Who Arthrosamid is for

Not every knee osteoarthritis patient is a strong candidate for Arthrosamid, and understanding where it fits — and where it does not — is the most useful starting point.

The clearest evidence for benefit comes from a 24-month cohort study of 269 patients (314 knees), which identified four patient factors independently associated with reaching a meaningful improvement across three validated outcome scores (VAS, Oxford Knee Score, and Lysholm): older age, lower Kellgren–Lawrence (KL) OA grade, absence of diabetes, and bilateral knee involvement. In practical terms, this points towards patients with mild-to-moderate disease — broadly KL grade II or III — as the group most likely to gain durable relief from a single injection.

Two groups should not receive Arthrosamid. Patients with an active joint infection are an absolute contraindication. Those with end-stage bone-on-bone disease (KL grade IV) are also excluded; in the same cohort, higher OA grade was the strongest predictor of eventually requiring total knee replacement (49 of 269 patients in the study reached TKR within two years).

If previous injections have been given, a waiting period is required before treatment can proceed: 6 weeks after a corticosteroid injection, and 3 months after hyaluronic acid or PRP. These washout intervals are standard across UK centres offering the procedure.

Arthrosamid is well suited to patients who want sustained relief from a single intervention rather than repeated short-cycle injections — and who are not yet at the stage where surgery is unavoidable. It is not available on the NHS and is accessed privately; UK pricing typically starts at around £3,000 inclusive of consultation, ultrasound, and the injection protocol.

How Arthrosamid works differently from other knee injections

Unlike hyaluronic acid, which temporarily supplements joint fluid, or corticosteroids, which dampen inflammation for weeks, Arthrosamid works through a different principle altogether. It is a 2.5% polyacrylamide hydrogel — a non-biodegradable, non-resorbable material that, once injected, is proposed to integrate with the synovial membrane lining the knee joint and form a lasting cushioning scaffold within the tissue.

Because the body does not break the material down, a single 6 ml dose is intended to remain in place indefinitely — which is the clinical rationale for studying it over five years rather than the months typical of conventional injection trials. The systematic review of 463 patients and both the original RCT (NCT04045431) and its 5-year extension were designed specifically to test whether that durability translates into sustained symptom relief, and the data so far suggest it does.

The precise mechanism is still being characterised. Histological data support the synovial-integration model, but the full picture has not yet been established — a limitation the published research acknowledges.

What happens on the day

The appointment is straightforward and takes place entirely as an outpatient procedure — no general anaesthetic, no overnight stay.

On arrival, a nurse will administer intravenous antibiotic prophylaxis approximately one hour before the injection. This is a routine infection-prevention step used consistently across UK specialist centres, not a response to any particular risk. The knee is then cleaned twice with an antiseptic solution before the procedure begins.

The injection itself is delivered under ultrasound guidance, which allows the clinician to confirm accurate placement within the joint space before the full 6 ml dose is released. Ultrasound guidance is a precision measure — it is standard practice for intra-articular procedures of this type, not an indication that something is difficult or uncertain.

Local anaesthetic is used at the injection site. The procedure itself is brief, and patients are able to walk immediately afterwards. A short observation period — typically around 30 minutes — follows before discharge.

Practical planning point: driving is not safe on the day of the procedure. Arrange a companion to accompany you or book alternative transport home in advance. Most patients return to light daily activity the following day.

The first 48 hours after the injection

Most of what you feel in the hours and days after the procedure is entirely expected. Mild soreness, stiffness, swelling, or bruising around the knee are a normal tissue response to the injection — not a sign that something has gone wrong. For most patients these sensations settle within 48 hours without any specific treatment.

Managing discomfort at home

• Ice — apply a wrapped ice pack for 10–15 minutes at a time; do not place ice directly on skin.
• Elevation — keep the leg raised when sitting or resting to help reduce any swelling.
• Paracetamol — appropriate for mild to moderate discomfort; follow standard dosing guidance.
• Dressing — keep the injection site clean and dry; avoid baths, swimming, or soaking the area until the dressing is removed.

Gentle flat-ground walking is fine from day one. Driving, however, should wait 24–48 hours. For the first one to two weeks, avoid anything that loads the knee heavily: running, gym sessions, heavy lifting, squatting under load, and twisting movements. Gradual return to normal daily activity is the right approach — not enforced rest, but not a return to full exertion either.

