Introduction

The appearance of rhytides and skin laxity are near certainties during the aging process. A number of modalities have been used to reduce the appearance of rhytides and skin laxity, including laser, mechanical, and surgical techniques. Over a decade ago, ablative resurfacing lasers were deemed the gold standard for facial skin tightening. Despite substantial clinical benefits, the technology was beset with significant downtime and an increased risk of side effects such as erythema, permanent pigmentary changes, infection, and scarring. Patients are now more accustomed to procedures with both reduced downtime and sufficient clinical improvement. This has led to a burgeoning number of non-ablative technologies with little to no recovery time and a more favorable risk–reward profile. Unlike ablative lasers, non-ablative technologies induce thermal injury to the dermis or subcutaneous tissues without epidermal vaporization. Epidermal protection is customarily achieved through the use of adjunctive surface cooling.

In terms of skin laxity specifically, the gold standard of treatment remains rhytidectomy or surgical redraping. The goal of this chapter is to review the major types of minimally invasive, non-ablative tissue tightening techniques, including radiofrequency-, light-, and ultrasound-based devices (Table 9.1). These devices are not a replacement for surgical procedures and appropriate patient selection remains key to overall satisfaction.

Table 9.1 Major types of skin-tightening technologies

Thermal collagen remodeling

All skin-tightening devices work by delivering heat in the form of energy to the skin or underlying structures. This creates mechanical and biochemical effects that lead to both immediate contraction of collagen fibers and delayed remodeling and neocollagenesis via a wound-healing response (Box 9.1).

Collagen fibers are composed of a triple helix of protein chains linked with interchain bonds into a crystalline structure. When collagen fibers are heated to specific temperatures, they contract due to breakage of intramolecular hydrogen bonds. Contraction causes the crystalline triple helix structure to fold, creating thicker and shorter collagen fibers. This is thought to be the mechanism of action of immediate tissue tightening seen after skin-tightening procedures. Studies have also found selective contraction of fibrous septae in the subcutaneous fat, which is thought to be responsible for the inward (Z-dimension) tightening (Fig. 9.1).

Figure 9.1 Human skin (A) before and (B) 4 months after treatment with the ThermaCool^® TC, showing epidermal thickening and increased dermal density.

Photographs courtesy of Solta.

Problems can arise if too much heat is delivered as the collagen fibrils will denature completely above a critical heat threshold. This can lead to cell death, denaturation, and scar formation. If too little heat is delivered, there will be no tissue response, although it appears that mild thermal injury gives rise to new dermal ground substance and tissue remodeling of photodamaged skin over time. The optimal shrinkage temperature of collagen has been cited as 57–61°C; however, contraction is in actuality determined by a combination of temperature and exposure time. For every 5°C decrease in temperature, a tenfold increase in exposure time is needed to achieve an equivalent amount of collagen contraction. Studies show that for exposure times in the millisecond domain the shrinkage temperature is greater than 85°C, whereas for exposure times over several seconds the shrinkage temperature is at a lower range of 60–65°C.

The other main mechanism in skin rejuvenation is a secondary wound-healing response that produces dermal remodeling over time. The wound-healing response entails activation of fibroblasts to increase deposition of type I collagen and encouraging collagen reorganization into parallel arrays of compact fibrils.

Radiofrequency devices

Radiofrequency devices have been used for hemostasis, electrocoagulation, and endovenous closure in medical dermatology. In the aesthetic arena, the technology has been used for skin resurfacing and non-invasive tissue tightening.

Radiofrequency energy is energy in the electromagnetic spectrum ranging from 300 MHz to 3 kHz. Unlike most lasers, which target specific absorption bands of chromophores, heat is generated from the natural resistance of tissue to the movement of electrons within the radiofrequency field as governed by Ohm’s law (Box 9.2). This resistance, called impedance, generates heat relative to the amount of current and time by converting electrical current to thermal energy. Consequently, energy is dispersed to three-dimensional volumes of tissue at controlled depths.

The configuration of electrodes in a radiofrequency device can be monopolar or bipolar, and both have been used for cutaneous applications. The main difference between the two is the configuration of electrodes and type of electromagnetic field that is generated. In a monopolar system, the electrical current passes through a single electrode in the handpiece to a grounding pad (Box 9.3). This type of electrode configuration is common in surgical radiofrequency devices because there is a high density of power close to the electrode’s surface with the potential for deep penetration of tissue heating. In tissue-tightening applications, surface cooling is used to protect the outer layers of the skin and heat only deeper targets. In a bipolar system, the electrical current passes between two electrodes at a fixed distance (Box 9.4). This type of electrode configuration has a more controlled current distribution; however, the depth of penetration is limited to approximately one-half the distance between the electrodes.

With radiofrequency technologies, the depth of energy penetration depends upon not only the configuration of the electrodes (i.e. either monopolar or bipolar), but also the type of tissue serving as the conduction medium (i.e. fat, blood, skin), temperature, and the frequency of the electrical current applied (Box 9.5). Tissue is made up of multiple layers, including dermis, fat, muscle, and fibrous tissue, all of which have different resistances to the movement of radiofrequency energy (Table 9.2). Structures with higher impedance are more susceptible to heating. In general, fat, bone, and dry skin tend to have low conductivities such that current tends to flow around these structures rather than through them. Wet skin has a higher electrical conductivity allowing greater penetration of current. This is why, in certain radiofrequency procedures, improved results can be seen with generous amounts of coupling fluid and increased hydration of skin. The structure of each individual’s tissue (dermal thickness, fat thickness, fibrous septae, number and size of adnexal structures) all play a role in determining impedance, heat perception, and total deposited energy despite otherwise equal parameters.

