Rheologic and Physicochemical Properties Used to Differentiate Injectable Hyaluronic Acid Filler Products

Abstract

Background: Injectable hyaluronic acid fillers are routinely used for correction of soft-tissue volume loss and facial rejuvenation. Product differentiation has primarily been based on the rheologic parameter known as elastic modulus (G′), although other physicochemical properties are being explored to characterize potential product performance. As clinical data regarding product performance are lacking, the practical experience of injectors provides a valuable bridge in the knowledge gap between product rheologic data and product use. Methods: Rheologic and physicochemical measurements (swelling factor and cohesion) were collected for 18 products. To observe the impact of G′ and hyaluronic acid concentration on swelling factor and cohesion, proportional relationships were evaluated. Contributing authors were queried regarding their G′-based selection of products when considering skin quality, degree of correction, injection depth, and anatomical location. Results: Relationships were observable between G′ and swelling factor and G′ and cohesion only when limited to products manufactured by the same crosslinking technology and the same concentration. No relationship between isolated hyaluronic acid concentration and swelling factor or cohesion was apparent. Although rheological parameters and the assumptions of ex vivo data translating to in vivo performance are oftentimes not completely aligned, in the clinical experience of the authors, in general, higher G′ products are better suited for thicker skin and deeper injection planes, whereas lower G′ products are better for more superficial planes, although exceptions to these trends are also made based on technical experience. Conclusions: While rheologic and physicochemical characteristics can vary widely between products and the methods and measurements of these parameters are often difficult to correlate, G′ represents a useful and consistent parameter for product differentiation. Understanding how to select products based on G′ is valuable knowledge for customizing injection plans and contributes to an optimal aesthetic outcome.

Evaluation of the Rheologic and Physicochemical Properties of a Novel Hyaluronic Acid Filler Range with eXcellent Three-Dimensional Reticulation (XTR™) Technology

“…Fillers with intermediate cohesivity distribute with a transitional pattern and fillers with low cohesivity have a higher tendency to have intradermal microbolus dispersion. Fagien et al presented data that suggest that as G decreases, the gel may exhibit more cohesive properties [31]. The same inverse relationship was noted by Edsman et al [44] further supporting the observation that this relationship appeared to exist only among products made by the same technology [31].…”

“…Exclusive manufacturing processes of HA fillers are used to alter HA molecular structure as well as their physicochemical and mechanical behaviors. These varied behaviors lend the product their unique characteristics and are perceived to affect their overall product performance [31]. Crosslinked HAs are viscous, so in order to facilitate extrusion of this material through a thin gauged needle, manufacturers have purposely produced low viscosity HA fillers with high shear rates.…”

“…G* measures the overall resistance to deformation of a gel, which comprises a recoverable component from the gel's elasticity (G ) and a non-recoverable component from the gel's viscosity (G ) [32]. In general, the higher the G , the higher is the elasticity and resistance to deformation and ultimately its capability to restore the soft tissue volume [31,33,34]. G also indicates filler stiffness or hardness and determines the injection at the appropriate anatomical plane.…”

“…These fillers have good volume restoration and can be injected deep into the soft tissue where they are neither palpable nor visible. Filler ranges have typically fulfilled this indication for volume correction of deep planes [31]. A potential advantage of high G is the capability to obtain a good prominent tissue projection and, with minimal amount of product, can provide structure to support the parts of the soft tissue with volume loss [3,35].…”

Better Results in Facial Rejuvenation with Fillers

“… G*, n*, and cohesivity obtained from Refs. 53 and 54 measured at 0.1 and 0.7 Hz. DW, mg, drop-weight in milligrams; NA, not available.…”