What Arthrosamid is and how it differs from other knee injections

Three types of injection are commonly used for knee osteoarthritis: corticosteroids, hyaluronic acid, and — more recently — polyacrylamide hydrogel. Arthrosamid belongs to the third category. It is composed of 2.5% cross-linked polyacrylamide and 97.5% water, and it is classified as a medical device rather than a pharmaceutical drug — a distinction that reflects how it works rather than any regulatory technicality. It received its CE Mark in April 2021 for the symptomatic treatment of knee osteoarthritis.

The key difference from the injections most patients will have encountered lies in what happens once it is inside the joint. Corticosteroids reduce inflammation and are typically active for a matter of weeks. Hyaluronic acid supplements the natural fluid in the knee but is gradually metabolised, usually over a similar timescale. Arthrosamid is non-biodegradable: rather than dissolving or being resorbed, it adheres to and integrates with the synovial membrane — the tissue lining the joint capsule — where it is thought to act as a durable mechanical scaffold and cushion.

A useful way to picture this is a filler that settles into and supports the joint lining, rather than one that washes through it. The treatment is delivered as a single 6 mL injection under ultrasound guidance. Whether and for whom that structural difference translates into meaningful, lasting benefit is what the clinical evidence addresses.

What the clinical trial data shows at 1, 3, and 5 years

The strongest evidence for Arthrosamid comes from a multicentre randomised controlled trial (registered NCT04045431) and two subsequent long-term extensions published in Clinical and Experimental Rheumatology in 2025.

The original RCT compared a single iPAAG injection against Synvisc-One (a hyaluronic acid product) in adults with moderate-to-severe knee osteoarthritis. At one year, the two treatments performed comparably on pain and function — establishing that iPAAG was at least as effective as an established standard-of-care injection over that timeframe.

What sets the data apart is what happened next. In the 3-year extension, 75 of the original 119 iPAAG-treated participants completed follow-up. Their WOMAC pain score had improved by a mean of −13.1 points from baseline (95% CI −17.9 to −8.4; p<0.0001). WOMAC pain is measured on a 0–100 scale, where higher scores represent worse pain; a reduction of around 10–12 points is generally considered clinically meaningful. By year five, 58 participants remained in follow-up, and the mean improvement had reached −16.2 points (95% CI −20.0 to −12.4; p<0.0001). No adverse events in either extension period were attributed to the device itself.

A separate and independently conducted open-label multicentre study, published in the Journal of Orthopaedic Surgery and Research in 2025, followed 49 participants from a different cohort for five years. Of these, 27 completed follow-up. The findings were closely aligned: WOMAC pain −14.6 (p=0.0002), stiffness −19.6 (p=0.0006), and physical function −12.5 (p=0.0015) — all sustained at the five-year mark.

The convergence of two independent five-year datasets is the most persuasive signal currently in the literature. That said, the completer numbers — 58 and 27 — are modest, and both studies carry the attrition risk common to long-term follow-up work. The results are encouraging, but they come from relatively small samples and should be read accordingly.

How it compares to corticosteroid and hyaluronic acid injections

For patients who have already tried a steroid or hyaluronic acid injection that eventually wore off, the most clinically relevant question is not which injection works fastest — it is which one holds.

A 2025 retrospective cohort study addressed this directly by following 150 patients with Kellgren–Lawrence grade II–IV knee osteoarthritis across three equal groups: iPAAG (Arthrosamid 6 mL), hyaluronic acid (Artroaid), and corticosteroid (methylprednisolone). At three months, all three groups had improved meaningfully on both VAS pain and WOMAC scores — no group pulled ahead in the short term. This is an important finding: it means Arthrosamid does not appear to produce a stronger initial effect than existing options.

