Re: Lift capabilities of hyaluronic acid fillers by Marcos Borrell, Dustin B. Leslie & Ahmet Tezel (J Cosmet Laser Ther. 2011;13:21–27)

In January 2011, a paper was published in the Journal of Cosmetic and Laser Therapy entitled ‘ Lift capabilities of hyaluronic acid fi llers ’ by Marcos Borrell, Dustin B. Leslie and Ahmet Tezel (2011;13:21 – 27). We have some comments about the results presented in that paper as well as some concerns about the methods and the scientifi c approach used. The authors state that the lift capability of a fi ller depends on the gel hardness (G ’ ) and the cohesivity. They compare two different fi llers: one manufactured by Allergan Inc., employer of the authors, and one fi ller from Medicis Aesthetics Inc., manufactured by Q-Med AB. In the paper, data are presented from measurements on the two fi llers and a comparison is made regarding the hardness and the cohesivity. The paper also describes the manufacturing processes of the fi llers and speculates on properties that could be commented on. However, we have focused on the scientifi c approach and the experimental data presented in the paper. We do agree that the gel strength (or ‘ gel hardness ’ , the term used in the paper) may be an important factor for the lifting capacity and that the gel strength could be measured by rheology. In fact, we presented and discussed this at the Anti-aging Medicine World Congress in 2010 (1). Rheology is usually perceived as diffi cult and therefore would a correct terminology have helped the readers to understand the presented data. Using the two invented terms ‘ linear viscosity ’ and ‘ gel hardness ’ for the scientifi c established term ‘ elastic modulus ’ will only confuse the readers, making it more diffi cult to compare the results with earlier published data (2 – 5). The term ‘ linear viscosity ’ does exist; it is sometimes used synonymously with ‘ zero shear viscosity ’ , which is measured by rotational viscometry. The linear viscosity or the zero shear viscosity may also be calculated from a frequency sweep if the Cox – Mertz rule (6) applies to the material, but it cannot be calculated from a strain sweep measurement at 5 Hz as the authors claim. Rheology is a useful tool for characterization of gel properties and several different rheological methods can be used to gain information about the material. The only rheological results presented in the paper are from a strain sweep measurement at 5 Hz (see their Figure 5), where the plateaus at low strains give the elastic modulus of the fi llers and a shorter plateau (a plateau ending at a lower strain) is usually found for stronger gels. Normally, a strain sweep is made to determine the linear viscoelastic region (LVR), which is used to fi nd the settings for the subsequent measurements. A frequency sweep, as used in the reference cited in the Methods section (4), is a rather common method that could have been used but with a different experimental set-up than used in reference 4 (7). The only information gained from the strain sweep shown in the paper is the elastic modulus at 5 Hz, which could have been given in a table or in the text. Presenting the curves (Figure 5) does not give any more information; probably it is only more confusing for those not engaged in rheology measurements. The authors do not help the readers to understand the fi gure; rather they confuse the readers further by stating that their product “ provides lift because it shows less susceptibility to yield to a given strain ” . The interpretation of the rheology section in the paper is that the authors have a limited knowledge in the fi eld. Journal of Cosmetic and Laser Therapy, 2011; 13: 123–125