Michele Zocchi in Italy and Ralph Kloen in the United States are credited with developing and introducing internal ultrasound–assisted liposuction (UAL). [1, 2] Industrial applications of high-energy ultrasound, as well as medical imaging applications of low-energy ultrasound, have been used for many years.
The use of ultrasound as an ablative tool in surgery has been well established in neurosurgery and general surgery applications. Zocchi's interest in ultrasound was originally for harvesting collagen from aspirated fat. The observations that adipose tissues were effectively emulsified while connective tissue structures were preserved in vitro led to the concept of using ultrasound adjunctively in vivo. This led to clinical use of first-generation devices (eg, SMEI, Casales), which used blunt solid probes 4-6 mm in diameter followed by aspiration with standard cannulas.
The promise of enhanced fat removal with minimal blood loss, claims of improved skin retraction, and safer large-volume procedures resulted in intense interest in UAL. However, enthusiasm was tempered by reports of skin burns, dysesthesias, and high seroma rates. Although considerable debate remains concerning the long-term effects and clinical use of UAL, it has become a well-established tool in the body-contouring armamentarium.
Physics of Ultrasound
Ultrasonic energy consists of sound waves traveling through a liquid medium at a frequency of greater than 16,000 cycles per second or Hertz (Hz). The study of the physical effects of sound waves in a liquid medium is called phonochemistry.
Ultrasound for practical applications can be produced by passing electrical energy to a piezoelectric crystal, resulting in mechanical vibration. Sound waves consist of alternating expansion and compression cycles. The compression phase exerts positive pressure in the medium, while expansion results from the negative pressure phase.
Ultrasound of sufficient intensity can produce microcavities in a liquid or semiliquid medium during the expansion cycle. This property of microcavitation is harnessed for UAL. Ultrasound also may work by direct micromechanical and secondary thermal effects. Microcavitation can be observed at the tip of a UAL cannula, although the energy is rapidly dispersed.
Therefore, the mechanism of action for internal UAL is presumed to be more related to this process than to micromechanical or thermal effects, which may play a greater role in external UAL. The precise effects of external UAL have not yet been well documented.
Effects of Ultrasound in Biologic Media
Zocchi states that the susceptibility of a liquid or biologic tissue to microcavity formation depends upon the molecular cohesion of the material. The negative pressure required is related to the density of the tissue. Low-density tissues such as fat cells have low molecular cohesion, which favors the production of microcavities. Sufficient negative pressure during the expansion phase results in microcavities large enough to form gas bubbles. However, a bubble in adipose tissue is inherently unstable, tending to either explode or dissolve. Ultrasound of the right frequency and amplitude (energy) may enable the bubbles to continuously expand and contract and reach a dynamic equilibrium. This process is facilitated by a moist environment, so the use of a wetting solution is important.
However, the ability of the bubble to contract during the compression phase is less than its ability to expand, because of differential diffusion of gas across the surface of the bubble, which is related in part to the surface area. This process culminates in increasing absorption of energy until a critical size is reached and the bubble implodes. Denser materials such as connective tissue and muscle are essentially unaffected by this process; however, continued application of ultrasound after complete emulsification by microcavitation results in accumulation of secondary thermal energy and micromechanical trauma.
Concern has been raised about potential effects related to other phenomena occurring with acoustic energy in an aqueous medium. With frequencies and amplitudes used in UAL, experimental work has shown that extreme temperature variations can occur in the inside of the microbubble. As the bubble collapses during the compression phase, core temperatures may momentarily reach 72,000 Kelvin or higher. This is manifested as a pulse of electromagnetic radiation in the visible, ultraviolet, or soft x-ray range. This effect is termed sonoluminescence.
Damage to DNA and other biomolecules may be the result of sonoluminescence, but this has not been demonstrated, and radiation levels generally are so low as to be barely detectable even under ideal conditions. Other sonochemical effects probably are more directly related to the high temperatures created during bubble collapse. Disruption of chemical bonds results in the formation of free radicals and other reactive ions. However, clinically significant levels of free radicals in aspirates of sonicated tissue have not been documented.
The histologic effects of UAL have been studied in animal and human models. In the pig model, Kenkel et al showed lower levels of hemoglobin in the lipoaspirate with UAL incorporating a cooling sheath as compared to standard tumescent technique; UAL without the cooling sheath had blood loss comparable to the standard technique.  More importantly, the hemoglobin-to-triglyceride ratio was lower in both UAL groups than in the standard liposuction group. Perfusion radiograms documented less vascular disruption with UAL. The human model can be evaluated by treating abdominoplasty specimens in situ prior to excision. These studies also have documented preservation of vascular and connective tissue structures.
Excessive application can result in "micromaceration" and fragmentation of connective tissues. Ultrasonic energy application at amplitudes and frequencies used in lipoplasty is reasonably selective for adipose tissue.
