Radiofrequency in Cosmetic Dermatology

M. Lapidoth, S. Halachmi

• Éditeur: S. Karger
• Année: 2014
• ISBN:
  9783318023176
• Sujet: Medicine
• Sous-sujet: Medical Technology & Supplies

Radiofrequency (RF) has been used in many areas of medicine for some time, but only gained popularity in aesthetics in the early 2000s. Although initial procedures provided variable results, the field of RF has grown dramatically. Currently, a multitude of devices which offer an array of technologies, each geared to specific applications, are on the market. In this book, a comprehensive discussion of the evolving area of RF is provided, and the current and cutting-edge technologies which span from noninvasive treatments of facial wrinkles to invasive body contouring procedures are discussed in detail. The methodologies covered include thermal and ablative modalities, volumetric and fractional approaches, as well as practical aspects of application in clinical practice. Included are also pearls on patient management and prevention and treatment of complications. Finally, RF-based procedures are compared with equivalent laser and surgical approaches in a risk-benefit evaluation. Reviewing the standard of care for RF-based treatments and providing information relevant for clinical decision-making and practice management, this publication is a valuable tool for dermatologists, plastic surgeons and all practitioners who use or are contemplating the introduction of RF into practice.

Lapidoth M, Halachmi S (eds): Radiofrequency in Cosmetic Dermatology.
Aesthet Dermatol. Basel, Karger, 2015, vol 2, pp 1-22 (DOI: 10.1159/000362747)

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Basic Radiofrequency: Physics and Safety and Application to Aesthetic Medicine

Diane Irvine Duncan^a· Michael Kreindel^b

^aPlastic Surgical Associates of Fort Collins, Fort Collins, Colo., USA; ^bInvasix Corp., Richmond Hill, Ont., Canada

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Abstract

This chapter summarizes the basic science of radiofrequency (RF) and its application in aesthetic medicine. The main parameters of RF including RF frequency, waveform, power, pulse duration, and penetration depth are described, and its application for treatment is analyzed. Monopolar and bipolar devices are described in detail for different clinical applications. The effect of RF electrode geometry on tissue heating is shown, and tissue-specific electrical parameters are summarized. The chapter discusses which RF parameters are required to reach therapeutic temperatures for tissue ablation, coagulation, or subnecrotic heating. RF parameters used for noninvasive, minimally invasive, and fractional treatment are compared. Finally, the chapter explains the main safety concerns associated with RF treatments and details the most common causes of adverse events.

© 2015 S. Karger AG, Basel

The term radiofrequency (RF) was first introduced with the invention of radio and was applied to electromagnetic radiation or current ranging from 3 kHz to 300 GHz. Since then, the field of medicine has used the relatively narrow band of this spectrum from 200 kHz to 40 MHz in many different applications. The main advantage of RF energy in medicine is a low or negligible reaction of nerves to high-frequency alternating current (AC) in comparison to lower frequencies.

William T. Bovie invented the first electrosurgical device while working at Harvard [1]. This device was used by Dr. Harvey Williams Cushing on October 1, 1926, at Peter Bent Brigham Hospital in Boston, Mass., to remove a tissue mass from a patient's head [2]. Since then, RF electrosurgical devices have become one of the most useful surgical instruments. Recently, RF has experienced a resurgence in aesthetic medicine with applications for ablative and nonablative applications. RF energy has become an irreplaceable tool in almost every field of medicine including dermatology, plastic surgery, and aesthetic medicine, the primary interest of this book. The tissue effects achievable using RF energy are based on a versatile thermal end point and are dependent on the applied energy density.

Several RF-induced thermal changes of tissue are commonly used in medicine:

(1) Ablation of tissue. This effect is generally used for cutting or removing tissue and is based on thermal evaporation of tissue. Ablation requires very high energy density, allowing conversion of tissue from a solid state to vapor with minimal thermal damage to the surrounding tissue [3]. A new use for RF ablation is for cautery of tumors.

(2) Coagulation. When applied to blood vessels, coagulation provides hemostasis for controlling bleeding during surgery. The same mechanism is effective for vascular lesion treatment [27]. Coagulation may be applied to soft tissue as well, to induce necrosis when immediate tissue removal is not required or not practical.

