3. History of energy-based devices

Ultrasound-assisted liposuction (UAL) was introduced by Zocchi in 1990 [24]. Reports of complications including seroma, burns, and contour irregularities caused a loss of popularity of this particular device. A current ultrasound-based device, Vaser, is still in regular use but is not targeted toward causing skin contraction. Power-assisted liposuction (PAL) is used by many practitioners who like the vibratory nature of the handpiece for treatment of fibrous areas and in secondary cases. Water-jet-assisted liposuction is excellent for fat grafting harvest, but does not improve the overall liposuction outcome. Laser-assisted liposuction (LAL) was shown to create a 13–17% skin surface area reduction at 3–6 months post-op [12]. Lack of a long-term outcome has led to a loss in popularity of this device. Radiofrequency-based tissue tightening has increased in popularity as longer-term improvement has been shown [23]. A bipolar device with indwelling liposuction capability was introduced in 2008 (Invasix) [16], and a monopolar device was cleared for soft tissue coagulation in 2012 (Thermi) [25]. This device utilized a subcutaneous cannula with an internal temperature probe. In 2012, a new version of the bipolar device (InMode) was introduced that contained both internal and external temperature monitors. A 50% improvement in upper arm pendulosity and a 36% skin surface area reduction at 1 year were seen with this device [26]. In 2016, the J-Plasma device (Bovie Medical) was introduced for the purpose of subdermal coagulation. The device was originally developed as a general surgical/gyn/urological laparoscopic cautery device. In esthetic cases, the cannula with blade retracted has been used in the subcutaneous plane for the purpose of collagen coagulation.

## 4. Mechanical effects of liposuction

Traditional SAL removes fat. However, the procedure is also a mechanical, nonthermal FSN stimulant. If simple fat reduction in a young patient with no soft tissue laxity is the goal, then fat removal and mechanical stimulation of the soft tissue will cause some reduction of the distended skin and soft tissue mass. Nonthermal trauma to the tissue induces an inflammatory response, which as we know causes inosculation of new blood vessels, generation of fibroblasts, and formation of new collagen or scar tissue within the treated space. Chemokines, cytokines, and growth factors all influence tissue response to mechanical injury [27]. Skin contraction with SAL measures about 10% at 6–8 weeks posttreatment [23] and then relaxes to a measured 8% skin surface area contraction at 1 year.

Practitioners such as specialists like Gasparoni and Toledo used superficial liposuction to optimize skin shrinkage [28]. However, reports of complications resulting from treatment by other surgeons have diminished the popularity of this procedure [29]. Full-thickness skin loss due to aggressive superficial liposuction is still a concern, especially in Central and South America.

### 5. Thermal effects of heating soft tissue

When heat is added to mechanical stimulation, the soft tissue response changes. Thermal effects of radiofrequency energy can include ablation, coagulation, and collagen contraction [30]. While ablation without adjacent tissue damage is a desired goal in vaporizing tumors or lesions, the esthetic practitioner has more commonly used RF to cauterize blood vessels or to directly shrink soft tissue. Common use of a pencil-type cautery unit includes shrinkage of the SMAS in face lifting or a "popcorn" capsulorrhaphy in breast implant surgery.

A newer concept is that of contraction of collagen fibers, a microeffect of RF energy on the tissue. Genin [31] notes that the native state of triple helical collagen strands change to a transitional state and then become denatured when temperature from the device over time causes the protein to unfold. In a study by Rossman [32], he showed that average collagen fibers measure 290 nm shrank to 101.5 nm when denatured, a contraction of 65%.

The electric properties of tissue are governed by impedance or "resistivity" to energy conductance [33]. When a uniform tissue type is exposed to radiofrequency energy, impedance can be measured using a certain cross section with a measured distance between the electrodes [30]. Blood is highly conductive to RF energy with a slight lowering of impedance between 0 and 6 MHz. Its conductivity coefficient is 0.7 (low impedance), while fat has high impedance with a conductivity coefficient of 0.03. The value of adding tumescent fluid to fat is illustrated by contrasting the impedance of dry and wet skin. Dry skin conducts RF energy at a 0.03 coefficient the same as fat, while wet skin has a 0.25 coefficient.

If tissue is heating slightly—elevating the temperature from 37°C to 44°C, the metabolic process quickens [34]. At 45°C, there is a structural change in the collagen helix which leads to hyperthermic cell death. At 60–80°C, collagen proteins denature and unfold. At 90°C, tissues become dessicated, and at 100°C, they become thermally ablated.

In treating patients, factors to consider include the "permittivity" of the tissue to heat, and the temperature achieved in the treatment region. These two factors, as well as duration of energy exposure, directly influence clinical outcome. The vascular perfusion to the area is another very important consideration. This can be

compromised in patients who have had previous procedures in the treatment zone. The frequency and type of energy source will also affect tissue response.
