally into the intervertebral space, but this had no clinical relevance. While the device does not have full technical and clinical support in the USA, it is widely used in Europe due to its ease of use and the wide safety profile.

In 2014, Anchala et al. [24] reported the first multicenter retrospective study of radiofrequency ablation of spinal metastatic diseases using a novel navigational bipolar radiofrequency ablation device (STAR ablation device, DFine Inc, San Jose, CA) purposely built for bone lesion ablation. The STAR Tumor Ablation System (comprised of the SpineSTAR electrode and the MetaSTAR generator) contains a pair of active thermocouples positioned along the length of the electrode at 10 and 15 mm from the center of the ablation zone. The ellipsoid ablation volume is approximately 20 × 15 × 15 mm when the thermocouple located 10 mm from the center of the ablation zone reaches 50 C and 30 × 20 × 20 mm when the thermocouple 15 mm from the center of the ablation zone reaches 50 C. The generator displays the two thermocouple readings permitting real-time monitoring of the peripheral edge of the ablation. Two versions exist: a smaller one where the thermocouples are located at 5 and 10 mm from the center of ablation and a larger one where the thermocouples are located at 10 and 15 mm from the center of ablation. It is important to understand that in the larger variant, the distal end of the ablation zone extends distal to the electrode tip for about 5 mm. The system is designed in a way that once the temperature of the proximal thermocouple (the one that is located near the anterior epidural space) reaches 50 ⁰C, the whole system will shut down completely as a major safety feature. Giving the articulating flexibility of the electrode, a unipedicular approach can be employed to cross the midline and reach practically any area in the vertebral body for targeted ablation. These navigational properties allow ablation of multiple overlapping zones to cover larger tumors. The MetaSTAR generator allows the use of four different power levels: 5, 10, 15, and 20 W settings. It also displays ablation time, impedance, and the two thermocouple temperature readings that permit real-time monitoring of the peripheral edge of the ablation zone. Cement augmentation can be performed via the same guiding cannula (Fig. 11.5).