How Arthrosamid works in the knee joint

Most patients who reach the point of considering Arthrosamid® have already tried a steroid or hyaluronic acid injection — and found the relief shorter than expected. Understanding why Arthrosamid® behaves differently starts with what it actually is: a single injection of 97.5% water and 2.5% cross-linked polyacrylamide hydrogel, known as iPAAG. It is not a drug, a hormone, or a biological agent.

The defining difference is permanence. Corticosteroids are absorbed within days to weeks; hyaluronic acid (sometimes called a 'gel injection') degrades over a similar timeframe. Arthrosamid® does neither. Once injected into the knee, the hydrogel micro-integrates with the synovial membrane — the tissue lining the inner capsule of the joint — and remains there as a structural scaffold. Rather than coating cartilage surfaces or acting as a temporary lubricant, it becomes part of the joint lining, providing cushioning that does not wash out.

Beyond the mechanical effect, biomarker studies have detected increased anti-inflammatory mediators in the joint at three months post-injection, suggesting a biochemical component to symptom relief; however, the precise mechanism is still an area of active investigation rather than settled science.

Arthrosamid® is manufactured by Contura International Ltd and is delivered as a single-use, 6 ml intra-articular injection — typically ultrasound-guided in an outpatient setting.

Which patients respond best

Radiographic severity is the most reliable predictor of response across the available evidence, and the clearest clinical steer is straightforward: Kellgren-Lawrence (KL) grade 2–3 osteoarthritis produces the strongest and most consistent benefit. At grade 4 — end-stage disease with extensive joint space loss — the subgroup data from the randomised controlled trial (n=239) actually favoured hyaluronic acid over Arthrosamid®, making appropriate staging central to the decision.

The RCT subgroup analysis adds useful granularity beyond grade alone. Statistical superiority over hyaluronic acid at 52 weeks emerged in three overlapping patient profiles: those aged under 70 (p=0.019), those with a normal BMI (p=0.011), and those with KL grade 2–3 OA (p=0.033). Older age, higher BMI, and more advanced disease each individually attenuated the advantage.

The 24-month PROMs cohort (314 knees) tells a slightly different story on age: there, older age, lower KL grade, absence of diabetes, and the presence of bilateral OA were independent predictors of reaching the minimal clinically important difference across all three outcome scores. The two datasets are not contradictory — the shared, consistent signal is that lower radiographic OA severity reliably predicts better response; the age and BMI findings diverge slightly and should be read as hypothesis-generating rather than definitive.

Arthrosamid® is not appropriate where inflammatory or rheumatoid arthritis is the primary diagnosis, where there is active local infection, or where disease is so advanced that knee replacement is the more suitable pathway.

Accurate staging before injection matters. AI-assisted MRI analysis — available through the MSK Doctors group's onMRI™ platform, which provides automated cartilage segmentation and T2 mapping — can support objective characterisation of remaining cartilage and help confirm KL grade, reducing reliance on plain-film estimation alone.

What the clinical evidence shows

The evidence builds from smaller open-label datasets toward larger comparative studies, and where a study sits in that arc matters for how much confidence to take from any single result.

The strongest individual dataset is Bliddal et al. (2024), an open-label, 52-week study of 49 patients in which a single iPAAG injection produced a WOMAC pain reduction of −17.7 points (95% CI −23.1 to −12.4; p<0.0001). To contextualise that figure: WOMAC pain is scored on a 0–100 scale, so a reduction of nearly 18 points represents a meaningful shift in day-to-day discomfort rather than a marginal statistical signal. At 52 weeks, 62.2% of participants met the OMERACT-OARSI responder threshold — a composite definition of clinically meaningful, multi-domain improvement.

A systematic review pooling 463 patients confirmed statistically significant outcomes at 52 weeks, 13 months, and two years, with no long-lasting adverse events reported across the included studies. The randomised controlled trial (n=239, iPAAG versus hyaluronic acid Synvisc-One) showed non-inferiority overall and approached superiority at 52 weeks (p=0.0572), with the subgroup advantages already described in the previous section.

The LUNA trial, presented in November 2025, is the largest multicentre observational study to date: 199 participants with moderate-to-severe knee OA, one-year interim data confirming tolerability and sustained relief, and a five-year follow-up still ongoing. Its findings will be important for understanding durability beyond the current evidence horizon.

