Radiofrequency
Radiofrequency has become an important and frequently used modality in cosmetic dermatology.24 This modality differs from cryolipolysis in that it relies on exploiting the difference in water content and impedance between tissues: the skin has low impedance, whereas fat tissue has high impedance. Radiofrequency induces thermal injury to targeted tissue layers, rather than the cold-induced damage seen in cryolipolysis, through devices that focus thermal energy on tissues with high impedance, inducing apoptosis of cells in the subcutaneous adipose tissue with minimal risk of damaging the epidermis, dermis, and muscle.9,25 Ultimately, thermal exposure to 43°C to 45°C over several minutes results in a delayed adipocyte death response.4 In addition to adipocyte death, RF has been shown to cause denaturation of collagen fibrils, leading to subsequent remodeling, neocollagenesis, and skin tightening.26
Radiofrequency devices can be broadly classified as monopolar or bipolar.24,27 Bipolar devices generally require more frequent treatments, whereas monopolar devices tend to require fewer treatment sessions with superior circumference and fat reduction.28
Overall, RF devices have a favorable side effect profile. The most common side effects are erythema and edema at the treatment site lasting less than 24 hours after the procedure.25 The absence of complications such as abdominal discomfort, erythema, and burning during treatment have been reported,27 with the exception of 1 case of hyperesthesia on the abdomen that lasted for 3 days after a treatment session.5 Although RF has beneficial effects on circumference reduction in the abdomen and thighs and can improve the appearance of cellulite, an increase in body weight may occur during treatment. When a localized area of fat such as the thigh is targeted for treatment but the remaining fat cells in the body are not affected, the remaining cells can continue to grow and expand; for instance, although fat cells destroyed with RF will not continue to expand, fat cells in untreated areas may continue to grow due to continued weight gain (eg, from excessive eating), leading to overall weight gain. Thus, patients must understand that weight gain is not an indication of treatment failure after RF or any other method of irreversible fat destruction.5
High-intensity Focused Ultrasound
High-intensity focused ultrasound recently was introduced as a new treatment modality for body contouring, specifically for skin tightening and rejuvenation.5 The mechanism of HIFU is similar to that of RF in that it also relies on heat to cause adipocyte apoptosis; however, it utilizes acoustic energy rather than electric energy. High-intensity focused ultrasound devices can deliver energy to the deep dermis, subdermal connective tissue, and fibromuscular layers in precise microcoagulation zones without damage to the epidermis. The focused energy induces a high temperature (>65°C) within 1 to 3 seconds, causing cell protein coagulation in the targeted area. In addition to its thermal effects, HIFU induces a mechanical effect that disrupts cell membranes immediately, which contributes to the coagulation necrosis process, further promoting necrosis and apoptosis. The effects of these devices can be visualized, as there always is a sharp demarcation between the targeted and untargeted tissue.29 Additionally, microcoagulation is thought to cause gradual skin tightening through collagen contraction and remodeling.30
High-intensity focused ultrasound first received FDA approval for eyebrow lifting and has been used safely and effectively to treat facial and neck skin in a variety of skin types as well as to improve the clinical appearance of the abdomen and thighs.31 This technique is best suited for patients with mild to moderate laxity of the skin or soft tissue who have a body mass index less than 30 kg/m2 and are seeking mild body contouring.32 The ideal patient is young with normal wound healing, since the clinical response to treatment is partly dependent on new collagen synthesis.33 Older patients with extensive photoaging or severe skin laxity are not good candidates for HIFU.
There are a variety of available HIFU devices,34 which utilize special transducers that direct ultrasound energy to a small focal point in the subcutaneous tissues that harmlessly passes through the skin.35 By using newly developed transducers with different energy outputs and focal depths, dermatologists can tailor HIFU treatment to meet the unique physical characteristics of each patient.31
Adverse effects of HIFU are limited to transient pain in most patients and occasional erythema and ecchymosis in some cases.31 In general, most adverse effects resolve spontaneously within 4 weeks and all by 12 weeks posttreatment. Studies also have reported hard subcutaneous nodules, discomfort, burning sensation, mild blisters, and one case of purpuric lesions, all at the treatment site.36-39 There is no evidence that HIFU can cause abnormalities in serum lipids or liver function tests.