Why single-injection format matters in knee OA

For many people with knee OA, a single injection visit enters the conversation once physiotherapy and simple analgesics have reached their limits but surgery still feels premature. At that decision point, the practical question is rarely "should I have an injection?" — it is "which one, and why does it matter?"

The answer turns on mechanism. Four mechanistic categories sit within the single-injection format: corticosteroid (anti-inflammatory), hyaluronic acid or HA (viscoelastic support), platelet-rich plasma or PRP (biological), and polyacrylamide hydrogel or PAAG (physical and structural). A fifth option, Cingal, combines HA with a corticosteroid in a single product; it is covered alongside HA rather than as a separate mechanistic family. Reduced injection frequency matters beyond convenience: it also affects cumulative exposure risk, clinic capacity, and patient adherence over the years a person lives with OA.

These five options do not carry equal evidence. No single randomised controlled trial has placed all four mechanistic families in direct head-to-head comparison in single-injection format, and the depth of the evidence base ranges from decades of guideline-supported use to early observational series. The sections that follow examine each category on its own merits, based on what the evidence actually shows.

Corticosteroids — fast relief, finite window

Corticosteroids — triamcinolone, betamethasone, and methylprednisolone among them — have the longest clinical record of any intra-articular treatment for knee OA. Their mechanism is pharmacological: injected into the joint space, a corticosteroid dampens the inflammatory response, reducing pain and swelling as the drug is gradually metabolised. Relief typically spans weeks to a few months, after which the effect tapers as the medication clears. No systematic review has identified one formulation as consistently superior to the others.

Because of this long track record, corticosteroids carry stronger guideline recognition than any other injection category. That familiarity makes them a common first choice when a patient has a defined, short-term goal — managing a flare before a holiday, reducing pain ahead of a rehabilitation programme, or buying time during a period of higher-than-usual demand on the joint.

The critical clinical consideration is cumulative use. Repeated injections into the same joint raise documented concerns around cartilage and tendon integrity, and this constrains their role in long-term OA management. The clinical consensus is not that corticosteroids should be avoided, but that their frequency requires careful judgement rather than routine top-up. It is also worth stating clearly that a single corticosteroid injection is not a disease-modifying intervention: it addresses symptoms rather than the underlying structural changes of OA, and there is no evidence that it slows disease progression.

Hyaluronic acid — what the single-dose evidence actually shows

Hyaluronic acid works differently from a corticosteroid. Rather than suppressing inflammation pharmacologically, it aims to restore the viscoelastic properties of synovial fluid — acting as a mechanical lubricant and shock absorber within the joint. This distinction matters when interpreting the evidence, because HA's benefits are not expected to appear immediately; the mechanism depends on sustained presence in the joint space.

The evidence base for single-injection HA is genuinely contested, and presenting it honestly means acknowledging three findings that do not fully align. One meta-analysis — Bannuru et al.'s 2015 network analysis — found that only multi-dose regimens of two to four or five or more injections produced pain relief significantly superior to intra-articular saline; the single-injection subset did not reach significance. Set against that, a 2019 meta-analysis focused specifically on single-injection HA products (Vincent P., Curr Ther Res Clin Exp, 2019) found clinical benefit for symptomatic knee OA, supporting single-dose HA as an evidence-backed option in its own right. A third source of evidence — a systematic review of 11 trials comparing single- with multiple-injection formulations — found no consistent difference in patient-reported outcomes between the two approaches, and five-injection regimens were not found superior to three-injection ones. These three positions cannot be fully reconciled. The practical implication is that blanket conclusions about HA as a category are less reliable than evidence cited at the product level.

Cross-linked single-injection products

The products most commonly used in a single-shot format — Durolane, Monovisc, Synvisc-One, and Ostenil Plus among them — are reticulated or cross-linked HA derivatives. Cross-linking increases molecular weight and resistance to enzymatic breakdown, extending joint residence time compared with standard HA. This is the pharmacological rationale for the single-injection reformulation: a higher-viscosity molecule that remains in the joint longer is intended to compensate for the reduced injection frequency.

Cingal — combined mechanism in one visit

Cingal occupies a distinct position in this category. It is a single-injection product combining hyaluronic acid with triamcinolone acetonide, the corticosteroid discussed in the previous section, aiming to deliver both mechanical lubrication and acute anti-inflammatory effect in one visit. The theoretical rationale is that the corticosteroid component addresses early inflammation while HA provides sustained joint support. Direct head-to-head trial data comparing Cingal with standalone HA or standalone corticosteroid is limited, so it is best understood as a clinically plausible hybrid rather than a format with a fully independent evidence dossier.

