Irvine, Calif. — To understand the injection characteristics of a hyaluronic acid (HA)-based dermal filler and how the material performs clinically, one must understand how it was made, an expert says.

To help dermatologists understand the concept of gel hardness, Dr. Tezel explains that if one sandwiches an HA gel between two plates, the higher the gel's G value, the more force it will take to push the top plate a small distance horizontally while holding the bottom one stationary. More specifically, he says G denotes the ratio of shear stress (force per unit area of the plate) to the shear strain imposed (the ratio of the horizontal displacement to the vertical distance between plates).

When manufacturers design HA fillers, he says, "One of the limitations they face is injectability, because HA dermal fillers are very viscous materials."