Filler rheology made simple: G prime and cohesivity
G prime, viscosity and cohesivity explained in terms of what they actually predict about how a gel behaves once it is in tissue.
Filler rheology is often presented as a table of numbers with no obvious connection to clinical choice. The numbers do mean something, but only two questions matter: how does this gel move through a needle, and how does it behave once it has stopped moving.
G′ — elastic modulus
G′ (G prime) is the storage modulus: resistance to deformation under shear. It is the most commonly used parameter to distinguish products, and higher values track with higher hyaluronic acid concentration and molecular weight.
Practically, G′ predicts whether a gel holds its shape against the tissue pressing on it. High G′ gels resist deformation and are used where projection has to survive compression — deep support over bone, structural work. Low G′ gels deform readily, which is what you want in mobile, superficial tissue where a stiff gel would read as a ridge.
The common error is treating G′ as a quality ranking. It is a behaviour descriptor. High G′ in a superficial plane produces a visible, palpable lump; low G′ in a deep structural position simply fails to hold anything up.
Viscosity
Viscosity describes resistance to flow. It largely governs injectability — how hard the syringe is to push, and how readily the gel spreads on delivery. High-viscosity products stay where placed; low-viscosity products distribute.
The literature draws a useful distinction here: viscosity best describes the gel during injection, while G′ and cohesivity better describe what it does after.
Cohesivity
Cohesivity is the tendency of the gel to hold together as one mass rather than fragment. Cohesive gels integrate more homogeneously in tissue and distribute lift across a broader area; low-cohesion gels can separate into pockets.
It is also the least settled of the three. Reviews note that cohesion lacks a standardised measurement method, which is why published cohesivity figures are not straightforwardly comparable between manufacturers. Treat cohesion claims as directional rather than absolute.
Hydrophilicity
Not strictly rheology, but it belongs in the same decision. Hyaluronic acid binds water, and products differ in how much. A strongly hydrophilic gel in a region with poor lymphatic drainage — the infraorbital hollow being the standard example — produces sustained swelling that the patient experiences as a poor result.
Reading a product from its numbers
A workable heuristic: high G′, high cohesivity, low hydrophilicity for deep structural support; low G′, lower cohesivity for superficial refinement in mobile tissue; and check hydrophilicity specifically before anything goes near the tear trough.
The numbers describe behaviour, not merit. The right question is always which behaviour the region requires.
