What Arthrosamid is and why it differs from a steroid or HA injection

Arthrosamid sits in a different category from the corticosteroid and hyaluronic acid (HA) injections that most patients with knee osteoarthritis will already have been offered. It is a polyacrylamide hydrogel — branded iPAAG — composed of 2.5% cross-linked polyacrylamide and 97.5% water, delivered as a single 6 ml injection directly into the knee joint. Unlike corticosteroids, which reduce inflammation temporarily, or HA preparations, which supplement the joint's natural lubricating fluid, Arthrosamid is non-biodegradable: once injected, it is not reabsorbed by the body.

The regulatory classification matters practically. Arthrosamid is approved as a medical device — CE-marked in Europe and the United Kingdom since 2021 — rather than as a pharmaceutical drug. That distinction changes the type of evidence required before approval and explains why the clinical dataset looks structurally different from a standard drug trial. It does not currently hold FDA approval, which limits the availability of large, US-based randomised trial data.

Manufactured by Contura Orthopaedics Ltd, Arthrosamid is described as acting by integrating into the synovial membrane lining the joint capsule — forming a durable scaffold rather than simply supplementing joint lubrication. That structural difference from HA viscosupplementation is what underpins the claim of longer-lasting effects from a single treatment.

How it is thought to work inside the knee joint

Synovitis — inflammation of the synovial membrane lining the inside of the joint capsule — is closely linked with pain intensity in knee osteoarthritis. That connection is central to how Arthrosamid is thought to work. Rather than remaining free in the synovial fluid as HA does, the hydrogel integrates into the synovial tissue itself after injection, forming a durable mechanical cushion within the membrane.

By embedding in this tissue, the treatment is proposed to reduce both the physical stress on the synovial lining and the inflammatory signalling associated with synovitis. Because the material does not degrade, this scaffolding effect persists without repeat dosing — the basis for the longer-duration outcome data described in the next section.

The precise pathway, particularly the relative contribution of mechanical versus anti-inflammatory effects, remains under active investigation. An ongoing NHS-funded study at The Robert Jones and Agnes Hunt Orthopaedic Hospital (RJAH) is examining exactly this question; it is discussed further in the evidence section below. For now, the synovitis-targeted rationale is clinically plausible and meaningfully distinct from the lubrication model — but should not be treated as settled mechanistic science.

What the clinical studies actually show

Four studies form the core of the published evidence base, and they vary considerably in design — a distinction that shapes how much weight each can carry.

TACIT RCT (Denmark). The most methodologically rigorous study is the TACIT trial, a randomised controlled trial conducted in Denmark. It compared Arthrosamid against a control group and demonstrated statistically significant reductions in knee pain and stiffness, with follow-up extending to two to three years. Because participants were randomly allocated, the TACIT trial offers stronger protection against bias than the observational and open-label studies described below.

IDA open-label study (49 patients, up to 5 years). This prospective study, though non-randomised, provides the longest published follow-up data — out to five years from a single injection. Open-label means both clinicians and patients knew what treatment was administered, which can inflate perceived benefit. Responder rates and age-related patterns from this study are examined in the following section.

Bliddal et al. 2024 (J Orthop Surg Res, PMC11064315). This 12-month open-label study, published in the Journal of Orthopaedic Surgery and Research, confirmed that a single 6 ml iPAAG injection maintained meaningful pain reduction one year after treatment. It represents a key post-approval effectiveness dataset, though the open-label design is again a limitation.

LUNA Study (199 participants, 2025). The largest multicentre trial to date, LUNA enrolled 199 participants with moderate-to-severe knee OA. Its 1-year interim data, first presented at ISIAT 2025 in Bucharest, showed an average WOMAC pain score improvement of approximately 17 points and no serious treatment-related adverse events. Crucially, LUNA is an observational study — there is no randomised control group — so confounding cannot be excluded.

