Monopolar radiofrequency

Heat from the resistance of the tissue

A laser needs a chromophore: a substance that absorbs the light. Radiofrequency does not. It is an alternating current of hundreds of thousands to a few million oscillations per second that flows through the tissue. How much heat that current generates depends on the electrical properties of the tissue, not on its colour ^[1].

At these frequencies, tissue behaves mainly as a resistor. The ions in the tissue fluid move with the alternating field, and the friction they encounter is converted into heat. That heat is generated mainly where the current is most concentrated: just beneath the active electrode ^[2].

How the heat is distributed across the skin layers also depends on how those layers are arranged. When two layers lie one above the other, such as the dermis above the fat, the same current flows through both and the less conductive layer heats up more. When two paths run side by side, such as the connective tissue septa between the fat lobules, the current mainly takes the more conductive path, and that path heats up more.

The electrodes determine where the heat goes

In monopolar RF, a single active electrode sits in the handpiece. The current flows through the body to a neutral electrode: an adhesive pad placed elsewhere on the body ^[3]. This pad is much larger than the active electrode, so the current spreads over a large area there and the skin beneath it barely warms up ^[2]. Monopolar RF reaches deeper than bipolar RF ^[3] and heats a volume of tissue beneath the electrode: the dermis and, deeper down, the connective tissue septa in the subcutaneous fat.

In bipolar RF, both electrodes sit in the handpiece and the current flows only between the two. The heat therefore stays more superficial: in theory, it reaches at most about half the distance between the electrodes ^[4].

In RF microneedling, needles, often insulated down to the tip, deliver the energy at a chosen depth in the dermis. A small coagulation zone forms around each needle, while the epidermis is largely spared. RF microneedling usually works in bipolar mode, Genius included. RF microneedling is a fractional treatment with tiny wounds; monopolar RF does not open the skin.

What determines the depth

The lower the frequency, the deeper RF energy penetrates ^[3]. Classic monopolar devices work at 6.78 MHz ^[5]. Tip size also matters: small tips let you treat small areas such as the eye area, but the energy penetrates less deeply with them ^[6].

XERF has three depth settings: Shallow, Middle and Deep. In an animal study, the Shallow and Middle settings delivered 6.78 MHz only ^[7]; in the Deep setting, a single shot delivers 6.78 MHz followed by 2 MHz ^[8]. The idea behind the combination is that each frequency works at a different depth. In a computer model, partly developed by employees of the manufacturer, 2 MHz at the same energy heated a wider and deeper area of the fat; 6.78 MHz concentrated the heat more along the connective tissue septa ^[7]. That is a model calculation: in the animal study, 2 MHz could not be tested separately from 6.78 MHz ^[7].

According to the Cynosure Lutronic brochure, the Deep setting treats all layers down to the SMAS, the layer of muscle and fascia beneath the subcutaneous fat. How deep the heat actually reaches in humans has not been measured in the published studies.

Collagen contracts and is renewed

Research on animal tendon tissue and human abdominal skin shows how heat acts on collagen: the fibres contract immediately, and the controlled thermal injury then triggers the production of new collagen ^[9].

Clinically, this matches what researchers saw in a multicentre study with a classic monopolar device: early tightening that increased further over the following months. They explained this as a direct effect of the heat, followed by tissue repair ^[10]. In a Japanese study, the result was clearer after six months than after three ^[11].

Cooling, contact and heat build-up

Because the current is most concentrated just beneath the electrode, that is also where most of the heat is generated ^[2]. Monopolar devices therefore cool the skin surface during treatment, so that the epidermis stays protected while the deeper layers heat up. XERF cools the electrode, and through the electrode the skin, with short bursts of cryogen gas ^[7]; Cynosure Lutronic calls this integrated cryogenic cooling. According to the manufacturer, the device also continuously measures the resistance of the skin and adjusts energy delivery accordingly (Accurate Impedance Feedback). It also monitors the skin surface temperature in real time. According to the clinical study, it automatically interrupts RF delivery if the skin surface exceeds 43 °C; no treatment reached that limit ^[8].

Contact between electrode and skin also matters. If the tip is not fully in contact with the skin, the current concentrates on a smaller area. In a Japanese study, two cases of blistering with pigmentary change were probably due to poor contact ^[3].

Building up heat over several passes

The first monopolar treatments used small tips, high energy and a single pass. With Thermage, the technique later shifted to larger tips, lower energy and several passes, guided by the heat the patient feels. In a survey of fourteen physicians, without statistical analysis, the new technique more often produced immediately visible tightening, and far fewer patients found the treatment too painful ^[12].

In the clinical study of XERF, gliding and stamping passes alternated ^[8]. The treatment protocols set out the number of passes, the overlap and the treatment endpoint.

Who benefits most

A systematic review found the highest satisfaction among patients seeking modest rejuvenation ^[13]. A review of RF in cosmetic dermatology concludes that RF can be an alternative to surgery, but is not equivalent to it ^[14]. Monopolar RF is therefore best suited to mild to moderate laxity, in patients who want little downtime and no surgery.

