Concepts and Terms

Researching hyaluronic acid injections can introduce a lot of unfamiliar words. These short explanations will help you understand treatment information and prepare questions for your healthcare professional.

Understanding the treatment

Hyaluronic acid (HA), also called hyaluronan

A substance your body makes naturally. In a joint, it helps the synovial fluid lubricate and cushion movement. HA is also used in some injections for osteoarthritis symptoms. You may also see it called “hyaluronan” or “hyaluronate”. [1,2]

Sodium hyaluronate

The sodium salt of hyaluronic acid. A product label may say “sodium hyaluronate” where patient information says “HA injection”. [3]

Viscosupplementation and viscosupplement

Viscosupplementation means injecting HA into a joint with the aim of improving lubrication and easing osteoarthritis symptoms. The injected product is called a viscosupplement. It is often described as a clear, gel-like material. [1,2] Some viscosupplements also contain an added corticosteroid; others contain HA without one. [1]

Intra-articular injection

An injection given directly into the space inside a joint. “Intra-articular” describes where the injection goes, not what it contains. Different types of treatment can be given this way. [2]

Single injection or a series

HA treatments are given either as one injection or as a series of weekly injections, usually over three to five weeks. The schedule depends on the joint being treated and the product used. [1,2] Product information may call the full course a treatment cycle. [3]

Understanding HA product descriptions

Molecular weight

A measure of the average size of the HA molecules. HA is made of long chains; longer chains have a higher molecular weight. You may see it expressed in daltons (Da), kilodaltons (kDa) or megadaltons (MDa). One MDa equals 1,000 kDa, or one million daltons. [4]

Concentration

How much HA is present in a given amount of fluid. For example, 20 mg/mL means each millilitre contains 20 milligrams of HA. Concentration is sometimes written as a percentage: 1% means 10 mg in each millilitre. Concentration describes the formulation, not how much is injected. [3]

Total dose

The total amount of HA in one injection. It depends on both the concentration and the volume in the syringe. For example, 2.4 mL at 20 mg/mL contains 48 mg of HA. [3] Total dose is a different number from molecular weight: one measures how much HA there is, the other how large its molecules are.

Linear or non-cross-linked HA

HA whose chains have not been chemically joined together through cross-linking. “Linear” describes the chain structure; the chains can still bend, coil and overlap. [4]

Cross-linked HA

HA that has been modified to create chemical bonds between its chains. These bonds change physical properties such as viscosity. “Cross-linked” describes a product’s structure; it is not, by itself, a rating of treatment quality. [4]

Physical entanglement

When long HA chains overlap and become intertwined, a little like strands of cooked spaghetti. This affects how the chains move and how the fluid flows. The chains can become entangled without being chemically cross-linked. [5]

Avian and non-animal origin

These terms describe where the HA in a product comes from. Avian-derived HA comes from birds; other products are non-avian. [6] Non-animal HA can be made by bacterial fermentation: bacteria produce the HA, which is then purified. [3] These terms describe the source and production method, not how well a product works.

Understanding how HA behaves

Rheology

The study of how a material flows and changes shape when forces act on it. In HA research, rheology covers measurements such as viscosity and elasticity. These tests describe how a material behaves; they do not, on their own, show how much pain relief someone will experience. [5]

Viscosity and zero-shear viscosity

Viscosity means resistance to flowing: honey has a higher viscosity than water. HA’s viscosity changes as it moves. Zero-shear viscosity is its viscosity when the fluid is barely moving. It describes how well the fluid resists being permanently pushed out of shape under a slow, steady load. [4]

Viscoelasticity

The ability to behave partly like a fluid and partly like an elastic material. A viscoelastic material can flow, but it can also resist changes in shape and partly spring back. This helps explain how HA-containing fluid responds to movement and pressure. [5,7]

Shear thinning

When a fluid becomes less resistant to flowing as it is made to move faster. HA solutions behave this way. “Thinning” describes a change in flow behaviour, not a loss of HA from the fluid. [7]

Understanding study terms

Kellgren-Lawrence (KL) grade

A way of rating how much osteoarthritis shows on an X-ray, from grade 0 (no signs of osteoarthritis) to grade 4 (severe). Higher grades mean more narrowing of the space between the bones and more bony changes. [8] Grades are often written in Roman numerals, for example “grade II–III”.

