An Overview of Radiofrequency Technology
Historically, ablative and nonablative laser devices have been used to improve facial
skin laxity, and recently novel technologies have emerged that use energy sources
other than light and laser. RF is a novel nonablative technology that uses an electric
current rather than light to deliver energy to the tissue. RF energy is a form of
electromagnetic energy ranging from 300 MHz to 3 kHz that has been used in many areas
of medicine including cardiology, urology, and sleep medicine. It was initially developed
in the 1920s for electrocautery,[8] but is now widely used in aesthetic applications. In 2002, the U.S. Food and Drug
Administration (FDA) approved the first RF device for facial wrinkle reduction.[9] Since the development of this monopolar RF device (ThermaCool; Thermage, Inc., Hayward,
CA), many other RF devices have been developed and combined with laser and light sources.
Similar to laser and light energy, RF energy interacts with tissue to induce thermal
changes. However, it does not follow the principles of selective photothermolysis,
and thus does not target a specific chromophore in the skin. Instead, RF devices generate
heat as a result of tissue resistance to the movement of electrons within the RF field
(Ohm's law). This resistance, also called impedance, creates heat relative to the
amount of current (A) and time (seconds).[10]
[11] Heat is then produced when the tissue's inherent impedance converts the electrical
current to thermal energy, indicated by the following formula: Energy (J) = I
2 × R × T (where I is the current, R is the tissue impedance, and T is the time of application).[12]
Like other skin tightening treatments, RF treatment is based on lessening skin laxity
through contraction and thickening of collagen fibers. The application of RF energy
produces resistance within the various layers of the skin. The resistance within the
tissue creates an electrical current that is converted to thermal energy. This resistance,
also known as impedance, varies according to the tissue's size, depth, and various
layers (dermis, muscle, fat, and fibrous tissue). Unlike laser and light-based treatments,
this electrical current does not selectively target epidermal melanin. Thus, patients
of all skin types can be more safely treated with RF. However, although RF is more
theoretically safe for all skin types, there is a risk of dyspigmentation if thermal
injury occurs due to overheating of the skin.
With RF treatments, depth of penetration is not the only consideration. The fact that
soft tissue is made up of multiple layers, including dermis, fat, muscle, and fibrous
tissue, all with varying resistance to the movement of RF energy, should also be considered.[11] Impedance is the principle that allows the heat to reach a larger volume of tissue,
and structures with higher impedance are more susceptible to heating and thus to tissue
injury.[13]
There are two major electrode configurations available in current RF devices: monopolar
and bipolar. Although the interaction between the emitted energy and the target tissue
is similar, the energy field created by each configuration differs.
Monopolar Radiofrequency
Monopolar RF devices deliver current using one active electrode that contacts the
skin and another that acts as a grounding pad.[11] The active electrode delivers the current to the skin, tightening it via volumetric
heating.[14] A cooling spray is applied to protect the epidermis, which also creates a reverse
thermal gradient.
The dermis is heated uniformly and volumetrically, sparing the cooled epidermis. The
partial collagen denaturation, through breaking hydrogen bonds in the collagen triple
helix, leads to collagen contraction and thickening.[15]
[16] Some collagen contraction may occur immediately due to fibril denaturation.[17] This collagen denaturation occurs at a threshold temperature of approximately 65°C.[18]
[19]
[20] Additional tightening then follows due to an inflammatory wound healing response
that triggers neocollagenesis and further skin contraction.[11]
[15]
[17] Collagen-based fibrous septae that separate fat lobules in the subcutaneous tissue
are also preferentially heated, which leads to further collagen denaturation and contraction
of the subcutaneous tissue. This accounts for the immediate tightening and lifting
effects noted after treatment.[11]
[17] It is also thought to be responsible for inward (Z-dimensional) tightening.[21]
The first monopolar RF device used for skin tightening was the ThermaCool device (Thermage,
Inc.), which was introduced in 2001 and received FDA approval for the noninvasive
treatment of periorbital rhytides and wrinkles in 2002 and for full face treatment
in 2004.[22] The ThermaCool device uses capacitive coupling to deliver RF energy to the skin
through a thin membrane in the treatment tip. The ThermaCool device has four main
components: a monopolar RF generator producing 6 MHz of alternating current, a handpiece,
a disposable treatment tip, and a cryogen cooling module. Using a unique capacitive
coupling membrane, RF is dispersed uniformly across the thin dielectric material on
the treatment tip. The RF generator changes the polarity of the electric field in
the tissue 6 × 106 times per second, causing charged molecules to move with the electric field at the
same frequency. Heat is then generated from the dermal tissue's natural resistance
to electron movement.
