There are many physical properties that should be taken into consideration when choosing the right filler for the right area. The study of such properties or rheology is an important concept that needs to be explained and understood for any serious injector.

Fillers as mentioned earlier are divided into simple, temporary ones that simply “fill” a given area, such as hyaluronic acid , or biostimulatory fillers , that enhance the production of collagen . These include calcium hydroxylapatite , poly l lactic acid, and polycaprolactone . The final class are the permanent fillers such as silicone and polymethylmethacrylate.

No matter the type of filler, the basic physical property that the material elicits on the tissue remains the same. The ability of the filler to withstand outside forces that include the movement of the overlying tissue and external pressure forces is known as the G′ of the filler or the elastic modulus . The G′ is calculated in vitro by using an oscillation instrument that exerts pressure on the material to calculate its stiffness or smoothness and thus immediate volume creation. Stiffness, contrary to popular belief, does not translate into the filler’s “lifting capacity.” It does not imply resistance of the gel. This property is known as cohesivity and has to do with both the filler’s resistance to degradation in vivo. For example, biphasic HA fillers are quite stiff owing to their high G′. These fillers during production pass through a mesh of a certain size and thus are dependent on their particle size in their final form. These particles are not adherent to one another when compared to the monophasic materials. Biphasic fillers are not cohesive, thus are more susceptible to degradation. These fillers provide immediate filling capacity, but may not have great “lifting capacity” as those with lower G′ but higher cohesivity. In summary, a filler’s lifting capacity is dependent on both the G′ and the cohesivity of the material.