WOMAC score

The Western Ontario and McMaster Universities Osteoarthritis Index: a questionnaire used in osteoarthritis research. People answer 24 questions about their pain, stiffness and difficulty with daily activities over the past 48 hours. Higher scores mean worse symptoms. [9] The questionnaire has more than one version, each with its own score range, so check which version a study used before comparing scores. [9]

Putting the terms into context

These words help describe HA treatments, but no single property tells the whole story. Your healthcare professional can explain how the evidence, potential benefits, risks and your circumstances relate to the options you are considering.

Explore patient information · Download the conversation guide

Sources

1. Arthritis Society Canada. Viscosupplementation. Updated June 2024. https://arthritis.ca/treatment/medication/reference-guide/viscosupplementation/ (accessed 2026-10-06)
2. Arthritis Society Canada. Injection treatments for osteoarthritis: what you need to know. https://arthritis.ca/healthy-living/injection-treatments-for-osteoarthritis-what-you-need-to-know/ (accessed 2026-10-06)
3. Reviscon® Mono 2.0% Instructions for Use (Canada). REVMIFU-CA-001, 12.09.2023.
4. Nicholls M, Manjoo A, Shaw P, et al. A comparison between rheological properties of intra-articular hyaluronic acid preparations and reported human synovial fluid. Adv Ther. 2018;35(4):523-530. doi:10.1007/s12325-018-0688-y https://pmc.ncbi.nlm.nih.gov/articles/PMC5910459/
5. Nicholls M, Manjoo A, Shaw P, et al. Rheological properties of commercially available hyaluronic acid products in the United States for the treatment of osteoarthritis knee pain. Clin Med Insights Arthritis Musculoskelet Disord. 2018;11:1179544117751622. doi:10.1177/1179544117751622 https://pmc.ncbi.nlm.nih.gov/articles/PMC5757428/
6. Ferkel E, Manjoo A, Martins D, et al. Intra-articular hyaluronic acid treatments for knee osteoarthritis: a systematic review of product properties. Cartilage. 2023;14(4):424-432. doi:10.1177/19476035231154530 https://pmc.ncbi.nlm.nih.gov/articles/PMC10807741/
7. Cowman MK, Schmidt TA, Raghavan P, et al. Viscoelastic properties of hyaluronan in physiological conditions. F1000Res. 2015;4:622. doi:10.12688/f1000research.6885.1 https://pmc.ncbi.nlm.nih.gov/articles/PMC4648226/
8. Alshahrani AS, Aljaffar AB, Albin Ahmed BJ, et al. Correlation between Kellgren-Lawrence classification of osteoarthritis and Knee Injury and Osteoarthritis Outcome Score. World J Orthop. 2025;16(11):111953. doi:10.5312/wjo.v16.i11.111953 https://pmc.ncbi.nlm.nih.gov/articles/PMC12679146/
9. Collins NJ, Misra D, Felson DT, et al. Measures of knee function: International Knee Documentation Committee (IKDC) Subjective Knee Evaluation Form, Knee Injury and Osteoarthritis Outcome Score (KOOS), Knee Injury and Osteoarthritis Outcome Score Physical Function Short Form (KOOS-PS), Knee Outcome Survey Activities of Daily Living Scale (KOS-ADL), Lysholm Knee Scoring Scale, Oxford Knee Score (OKS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Activity Rating Scale (ARS), and Tegner Activity Score (TAS). Arthritis Care Res (Hoboken). 2011;63(Suppl 11):S208-S228. doi:10.1002/acr.20632 https://pmc.ncbi.nlm.nih.gov/articles/PMC4336550/