What the evidence actually shows — the short answer

Three intra-articular injections, meaningfully different evidence bases, and no single trial that puts them head-to-head — that is the honest starting point. Because no three-arm RCT has compared Arthrosamid, PRP, and HA directly, any ranking draws on separate studies and indirect inference.

The clearest finding concerns PRP versus HA: Belk et al.'s 2021 meta-analysis (514 citations) and Li et al.'s 2025 replication both show PRP producing statistically significantly better WOMAC, VAS, and IKDC scores at 6 and 12 months. Arthrosamid sits differently in the evidence: the Bliddal 2024 RCT found it non-inferior to HA at six months, and statistically superior in pre-specified subgroups — patients under 70, those with normal BMI, and those with Kellgren-Lawrence grade 2–3 disease — at 12 months. HA has the longest clinical pedigree, with studies dating to 1974, but its symptom window is the shortest of the three.

Duration is where the options diverge most sharply. HA requires repeat courses at intervals of a few months; PRP effects may persist for one to two years; Arthrosamid, as a single permanent implant, has demonstrated sustained benefit at five-year follow-up. For many patients, that difference in treatment burden is as relevant as the comparative pain scores.

How each injection works in the knee

The difference in how long each injection lasts comes down to what happens inside the joint once the needle is withdrawn.

Hyaluronic acid (HA) works by topping up the knee's natural shock-absorbing fluid. In an osteoarthritic joint, synovial fluid becomes thinner and less viscous; injecting HA temporarily restores that cushioning quality and may also dampen local pain signals. The body gradually clears the injected HA over weeks, which is why the effect fades and repeat courses are needed.

Platelet-rich plasma (PRP) is spun from the patient's own blood to concentrate platelets, which then release growth factors and anti-inflammatory mediators into the joint space. The aim is to shift the OA environment towards less inflammation and, in some cases, to support tissue. As those biological signals dissipate over months, the benefit slowly wanes.

Arthrosamid takes a different approach entirely. The 2.5% polyacrylamide hydrogel — 97.5% water, 2.5% cross-linked polymer — physically integrates into the synovial lining of the knee rather than dissolving into the joint fluid. Because it is not metabolised or absorbed, its cushioning effect does not depend on the body clearing or replacing it. That permanence explains the multi-year durability data; it also means the decision to proceed deserves careful discussion with a consultant, since the implant is designed to remain in situ indefinitely.

PRP versus hyaluronic acid: the clearest comparative evidence

Multiple independent meta-analyses agreeing on the same direction of effect carries more weight than a single trial — and PRP versus HA is the one comparison in this trio where that convergence exists.

WOMAC (Western Ontario and McMaster Universities Osteoarthritis Index) measures pain, stiffness, and physical function on a standardised patient-reported scale; VAS records pain intensity on a numerical line; IKDC reflects overall knee function and activity. Across all three, the Belk et al. 2021 meta-analysis — now cited more than 514 times — found PRP producing statistically significantly better results than HA at six and twelve months. The Li et al. 2025 meta-analysis independently reached the same conclusion, specifically confirming lower WOMAC pain scores in the PRP group across randomised controlled trials.

Not every review aligns neatly. Ivander et al.'s 2024 systematic review judged both PRP and HA effective but found the evidence insufficient to confirm PRP superiority — a useful counterweight against reading the overall literature as settled.

Preparation quality is a genuine practical variable. The trials generating the clearest benefit used leukocyte-poor PRP processed with a double centrifuge spin to achieve therapeutic platelet concentrations; a clinic following that protocol is operating closer to the conditions where the positive results were produced. Single-spin or leukocyte-rich preparations may not replicate those outcomes. PRP courses typically involve one to three injections, with benefit estimated to last twelve to twenty-four months before repeat treatment becomes relevant.

For a patient considering PRP, asking which preparation protocol a clinic uses is clinically meaningful — the answer indicates how closely the treatment matches the evidence behind it.

Arthrosamid versus hyaluronic acid: the pivotal trial and durability data

The pivotal comparative study for Arthrosamid is the Bliddal et al. 2024 RCT (Clin Exp Rheumatol, 34 citations), which set a single 6 mL iPAAG injection against hyaluronic acid in patients with knee OA. At six months, iPAAG was non-inferior to HA — meeting the pre-specified non-inferiority threshold. By twelve months, iPAAG was numerically better than HA across the overall population, though that gap did not reach statistical significance.