When to seek urgent advice

Contact your treating clinician promptly if you notice any of the following:

• Severe or rapidly worsening knee pain
• Marked warmth, redness, or swelling that is increasing rather than settling
• Fever or chills

These signs are uncommon, but they can indicate joint infection, which requires prompt assessment rather than a wait-and-see approach.

Recovery in the weeks and months ahead

Recovery from Arthrosamid is measured in weeks and months, not days — and keeping that timeline in mind from the outset helps avoid the most common source of disappointment: judging the treatment before it has had time to work.

Meaningful pain reduction typically begins at around four weeks, as the polyacrylamide hydrogel integrates with the synovial membrane and begins providing its cushioning effect. Maximum clinical improvement, however, generally emerges somewhere between three and twelve months post-injection. Patients who notice only modest change in the first few weeks should not conclude that the treatment has failed; the benefit builds gradually.

Rehabilitation and weight management

Once the initial recovery window has passed, a structured exercise programme focused on knee muscle strengthening — such as the dedicated Arthrosamid Rehabilitation Programme — is recommended. Stronger muscles around the knee reduce the load borne by the joint itself, complementing what the hydrogel provides mechanically. This is something patients can actively pursue, rather than a passive waiting period.

Weight management works on the same principle: reducing body mass lowers joint load and helps sustain treatment benefit over time. For patients carrying excess weight, even a modest reduction can make a measurable difference to knee symptoms.

A note for patients with more advanced OA

As discussed in the candidacy section above, higher-grade knee OA predicts greater risk of eventually requiring knee replacement. Improvement in symptoms following Arthrosamid does not remove that possibility; patients with more advanced disease should discuss long-term joint management with their clinician from the outset.

How durable the results are and what the evidence shows

The strongest evidence for Arthrosamid's durability comes from two independent five-year studies. To anchor the figures: a 10-point improvement on the WOMAC pain scale is generally regarded as clinically meaningful to patients, which puts the findings in proportion. A prospective open-label extension (27 completers from an initial cohort of 49) recorded mean WOMAC pain improvements of −14.6 points at five years, with sustained gains in stiffness (−19.6) and physical function (−12.5). An RCT extension (NCT04045431, 119 participants) found a mean WOMAC pain improvement of −16.2 points at year 5 (p<0.0001). In neither extension study were any adverse events between years one and five attributed to the device.

A systematic review pooling outcomes across 463 patients confirmed efficacy at 52 weeks and two years, with RCT results numerically superior to hyaluronic acid. A retrospective cohort study (50 patients per group, KL grades II–IV) compared iPAAG directly with hyaluronic acid and corticosteroid at 12 months: VAS pain scores in the HA and corticosteroid groups had returned to baseline by that point, while the iPAAG group remained slightly improved — a durability difference rather than a peak-effect difference.

The evidence has real limits that deserve plain statement rather than a footnote. The large 24-month cohort study (314 knees) had no randomised control group. Only 58 of 119 participants completed five-year follow-up in the RCT extension — a modest number from which to generalise. The HA-and-steroid comparison is retrospective, not randomised. No head-to-head data against PRP exist, and outcome data specifically in patients under 60 remain limited. Taken together, the evidence supports iPAAG as a durable option for carefully selected patients while leaving several clinical questions unanswered — including precisely how long benefit persists across the full range of OA severity.

Arthrosamid is not currently NHS-funded and is accessed privately in the UK, with pricing from approximately £3,000 inclusive of consultation, ultrasound guidance, and the injection protocol.

1. [1] Polyacrylamide hydrogel injections in knee osteoarthritis: A PROMs-based 24 month cohort study. (2025). https://doi.org/10.1016/j.jcot.2025.103136 https://doi.org/10.1016/j.jcot.2025.103136
2. [2] Sustained symptom relief and safety over five years following a single intra-articular injection of 2.5% iPAAG in patients with knee osteoarthritis. (2025). https://doi.org/10.55563/clinexprheumatol/bsper8 https://doi.org/10.55563/clinexprheumatol/bsper8
3. [3] A prospective, open-label clinical investigation of a single intra-articular iPAAG injection: 5-year extension study. (2025). https://doi.org/10.1186/s13018-025-06526-0 https://doi.org/10.1186/s13018-025-06526-0
4. [4] Comparative efficacy of polyacrylamide hydrogel versus hyaluronic acid and corticosteroids in knee osteoarthritis: A retrospective cohort study. (2025). https://doi.org/10.1097/MD.0000000000044655 https://doi.org/10.1097/MD.0000000000044655