How rheological properties guide product choice, injection technique, and decision-making in aesthetic medicine
In aesthetic medicine, the success of a treatment with hyaluronic acid does not solely depend on mastery of facial anatomy or injection technique. Two fillers injected in the same anatomical plane can produce very different results, both aesthetically and functionally.
Why?
The answer lies in rheology, the science that studies the behavior of materials when subjected to mechanical stress.
For healthcare professionals, understanding the rheological properties of cross-linked hyaluronic acid fillers has become essential. This knowledge allows for selecting the most suitable product for each clinical indication, optimizing results, and enhancing the safety of injections.
What is Rheology?
Rheology is the discipline that analyzes how a material deforms, resists, or flows when subjected to a force. est la discipline qui analyse la manière dont un matériau se déforme, résiste ou s'écoule lorsqu'il est soumis à une force.
Applied to injectable fillers, it helps to understand the behavior of the product:
during injection,
immediately after implantation,
during its integration into the tissues,
and throughout its duration of action.
Thus, the real question is no longer:
"What is the best filler?"
But rather:
"Which filler has the most suitable rheological properties for this anatomical indication?"
Cross-Linked Hyaluronic Acid Fillers: Much More Than a Simple Gel
The hyaluronic acid fillers consist of a hydrogel made of cross-linked hyaluronic acid, suspended in a physiological or phosphate-buffered solution.
The cross-linking creates a three-dimensional network that increases the stability of the gel, improves its resistance to enzymatic degradation, and prolongs its duration of action after injection.
Today, the most commonly used cross-linking agent is 1,4-butanediol diglycidyl ether (BDDE), known for its effectiveness and safety profile.
Other cross-linking agents have also been developed, including:
1,2,7,8-diepoxyoctane (DEO),
divinyl sulfone (DVS),
hexamethylenediamine (HMDA),
polyethylene glycol diglycidyl ether (PEGDE).
The type of cross-linking as well as its degree directly influence the mechanical properties of the filler and, consequently, its clinical behavior.
Why is Rheology Essential in Clinical Practice?
Each anatomical region of the face presents specific biomechanical constraints.
A filler intended to restore the projection of the cheekbones or chin will not have the same characteristics as a product designed to treat superficial fine lines or restore lip volume.
Rheology allows for anticipating several essential parameters:
the supportive capacity of the tissues,
the projection power,
the malleability of the product,
its integration into the tissues,
its behavior during facial movements,
its resistance to mechanical stress.
The goal is therefore not to choose the most "performant" filler, but the one whose properties best match the anatomical needs of the patient.
The Elastic Module (G′): The Supportive Capacity of the Filler
The elastic module (G′) measures the ability of a gel to return to its shape after undergoing deformation.
In practice, it reflects the filler’s ability to resist the compressive forces exerted by the surrounding tissues.
A filler with a high G′ generally offers:
a better supportive effect,
a greater projection capacity,
a better resistance to compression.
These characteristics make it a preferred choice for indications requiring structural support, such as the cheekbones, chin, jaw, or temples.
In contrast, products with a lower G′ may be more suitable for areas where flexibility and tissue integration are desired.
It is, however, essential to remember that a high G′ does not mean that a filler is "better," but simply that it has different mechanical properties.
The Viscous Modulus (G″): The Capacity to Absorb Stresses
The viscous modulus (G″) represents the filler’s ability to dissipate energy when subjected to stress.
The greater this component, the more the product exhibits fluid behavior during deformation.
Clinically, this property influences:
the ease of injection,
the ability to adapt to tissues,
the behavior of the filler after implantation.
The analysis of G″ complements that of G′ to better understand the overall mechanical response of the product.
Cohesiveness: Maintaining the Integrity of the Gel
The cohesiveness corresponds to the internal adhesion forces that keep the cross-linked hyaluronic acid chains together.
It reflects the filler’s ability to remain compact after injection rather than dispersing into the tissues.
This property particularly influences:
the stability of the product,
its tissue integration,
the maintenance of volumes,
the definition of contours,
its resistance to compression and stretching.
Depending on the clinical indication, a high or more moderate degree of cohesiveness may be sought.
The Tan Delta (tan δ): The Balance Between Elasticity and Viscosity
The tan delta (tan δ) corresponds to the ratio between the viscous modulus (G″) and the elastic modulus (G′).
This index allows for the evaluation of whether a filler exhibits a behavior that is more elastic, comparable to a gel, or more viscous, close to a liquid.
A low tan δ indicates a primarily elastic material.
A high tan δ indicates a relatively more viscous behavior.
This value provides a comprehensive view of the mechanical behavior of the filler and complements the analysis of other rheological parameters.
Combining Rheology with Facial Anatomy
The choice of a filler can never rely solely on its mechanical characteristics.
Rheological properties must always be interpreted considering:
facial anatomy,
the injection plan,
the characteristics of the tissues,
therapeutic objectives,
the dynamics of the face.
It is this approach combining anatomy and biomaterials science that allows for harmonious, natural, and lasting results.
Beyond Rheological Data: The Importance of Clinical Reasoning
Rheological parameters are valuable decision-making tools, but they are not sufficient on their own.
The clinical outcome also depends on:
the injection technique,
the chosen anatomical plan,
of the concentration in hyaluronic acid,
of the manufacturing technology,
of the characteristics specific to each patient.
Rheology should therefore be considered as a complementary element to the clinical judgment of the practitioner.
From Science to Practice: The Philosophy of ISTYA Aesthetic
At ISTYA Aesthetic, we believe that excellence in aesthetic medicine relies on the simultaneous mastery of facial anatomy, of medical simulation, of facial ultrasound and the properties of injectable biomaterials.
Understanding the mechanical behavior of fillers allows practitioners to select the most suitable product, secure their injections, and achieve reproducible results based on scientific data.
Conclusion
Rheology is today an essential pillar of modern aesthetic medicine.Understanding the
elastic modulus (G′), the viscous modulus (G″), the cohesiveness and the tan delta allows healthcare professionals to better anticipate the behavior of hyaluronic acid-based fillers, adapt their therapeutic strategy, and improve patient safety.At the intersection of
anatomy, of the science of biomaterials and et du clinical reasoning, rheology is a true lever of excellence for a more precise, safer, and more personalized practice of aesthetic medicine.