Three injections, one question: what are you actually choosing between?

If a knee specialist has suggested an injection — and specifically a single one — the practical question is rarely 'should I have one?' but 'which one, and why does it matter?'

Three mechanistically distinct options share the single-injection format: a corticosteroid (CSI), a hyaluronic acid (HA) viscosupplementation product such as Monovisc, and the newer polyacrylamide hydrogel sold as Arthrosamid®. They work in entirely different ways, their effects last for markedly different lengths of time, and the evidence base behind each one varies considerably in design and robustness.

The format — one clinic visit, one injection — is the same, but that is roughly where the similarities end. How strongly each treatment targets inflammation versus mechanical wear, how long any benefit is likely to persist, and how each sits within NHS access and private funding all differ enough to make the choice genuinely consequential.

The sections that follow examine each option in turn, then address the questions most patients at this stage find hardest: how long relief is likely to last, and how confident the evidence allows us to be in that estimate. Those two considerations — durability and evidential confidence — turn out to be the most useful lens for comparing three treatments that, on the surface, look reassuringly similar.

How each injection works — and why mechanism matters

Each of these three injections enters the same space inside the knee, yet once inside, they do entirely different things — and that difference shapes everything from onset of relief to how long any benefit is likely to last.

Corticosteroid (CSI) is an anti-inflammatory agent. It works by suppressing the inflammatory cascade within the joint, reducing swelling, heat, and pain in a matter of days. Its effect is real and rapid, but it targets the symptom of inflammation rather than the underlying joint environment; once the drug is cleared, the stimulus for inflammation remains unchanged.

Hyaluronic acid (HA) works on a different principle: viscosupplementation. Healthy synovial fluid is rich in HA and provides the joint with its natural lubrication and shock absorption. In OA this fluid becomes thinner and less viscous; HA injection supplements what the joint has lost, restoring mechanical function rather than suppressing a biochemical signal.

Polyacrylamide hydrogel (iPAAG/Arthrosamid®) is categorically different from both. Rather than dispersing into the joint space, it integrates with the synovial capsule lining as a permanent, non-biodegradable scaffold. The proposed mechanism centres on a reduction in synovitis — the chronic inflammation of the synovial tissue strongly linked to OA pain — alongside mechanical cushioning. Because it is non-pharmacological and permanent, it is regulated as a CE-marked medical device rather than a drug, which has direct implications for NHS funding and future treatment planning.

These mechanistic distinctions matter clinically: different dominant pain drivers call for different targets. Here is what the evidence actually shows about how each performs in practice.

What the clinical evidence shows for each option

Evidence quality varies considerably across the three options — and that difference is as clinically important as the results themselves.

Corticosteroid injection

The longest clinical track record here is supported by RCTs and meta-analyses. Deyle et al. (NEJM, 2020) found that physical therapy outperformed glucocorticoid injection at one year in a direct comparative RCT, though corticosteroid produced faster short-term relief in the first weeks. Effect typically fades within three months. Concerns exist that repeated courses may carry chondrotoxicity risk and could complicate total knee replacement if one becomes necessary later.

Hyaluronic acid

A 2019 meta-analysis by Vincent P (Curr Ther Res Clin Exp) evaluated single-injection HA products specifically and found meaningful symptomatic efficacy — confirming that a one-visit course has a justifiable evidence base. Bannuru et al.'s network meta-analysis (Ann Intern Med, 2015) placed HA above placebo for pain, though the clinical significance of that margin remains contested. A further systematic review (Concoff et al., BMC Musculoskelet Disord, 2017) found no clear advantage for multi-injection courses over a single injection. Guideline bodies remain split: OARSI is more supportive, while AAOS and ACR are more reserved.

Polyacrylamide hydrogel (iPAAG)

The evidence here is newer and less mature in study design. Bliddal et al.'s 6-month prospective study (2021) and a 12-month open-label follow-up (J Orthop Surg Res, 2024) both reported sustained effectiveness and safety. A UK case series (Maulana, Cole, Lee, 2022) noted a reduction in patellofemoral bone marrow lesions after a single iPAAG injection — a possible signal of structural benefit, though controlled trial confirmation is still needed. iPAAG's current evidence base is observational and open-label, placing its evidence maturity below that of CSI and HA.

The placebo context

Intra-articular injection itself relieves knee OA symptoms meaningfully, independent of the active agent — confirmed in meta-analyses of sham-injection arms (Previtali et al., Cartilage, 2020; Qvistgaard et al., Osteoarthr Cartil, 2006). This does not invalidate any of the three treatments, but it is a reason to interpret effect sizes from uncontrolled cohorts with appropriate caution.

How long each injection lasts — the key differentiating axis

For most patients weighing these options, the most practical question is simply: how often would I need to come back?

Corticosteroid offers the shortest window. Relief typically arrives within days but tends to fade within weeks to around three months. It is best suited as a short-term bridge — calming an acute inflammatory flare, enabling rehabilitation to begin, or preparing a joint ahead of surgery.

Hyaluronic acid provides intermediate durability. Meta-analysis evidence, including Vincent P's 2019 review of single-injection products and Bannuru et al.'s network analysis (Ann Intern Med, 2015), supports around three to six months of symptomatic benefit for most patients. Some data suggest that higher-molecular-weight single-shot formulations may extend that window towards twelve months in a proportion of cases.

iPAAG (Arthrosamid®) carries the longest claimed duration. Open-label follow-up data from Bliddal et al. (J Orthop Surg Res, 2024) indicate sustained effectiveness at twelve months, with preliminary findings pointing towards benefit lasting up to three years. The proposed explanation is the hydrogel's permanent, non-biodegradable structure: unlike CSI or HA, it is not cleared by the body over time.

One important caveat applies to this entire comparison. CSI and HA durability figures are drawn from controlled trials with placebo arms; iPAAG's longer-duration data come from open-label cohorts without a comparator group. Until head-to-head RCT evidence emerges, the ranking remains tentative — the true durability advantage of iPAAG may prove larger, smaller, or different in character from what observational data currently suggest.

Guideline positions, access, and the cost gap

Access and cost differ sharply across the three options — and for UK patients, those practical realities can be as decisive as the clinical evidence.

Corticosteroid injection sits firmly within NHS care. A single course for a suitable patient carries no guideline controversy, and it is routinely offered in primary and secondary care settings. Repeated long-term courses attract more caution in clinical guidelines, given concerns about potential chondrotoxicity, but a one-off injection is uncontroversial.

Hyaluronic acid occupies more contested ground. OARSI guidelines support HA for knee OA; AAOS and ACR are more reserved and do not endorse it as standard of care. NICE in the UK has historically not recommended routine HA on the NHS, which means most patients accessing it in the UK will do so privately. Different professional bodies are weighing the same body of evidence and reaching different conclusions — a live debate rather than a settled one.

iPAAG (Arthrosamid®) is a CE-marked medical device, not a licensed drug, and is not NHS-funded. Private access costs from approximately £3,000 per joint in the UK; major insurers, including Bupa and AXA, do not currently cover it. No major clinical guideline endorses iPAAG as standard of care — it sits in the emerging-evidence category.

No published cost-effectiveness analysis currently compares all three options over a two-to-three-year horizon — a genuine gap for patients trying to weigh total cost against anticipated benefit, particularly when factoring in the likelihood of repeat injections.

Which injection suits which patient — and what to ask at assessment

Matching the right injection to the right patient depends less on which option sounds most advanced and more on what the joint is actually doing — and what the patient needs from treatment over the next six to thirty-six months.

When corticosteroid makes most sense. A presentation dominated by warmth, swelling, and rapidly worsening pain — hallmarks of an acute inflammatory flare — is the clearest case for CSI. Its onset is fast and its anti-inflammatory effect is direct. It is also the most practical choice when a short-term bridge is the goal: settling symptoms enough to engage meaningfully with physiotherapy, or reducing intra-articular inflammation ahead of surgery.

When single-injection HA is a reasonable starting point. Patients with mechanical, activity-related pain — stiffness after rest, discomfort on stairs, no particular joint warmth — and without a strong inflammatory picture are closer to the HA profile. A three-to-six month relief window is realistic, and for patients where cost or NHS access is a constraint, single-injection products offer a supported, lower-barrier option.

When iPAAG enters the conversation. Persistent OA pain that has not responded adequately to corticosteroid or HA, particularly where synovitis is suspected as a pain driver, is the scenario most clinicians point to when discussing Arthrosamid®. The longer claimed duration may appeal to patients who want to reduce the frequency of intervention. Expectations should, however, be grounded in the current evidence standard: the durability data come from open-label cohorts rather than placebo-controlled RCTs.

Questions worth raising at consultation

• Based on my imaging and examination, is there active synovitis — and does that change which injection is most appropriate?
• If I have had a corticosteroid or HA injection before, what does the duration of my previous benefit tell us about which pathway to try next?
• What is the plan if this injection provides inadequate relief — and at what point would a different option or a different approach be considered?
• How should I structure physiotherapy or exercise alongside whichever injection we choose?

That final question matters regardless of which option is selected. None of the three injections has been shown to perform optimally as a standalone treatment; the evidence base for all of them sits alongside — not instead of — supervised exercise and load management. An injection that buys three months of reduced pain is most valuable when those three months are used to build the conditioning that sustains improvement beyond them.

1. [1] Joint injection — Wikipedia. https://en.wikipedia.org/?curid=1425459 https://en.wikipedia.org/?curid=1425459