RESUMEN
BACKGROUND: The terms "biphasic" and "monophasic" have been used frequently as a means of differentiating hyaluronic acid (HA) fillers. This type of categorization is based on misinterpretations of the term "phase" and provides no help to the practitioner when selecting the most appropriate product for each indication, patient, and injection technique. OBJECTIVE: The purpose of this study was to analyze the properties of 2 HA filler families; Juvederm (JUV) (Allergan), often stated to be monophasic and Restylane (RES) (Galderma), often stated to be biphasic, and discuss what properties may have led to the use of the terms monophasic and biphasic. MATERIALS AND METHODS: Three different methods were used for JUV and RES: determination of extractable HA; determination of water uptake; and microscopy. RESULTS: The analyzed products were shown to contain both observable gel particles and extractable HA and have the ability to absorb added water. CONCLUSION: The categorization of HA fillers as biphasic or monophasic was shown to be scientifically incorrect and should therefore be avoided. Further analytical measurement of the properties leading to this misinterpretation can provide information to discriminate and categorize HA fillers on a sounder scientific basis.