**8. Effect of spot size**

In order to create tissue ablation, very high energy density is required. In electro-surgical cutting instruments, a very small electrode, or needle type electrode is used to concentrate electrical current to very small area, which increases the energy density to ablative levels. Coagulation instruments, which create energy and thermal profiles coagulating the cells and shrinking the collagen, usually have larger surface area electrodes than ablative devices. Typically, the surface area of such electrodes is a few square millimeters to generate heat in larger volume but at a lower level to create coagulation rather then ablation. Sub-necrotic heating is usually used for treatments related to stimulation of natural processes in the tissue, such as collagen remodeling, revascularization, speeding fat metabolism. In this case the spot size is about 1 square centimeter or larger. Schematical illustration of spot size effect is shown in **Figure 11**.

Generally, the smaller the electrode, the higher the energy density and the effect tends to be ablative (e.g., cutting cautery tips), whereas larger sized electrodes, have a gentler tissue effect, either coagulation (hemostasis) or sub-necrotic tissue heating [16].

### **9. Monopolar RF systems**

RF current always flows between two electrodes having opposite polarity. The FDA definition of monopolar devices relates to the size and position of electrodes in respect to patient during the treatment. According to FDA guidance [11], monopolar is an electrosurgical technique in which the current flows from a single active electrode at the surgical site, through the patient, to a relatively distant return electrode.

The most common feature of a mono-polar device is a single electrode applied in the treatment area while the return electrode has a much larger contact surface and is placed outside of the treatment zone, usually in the form of a grounding pad. In this electrode geometry, the high RF current density is created near the active electrode and RF current diverges toward the large return electrode. The heat zone for this geometry can be estimated using analytic spherical model for continuity equation stating that electrical current flows continuously from one electrode to another.

$$\nabla\_{\mathbf{r}} \mathbf{j} = \mathbf{0} \tag{7}$$

Taking into account Ohm's law in differential form (Eq. 3) and the definition of an electric field, Eq. 5 can be rewritten as:

$$\frac{1}{r^2}\frac{\partial}{\partial r}r^2\frac{\partial \rho}{\partial r} = \mathbf{0} \tag{8}$$

Where *φ* is the potential of the electric field. The solution for this equation provides RF current density distribution between electrodes.

$$j = \frac{\sigma V r\_0 R}{r^2 \left(R - r\_0\right)}\tag{9}$$

Where *σ* is tissue conductivity, *V* is voltage between electrodes, *r0* is radius of small electrode and *R* is the radius of the large electrode.

For the instance when the return electrode is much larger than the active electrode, the equation can be simplified as:

$$j = \frac{\sigma V r\_0}{r^2} \tag{10}$$

Correspondently, heat power according to Joule's law can be estimated as:

$$P = \frac{\sigma V^2 r\_0^2}{r^4} \tag{11}$$

This simple equation leads to a few interesting conclusions:

Heat generated by RF current near the active electrode does not depend on position of the return electrode when return electrode is much larger in size than the active electrode and located at a distance which is much larger than the active electrode size.

Heating decreases dramatically as distance increases from the active electrode. As was shown before, RF energy penetration depth is about one third of electrode radius (**Figures 5** and **6**). However, heating temperature on the electrode surface may reach hundreds of degrees centigrade and coagulation effect may be extended much larger than RF penetration depth. The other factor enlarging thermal zone is heat conductivity spreading heat around.

#### **Figure 12.**

*Schematic RF current distribution between electrodes for monopolar system.*

#### **Figure 13.** *Electrical current flowing through the patient and monopolar electrosurgical device.*

RF current behavior in the body for monopolar systems is visualized schematically in **Figure 12**.

RF current is concentrated on the active RF electrode and rapidly diverges toward the return electrode.

Monopolar devices are most commonly used for tissue cutting. Schematically, the RF current flow through the patient for monopolar devices is shown in **Figure 13**.

The RF current is always flowing through a closed loop via the human body. As we showed above, the current density out of the vicinity of the return electrode is negligible. However, a malfunction where some low frequency current escapes out of a monopolar configuration holds high risk because the entire body is exposed to the electrical energy. Most commercially available devices have isolated output to avoid any unexpected RF current path to the surrounding metal equipment.

Treatment effects with monopolar devices depend on RF power and size of electrode. The classic use of monopolar technique is tissue cutting and ablation while occasionally it is used for soft tissue coagulation or sub-necrotic heating [9, 17–19].

The main features of monopolar devices are:

