Four injections, four different jobs

'I've been told I could have an injection — but which one?' That question sits at the centre of most knee osteoarthritis consultations once conservative care has reached its limits, and the answer is rarely straightforward: corticosteroid, hyaluronic acid (HA), platelet-rich plasma (PRP), and polyacrylamide hydrogel (iPAAG) each occupy a different position on a duration-and-mechanism spectrum. They are not interchangeable versions of the same treatment.

The central trade-off running through all four is onset speed versus durability. Corticosteroid works within 24–72 hours but typically lasts 6–12 weeks. HA takes longer to produce relief but holds it for around six months. PRP works through biological signalling and may sustain benefit for roughly a year, though with considerable variation. iPAAG is mechanistically distinct from all three — a non-biodegradable hydrogel that integrates into the synovial membrane and provides physical cushioning rather than any biological effect, with published data supporting multi-year duration from a single injection.

Patient factors matter just as much as mechanism: disease severity, whether pain is an acute flare or chronic background ache, insurance coverage, budget, and willingness to repeat injections all shape which agent fits best.

One important caveat before each is examined in detail: no published randomised controlled trial has compared all four agents head-to-head in a single study. The evidence base, though growing, remains incomplete.

Corticosteroids: fastest relief, shortest window

Speed is corticosteroid's defining advantage. Within 24–72 hours of injection — faster than any other agent in this group — most patients notice a meaningful drop in pain, making it the appropriate first choice when an acutely inflamed knee is the clinical picture: warm, swollen, and difficult to weight-bear at short notice.

That speed comes with an honest caveat about duration. Relief typically lasts 6–12 weeks rather than months, and a response at that length is a normal outcome rather than a sign the treatment has failed. For a patient managing a short-term flare before a planned event, or who needs to engage with physiotherapy that pain has been blocking, six good weeks can be clinically worthwhile. Where corticosteroid is less well matched is as a long-term strategy for the chronic, background ache that characterises most knee OA: the symptom curve tends to return to baseline, and the next flare may not be far behind.

The structural concern with corticosteroids deserves a measured reading of the evidence. Studies linking repeated intra-articular injections to accelerated cartilage loss are the primary source of caution in the literature; the risk from a single, appropriately timed injection is less clearly established. The clinical consensus is that occasional use in the right context remains reasonable, but that multiple injections over a short interval warrants careful shared decision-making.

Systemic absorption is also clinically real. Triamcinolone — a commonly used corticosteroid preparation — has been associated with elevation of intraocular pressure following intra-articular injection, an effect not observed with hyaluronic acid. For patients with glaucoma or poorly controlled diabetes, this is a relevant factor to raise with a clinician before proceeding.

Hyaluronic acid: moderate gains with mixed guideline support

Hyaluronic acid has a more nuanced biological story than its reputation as a 'joint lubricant' suggests. Injected into the knee, it acts on at least two levels: restoring the viscosity of synovial fluid, which becomes thinner and less effective as a shock absorber in OA, and exerting anti-inflammatory effects on the joint environment. Framing it as simple lubrication undersells its mechanism, though that shorthand persists widely in patient-facing materials.

Onset is slower than corticosteroid — benefit typically builds over several weeks rather than days — with duration in responsive patients of around six months.

Where the evidence becomes genuinely contested is in the question of how many injections are needed. A systematic review of 11 studies comparing single versus multiple injection regimens found no consistent advantage to multi-dose courses: five-injection schedules were not consistently superior to three-injection ones. However, a separate meta-analysis reached a more granular finding — two-to-four and five-or-more injection regimens each produced pain relief over saline, while single injection alone did not reach statistical superiority. The honest clinical summary is that this debate is unresolved, and neither framing — 'HA works' or 'HA doesn't work' — fully reflects the literature.

Those conflicting results have fed divergent guideline positions. Some European rheumatology bodies endorse HA as an adjunct treatment; others, including certain iterations of NICE guidance and the American Academy of Orthopaedic Surgeons, have issued neutral or cautionary positions at various points. Country-level reimbursement decisions often influence which guideline stance is most visible to patients and clinicians.

A growing number of practitioners now use higher-molecular-weight or cross-linked — sometimes called reticulated — single-injection formulations. These are engineered to remain in the joint longer than standard preparations, reducing repeat visit burden without changing the underlying agent.

For many UK patients, one practical point carries real weight: HA viscosupplementation is frequently covered by private health insurance, making it accessible where PRP or polyacrylamide hydrogel — rarely if ever reimbursed — would require full out-of-pocket expenditure. That access difference is not a clinical argument for HA, but it is a genuine factor in the shared decision-making conversation.

PRP: a biologic option with a heterogeneity problem

Few injection treatments in orthopaedics carry as much conflicting baggage as PRP, and for patients who have read around the subject, the contradictory headlines are genuinely confusing. Both the optimism and the scepticism have evidence behind them — and understanding why requires looking at what 'PRP' actually means in practice.

The most structured recent evidence sits on the positive side. A 2025 meta-analysis of 12 RCTs comparing leukocyte-poor PRP (LP-PRP) with hyaluronic acid found LP-PRP outperformed HA on WOMAC total score, physical function, and VAS pain at 3, 6, and 12 months, with no significant difference in adverse event rates between the two groups. A broader 42-study 2025 meta-analysis placed PRP above both HA and corticosteroid for medium-term pain relief. Those are reasonably consistent signals, not easily dismissed.