Post-market TKR data. Longer-term surveillance data suggest that up to 56% of patients treated with a single iPAAG injection did not go on to require total knee replacement within 10 years. This is a striking figure, but it comes from observational follow-up rather than a controlled trial; multiple factors influence whether a patient eventually needs joint replacement, and a causal link to the injection cannot be established from this data alone.

Which patients are most likely to be suitable

The studies described above have been conducted in a fairly consistent patient group: adults with symptomatic knee osteoarthritis who retain meaningful joint space, have exhausted standard conservative measures — including physiotherapy and hyaluronic acid or corticosteroid injections — and are either not yet surgical candidates or are actively looking to defer knee replacement. That is the population in whom the treatment has been evaluated, and it remains the appropriate frame for thinking about suitability.

Within that group, age appears to be one distinguishing factor. The IDA data, as discussed in the previous section, recorded a roughly 80% response rate among participants under 70, a signal that — while drawn from an open-label study of 49 patients — is clinically worth noting when discussing likely benefit with a patient in their fifties or sixties.

Which other characteristics predict a stronger response remains genuinely uncertain. The NHS-funded RJAH study was designed specifically to address this question alongside its mechanistic aims. Until those data are available, patient selection rests on the criteria visible in the published trials rather than on a validated responder profile.

Arthrosamid is not an alternative to surgical assessment in advanced disease. Patients with severe tri-compartmental OA, significant varus or valgus deformity requiring correction, or bone-on-bone changes affecting the whole joint remain candidates for joint-preservation surgery or knee replacement — and that clinical conversation should not be bypassed on the basis of an injection that targets synovial symptoms rather than structural failure.

Gaps and limitations in the current evidence

Taken together, the four studies reviewed above point consistently in the same direction: measurable pain relief, a reassuring short-term safety record, and durability beyond one year. What they do not yet collectively establish is whether those findings would survive the most rigorous scrutiny available — a large, independently funded, placebo-controlled phase III randomised trial.

That gap matters in practice because intra-articular knee injections are associated with a well-documented and substantial placebo response. Without adequately blinded control arms, it is difficult to quantify precisely how much of the observed improvement is attributable to the hydrogel itself versus the act of injection, clinical attention, and patient expectation. The TACIT RCT provides the strongest protection against this confound, but the evidence base as a whole remains skewed towards open-label and observational designs.

Arthrosamid also lacks FDA approval, meaning no large US-based phase III trials have been conducted; the evidence base is predominantly European and, to varying degrees, connected to Contura Orthopaedics' funding — a position standard for a CE-marked medical device at this stage of post-market development, but appropriate context for any reader weighing the literature.

Perhaps the most important clinical distinction is this: the available data demonstrate symptom relief, not structural modification of osteoarthritis. Arthrosamid has not been granted a disease-modifying designation under any regulatory framework, and the published evidence does not support that claim. Whether the treatment influences joint structure over time is one of the questions the RJAH NHS study is designed to investigate in a real-world UK setting.

Access, cost, and what to expect in the UK

Arthrosamid is not routinely funded by the NHS and is not currently reimbursed by major UK private medical insurers including Bupa and AXA; patients access it on a self-funded basis. UK pricing typically ranges from approximately £3,000 for a single-box protocol to around £8,000 for three boxes, depending on clinic and treatment plan.

Price variation between providers reflects what is included. Accurate intra-articular delivery requires ultrasound image guidance to confirm needle placement; intravenous antibiotic prophylaxis and structured follow-up are also standard in published protocols. A quote that excludes any of these elements is not directly comparable to one that does not.

A thorough pre-injection assessment — reviewing imaging, functional status, and whether the clinical picture matches the patient profile in which the treatment has been evaluated — is the appropriate first step before any injection is arranged. Patients considering Arthrosamid should ask their GP or referring clinician whether their joint status, symptom duration, and prior treatment history are consistent with the criteria applied in the published trials described above: meaningful retained joint space, failure of standard conservative measures, and no surgical indication that a single-injection approach would not address.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without a referral; consultant-led assessments are available at Sleaford NG34 and Grantham NG31 — lincolnshireknee.co.uk.