The Basics Behind Filler Rheology & Product Choice

Go with the right flow.

Soft tissue augmentation with Hyaluronic Acid (HA) fillers has become one of the most commonly performed procedures in North America.  Already found in the extracellular matrix of many tissues, especially in the skin where it plays an important role in providing structure and maintaining moisture, it is a natural fit. Further advantages of HA dermal fillers include an excellent safety profile, durability, reversibility with hyaluronidase, and it has even been demonstrated that the injection of a small gel particle HA filler stimulates natural collagen production. However, the physical characteristics of different hyaluronic acid products is quite variable, and a good understanding of the differences is necessary for practitioners to tailor procedures accordingly, procuring the most desirable final results. 

Based on the manufacturing methods used, HA dermal filler may differ in their physical characteristics and therefore behave clinically in different ways. Common characteristics ascribed to the properties of fillers include “bi-phasic” fillers – containing different sizes of particles, “mono-phasic” pertaining to filler that contain all of the same sized particles, “size of particle,” “degree of cross-linking,” and “G-prime” – used to describe the degree of hardness in filler and its suitability for volumizing. But other more nuanced characteristics also contribute to the aesthetic results and before filler is injected into the target tissue it is important to consider:

1. Shear Deformation: occurring when a force is applied along the surface with a vertical vector, resulting in lateral shearing or torsion of the surface,
2. Compression/Stretching: occurring when a force is applied vertically, as in compression or stretching
3. Viscoelasticity: the viscosity and elasticity in an HA filler when shear deformation occurs
1. Elasticity referring to the degree of recovery when a shear force occurs and is then removed; a stronger shear force is required to deform a material with high elasticity,
2. Viscosity referring to the internal friction of a substance or “thickness”, and;
4. Cohesivity: the internal adhesion forces among individual cross-linked HA units within an HA gel, measured by resistance after undergoing compression or vertical stretching.

Note: that cohesivity depends on the concentration of HA and some specific steps in the cross-linking process, but is not related to the level of cross-linking, “a highly cohesive gel has a higher molding capacity just after injection, but less cohesive gels are more flexible” (Choi, 2020).

For example, a sample of five HA dermal fillers contain HA derived from similar bacterial cultures and are stabilized with the same BDDE cross-linker, but are manufactured using different patented technology. Regardless of the manufacturing methods employed, each produces gels containing particulate HA, and differences include “particle size distribution, total HA concentration, and the extent of crosslinking” (Stocks et al., 2011). Together, these properties may alter the firmness, elasticity and viscosity of the gels and impact the way they behave in the skin, including the amount of structural support or “lift” they provide. Further considerations include the “gel swell”; HA fillers are referred to as hydrophilic or “water-loving” – this characteristic is related to the level of gel hydration. “Fully hydrated gels that have reached their maximum water-binding capacity will not swell following injection while there is a tendency for less hydrated gels to swell after injection” (Stocks et al., 2011).

In rheology, we study how materials behave in response to applied forces. The four rheological parameters used to describe viscoelastic properties are: 

1. G* or ‘G-prime” (overall viscoelastic properties) representing the total resistance to deformation of a HA gel
2. G’ (elastic properties) representing the ability of a material to recover its shape after shear deformation
3. G” (viscous properties) representing the viscous value of the energy fraction that is lost under shear deformation, and;
4. Tan δ (the ratio between viscous and elastic properties): G”/G’.       

Taking the above into consideration, it would naturally figure, and tests have confirmed, that different HA dermal filler products are better suited to address different issues and areas of treatment. In a study conducted by D. Stocks, H. Sundaram, J. Michaels, et al. titled “Rheological Evaluation of the Physical Properties of Hyaluronic Acid Dermal Fillers”, it was found that “the structural support and lifting capacity of HA fillers that possess a high degree of firmness (G’) and viscosity may be more desirable in treating deep folds and creating lift and volume while products that are less firm and viscous may be better suited for treating shallower folds and lines. Also, dermal fillers with greater viscosity and elasticity will tend to resist spreading after implantation.”(Stocks et al., 2011). Generally speaking, firmer fillers should be used for areas where “structure” is needed such as cheeks, jawline, or temples and softer fillers would be more beneficial in more fragile areas such as lips, periocular and perioral areas.

For quick reference, consider:

In conclusion, the various dynamics of the face alongside the different characteristics of HA dermal fillers should be taken into consideration for desired, patient outcomes and best results!

References

Choi, Moon Seop. “Basic rheology of dermal filler.” Archives of plastic surgery vol. 47,4 (2020): 301-304. doi:10.5999/aps.2020.00731 

Stocks, David BSc (Hons), Sundaram, Hema MD, Michaels, Jason MD, Durrani, Mazer J. PhD, Wortzman, Mitchell S. PhD, Nelson, Diane B. BSN MPHd. “Rheological Evaluation of the Physical Properties of Hyaluronic Acid Dermal Fillers.” Journal of Drugs in Dermatology, September 2011: Vol 10, Issue 9.