The difference emerged over time. At six months, iPAAG outperformed corticosteroid (VAS p<0.001; WOMAC p=0.008), while scores in the steroid group had already begun declining. By twelve months, both the HA and corticosteroid groups had returned to near-baseline levels, whereas iPAAG patients remained slightly improved. The twelve-month inter-group comparison showed no statistically significant difference between iPAAG and HA (p=0.128) — an honest result worth stating plainly.

The picture that emerges is one of durability rather than potency: a single iPAAG injection may sustain the benefit that other injections achieve but cannot hold. The retrospective design of this study limits certainty, and a prospective controlled trial would be needed to confirm the finding, but the durability signal is consistent with the longer-term data reviewed in the previous section.

Which patients tend to respond best

The best-responder profile emerging from current data points towards older patients with moderate knee osteoarthritis — Kellgren–Lawrence grade II or III — who do not have diabetes. A 24-month PROMs cohort study of 314 knees identified these factors, along with bilateral disease, as the characteristics most consistently linked to achieving a clinically meaningful improvement following injection. Notably, the same cohort found that patients with higher KL grades were significantly more likely to go on to total knee replacement within 24 months — a finding consistent with the broader clinical principle that joint replacement remains the standard of care for end-stage disease.

These are population-level signals rather than a rigid eligibility checklist. Ongoing synovial biomarker analyses and bone marrow lesion imaging studies are investigating why some patients within this broad profile respond better than others. Even without that mechanistic detail resolved, the directional picture is useful: moderate structural disease, older age, and the absence of conditions such as diabetes appear to favour a meaningful response.

Response is not guaranteed even within a favourable profile, and patients with more advanced OA may find a different pathway — including a surgical conversation — more appropriate. Establishing which group a patient falls into requires a proper clinical assessment; population patterns are a guide, not a prediction, and individual factors such as prior treatments, activity demands, and realistic expectations shape the recommendation.

What the evidence does not yet answer

The most significant gap is the absence of a placebo arm in the long-term extension phases. Both 5-year datasets compared iPAAG against hyaluronic acid, not a sham injection, so the precise magnitude of treatment effect — separated from natural disease variation and the general response to any intra-articular injection — remains uncertain. That two independent datasets converge on similar improvements at five years is a meaningful signal; it is not, however, the same as a placebo-controlled demonstration of effect size.

Attrition limits how far that signal extends. The RCT extension followed 119 iPAAG participants to five years, with 58 completing; the separate open-label study started with 49 participants and 27 reached the endpoint. Patients who withdrew include those who did not respond or who required joint replacement, so completers are likely to represent better-than-average outcomes rather than the full range.

No head-to-head trial yet exists against platelet-rich plasma or other orthobiologics — a clinically relevant gap as interest in those options grows. On safety, published extension data consistently reported no serious device-related adverse events, though real-world reporting across larger populations is still accumulating and a clearer picture will take time to form.

Because Arthrosamid is a CE-marked medical device rather than a licensed pharmaceutical, it followed a regulatory approval pathway with different evidence requirements from drug licensing. Some clinical guideline bodies apply stricter evidence thresholds when evaluating treatments approved under medical device legislation, which partly explains why it has not yet appeared in mainstream OA treatment guidelines.

What the current evidence supports is a reasonable confidence that sustained symptom relief is achievable in appropriate candidates following a single injection. What it cannot yet quantify with precision is how much of that benefit derives specifically from the hydrogel, rather than from the act of injection itself — and that distinction matters for setting realistic expectations.

Getting assessed for Arthrosamid at Lincolnshire Knee

For patients in Lincolnshire and the surrounding region, a consultant assessment is the appropriate starting point — not a self-reported match to any published responder profile. A clinical evaluation would typically review knee OA grade, symptom history, and prior treatments, with imaging reviewed or arranged as needed. Whether Arthrosamid is appropriate depends on the full picture established at that assessment, not on a checklist.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral, avoiding NHS waiting lists. Consultations are available at Sleaford NG34 (head office, with Open MRI) and Grantham NG31 (consultation and diagnostics). Book an assessment at lincolnshireknee.co.uk.

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