Comparative Rheology of Hyaluronic Acid Fillers, Poly-l-lactic Acid, and Varying Dilutions of Calcium Hydroxylapatite

Dear Editor, McCarthy et al1 aimed to describe the range of rheological properties exhibited by undiluted and unapproved experimental dilutions of calcium hydroxylapatite in a carboxymethyl cellulose carrier (CaHA-CMC) and how it compares to poly-l-lactic acid and the wide range of hyaluronic acid (HA) fillers on the market. Although we commend the authors for attempting to use rheological measurements to describe the dilutional versatility of CaHA-CMC, we strongly believe that these conclusions are erroneous and irrelevant, and the clinical implications are scientifically misleading. Starting with the nature of the product the authors are characterizing, CaHA-CMC is a biphasic filler comprised of calcium hydroxylapatite microspheres suspended in an aqueous CMC solution carrier, in a 30%:70% mixture, respectively. The CMC carrier has gel-like properties that can be measured with a rheometer, but the CaHA microspheres are hard bioceramic materials that do not exhibit viscoelastic properties; this intrinsically confounds any results collected (eg, like adding rocks to a gel).2,3 In a previous study, the CaHA microspheres seem to increase the product’s observed G′ compared with the CMC carrier alone.4 However, these rheological parameters (G*, G′, Gʺ, and tan δ) are characteristics of viscoelastic materials (eg, gels) and cannot accurately describe the physical behavior of CaHA-CMC, as this biphasic filler is not a viscoelastic material.5 The same can be said for poly-l-lactic acid, which is also not a viscoelastic material, and the authors should not have included it in the graph and table comparisons. Applying measurements intended for viscoelastic materials to nonviscoelastic materials results in fundamentally flawed data, making any conclusions or clinical interpretations invalid. Furthermore, juxtaposing measurements from different studies, despite identical conditions, is problematic. Although this is acknowledged as a methodological limitation by the authors, it still forms the basis for all conclusions. Despite the inappropriate application of rheological measurements, the most troubling conclusion from this study is the clinical implication that CaHA-CMC can be optimized to reflect the full range of HA filler properties using the “dilutional rheomodulation” recommendations in Table 4 of the work of McCarthy et al.1 The CMC carrier is the only gel-like component of the product, which provides the initial volumization effects, and thus can reflect the rheological measurements (eg, G′ lifting capacity).2,3 However, CMC only acts as a carrier and is “rapidly removed from the body within 6 to 8 weeks,” as described by Aguilera et al.2 Therefore, the rheological measurements of undiluted and experimental dilutions of CaHA-CMC presented in this article only have short-term clinical implications. After the CMC carrier disappears within 6–8 weeks, the only component of the product remaining is the CaHA microspheres, which do not resemble or behave like HA filler gels. Therefore, it is misleading for the authors to suggest that various dilutions of CaHA-CMC can be used to mimic the effects of the wide range of HA fillers available. Given the significant concerns we have with the methodology and the misleading and biased conclusions presented in this article, we advise readers against accepting its findings without critical evaluation and further verification from reliable sources. DISCLOSURES Dr. Hicks is an employee of Galderma, Dallas, TX. Öhrlund is an employee of Galderma, Uppsala, Sweden.