PRP — what the growing evidence base does and does not resolve

PRP sits apart from corticosteroids and hyaluronic acid in a fundamental way: it is not a pharmaceutical compound or a synthetic material but a concentrate derived from the patient's own blood, enriched in platelets and the growth factors they carry. The biological rationale is that delivering these factors directly into the joint may modulate the local inflammatory environment and support repair at a tissue level — though the precise mechanism remains an area of active investigation.

On efficacy, the meta-analytic picture is more consistent here than for single-injection HA. Multiple analyses converge on PRP being superior to placebo for pain relief, with reported benefits reaching 12 months; some studies have shown sustained improvement beyond 24 months, after which effects appear to taper gradually.

The principal limitation is one of standardisation. No universal PRP preparation protocol exists: platelet concentration, leucocyte content, activation method, and injection volume differ between studies, between products, and between clinics. PRP administered in one setting may differ substantially from that used in a published trial, which limits the reliability of cross-study comparison and makes it difficult to translate published outcomes directly into clinical practice.

Guideline bodies have arrived at different conclusions from the same evidence pool. AAOS and ACR have declined to recommend PRP for knee OA, citing the quality of available evidence. OARSI has endorsed it. This divergence does not reflect a fault in the evidence so much as a mismatch of timing: an active evidence base that is accumulating faster than the slower cycle of formal guideline revision can accommodate.

The placebo caveat applies here with particular force. Published data suggest the placebo response in PRP trials is larger than in many other intra-articular injection categories — a finding that does not invalidate positive trial results but does mean that uncontrolled or open-label studies should be interpreted with specific caution. Meta-analytic convergence on PRP superiority over placebo accounts for this concern rather than ignoring it, which is what gives that finding its relative weight.

Arthrosamid — a physically acting option with observational data

Arthrosamid (iPAAG — injectable polyacrylamide hydrogel) is unlike everything else in this comparison. It is not a drug, not a biologic, and not a substance the body absorbs over time. A non-absorbable hydrogel, it integrates into the synovial membrane after a single injection and acts mechanically — providing cushioning and lubrication through its physical presence in the joint rather than through any pharmacological or biological pathway. Because it does not degrade, the duration question takes on a different character than it does for corticosteroids or HA: the material is designed to remain, not to be metabolised away.

The evidence base is observational rather than trial-controlled. The most substantial available data comes from a 24-month cohort study of 269 patients across 314 knees, published in the Journal of Clinical Orthopaedics and Trauma, which tracked patient-reported outcomes without a randomised control group. Open-label prospective studies at six months (Bliddal et al., J Orthop Res Ther, 2021) and 12 months (Bliddal et al., J Orthop Surg Res, 2024) add further support, both reporting sustained symptomatic improvement over their respective follow-up periods. The absence of a control arm across this evidence base is a genuine constraint: without randomisation, treatment effect cannot be cleanly separated from placebo response or natural variation in symptoms over time. No published randomised controlled trial data for Arthrosamid in knee OA currently exists.

None of this positions Arthrosamid as unproven in a pejorative sense — a 24-month observational cohort with consistent patient-reported outcomes is a meaningful evidence foundation for a newer product category. It is simply a different tier from RCT data, and that distinction is the appropriate lens through which to evaluate it. Arthrosamid has not been shown to delay surgical intervention or slow the underlying progression of OA.

Choosing between options — what the evidence gap means in practice

No single published trial places all four treatment categories in direct head-to-head comparison in a single-injection format. Bannuru et al.'s 2015 network meta-analysis compared pharmacological interventions broadly but predates PAAG's clinical use — meaning the most mechanistically distinct option in this group has never been formally ranked against the others in a controlled setting. That gap is not a reason to suspend clinical decision-making; it is a reason to hold any ranking loosely.

Patient goal and mechanism fit do more practical work than evidence hierarchies alone. Managing recurrent inflammatory flares with a near-term functional objective — a return to sport, a planned procedure — is a different clinical conversation from seeking sustained relief over 12 months or more. Corticosteroid's pharmacological window fits the first; the PRP meta-analytic data, converging on 12-month outcomes, better informs the second. Where repeated corticosteroid use raises cumulative concerns and a physical mechanism is preferred, Arthrosamid's observational profile enters the discussion. Cross-linked single-injection HA products sit across a broad middle ground — suited to patients who want a single-visit approach without committing to a biologic or a non-absorbable material.

Guideline positions on all of these options continue to evolve as evidence accumulates — no single body's current stance should be treated as the final word. The appropriate response is assessment by a clinician current with that literature, one who can weigh OA severity, prior treatment history, and individual goals together. An injection is one component of a broader management plan; the clinical conversation that precedes it is as important as the intervention itself.