What the research actually shows about Arthrosamid

A single, non-surgical injection for knee arthritis, followed in patients from six months out to ten years. This guide walks through every study — what was found, and where the evidence runs thin. Every number is sourced. Where the research cannot answer a question, we say so.

Why your knee actually hurts

Almost everyone arrives having been told the wrong story. The right one explains why an injection can still work in a knee that is “worn out”.

The story you have heard: arthritis wears away the cartilage, bone grinds on bone, and that grinding is the pain. It is the picture nearly every patient carries into the room.

Here is the part that does not fit. Cartilage has no nerve supply. It cannot generate pain. Whatever is hurting in your knee, it is not the cartilage itself.

So where is the pain coming from?

Osteoarthritis was never only a cartilage disease. It involves the whole joint — the cartilage, the bone underneath it, and the synovium, the thin lining that wraps the joint and produces its fluid.²⁶

In arthritis that lining becomes inflamed, and unlike cartilage the synovium is richly supplied with nerves. When researchers put patients through contrast MRI scans, the amount of inflammation in the joint lining tracked with how much pain they were in.¹⁴ Studies looking at which specific tissues correlate with pain severity keep landing on the same tissue.^15,16

This is why the X-ray never quite matches the person. Every family doctor has seen a patient with a frightening X-ray who walks in comfortably, and another with a mild-looking one who can barely manage the stairs. If the grinding were the pain, that would not happen.

And it points somewhere useful. If a meaningful share of the pain comes from an inflamed joint lining, then a treatment that acts on that lining has something to work with — even in a knee where the cartilage is long gone.

What Arthrosamid is

A hydrogel — 2.5% polyacrylamide, 97.5% sterile water. A single 6 mL injection into the joint.

The critical difference from everything else you may have had is that it does not biodegrade. Cortisone washes out. Gel injections are broken down by the body. This does not. It becomes part of the joint.

What happens after the injection

The gel is drawn into the joint lining and incorporated into it, forming a stable cushioning layer within the synovium itself. This has been shown directly in tissue studies.¹² It is not a lubricant sitting in the joint space waiting to be absorbed. It becomes a permanent part of the lining and stays.

And the lining, as above, is a major source of the pain. When researchers scanned knees with ultrasound after treatment, markers of inflammation in the joint were reduced compared with a standard gel injection.¹³

What the appointment involves

• An assessment first. We go through your history, examine the knee, and tell you honestly whether this is likely to help. If it is not, we say so.
• Antibiotics before the procedure. Standard, and we do not skip it.
• The injection is ultrasound-guided. We watch it go in. No guesswork about placement.
• It takes minutes, and you walk out of the room on your own. No sedation, no crutches.
• Soreness and swelling for a few days is normal. It settles.
• One injection. No course, no series, no monthly appointments.

What you have probably already tried

Most people arrive having had cortisone, or the gel, or both, and having watched them stop working. It is worth knowing what the evidence says about why.

Cortisone

Relief comes fast and it fades. Most patients already know that from experience. What is less well known is the rest of the picture.

In a randomised trial, patients given cortisone every three months for two years had significantly more cartilage loss than those given saline, with no better pain relief to show for it.¹⁷ A separate analysis found repeated cortisone injections were associated with an increased risk of eventually needing a knee replacement.¹⁸

This does not mean cortisone is useless. It has a place, particularly for a bad flare. It means it is not a long-term plan, and repeated injections carry a cost.

Hyaluronic acid, the “gel”

Biodegradable by design: the body breaks it down, which is why it needs repeating. The evidence for it is genuinely mixed. A large meta-analysis in the BMJ found the pain benefit small enough to question whether patients would notice it.²⁰ A long-term cohort study following real patients rather than trial participants found no clear lasting benefit from either cortisone or hyaluronic acid compared with people who had neither.¹⁹

The head-to-head test. Arthrosamid was put up against a standard gel injection (Synvisc-One) in a randomised controlled trial. Over one year it performed comparably overall and better in patients under 70.^1,2 And it kept working for years after the gel would have been broken down and gone.

Does it work? The ten-year trail

Most treatments get studied for a few months and then marketed forever. This one has been followed in patients continuously, and published at every step.

The threshold researchers use for a clinically meaningful improvement is 9 points. This is comfortably past it, and it held for five years (p<0.0001). It is the peer-reviewed number, and if you take one figure from this guide, take this one.³

The number you will see quoted elsewhere

If you have been told you are bone-on-bone

This is the page other clinics skip.

