INTRODUCTION
Due to the large and increasing number of available dermal filler products, health care providers (HCPs) face a great challenge when selecting the appropriate product suitable for each patient, indication and injection technique. Since head to head clinical studies comparing products from different manufacturers are rare and difficult to perform, much of the discussion in promotional materials is based on physical properties. However, the information is usually less detailed and often more commercially than scientifically based. For this reason, there is an increased demand for scientifically based comparisons, and a number of publications have been presented where hyaluronic acid (HA) fillers are compared according to their physicochemical properties.1-10 In these papers, many different analytical techniques are used, one of the more popular being rheometry. Rheology denotes the science of studying the deformation of materials while rheometry is the science of how to measure the rheological properties. Rheometry techniques are well established, the theory foundation for rheology is mature, and many high quality test instruments are available. The rheological properties are often in focus when trying to relate HA-filler properties to the clinical outcome. As with all analytical techniques, however, the results will depend not only on the samples, but also on the instrument settings. Although most investigators try to state the exact settings, there has been little discussion on what settings are appropriate, and how different settings may affect the results. Another point of interest is which rheological properties to present. Although many parameters can be calculated from one rheological measurement, presenting too many may be more confusing than informative, so selecting the parameters most descriptive and most relevant to clinical use of a HA-filler is important. As the work to understand the correlation between the physicochemical properties of dermal fillers and their clinical performance continues, there is a greater need for consensus on how these properties should be measured in order to make comparing the results from different investigators more useful. The aim of this paper is to clarify different terms and to discuss factors affecting the results from rheometric measurements on HA-fillers, and to suggest suitable measurement settings and rheological parameters to include in presented results.
Rheology
Basic rheological concepts include elasticity and viscosity. An elastic or solid-like material returns to its original shape when the deforming force is removed, e.g. a rubber ball. A viscous or liquid-like material does not return to its previous shape, i.e. stays deformed, when the deforming force is removed, e.g. clay. Many semisolid materials exhibit a mix of elastic and viscous properties. This can be recognized for pizza dough, which