Who benefits most: the subgroup findings

A pre-specified subgroup analysis told a more precise story. Patients aged under 70, those with a normal BMI, and those with Kellgren-Lawrence grade 2–3 knee OA all showed statistically significantly better outcomes with iPAAG than with HA at twelve months. For a younger, non-obese patient with mild-to-moderate disease — the profile most consultants weigh when considering long-term options — this distinction carries practical weight.

Durability: the evidence in steps

The durability data extend further than any comparable HA dataset. The IDA open-label study's three-year extension, presented at OARSI 2023, demonstrated statistically significant reductions in pain, stiffness, and physical function maintained three years after a single injection. The Bliddal 2025 five-year follow-up (PMC) confirmed continued improvement across WOMAC pain, stiffness, physical function, and Patient Global Assessment — five-year single-injection data of this kind are unusual for this class of treatment and are clinically meaningful.

The LUNA trial (N=199, multicentre European, 2025) added observational breadth: a mean 17-point improvement in WOMAC pain at twelve months, with approximately 70% of participants achieving a clinically meaningful response from one injection and no serious device-related adverse events reported. It is worth noting that the IDA study is open-label and LUNA is observational; neither carries the blinding of the Bliddal RCT, so both should be weighted accordingly.

Arthrosamid is currently available in private UK clinics and is not an NHS-commissioned treatment, meaning access differs materially from HA — which is available on the NHS — and PRP.

Duration and repeat injections: how the three options compare over time

Consider a patient in their late 50s with Kellgren-Lawrence grade 2–3 knee OA who is not yet a surgical candidate. Managing symptoms for a further decade before arthroplasty becomes appropriate is a realistic prospect — and the cumulative treatment burden over that period differs substantially between the three options.

HA requires a fresh course every four to six months. Over ten years, that translates to roughly fifteen to twenty injection episodes — a meaningful commitment of clinic visits, recovery days, and repeat out-of-pocket cost for patients accessing treatment privately.

PRP cuts that frequency. A course typically runs to one to three injections, with benefit estimated to last twelve to twenty-four months; a patient might expect treatment every one to two years rather than two to three times annually.

Arthrosamid sits in a different category. Its design as a permanently integrating implant means a single injection is the treatment — not a course. The five-year durability data already discussed place this in a different administrative register entirely for patients planning ahead over a long horizon.

The critical caveat is worth stating plainly: no RCT has directly compared Arthrosamid and PRP against each other. Their relative positions are triangulated — PRP outperforms HA in multiple meta-analyses; Arthrosamid is non-inferior to HA in the Bliddal 2024 RCT. Where the two sit relative to one another remains an open question, and any ranking of Arthrosamid against PRP is an inference from separate data, not a directly measured finding.

Which injection is right for your knee

No single injection wins outright — the better question is which option fits a particular patient's OA grade, age, lifestyle, and tolerance for repeat treatment.

Three rough profiles illustrate where the evidence points.

Active patient, late 50s, KL grade 2–3, wants comparative biological evidence. PRP has the strongest head-to-head data against HA across multiple meta-analyses, and leukocyte-poor preparations appear to sustain benefit for up to two years. A course of one to three injections suits someone comfortable returning periodically for treatment and prioritising the comparative trial record.

Patient seeking long-term relief from a single procedure. Arthrosamid's five-year durability data, combined with subgroup findings from the Bliddal 2024 RCT showing statistically significant benefit in patients under 70 with normal BMI and KL2–3 disease, makes it a clinically coherent choice where that profile matches. One injection rather than a recurring course is a meaningful practical distinction over a ten-year management horizon.

Where access or cost is the primary constraint. HA remains appropriate. It is widely available, has a long safety record, and provides genuine short-to-medium-term relief for mild-to-moderate OA — functioning usefully as a step in a staged clinical pathway even if effects require repeating every four to six months.

End-stage KL4 disease sits outside the evidence base for all three options; at that point, the conversation typically shifts towards joint preservation surgery or arthroplasty, guided by a consultant assessment of individual anatomy and goals.