Table 9.2 Dielectric properties for human tissue at 1 MHz and room temperature

Temperature also influences tissue conductivity and the distribution of electrical current. Generally, every 1°C increase in temperature lowers the skin impedance by 2%. Surface cooling will increase resistance to the electrical field near the epidermis, driving the radiofrequency current into the tissue and increasing the penetration depth. Conversely, target structures that have been pre-warmed with optical energy will, in theory, have greater conductivity, less resistance, and greater selective heating by the radiofrequency current. This is the theoretical advantage touted by hybrid skin-tightening devices that use a combined approach of light and radiofrequency energy together giving synergistic results.

Monopolar radiofrequency

The first monopolar tissue-tightening device on the market was the ThermaCool^® device (Solta Medical, Hayward, CA), introduced in 2001. It remains the most exhaustively studied and published apparatus. The ThermaCool^® device uses a capacitive coupled electrode at a single contact point and a high-frequency current at a frequency of 6 MHz. A disposable membrane tip is used to deliver the energy into the skin, with an accompanying adhesive grounding pad serving as a low-resistance path for current flow to complete the circuit. The use of capacitive rather than conductive coupling is important because it allows the energy to be dispersed across a surface to create a zone of tissue heating. With conductive coupling, the energy is concentrated at the tip of the electrode, resulting in increased heating at the contact surface and an increased risk of epidermal injury (Fig. 9.2).

Figure 9.2 (A) Thermage ThermaCool^® device; (B) capacitively coupled electrode treatment tip.

Photographs courtesy of Solta.

In the early clinical experience, one of the main drawbacks to the ThermaCool^® procedure was a high degree of discomfort during the procedure, requiring heavy sedation or frank anesthesia. The protocol at that time was to perform 1–2 passes at higher energies. The treatments were quite painful, results tended to be inconsistent from patient to patient, and some adverse events such as fat necrosis and atrophic scarring were noted. Over the years, treatment protocols have evolved to a paradigm utilizing lower energies, multiple passes, and patient feedback on heat sensation as the end point of therapy. This has all but eliminated the risk of unacceptable side effects and has greatly reduced the pain involved such that most procedures can be performed without any anesthesia. Monopolar radiofrequency energy is now commonly used to accomplish skin tightening of the face, eyelids (Case study 1), abdomen (Fig. 9.3), and extremities.

Figure 9.3 Tightening of abdominal skin with the ThermaCool^® TC: (A) before, and (B) 1 year after one pass at 15.5 J.

Case Study 1

A 47-year-old woman presents for a consult regarding excess skin on her upper eyelids. She states she has noticed a gradual increase in drooping over the last several years and she is finding it difficult to wear eye shadow. She has her thirtieth high school reunion in 4 months and states she wants improvement by then. She tells you she is not trying to look 18 again, but just wants to look as good as she feels. On examination, the patient has mild to moderate excess skin laxity on the upper eyelids with minimal bulging of the fat pads. Her brows are in a normal position without significant ptosis. This patient would be a candidate for either radiofrequency skin tightening or a surgical blepharoplasty. She may be a better candidate for non-surgical tightening because of her mild to moderate skin laxity without underlying structural deficits. She also has realistic expectations about results and has several months post-procedure for the skin tightening to take effect before her goal event. Most of the skin-tightening technologies can be used over multiple areas of the body; however, there are a few locations that favor some devices over others. The ThermaCool^® device is an excellent choice for skin tightening of eyelid skin because it has a small 0.25 cm² tip, high eye safety profile and lack of significant discomfort during treatment. When the eyelids are being treated, plastic corneoscleral lenses must be put in place. These should be gently inserted and removed so as not to cause erosions of the corneal surface. In addition, the operator should be careful not to deliver too much pressure on the globe, as this can result in vasovagal stimulation and bradycardia. Practitioners should never use ThermaCool^® tips larger than the 0.25 cm² eyelid tip owing to the depth of penetration.

The clinical results of non-ablative radiofrequency skin tightening were first reported by Fitzpatrick and colleagues for the periorbital area in 2003. At least some degree of clinical improvement was reported in 80% of subjects (Figs 9.4–9.6) In 2006, Dover and colleagues compared the original single-pass, high-energy technique with the updated low-energy, multiple-pass technique using immediate tissue tightening as a real-time end point. With the original treatment algorithm, 26% of patients saw immediate tightening, 54% observed skin tightening at 6 months, and 45% found the procedure overly painful. With the updated protocol, 87% had immediate tissue tightening, 92% had some degree of tightening at 6 months, only 5% found the procedure overly painful, and 94% stated the procedure matched their expectations (Figs 9.7, 9.8). The low-energy, multiple-pass protocol has also been reported to be significantly safer, lowering the incidence of adverse events to less than 0.05%.

Figure 9.4 Eyebrow lift following Thermage^® treatment: (A) baseline, and (B) 4 weeks post-treatment with a mean lift of 3.42 mm (right brow) and 3.41 mm (left brow).

Photographs courtesy of Solta.

Figure 9.5 Periorbital rejuvenation following Thermage^® treatment: (A) baseline, (B) 2 months post treatment, and (C) 4 months post-treatment.

Photographs courtesy of Solta.