(3) Collagen contraction. High temperatures induce immediate transformation in the tertiary structure of proteins. When applied to collagen, heating allows tissue shape to change for medical and cosmetic purposes. Immediate, predictable collagen contraction occurs at a temperature range of 60-80°C in orthopedic procedures [4] and ophthalmology [5]. For noninvasive cosmetic procedures, this effect is produced with lower temperatures in order to avoid skin necrosis. However, due to the lower temperatures, the outcome of the procedure is often less consistent, requires multiple procedures, and takes a longer time to show results [6, 7].

(4) Tissue hyperthermia. Heating of tissue to superphysiologic temperatures is a popular method of skin treatment using subnecrotic temperatures to stimulate natural physiological processes in attempts to modify skin appearance and to reduce subcutaneous fat [8, 9]. This heating does not induce immediate effects of coagulation but can stimulate fibroblasts to synthesize collagen and may alter the metabolism of adipocytes in favor of lipolysis.

Radiofrequency Energy Characteristics

The clinical effects of RF depend on a combination of the RF parameters and on the method of its application to the tissue.

Radiofrequency Frequency

The frequency of electrical current characterizes how many times per second an electrical current changes its direction and is reported in hertz. This change in direction is associated with a change of voltage polarity. Direct current has a frequency of 0 Hz, which is typically used in battery-powered devices. Standard AC in the range of 50-60 Hz is used for most home appliances. AC current causes nerve and muscle stimulation and at high powers is very dangerous. It can cause acute pain, muscle spasms, and even cardiac arrest.

Fig. 1. Frequency spectrum of electrical current.

At a frequency of 100 kHz and higher, the muscle- and nerve-stimulating effects decrease. In this range, higher power can be applied to the tissue safely to create the desired thermal effect (fig. 1). Although at frequencies above 100 Hz nerve reaction from electrical current is dramatically diminished, at high amplitudes skin reaction can be observed even at a frequency above 1 MHz. RF energy propagates in the tissue in the form of electrical current between applied electrodes and in the form of radiation at higher frequencies. Frequencies in the range of 200 kHz to 6 MHz are the most common in medicine, but there are devices with frequencies up to 40 MHz [10]. The higher frequency electrical oscillations are used mostly for communication.

Radiofrequency Waveform

Typically, sine RF voltage is used in medical devices. The RF energy can be delivered in continuous wave (CW) mode, burst mode and pulsed mode (fig. 2). For gradual treatment of large areas, the CW mode is most useful as it allows a slow increase in temperature in bulk tissue. This approach is applied for targeting cellulite, subcutaneous fat, and skin tightening. The burst mode delivers RF energy with repetitive pulses of RF energy. It is used in applications where peak power is important while average power should be limited. This application is used in blood vessel coagulation. Pulsed mode is optimal when the goal is to heat a small tissue volume while limiting heat conduction to the surrounding tissue, similar to the rationale of applying short pulse duration in laser treatments. Pulsed mode is effective for fractional skin ablation and is characterized by pulse durations which do not exceed the thermal relaxation time (TRT) of treated zone.

Fig. 2. Typical RF waveforms.

Radiofrequency Power

The most important characteristics of RF energy are its peak and average power. Peak power is important to estimate the thermal effect produced, while average power affects the speed at which the heating is induced. For CW operation mode, the peak and average power are the same. For pulsed or burst mode, the average power is the total power delivered divided by the time the device is applied, including the ‘off’ cycles.

Another important characteristic of RF is power density. High power applied to a large skin surface may create only gentle warming, but when applied through a needle electrode, the same power is applied over a small contact point, leading to high power density. At high power densities, RF may create intense tissue ablation rather than warming or coagulation.

Thermal Effect of Radiofrequency Current

The heat power (P) generated in a tissue volume by electrical current during a period of time (t) is described by Joule's law:

The heat generated is measured in joules/cm³. As the equation describes, power increases as a square function of the RF current density (j). Conversely, heating power changes in inverse proportion to tissue conductivity (σ).

Taking into account that current density according to Ohm's law is proportional to the electric field strength and tissue conductivity (equation 2),

we can rewrite the equation (1) as

In other words, the higher the tissue conductivity, the greater the heat that will be generated when constant RF voltage ...