Two injections, one decision

When a knee specialist mentions both Arthrosamid and PRP in the same conversation, the natural question is: which one should I have? The short answer is that it depends — and the reason it depends is worth understanding before booking anything.

Both are injections given directly into the knee joint, typically after rest, physiotherapy, and simple pain relief have not provided sufficient relief. That is where the similarity ends. Arthrosamid is a synthetic hydrogel that physically integrates into the joint lining and stays there; PRP draws concentrated growth factors from the patient's own blood to try to modify the joint environment biologically. Different materials, different goals, different schedules — and, in practice, different patient profiles.

Neither treatment reverses structural cartilage damage. Both target pain and function, not disease reversal, and patients should go in with that understanding.

What is also worth knowing upfront: no published randomised controlled trial has put the two injections head-to-head. Every comparison currently available — including this one — is built from separate trial programmes. The article that follows sets out what each treatment does, what the evidence shows individually, and which situations each tends to suit best.

How each injection works inside the knee

The physical difference between these two injections runs deeper than brand or category — they work through entirely separate mechanisms.

Arthrosamid is a 2.5% polyacrylamide hydrogel (iPAAG) delivered as a single 6 ml injection under ultrasound guidance. Rather than floating freely inside the joint, it adheres to and integrates into the synovial membrane — the tissue that lines the knee capsule. Think of it less as a lubricant and more as a permanent internal scaffold: the hydrogel bulks up the joint lining, providing mechanical cushioning that persists because the material is non-biodegradable and is not metabolised by the body. Once in place, it remains in situ; there is no gradual breakdown and no repeated dosing required to sustain the effect.

PRP works through a fundamentally different route. A sample of the patient's own blood is centrifuged to concentrate the platelets, which carry a range of growth factors including VEGF. This concentrated plasma is then injected into the knee, where those growth factors are thought to modulate the local inflammatory environment and support tissue signalling. The mechanism is biological — and that is both its appeal and its limitation. Because PRP is prepared from each patient's blood, the effective platelet dose, the preparation technique, and individual biology all shape what actually ends up in the joint. A 2024 systematic review of 29 RCT arms found that studies using a mean platelet dose of approximately 5,500 ×10⁶ achieved significantly better outcomes than those using around 2,302 ×10⁶ (p<0.01) — a difference driven entirely by how the product is prepared, not by the drug itself. It is not a standardised pharmaceutical product in the way a synthetic hydrogel is.

Both are delivered as intra-articular knee injections, but the downstream biology is categorically distinct: one is mechanical scaffolding, the other an autologous biological signal.

What the clinical evidence actually shows

Arthrosamid: what the studies found

The clearest published data come from a 52-week open-label study by Bliddal and colleagues (n=49), 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). A reduction of that magnitude typically represents a shift from pain that regularly interrupts daily tasks — climbing stairs, rising from a chair, walking to the shops — to something patients describe as manageable background discomfort. At one year, 62.2% of participants met OMERACT-OARSI responder criteria for clinically meaningful improvement, with sustained gains in stiffness and physical function scores.

Broader evidence extends this picture. A systematic review covering 463 patients confirmed statistically significant benefits through two years, including numerically superior outcomes versus hyaluronic acid in one RCT. A 2025 retrospective cohort study (n=150, Kellgren–Lawrence grade II–IV) found that iPAAG, hyaluronic acid, and corticosteroid all reduced pain similarly at three months — but only the iPAAG group maintained those gains at 12 months while the other two groups returned to baseline. Safety data presented at the World Congress of Osteoarthritis in April 2025 covers ten years of follow-up and found no unusual adverse reactions, including in patients who subsequently had a total knee replacement.

PRP: what the studies found

A 2025 meta-analysis of 18 RCTs (1,995 patients) confirmed PRP exceeded the minimal clinically important difference for pain (VAS ≥1.37) at three and six months, and for function (WOMAC ≥6.4) at all follow-up points. A VAS difference of ≥1.37 on a zero-to-ten scale roughly marks the boundary between pain that demands constant management and pain most patients can work around — a genuine, if moderate, shift in daily experience. Whether any individual reaches that threshold depends substantially on preparation: studies with positive six-month outcomes used mean platelet doses of approximately 5,500 ×10⁶ versus around 2,302 ×10⁶ in non-significant studies (p<0.01). That difference is not academic — two patients treated with "PRP" in different clinics may have received preparations of meaningfully different potency, which is why the evidence base is harder to read as a single coherent picture.

Neither treatment has been tested against the other in a published randomised controlled trial. All comparative observations, including those in this article, are drawn from separate study programmes — a limitation worth holding in mind when weighing the numbers above.