Eleven. That is far too few to build a percentage anyone should stand behind, so we are not going to give you one. They did improve on average, and the improvement crossed the threshold for meaningful pain relief. But eleven people is not a statistic about you.²

You will find clinics happy to quote you 80% regardless. We will not.

The distinction that actually matters

Bone-on-bone is a phrase, not a diagnosis. On an X-ray, arthritis is graded, and that phrase gets applied to two different grades. The gap between them is not cosmetic.

A great many people told they are bone-on-bone turn out to be Grade 3, where the evidence is genuinely strong. Which one you are is something we sort out at the assessment.

And the reason it can still work: if the pain comes substantially from the inflamed joint lining rather than from the missing cartilage, then a treatment acting on that lining still has a target, even when the cartilage is gone. That is not a marketing line. It is why advanced patients are treated at all.

Is it safe?

It is the question we are asked most, because the gel is permanent and permanent makes people uneasy. It is a fair question.

61 patients, 89 knees, average 9.9 years. No infections, no allergic reactions, no systemic effects traced to the injection. In the knees that later went on to replacement surgery, the surgical records showed nothing unusual.⁴

The five-year randomised follow-up found the same. Adverse events happened — people in their seventies have heart attacks and break ankles — but none were judged related to the treatment.³ A separate safety case series reached the same conclusion.²⁵

The part most people do not know

This material is not new. Polyacrylamide hydrogel has been used in humans for over two decades, in places well outside the knee, and followed for a long time in each.

So when it goes into a knee, you are not being injected with an unknown. You are being injected with a material that has a long human track record, into a new location.

What you should expect: a sore, stiff, swollen knee for a few days. That is the common one, and it settles.

Who this is not for

We would rather turn you away than take your money for something that will not help you.

• Knee pain that started suddenly, or after a fall. That needs an X-ray before anything else. It may not be arthritis at all, and an injection is the wrong move until someone knows what they are treating. See your family doctor first.
• A cortisone or gel injection in the last three months. The joint needs to settle before another goes in. Call anyway — we will simply date the appointment correctly.
• A knee that has already been replaced. There is no joint left to treat.
• Arthritis that is truly end-stage. If a knee replacement is what you actually need, we will tell you that. Some people are trying to delay surgery, and there is a real case for that. Some people are avoiding a surgery they should have. Those are different situations and they deserve different answers.

Where every number came from

We said we would show our work. Look any of these up.

