INTRODUCTION
Calcium hydroxylapatite microspheres in a carrier gel containing carboxymethyl cellulose (Radiesse® [CaHA]; Merz North America, Inc., Raleigh, NC) is FDA-approved, as well as approved in other countries, for the correction of moderate to severe facial wrinkles and folds. CaHA provides immediate volume correction, while also stimulating collagenesis at the injection site to provide long-lasting treatment results for up to a year or more in many patients.1-4 In 2007, anecdotal successes was reported in reducing pain during off-label injection of CaHA into the hand by manually mixing lidocaine with the CaHA immediately prior to use.5 One year later, an article exploring the rheological properties of CaHA manually mixed with lidocaine was published.6 In 2009, the FDA approved the practice of mixing of lidocaine with CaHA.7 CaHA (+), a new formulation of CaHA developed with the convenience of integral 0.3% (nominal) lidocaine hydrochloride, received FDA approval in 2015 and CE mark approval in 2016. CaHA (+) has been shown to be associated with significant pain reduction when compared with CaHA without lidocaine.8-10
CaHA without lidocaine previously demonstrated higher complex viscosity (η*) and elastic modulus (G’) when compared with hyaluronic acid (HA) fillers without lidocaine.11 The rationale for the study described herein was to determine whether similar rheological findings would be observed when lidocaine had been added to the CaHA during manufacturing.
Material/gel deformation is either recoverable (elastic) or non-recoverable (viscous). The relevant rheological properties often considered in discussions of soft-tissue fillers are complex viscosity (η*) and elastic modulus (G’). Each of these rheological properties was examined and the results are reported here. Complex viscosity (η*) is a quantification of resistance to deformation (flow) by shearing forces. In clinical settings, a high-viscosity filler is more likely to remain where it is placed and maintain a solid form.11,12 Peanut butter and room temperature butter have been used previously to illustrate the difference between high η* and low η*, respectively.11 Elastic modulus quantitatively measures gel stiffness, or the ability of the recoverable portion to resist deformation (eg, extrusion through a needle, ongoing muscle movement in the face). In clinical applications, high G’ fillers are considered more resistant to deformation and provide better structure and volume than fillers with low G’.11,12
In this study, investigators sought to compare the rheological properties η* and G’ of CaHA (+) lidocaine to CaHA without lidocaine, and then to compare the rheological measurements of CaHA (+) with 5 cross-linked hyaluronic acid (HA) fillers containing integral 0.3% lidocaine.
MATERIALS AND METHODS
The 2 CaHA formulations (CaHA and CaHA (+)) were manufactured and provided by Merz North America, Inc. The 5 cross-linked