The counterweight is the RESTORE randomised controlled trial, published in JAMA in 2021. In 288 patients with KL grade 2–3 knee OA, three weekly LP-PRP injections reduced pain by 2.1 points on an 11-point scale versus 1.8 points for saline placebo at 12 months — a difference that did not meet the threshold for minimum clinically important difference. MRI showed no significant benefit to medial tibial cartilage volume. This was a well-conducted trial in a comparable population to those in positive meta-analyses, which makes the finding hard to set aside.

The most likely explanation for the gap between RESTORE and the meta-analysis literature is that 'PRP' is not a single standardised product. Leukocyte content — specifically the distinction between leukocyte-poor (LP) and leukocyte-rich (LR) formulations — platelet concentration, and preparation protocol vary considerably between studies and between clinics. Outcomes from one protocol cannot reliably be extrapolated to another, which is why trials that group all PRP preparations together often produce noisy results.

In clinical practice, the realistic expectation is that benefit, when it occurs, lasts roughly up to 12 months — with some patients seeing less and others somewhat more. Repeat injections every 6–12 months are common and should be factored into planning from the outset, rather than treated as evidence of treatment failure. For patients with earlier-stage disease seeking a biologic option short of surgery, LP-PRP prepared under consistent protocols represents a reasonable pathway — but the evidence base is still resolving, and individual responses vary.

Polyacrylamide hydrogel: the longest-acting single-injection option

Unlike every other agent discussed so far, polyacrylamide hydrogel works through physical integration rather than biological or chemical activity. The product — a 2.5% non-resorbable, non-biodegradable hydrogel — is injected into the knee joint, where it integrates into the synovial membrane and creates a stable cushioning layer. It is not metabolised, does not degrade over time, and does not attempt to regenerate cartilage or modulate inflammation. That mechanical distinction matters for setting expectations: this is not a regenerative treatment in the tissue-repair sense.

Durability: comparative cohort findings

A 150-patient retrospective cohort (KL grade II–IV) comparing iPAAG, HA, and corticosteroid found all three groups reduced VAS pain scores from approximately 7 to 3 at three months — equivalent early performance across agents. The separation emerged over time: HA and corticosteroid scores had returned to near-baseline by 12 months, while iPAAG patients maintained their improvement. That directional finding is clinically meaningful, but the 12-month inter-group difference did not reach statistical significance — an important qualifier before drawing firm conclusions.

Published cohort data support an average durability of 2–3 years from a single injection, referenced in studies including a 52-week prospective dataset and Bliddal et al.'s 12-month open-label follow-up (2024). Company-reported data suggest benefit may extend to 4–5 years in some patients; independent long-term RCT evidence beyond two years remains limited.

Patient selection

A 24-month PROMs cohort of 314 knees identified four factors associated with reaching a meaningful clinical improvement: older age, lower Kellgren–Lawrence grade, absence of diabetes, and bilateral rather than unilateral OA. Patients with higher KL grades were more likely to progress to total knee replacement during the follow-up period — reinforcing that iPAAG is better positioned for mild-to-moderate disease than for end-stage OA.

Cost and access

Arthrosamid is not available on the NHS and is not routinely covered by major UK private insurers. Private treatment at specialist centres starts from approximately £3,000, typically encompassing consultation, ultrasound guidance, the product, and a follow-up appointment. For many patients, that figure is the deciding factor — not a clinical limitation of the treatment, but a real-world constraint that belongs in any honest comparison.

Matching the injection to your situation

Choosing between these agents is less a matter of ranking than of matching each one's strengths to the specific problem at hand.

If the primary issue is an acute flare — a joint that is swollen, warm, and has become suddenly limiting — corticosteroid remains the most appropriate starting point, with relief measured in weeks rather than months and no expectation of long-term disease modification.

If symptoms are persistent but moderate, and insurance coverage or access via the NHS is a practical consideration, HA provides a reasonable six-month window with a well-established safety record, particularly in single high-molecular-weight formulations.

For patients with earlier-stage disease seeking a biologic mechanism, LP-PRP is a reasonable pathway — provided they understand that outcomes depend heavily on the preparation protocol used and that repeat injections every six to twelve months are the norm rather than an exception.

iPAAG sits at the longer end of the duration spectrum. For patients with confirmed mild-to-moderate OA who are not yet at the surgical threshold and can meet the out-of-pocket cost, its durability profile is the defining advantage — though data beyond two years remain largely observational.

Three specific evidence gaps make individual assessment essential rather than optional: iPAAG's durability past two years is still being quantified in prospective studies; PRP outcomes remain protocol-dependent in ways that population-level meta-analyses cannot fully resolve; and no trial has placed all four agents in direct competition within the same patient population simultaneously. Where those gaps intersect with a person's disease stage, lifestyle, and priorities, published averages cannot determine the right choice — a clinical assessment can.

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] Intraarticular leukocyte-poor platelet-rich plasma injection is more effective than intraarticular hyaluronic acid injection in the treatment of knee osteoarthritis: a systematic review and meta-analysis of 12 RCTs. (2025). https://doi.org/10.1186/s43019-025-00266-5 https://doi.org/10.1186/s43019-025-00266-5
3. [3] Effect of Intra-articular Platelet-Rich Plasma vs Placebo Injection on Pain and Medial Tibial Cartilage Volume in Patients With Knee OA: The RESTORE Randomized Clinical Trial. (2021). https://doi.org/10.1001/jama.2021.19415 https://doi.org/10.1001/jama.2021.19415