The efficacy of monopolar RF has been widely investigated in various applications.
Fitzpatrick et al conducted a blinded, multicenter trial in which 86 patients received
a single treatment in the lateral canthal and forehead areas.[23] Treatment efficacy was evaluated using the Fitzpatrick Wrinkle Classification System
(FWCS), in addition to objective eyebrow position measurements. A total of 83% of
patients showed the improvement of at least one point on the FWCS, and 50% of patients
were satisfied with the improvement in periorbital wrinkling. Brow lift of at least
0.5 mm was noted in approximately 62% of patients. The authors concluded that there
was an objective and subjective reduction of periorbital wrinkles and changes in brow
position.[23] Side effects were uncommon, although there was an overall incidence of 0.36% of
second-degree burns.
Bassichis et al also evaluated the ThermaCool device for rejuvenation of the upper
third of the face by assessing changes in brow position.[24] Twenty-four patients received a single-pass treatment in the temporal and lateral
forehead areas. They found that treatment resulted in a statistically significant
brow elevation of at least 0.5 mm in 87.5% of patients. Despite this, 64% of patients
did not perceive a cosmetic benefit. No complications were reported.
Brow elevation was also studied by Nahm et al in a study involving ten patients.[25] One side of the face was treated with a single pass using the ThermaCool device.
By 3 months post-treatment, there was a statistically significant average elevation
of 4.3 mm of the mid-brow and 2.4 mm of the lateral brow, with a 1.9 mm increase in
the level of the palpebral crease.
Using a different monopolar RF device (Biorad, Guangdong, China), El-Domyati et al
treated patients for 3 months at 2-week intervals. All six patients exhibited notable
improvement in skin tightening in the periorbital and forehead regions, with continued
improvement for 3 months after treatment. Skin tightening improved from 35 to 40%
at the end of treatment to 70 to 75% at 3 months following treatment.[26]
A 4 MHz monopolar system (Pelleve, Ellman International, Inc., Oceanside, NY) has
recently been evaluated for the treatment of periorbital rhytides. Javate et al evaluated
patients 1, 3, and 6 months after treatment, and statistically significant changes
were noted clinically and according to electron microscopic evaluation.[27] Taub et al evaluated this device using a continuous motion technique, achieving
a surface temperature of 40 to 42°C and maintaining that temperature for an additional
four to six passes.[28] Two weeks after the first treatment, patients noted an overall 25 to 30% improvement.
Although there was a reduction in results after the second treatment, the treating
physician and patient rated an average improvement of 46 and 30%, respectively, 6
months after the final treatment.[28]
Treatment of laxity in the lower face and neck with the ThermaCool device has also
been investigated. Jacobson et al treated 24 patients with laxity of the neck, nasolabial
folds, marionette lines, and jawline, with 17 of the 24 patients showing notable improvement
up to 3 months following treatment.[29] Alster and Tanzi reported similar findings, with improvement in moderate cheek laxity
and nasolabial folds.[30]
Side effects are generally mild and self-limited following monopolar RF treatments.
Weiss et al published an extensive retrospective chart review that investigated the
rate and degree of side effects after treatment using the ThermaCool device.[31] Most side effects experienced were transient erythema and edema. However, there
were rare cases of superficial crusting, slight depression of the cheek, subcutaneous
erythematous papules, and neck tenderness that were noted. The overall rate of adverse
side effects was 2.7%, but none of these side effects were experienced when using
a lower energy multiple-pass treatment algorithm.[31]
Hybrid Monopolar and Bipolar Radiofrequency
A hybrid RF system takes advantage of two mechanisms of RF-induced tissue heating
by using two handpieces: one monopolar and one bipolar (Accent RF, Alma Lasers, Ltd.,
Caesarea, Israel). The monopolar handpiece achieves deep volumetric heating of the
skin (up to 20 mm) through the rotational movement of water molecules in the alternating
current of the electromagnetic field.[32] The bipolar handpiece is used for more superficial localized (nonvolumetric) heating,
at a depth of 2 to 6 mm, based on tissue resistance to the RF conductive current.[32]
[33]
A study investigating hybrid monopolar and bipolar RF treatments for the treatment
of facial rhytides and skin laxity found that 56% of participants had some degree
of improvement. And when divided into two age groups, the younger age group reported
statistically significantly higher satisfaction scores when compared with the older
group.[33] In the study, the authors supported their observation by citing a study that suggested
that heat-labile collagen bonds are progressively replaced by irreducible multivalent
cross-links as the tissue ages, rendering older skin less amenable to RF tissue tightening.[34] Patient satisfaction scores were also higher in the younger age group in this study.