Dark skin

RF does not target melanin in the epidermis and is therefore regarded as safe for all skin types ^[3]. The clinical data come mainly from skin types III–IV. In 85 Japanese women, monopolar RF produced good tightening ^[11][3], and in a study of XERF in twenty Korean women, no side effects were observed ^[15]. The second XERF study also included patients with skin types II and V, without separate results per skin type. We found no data for skin type VI. It is not without risk: blisters caused by poor contact can leave pigmentary change in dark skin ^[3].

What can go wrong

After treatment, redness and swelling are normal; they usually resolve within a day ^[16]. In a retrospective analysis of more than 600 treatments, 2.7% had a temporary side effect and none a permanent one ^[16].

Burns are the main risk. In the early studies, they occurred with a small proportion of pulses ^[10]; devices have since used larger tips, lower energy and several passes ^[12].

A second risk is loss of subcutaneous fat. It is mentioned as a risk of high-energy monopolar RF, especially in slim patients ^[17]. A systematic review of RF on the face and neck also describes a reduction in facial fat ^[13], but we could not find how often this happens unintentionally. In the analysis of 600 treatments, there was one mild indentation on the cheek, which had fully resolved after 3.5 months ^[16]. In the second XERF study, the researchers saw no contour irregularities or unwanted volume loss after three months. Measured at the skin surface, volume decreased in the medial parts of the face and initially increased in the lateral parts; after three months there was no significant net change. They interpret this as a redistribution of volume ^[8].

The treatment is not painless. In the second XERF study, the practitioners titrated the energy to mild to moderate pain. Six of the sixteen patients reported mild pain, eight moderate and two severe; four received a topical anaesthetic at their own request ^[8].

In the second XERF study, the contraindications were: an implanted electronic device such as a pacemaker or defibrillator, pregnancy and breastfeeding, an active infection or inflammation of the skin in the treatment area, and an active malignancy in that area ^[8]. An electronic implant is on the list because of how the technology works: the current flows through the body to the neutral electrode ^[3]. The full list is in the device manual.

Alongside other technologies

RF microneedling uses the same energy source, but delivers it through needles into small zones in the dermis. A systematic review found improvement in texture and tightening, with redness and swelling as the main side effects ^[18]. Monopolar RF heats a larger volume beneath the electrode and is aimed mainly at tightening. For scars and irregular texture, RF microneedling is the technology to weigh it against. XERF works in monopolar mode; Genius is an RF microneedling device.

Lasers act on water or on a chromophore. Fractional lasers such as CO₂, erbium and thulium treat a grid of narrow zones reaching from the skin surface into the dermis, and are used for texture, scars and fine wrinkles. Pigment did not improve in the second XERF study ^[8]. For pigment, Q-switched and picosecond lasers are among those used, at Nd:YAG or alexandrite wavelengths; the pigmentation page compares them.

Microfocused ultrasound, which we do not sell, places small coagulation points at a fixed depth in the tissue ^[3]; monopolar RF heats a volume beneath the electrode.

What the XERF studies show

Monopolar RF has been used to tighten the skin without surgery since the early 2000s ^[10][16], and much has been published on it. A review of non-ablative RF describes the technique as safe, well tolerated and effective, with modest results. It also notes that many studies are not randomised, have no comparison group and assess the result subjectively ^[14]. A later systematic review pooled 121 studies of all forms of RF on the face and neck and found a lifting effect, better elasticity and fewer wrinkles in objective measurements as well ^[13].

In the 2003 multicentre study, the result depended strongly on who assessed it and how. After one treatment, 62% of the measured brows were at least 0.5 mm higher. On blinded photographs, independent assessors saw an improvement of at least one point on a wrinkle scale in 83% of the periorbital areas. The treating physicians, who judged without the pre-treatment photographs, saw an improvement in 29% of the areas, and half of the patients were satisfied ^[10]. A one-point improvement is hard to see without a comparison photograph.

Research on XERF itself is still at an early stage. The first results are consistent with what is known about monopolar RF: two animal studies show the expected tissue changes. In a letter to the editor, employees of the manufacturer described thicker and shorter collagen bundles and thicker connective tissue septa in pigs, six hours after treatment ^[5]. In a second animal study, with three minipigs, the dermis and the connective tissue septa thickened in all three settings, most clearly after 30 days, and the changes reached deeper in the Deep setting ^[7]. The settings did not receive the same energy, and there is no statistical comparison.

Twelve weeks after a single treatment in twenty women with skin types III–IV, two blinded assessors correctly identified the post-treatment photograph in 80% of cases; pain was mild to moderate and there were no side effects ^[15]. In a second study of sixteen women, the brow and upper eyelid were measurably lifted after one and three months, and skin quality improved ^[8].

Our assessment: in many patients, monopolar RF produces visible, modest tightening that increases gradually. Downtime is short, and with correct technique there are few complications. XERF combines two frequencies in one shot. Whether that combination adds anything clinically compared with a single frequency has not yet been studied.