Drs. Nagy and Vanek report a 17 percent benefit in skin contraction for vibration amplification of sound energy at resonance (VASER) compared with an 11 percent improvement for traditional liposuction, a difference of 6 percent.1 Comparing percentages with a percentage, they cite a 53 percent relative benefit in skin tightening from this form of ultrasonic assistance. Titled a multicenter study, it involved two surgeons operating on 20 nonconsecutive patients.
The first question to ask is whether 20 women and 33 sites are sufficient to detect a 6 percent difference in treatment effects. Standard deviations would have to be unusually small. The measurements—the crux of the article—are not included in the authors' Table 1. The authors do illustrate bars representing 95 percent confidence intervals in Figure 2 of their study, deleting the bars on one side so they do not overlap. Such wide overlapping confidence intervals are difficult to reconcile with a p value of 0.003. A power analysis for 80 percent power, with an alpha level of 0.05, using a two-tailed, matched-pairs t test would require a sample size of 199 to detect a small treatment effect (Cohen's d = 0.20)2,3 or, in this case, a small difference in treatment effects.
The authors divide the measured skin retraction by the volume of aspirate and call the adjusted values “normalized.” No foundation is offered to justify this alteration that penalizes the non-VASER sides (because their mean aspirate volumes were greater), apart from a comment about achieving a cosmetic endpoint. Evidently, the authors believe a correction is needed because there should be more skin contraction if more volume is removed.
In contrast, the sides treated with liposuction alone may have been more aggressively treated compared with the VASER sides, causing more tissue trauma and possibly interfering with skin contraction. Indeed, the higher aspirate volumes, despite a lower infusion-to-aspirate ratio, and higher lipocrit values on the non-VASER sides are consistent with greater tissue trauma. Ideally, and this would not have been difficult to do, the infusion-to-aspirate ratios and aspirate volumes would have been equal on both sides, allowing an apples-to-apples comparison.
To justify the authors' adjustment of their measurement data, a proportionate relationship between aspirate volume and skin retraction would need to be established. None is provided. Clinical experience suggests that there is an ideal aspirate volume for a particular area, with optimal skin contraction, and either undertreatment or overtreatment (compromising elasticity) results in lesser degrees of skin contraction, making a proportionate relationship unlikely.
Another crucial study question is what parameter to measure. The correct parameter for evaluating skin tightening is skin area. Instead, the authors measure perimeters. Skin retraction is represented as the change in the perimeter of a tattooed equilateral triangle. The authors make no mention of the squared relationship between the area contained in a triangle and its perimeter (Fig. 1) and how this difference relates to their adjustment for aspirate volume. Their adjustment contemplates that the skin will contract to one-fourth of its original area if the aspirate volume is doubled—the consequence of reducing the perimeter by half. Such a dramatic reduction would be physically impossible in view of the unchanging musculoskeletal dimensions of the treated area. Even if a proportionate relationship between skin retraction and aspirate volume exists, the authors are imposing an exponential adjustment on changes in area based on linear changes in aspirate volume and, in so doing, are overcorrecting.
The authors state that the difference in mean aspirate volumes between the two groups (671 cc versus 781 cc) was a nonsignificant 6.4 percent. This difference is evidently a typographic error and should read 16.4 percent; however, not only is the difference incorrect, these means are also erroneous. My calculations based on the data in the authors' Table 1 reveal means of 702.27 and 757.58 and a significant difference (7.9 percent greater for the non-VASER group, p = 0.04 using a two-tailed t test). If the authors' adjustment is removed—whether it is 7.9 percent or 16.4 percent—the degree of skin retraction comes out in favor of the non-VASER sides.
One problem using triangles is that the area can vary even if the perimeter is unchanged, depending on the orientation of the three dots.4 This geometric fact makes it impossible to convert triangular perimeters to areas, unlike squares. The summation of three such measurements, evidently made by the surgeons using a ruler on the skin under ultraviolet light, increases the potential for error. Computer-assisted area measurements, possibly using the simpler geometry of squares, made from calibrated and size-matched photographs (Fig. 1) would likely make the measurements easier, avoid summation, reduce measurement error, and more accurately represent skin contraction as a two-dimensional process (setting aside the implications of three-dimensional contraction).
There are other issues to consider here: the use of different cannulae, unreported postoperative weights, insufficient sample sizes to justify interoperator comparisons and questionnaires, and no control group to assess measurement reproducibility in untreated areas. The control group here is really an alternative treatment group. There is no information on the inclusion rate. Did the 33 sites represent all tattooed sites, or were some excluded from evaluation? Conclusions regarding blood loss and early postoperative swelling are undermined by the differences in aspirate volumes and infusion-to-aspirate ratios between groups.
Although the financial disclosure states that the authors were not paid, the company sponsor paid for patient recruiting and surgeon fees and no doubt made these expensive machines available to the authors at reduced cost. As for commercial bias, Dr. Nagy has spoken extensively on behalf of VASER for 8 years.5 Both authors are advertised to patients as VASER physicians on the company Web site, www.vaser.com, which also highlights the study findings. According to Dr. Nagy's Web site, Grant Palmer, Ph.D., of Sound Surgical Technologies was originally credited as one of the study authors.5 Along with a commercial VASER video, the word VASER appears 20 times on a single page of Dr. Vanek's Web site.6
This study is an example of a high-level study design, but one with weaknesses in methodology, error, financial conflicts of interest, and commercial bias, all of which undermine the authors' conclusions. With adequate sample sizes, consistency of technique, statistical safeguards, and impartial investigators, this ultraviolet tattooing measurement technique holds promise for future studies. In the meantime, it is important to avoid making clinical recommendations (and major investments) on the basis of unreliable data.
Eric Swanson, M.D.
11413 Ash Street
Leawood, Kan. 66211
The author thanks Jane Zagorski, Ph.D., for statistical analyses.
The author has no conflicts of interest to disclose. There was no outside funding for this study.
1. Nagy MW, Vanek PF Jr. A multicenter, prospective, randomized, single-blind, controlled clinical trial comparing VASER-assisted lipoplasty and suction-assisted lipoplasty. Plast Reconstr Surg. 2012;129:681e–689e.
2. Cohen J. Statistical Power Analysis for the Behavioral Sciences. 2nd ed. Hillsdale, NJ: Lawrence Erlbaum; 1988.
3. Faul F, Erdfelder E, Lang AG, Buchner A. G*Power 3: A flexible statistical power analysis program for the social, behavioral, and biomedical sciences. Behav Res Methods 2007;39:175–191.
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