Hyaluronic acid fillers have become popular soft tissue filler augmentation agents for the non-surgical corrections of changes associated with ageing. In 1989, Balasz and Denlinger described first hyaluronic acid (HA) developed as a dermal filler which have been used for facial rejuvenation and restoration of lost volume and enhancement of facial contours.
HA is a natural humectant composed as a linear polysaccharide of repeating of disaccharide units (sugar) chains found in every tissue of the human body, as the body’s natural hydrator. This hydrophilic molecule acts like a sponge to attract and retain water. This amorphous substance fills the extracellular spaces between collagen fibers and bundles and acts as a scaffold for extracellular matrix, providing rigidity, hydration and the turgor of tissues, whilst allowing cellular movement and regeneration and protects the skin from free radical damage, partly against UVA and UVB4.
Human body also contains its natural hyaluronidase, and 80% of it is presents in the human skin, which is the reason why with time any hyaluronic acid is broken down. In the natural unmodified state, implanted exogenous HA is rapidly degraded by our one human hyaluronidase and has a half-life of 12-24 hours. To improve its resistance to enzymatic breakdown and increase tissue viability and in situ residency, the HA molecule must be stabilised through a chemical process of cross-linking.
Injectable hyaloronic acid is biodegradable and biocompatible with human hyaluronic acid. Soft tissue augmentation with this injectable filler does not require prior skin testing and provides robust tissues augmentation. Its ability to attract and maintain significant amounts of water accounts for the volume and duration of correction that persists until stabilized gel is completely metabolized.
There are 2 broad types of cross-linked injectable HA:
- ANIMAL-DERIVED HA
Extracted from rooster combs, manufactured by Genzyme Corp. (distributed as Hyalform (Hylan B)) in 3 concentrations for treatment of different depths of folds and wrinkles.
- NON-ANIMAL-DERIVED SYNTHETIC HA
Have been generated from Streptococcus bacterial fermentation.
Variability in methods used to manufacture HA fillers have given rise to differences in properties such as degree of cross-linkage, particle size and concentration.
WHAT IS RHEOLOGY?
Dermal filler rheology, the study of the flow of matter, can be used to help us practitioners/physicians to differentiate between dermal fillers targeted to certain areas of the face and to understand the way fillers will behave when injected into particular area or layer of the skin and to choose the most appropriate filler to achieve the desired aesthetic results.
For example: When treating the deep subdermal layers of the cheeks, it is important to choose the filler, which gives good VOLUME and PROJECTION without easily spreading through tissues.
On the contrary, by injecting into the superficial dermal layers, it is important to choose an easily spreading filler through the tight connective tissues to smoothly distribute in the upper layers of the skin.
THE FOLLOWING FACTORS AFFECT THE PHYSICAL CHARACTERISTICS OF HA DERMAL FILLERS:
- Elasticity modulus ( G ´ ) – ability to recover the original shape after shear deformation.
- Viscosity ( G“ ) – the inability to recover the original shape after shear deformation.
- Complex modulus ( G* ) – the total ability of material to resist deformation. This factor is the sum of ELASTICITY and VISCOSITY modulus.
- Cohesivity – the strength of cross-linking adhesion forces that hold the particular HA units together. COHESIVITY is determined by concentration of HA and the degree of cross-linking. High COHESIVITY of filler helps to maintain the vertical projection.
Injectable HA fillers are stabilized, among others with cross linking proteins usually 1,4 BDDE*. This makes the filler more resistant to biodegradation and has a clinical duration up to 2 years. The BDDE cross-linked HA fillers minimize the risk of inflammatory response of tissues, allergic – and delay onset reactions.
On the table below you can see the rheological properties of some of the fillers sold in Top Dermal online store.
Product Cross-linking Technology G’(Pa) Viscosity Concentration (mg/ml) Cohesivity** Restylane Lidocaine (Galderma Laboratories)
Restylane LYFT (Galderma Laboratories)
Juvéderm Voluma (Allergan, Inc.) Monophasic, monodensified, Vycross, BDDE
Juvéderm Volbella (Allergan, Inc.) Monophasic, monodensified, Vycross, BDDE
Belotero Balance (Merz Aesthetics) Monophasic, polydensified, CPM*, BDDE
Radiesse (Merz Aesthetics)
*BDDE – 1,4-butanediol diglycidyl ether
*CPM – Cohesive Polydensified Matrix
**On the Gavard-Sundaram Cohesivity Scale
CLASSIFICATIONS OF HA FILLERS:
HA fillers can be classified according to their particulate forms into MONOPHASIC and BIPHASIC fillers.
MONOPHASIC fillers consist of:
They are used, for example for volumization of lips (risk of filler migration), for augmentation of moderate folds and wrinkles, for reducing of mimic folds, increases the turgor of skin, correction of scars and post acne.
BIPHASIC fillers consist of a mix of cross-linked and non-cross-linked HA particles and have advantages like:
- higher resistance to biodegradation
- long duration in the tissues
- high stability
- low index of migration
- excellent lifting effect of sagged tissues and contouring of face
The BIPHASIC fillers have to be injected in the deep layers. For example, supraperiosteal. It has to be performed with high professionalism and high confidence of experienced practitioner, to avoid unwanted results or side effects, for example Tindal effect or visibility of filler superficial placed in the tissues.
Combining the objective factors that influence filler performance with clinical experience will provide the patient with the original product for achieving the best cosmetic result. A careful review of these filler characteristics is essential in determining filler selection, performance and patient’s expectations.