Alexiades-Armenakas et al compared the monopolar and bipolar handpieces in a split
face study for the treatment of facial rhytides and laxity. After four treatments,
the degree of improvement for each handpiece approached but did not achieve statistical
significance.[9] Although there was a slightly greater degree of improvement with the bipolar handpiece,
this difference did not achieve statistical significance.
As with other RF devices, side effects are uncommon, but there is still risk of burns,
skin breakdown, and scarring with the use of inappropriately high energies.[35] Using pain as a feedback mechanism and operator technique are both important for
optimal patient safety. The handpiece should be kept in continual motion when in contact
with the skin to prevent overheating.
Vacuum-Assisted Bipolar Radiofrequency
Vacuum-assisted bipolar RF (Aluma, Lumenis, Inc., Santa Clara, CA) combines bipolar
RF with a vacuum technology known as FACES (Functional Aspiration Controlled Electrothermal
Stimulation). This device uses a handpiece that incorporates a vacuum to suction a
segment of skin between two electrodes. This limits the volume of treated tissue to
the skin between the electrodes, allowing the use of lower overall energy. As only
targeted layers of skin and subcutaneous fat are suctioned between the electrodes,
nontarget structures such as muscle and bone are avoided.[35] Although not clinically proven, some have postulated that exposure to the vacuum
may cause mechanical stress on fibroblasts and increased collagen formation to increase
clinical efficacy.[16]
[36]
Gold et al studied vacuum-assisted bipolar RF in 46 patients with facial skin laxity.
Using the Fitzpatrick-Goldman Classification of Wrinkling and Degree of Elastosis
scale, the mean elastosis score decreased from 4.5 (pretreatment) to 2.5 (6 months
posttreatment). Despite the overall satisfaction of treatment outcome among the participants,
satisfaction levels declined during the follow-up period.[16] The authors noted that this declining satisfaction did not correspond with the progressive
improvement noted by the investigators, a phenomenon often observed with long-term
follow-up of aesthetic treatments. This decline in satisfaction is also thought to
be a common finding in RF skin treatments because of delayed neocollagenesis and the
long-term wound healing response, with slowly progressive, incremental changes.
Side effects that can occur after vacuum-assisted bipolar RF procedures include erythema,
burns, blistering, edema, purpura, crusting, and transient hyperpigmentation.[16]
Combined Bipolar Radiofrequency and Optical Energy
Although monopolar RF devices have only one active electrode contacting the skin,
the bipolar configuration consists of two active electrodes placed a short distance
apart that overlie the treatment area. The current flows between the two electrodes,
and the depth of penetration is approximately half the distance between the two electrodes.[15] Depth of penetration is the major limitation of the bipolar configuration. Though
offering a more shallow depth of penetration, this configuration does provide more
controlled distribution of energy and less pain.[37]
Bipolar RF devices are frequently combined with light-based technologies, termed electro-optical
synergy (ELOS).[12]
[38] The ELOS system (Syneron Medical Ltd., Yokneam, Israel) uses the synergistic effects
of light- and RF-based devices. The light energy preheats the target tissue through
photothermolysis, lowering the tissue's impedance. This in turn makes the tissue more
susceptible to the RF component. The advantage is that lower energies of both the
light and RF can be used to deliver a safer treatment with fewer side effects.[15]
[38] The RF also allows for deeper penetration into the dermis than nonablative lasers,
which are subject to scatter within the tissue.
Common ELOS systems incorporate intense pulsed light (IPL), a diode laser, or infrared
light. Earlier systems such as the Aurora SR and Polaris WR (Syneron Medical Ltd.)
used the bipolar configuration with an IPL and a 900-nm diode laser, respectively.
The optical and RF energies are delivered simultaneously through the treatment tip.
Recently, a newer generation ELOS Plus platform was made available that also incorporates
both an IPL and a diode laser with RF energy.
A clinical study by El-Domyati et al evaluated the histologic changes and clinical
outcomes using the Aurora SR system.[39] Six subjects were treated in the periorbital region over six sessions. Both clinical
photographs and punch biopsy samples were analyzed both immediately and 3 months after
treatment. At 3 months, improvements in skin tightening, skin texture, wrinkles, and
overall satisfaction were 75 to 80%, 70 to 75%, 95 to 100%, and 95 to 100%, respectively.
On histologic analysis, increased epidermal thickening, a 53% reduction in elastin
content and a 28% increase in newly synthesized collagen fibers were noted.