However, the potential for injury to myelin sheaths because of their high lipid content does exist. Histologically, effects can be observed in the rat sciatic nerve model with low to moderate settings. No functional impairment is seen until higher settings are applied. This correlates histologically with signs of neuronal injury and myelin breakdown and repair; functional recovery occurs after several weeks. Clinically, excessive application of ultrasound may be associated with postoperative numbness, paresthesias, and discomfort.
Indications and Clinical Application
Most plastic surgeons with experience with UAL have reported better results with less fatigue in treating fibrous areas, such as gynecomastia, the back, upper abdomen, and posterior hip rolls (see the images below).  Areas with less dense fat such as the inner knee and medial thigh generally do not benefit from application of ultrasound with lipoplasty. UAL is an adjunct to lipoplasty rather than an alternative methodology.
Second-generation devices (eg, Lysonix, Mentor) use hollow cannulas for simultaneous aspiration and ultrasound delivery, allowing the quality of the aspirate to be monitored. When the aspirate changes from pale yellow to pink or gray, emulsification is complete. This is a clinically significant end point that must be respected when using second-generation devices.
Second-generation devices have certain disadvantages, as well. They are relatively large in diameter (≥5 mm), which requires longer incisions in order to accommodate skin protectors. The lumen is small, resulting in inefficient aspiration; therefore, a 2-stage procedure is generally performed with a second aspiration-only component. Additionally, the ultrasound energy is focused in a longitudinal direction, directly away from the tip; this may increase the risk of burns.
Third-generation technology returns to the solid probe with some changes in probe design (Vibration Amplification of Sound Energy at Resonance [VASER], Sound Surgical Technologies LLC, Louisville, Colo).  Grooves in the probe near the tip result in radial dispersion of acoustic energy, yielding more effective emulsification with lower total energy application. These probes are 2.9 mm and 3.7 mm in diameter, so smaller incisions are required. Some evidence indicates that VASER liposuction reduces blood loss compared to similar cases without ultrasound, with lower hemoglobin content in the UAL aspirate.  As currently practiced, third-generation UAL has a low complication rate. 
Continuous cannula movement is recommended to minimize the potential for focal, secondary thermal injury. While some practitioners have adopted a slow, deliberate style, and others favor rapid movement with high amplitude settings, the speed of cannula advancement should be determined by the degree of resistance at a given energy level. Loss or resistance to cannula advancement is a strict end point, because it indicates complete tissue emulsification.
Multifrequency (eg, pulsed vs continuous) has been proposed as an advantageous technical improvement,  although supportive data are largely subjective. Another application is for the treatment of multiple systemic lipomatosis.  In this condition, the lipomas are nonencapsulated, so the specific benefits of adding ultrasound also remain subjective.
Some discussion remains as to whether UAL enhances skin retraction. Since this is a difficult phenomenon to measure objectively, claims that indications for the use of UAL extend to large-volume cases or to patients with lax skin have not been supported. Presumably, skin retraction would be stimulated by controlled thermal injury to dermal collagen. Current techniques and technology are not developed to the state that this can be accomplished safely and predictably.
The use of UAL for very large volume fat removal is also controversial. Since a wet environment is necessary for effective conduction of acoustic energy into adipose tissue, infusion of a wetting solution is required. The ratio of wetting solution infused to anticipated fat aspiration is 1:1.5-2, regardless of whether this results in tissue tumescence. Although it may be possible to accomplish massive fat aspiration with minimal blood loss, the requisite amounts of wetting solution are associated with substantial fluid shifts postoperatively.
The American Society of Plastic Surgeons (ASPS) recommends that outpatient lipoplasty be limited to 5000 mL of total aspirate, regardless of the technique. 
Early clinical series reported high rates of seromas associated with UAL. These can be avoided with attention to technique, especially amplitude settings and duration of application. If seromas do occur, they are usually treated with serial aspiration and postoperative compression.
Skin burn is another complication that can be avoided by the use of skin protectors or cooling sheaths. Some surgeons have found that skin protectors are unnecessary as long as fluid is oozing from the entry site and the cannula is kept moving without pressure against the adjacent skin. A greater risk may be "end hits" resulting from the tip of the cannula impacting the undersurface of the skin, particularly with second-generation devices. Vigilance is required to avoid "tenting" the skin from the cannula tip while applying ultrasound.
A systematic approach is important, working either from deep to superficial or vice versa. Application of UAL in the superficial plane may be associated with higher risk of skin burns, seromas, and uneven skin retraction.
Perform final contouring after an ultrasonic end point is reached. This stage is done with standard cannulas, and separate incisions for "fine tuning" with microcannulas may be used, if needed. The overall end point for the lipoplasty procedure is determined by satisfactory contouring rather than by indicators for stopping the ultrasonic portion. Because of the extensive fragmentation of lipocytes in the emulsion, the aspirate is not suitable for regrafting.