1. Bliddal H, et al. Polyacrylamide gel versus hyaluronic acid for the treatment of knee osteoarthritis: a randomised controlled study. Clin Exp Rheumatol 2024;42(9):1729–35. Peer-reviewed RCT. Trial registration NCT04045431
2. Bliddal H, et al. One-year performance in age, BMI and Kellgren–Lawrence subgroups: a subgroup analysis of a randomised study. Poster 496, OARSI 2022. Osteoarthritis and Cartilage 30(1):S373–74. Source of the age subgroups and the n=11 Grade 4 figure.
3. Bliddal H, Beier J, Hartkopp A, Conaghan PG, Henriksen M. Sustained symptom relief and safety over five years following a single intra-articular injection of 2.5% polyacrylamide hydrogel in patients with knee osteoarthritis. Clin Exp Rheumatol 2026;44(7):1410–17. Peer-reviewed. Free to view. Read the paper
4. Bliddal H, et al. 10-year follow-up after intra-articular injections of 2.5% polyacrylamide hydrogel for knee osteoarthritis. Presented at WCO-IOF-ESCEO 2025, Rome. 61 patients, 89 knees. Retrospective, no control group, not peer-reviewed. Congress report
5. Bliddal H, et al. Effectiveness and safety of polyacrylamide hydrogel injection for knee osteoarthritis: 12-month follow-up of an open-label study. J Orthop Surg Res 2024;19:274. Peer-reviewed.
6. Bliddal H, Hartkopp A, Beier J, Conaghan PG, Henriksen M. A prospective, open-label clinical investigation of a single intra-articular polyacrylamide hydrogel injection in participants with knee osteoarthritis: a 5-year extension study. J Orthop Surg Res 2026;21:43. Open access. Read the paper
7. Henriksen M, et al. Intra-articular 2.5% polyacrylamide hydrogel for knee osteoarthritis: an observational proof-of-concept cohort study. Clin Exp Rheumatol 2018;36(6):1082–85.
8. Bliddal H, et al. Polyacrylamide hydrogel injection for knee osteoarthritis: a 6-month prospective study. J Orthop Res Ther 2021;6(2):1188.
9. Bliddal H, et al. Results from 2 years after treatment. Poster, OARSI 2022. Osteoarthritis and Cartilage 30(1):S371–72.
10. Bliddal H, Beier J, Hartkopp A, Conaghan PG, Henriksen M. Three-year follow-up from a randomised controlled trial of a single intra-articular polyacrylamide hydrogel injection in subjects with knee osteoarthritis. Clin Exp Rheumatol 2026;44(5):996–1003. Free to view. Read the paper
11. Bliddal H, et al. Polyacrylamide hydrogel for knee osteoarthritis: four-year results from a prospective study. Orthop Procs 2024;106-B(SUPP_18):106.
12. Christensen L, et al. Synovial incorporation of polyacrylamide hydrogel after injection into normal and osteoarthritic joints. Osteoarthritis and Cartilage 2016;24(11):1999–2002.
13. Ellegaard K, et al. Inflammatory markers in the knee assessed with ultrasound: secondary analysis of a randomised trial of Arthrosamid vs Synvisc-One. Osteoarthritis and Cartilage 2024;32:745–836.
14. Baker K, et al. Relation of synovitis to knee pain using contrast-enhanced MRIs. Ann Rheum Dis 2010;69(10):1779–83.
15. Torres L, et al. The relationship between specific tissue lesions and pain severity in persons with knee osteoarthritis. Osteoarthritis and Cartilage 2006;14(10):1033–40.
16. Mathiessen A, Conaghan PG. Synovitis in osteoarthritis: current understanding with therapeutic implications. Arthritis Res Ther 2017;19(1):18.
17. McAlindon TE, et al. Effect of intra-articular triamcinolone vs saline on knee cartilage volume and pain: a randomized clinical trial. JAMA 2017;317(19):1967–75.
18. Wijn SRW, et al. Intra-articular corticosteroid injections increase the risk of requiring knee arthroplasty. Bone Joint J 2020;102-B(5):586–92.
19. Liu SH, et al. Long-term effectiveness of intra-articular injections on patient-reported symptoms in knee osteoarthritis. J Rheumatol 2018;45(9):1316–24.
20. Pereira TV, et al. Viscosupplementation for knee osteoarthritis: systematic review and meta-analysis. BMJ 2022;378:e069722.
21. Pallua N, Wolter TP. A 5-year assessment of safety after facial soft-tissue augmentation with polyacrylamide hydrogel: a prospective multicentre study of 251 patients. Plast Reconstr Surg 2010;125(6):1797–804.
22. Negredo E, et al. Ten-year safety with polyacrylamide gel used to correct facial lipoatrophy in HIV-infected patients. AIDS Res Hum Retroviruses 2015;31(8).
23. Brosche T, et al. Seven-year efficacy and safety outcomes of Bulkamid for stress urinary incontinence. Neurourol Urodyn 2021;40(1):502–08.
24. Walmod PS, et al. An injectable 2.5% cross-linked polyacrylamide hydrogel demonstrates no neurotoxicity in human stem-cell-derived neurons. Front Toxicol 2025;7:1585430.
25. Overgaard A, Bliddal H, Henriksen M. Safety of intra-articular polyacrylamide hydrogel for knee osteoarthritis: a retrospective case series. Osteoarthritis and Cartilage 30(1):S370–71.
26. Hunter DJ, Bierma-Zeinstra S. Osteoarthritis. Lancet 2019;393(10182):1745–59.

Also worth reading

We have written up the evidence on the two treatments patients ask us about most often, to the same standard as this page.

• Stem cell injections for knee arthritis — what the largest trial found, and whether you can get them in Canada or the United States.
• Can knee cartilage be regrown? — where cartilage regeneration genuinely works, and what happened when a drug did regrow cartilage in arthritic knees.

This guide is general information about a treatment. It is not a diagnosis, and it is not medical advice about your knee. Whether Arthrosamid is right for you depends on what is actually happening inside your joint, which is what an assessment is for. If the evidence does not support treating a knee like yours, we will tell you. Arthrosamid is approved by Health Canada and is not approved for use in the United States.