A second study using the Aurora SR system was conducted by Sadick et al, who reported
similar findings in 108 patients.[40] Each patient received five full-face treatments and were assessed according to photographic
evaluation and patient satisfaction. Although overall skin improvement was 75.3%,
which included wrinkle improvement, pore size, and pigmentation, among other factors,
skin laxity improved 62.9%. Patient satisfaction was 92% at 15 weeks following treatment.
The Polaris WR system was also evaluated for the treatment of facial rhytides and
skin laxity.[41] The succession of RF and diode laser energy led to heating at a maximal dermal depth
of 2 mm. Twenty-four patients underwent three treatment sessions. Improvements were
noted in both skin laxities in facial rhytides, most notably in the periorbital region.
Continued improvement in skin laxity was observed at 6 months posttreatment.
Fractional Radiofrequency
A recent nonablative approach involves fractional RF, which is currently delivered
in two ways. Formerly known as Matrix RF, Sublative Resurfacing (Syneron Medical Ltd.)
uses a series of electrodes to deliver RF energy; another device (ePrime, Syneron
Medical Ltd.) uses an array of microneedles arranged in pairs between which RF energy
is delivered.[42] Thermal wounds are created in a fractional manner directly to the reticular dermis.
The tissue directly beneath the electrodes or microneedles is selectively targeted
for deep dermal heating, while the surrounding areas are left intact.[43] The unaffected areas serve as a reservoir of cells that promote wound healing in
the treated areas.
Fractional RF devices contain an applicator with a disposable tip that contains parallel
rows of electrodes or microneedles that are arranged in a bipolar array. A closed
circuit of bipolar RF current is created between positively and negatively charged
electrodes or microneedles.
A prospective multicenter study was performed on 35 subjects who underwent three treatments
on their entire face with the Matrix RF device.[43] Eighty-seven percent of patients showed improvement in skin tightening, with a trend
toward less wrinkling and elastosis. Eighty percent of patients were satisfied with
their treatment. Other than one patient experiencing prolonged edema, side effects
were minimal.
Another study was undertaken to assess the efficacy and safety of the Matrix RF treatments
in photoaged Asian skin.[44] Moderate (26–50%) and incremental improvements were observed in each category of
physician evaluation, including smoothness and tightness. The degree of elastosis
also decreased significantly. Importantly, no pigment alterations or fat atrophy were
observed.
Intense Focused Ultrasound
Intense focused ultrasound (IFUS) has been investigated as a tool for treating solid
benign and malignant tumors for many decades and has recently emerged as a potential
noninvasive alternative in tissue tightening.[45] The primary mechanism of heat-induced tissue response is through coagulative necrosis
with precisely defined, sharp margins caused by absorption of acoustic energy.[46] The ultrasound waves induce a vibration in the composite molecules of a given tissue,
and the thermoviscous losses in the medium create tissue heating. As with RF, the
cellular changes depend on the increase in temperature and the exposure duration.
This can range from subtle ultrastructural cell damage with modulation of cellular
cytokine expression to total cell necrosis.[46] These findings parallel those found after laser or light-induced heat applications.
IFUS uses short, millisecond pulses with a frequency in the megahertz domain, using
significantly lower energies (0.5–10 J) than with traditional high-intensity focused
ultrasound. Initial experiments on postmortem skin using a prototype device (Ulthera
Inc., Mesa, Arizona) showed a focal depth of 4.2 mm below the skin surface.[45] This depth would potentially permit the targeting of surgical planes, such as the
superficial muscular aponeurotic system.
Initial clinical evaluation with the Ulthera device showed significant tightening
with a 1-mm eyebrow lift in more than 75% of study participants.[47] This helped lead to FDA approval in 2009 for tightening of the skin around the eyebrow.
Recent studies have investigated the effects of IFUS on tightening of the lower face
and neck. Suh et al treated 22 subjects, and improvements of the nasolabial folds
and jaw line were assessed. Objective improvement was seen in both areas, and 77 and
73% of patients reported improvement in the nasolabial folds and jaw lines, respectively.[48] Histologic evaluation of biopsy samples showed greater dermal collagen with thickening
of the dermis and straightening of elastic fibers in the reticular dermis after treatment.
Another study evaluated ultrasound tightening of skin laxity of the lower face and
neck using a two-pass protocol.[49] Ten subjects were treated using the two-pass approach using two different probes.
In this study, 80% of blinded clinicians and 90% of the subjects reported subjective
improvement. The safety of focused ultrasound has also been evaluated in Asian patients.[50] Focal bruising was present in up to 25% of treatment sessions. Interestingly, two
cases of postinflammatory hyperpigmentation were seen on the forehead at 1 month posttreatment.
The only other notable side effect was pain, which was recorded as severe in 54.4%
of treatment sessions.