Practical differences: schedule, consistency, and access

The practical shape of each treatment journey differs considerably, and for many patients these logistics matter as much as the clinical numbers.

Arthrosamid involves a single clinic visit: one injection, one consultation, and no scheduled return dose. Published evidence suggests the effect persists for two to three years on average, with emerging five-year follow-up data adding to that picture. There is no standard protocol for repeat injection if symptoms return, so the expectation going in is that one treatment covers a substantial window.

PRP typically requires repeat cycles. Most patients see symptomatic benefit for somewhere between six and twelve months, after which a further course — usually another series of injections — is needed to maintain the effect. That pattern carries a real implication for time commitment: multiple clinic visits per year, not a single episode of care. Cumulative cost compounds accordingly.

The consistency question is also worth raising plainly before comparing quotations from different providers. Arthrosamid is a pharmaceutical-grade synthetic product with a fixed formulation — every vial is the same. PRP is prepared fresh from each patient's blood, and preparation protocols vary between clinics. As s2 and s3 established, those differences in technique translate into differences in what actually reaches the joint. Two providers quoting for 'PRP' may not be offering equivalent treatments.

Neither injection is routinely funded by the NHS for knee osteoarthritis. Both are private-pay treatments in the UK, making comparative cost a legitimate and reasonable part of the decision.

Which patients are more likely to suit each approach

Deciding between the two comes down less to one being categorically better and more to which fits where a patient sits on the osteoarthritis spectrum — and what they are asking from treatment.

For earlier-stage knee OA (broadly Kellgren–Lawrence grade I–II), the joint's biological environment is more likely to respond meaningfully to growth factor signalling. PRP's mechanism — concentrated platelets modulating inflammation and supporting the local tissue environment — relies on that responsiveness. Where the cartilage and synovial lining are less degraded, the biological hypothesis has more to work with.

Arthrosamid tends to be positioned for patients with established, moderate-to-severe disease (KL grade II–IV). At that stage, the primary need is structural support and sustained symptom control rather than biological stimulation — which maps directly to what a non-resorbable hydrogel scaffold provides.

Beyond OA stage, two other factors regularly shape the conversation. First, treatment cadence: patients who want a single episode of care — one injection, no planned repeat visits — often find Arthrosamid better matched to that preference. Those comfortable with an autologous, biologically derived approach and periodic re-treatment may prefer PRP's flexibility. Second, patient-level biology: VEGF concentration in PRP has emerged as a potential response predictor, with one 2025 study finding patients with VEGF ≥120 pg/ml had a response rate of only 26.9% compared with 75% in the lower-VEGF group. No equivalent predictive biomarker currently exists for Arthrosamid. That asymmetry means personalised pre-injection assessment matters more for PRP than for a standardised synthetic product.

Clinical staging — MRI-based cartilage and structural assessment, alongside examination — is the basis on which a consultant determines candidacy. Both treatments sit firmly within the conservative, non-surgical tier: if joint degeneration has reached the point where surgery merits discussion, a thorough assessment will make that clear.

Getting a personalised assessment

The fork between these two treatments — broadly, disease stage and whether a single clinic visit or a repeat-cycle approach suits your life better — is one a consultant can resolve once your knee has been properly assessed. MRI-based cartilage mapping and clinical examination provide the grounding that no article can substitute for.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without a GP referral, with clinics in Sleaford (NG34) and Grantham (NG31). A consultant-led assessment can confirm your Kellgren–Lawrence grade, determine whether an injection therapy is appropriate, and clarify which option — or whether a surgical pathway merits discussion alongside — better fits your situation and goals.

Book directly at lincolnshireknee.co.uk. No referral letter is needed.

1. [1] Efficacy and Safety of Intra-articular PRP Versus Corticosteroid Injections in Knee Osteoarthritis: Systematic Review of RCTs. (2025). https://doi.org/10.7759/cureus.80948 https://doi.org/10.7759/cureus.80948
2. [2] PRP Injections for Knee Osteoarthritis: Improvement Is Clinically Significant and Influenced by Platelet Concentration – Meta-analysis of RCTs. (2025). https://doi.org/10.1177/03635465241246524 https://doi.org/10.1177/03635465241246524
3. [3] 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
4. [4] A Systematic Review of the Novel Compound Arthrosamid Polyacrylamide (PAAG) Hydrogel for Treatment of Knee Osteoarthritis. (2022). https://doi.org/10.18103/mra.v10i8.2950 https://doi.org/10.18103/mra.v10i8.2950
5. [5] A Higher Platelet Dose May Yield Better Clinical Outcomes for PRP in the Treatment of Knee Osteoarthritis: A Systematic Review. (2024). https://doi.org/10.1016/j.arthro.2024.03.018 https://doi.org/10.1016/j.arthro.2024.03.018