A 24-month PROMs cohort (314 knees) adds a durability signal: outcomes remained meaningful at two years, though 49 patients — concentrated in higher-grade disease — went on to require knee replacement, a reminder that Arthrosamid® is not a substitute for arthroplasty in advanced cases.

A 2026 review concluded that, despite this encouraging picture, routine clinical recommendation remains premature pending larger, longer randomised trials. The evidence is solid; it is not yet complete.

How Arthrosamid compares to other knee injections

Three injectable options currently occupy this part of the conservative knee pathway: corticosteroids, hyaluronic acid (HA), and Arthrosamid®. The comparative evidence is clearer than might be expected, though it does not point uniformly in one direction.

Against corticosteroid, the 2025 retrospective cohort (n=150, KL grade II–IV) provides the most direct data. Arthrosamid® outperformed steroid at 6 months (p=0.008). By 12 months, the steroid group had returned to baseline pain scores; the Arthrosamid® group retained a slight VAS improvement. For patients who have previously relied on repeated steroid injections and found the benefit shortening each time, that durability difference is a practical rather than abstract consideration.

Against hyaluronic acid, the picture is more nuanced. In the randomised controlled trial (n=239), Arthrosamid® was non-inferior overall. The subgroup advantages in younger patients, those with normal BMI, and those with KL grade 2–3 disease have already been described. The reverse is also important to state plainly: at KL grade 4, hyaluronic acid produced better outcomes — a finding that reinforces why accurate staging before any injection decision matters.

The same 12-month durability pattern holds here: the HA group also returned to baseline at 12 months in the retrospective cohort, while Arthrosamid® outcomes remained stable.

There are no head-to-head comparative data against PRP or other biologic injections in the published evidence retrieved for this article; any comparison with those agents would be speculative.

Arthrosamid® sits within the injection and conservative management pathway. It is not a replacement for knee arthroplasty, and the evidence does not frame it as one.

Safety and common patient concerns

One question patients frequently raise after researching Arthrosamid® online is whether the word "acrylamide" in its composition is a cause for concern. Acrylamide monomer is indeed a recognised neurotoxin — but the substance in Arthrosamid® is a cross-linked polymer, a structurally distinct form. A 2025 in vitro study exposed human iPSC-derived neurons to 2.5% iPAAG at concentrations up to 20% for 96 hours and recorded no statistically significant neurotoxic or cytotoxic effects. The monomer and the polymerised product are not biologically equivalent, and that distinction matters when weighing this concern.

The broader adverse-event profile is modest. Transient injection-site pain is the most commonly reported side-effect, occurring in approximately 16% of cases across the published literature; it is typically short-lived. No serious device-related complications have been reported across the systematic review dataset of 463 patients or in the trial literature.

Because Arthrosamid® is non-absorbable, it does not enter the systemic circulation — a relevant consideration for patients who have concerns about the systemic effects associated with repeated corticosteroid use.

Arthrosamid® is contraindicated in active local knee infection and inflammatory or rheumatoid arthritis; individual suitability requires a consultant assessment.

Recovery after the injection and realistic expectations

Most patients want to know, above everything else, when they can get back to ordinary life — and the answer here is faster than many expect for the initial return, slower for the full benefit.

Mobilisation is standard on the day of the injection itself. Mild swelling or stiffness may settle over a few days, and driving is generally considered safe after approximately 24 hours. For the first one to two weeks, specialist guidance consistently recommends light flat-ground walking while avoiding high-impact exercise and heavy weight-bearing — a short, bounded restriction rather than an open-ended one.

Full improvement, however, is not an early event. Because Arthrosamid® works through gradual integration into the synovial membrane, the benefit typically emerges over 6–12 weeks. Patients who judge the injection at two weeks and find it unremarkable are measuring too early.

On duration, clinical sources cite relief lasting 2–5 years, and the systematic review evidence extends to two years with consistent findings. It is worth being precise about what that means: the two-year figure comes from cohort and open-label data rather than long-term RCTs. The LUNA trial, which presented its one-year interim in November 2025, will follow participants for up to five years — so the upper end of the duration range will be tested more rigorously in time.

Outcomes appear better when the injection is part of a broader programme. Clinical consensus across published guidance holds that combining Arthrosamid® with structured physiotherapy and load management produces superior results to the injection alone; Contura provides a free online rehabilitation programme designed specifically for patients who have received it. That combination principle — addressing load, movement, and the joint environment together — reflects the same logic that underpins the patient-selection evidence discussed earlier in this article.

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