INTRODUCTION
The demand for minimally invasive aesthetic treatments, including hyaluronic acid (HA) fillers, has grown significantly in recent years, with the uptake of such procedures rising by more than 75% in the United States (US) over the past decade.1,2 Clinician experience, expertise, and confidence in handling and administering these products have subsequently grown.1,3 HA fillers provide a durable, yet non-permanent, non-surgical option to address facial volumetric changes associated with aging.2,4-9 HA filler treatments in general aim to provide volume so that the face appears lifted while looking even and natural.2,4-6
HA gel strength/firmness is usually expressed as the elastic modulus, or G prime (G'), while flexibility can be defined by the xStrain (the strain value for the G prime/G double prime [G"] crossover in the amplitude sweep).2,10,11 The xStrain represents the furthest point at which the gel can recover following deformation.2,11 Beyond this point, the gel begins to behave more like a liquid and will no longer be able to return to its original shape.2,11 Because G prime and xStrain are two separate properties and not necessarily linked, products with similar G primes may exhibit different xStrains and vice versa.12 Products with a higher G prime are stronger and more resistant to deformation than those with a lower G prime.12 Products with higher xStrain are more flexible than those with lower xStrain values.11,12
The NASHA® technology, used for Restylane®, Restylane Silk (R. Silk), and Restylane Lyft (R. Lyft), allows for the preservation of the naturally long HA chains resulting in strong gels with high G primes. In addition, the NASHA technology uses minimal modification and controlled particle sizing.11,13-17 HA fillers produced with NASHA exist both with and without lidocaine.13,16,17 The OBT™ technology (referred to as XpresHAn in the US) produces flexible HA fillers where the strength/firmness (G prime) is varied by applying different degrees of crosslinking.11,12,14,15,18-25 Fillers formulated using OBT include Restylane Refyne (R. Refyne), Restylane Kysse (R. Kysse), Restylane Defyne (R. Defyne), and Restylane Volyme (R. Volyme; Restylane Contour in the US).19-22,25
