Why a single injection — and who it actually suits

For many people with knee osteoarthritis, the prospect of returning to a clinic three, four, or five times for a course of injections is a genuine barrier — to treatment, to work, and to getting on with life. The reasonable question, then, is whether a single visit can achieve something meaningful. In most cases of mild-to-moderate OA, the answer is yes.

Three genuinely different product classes now carry the 'single injection' label, and the differences between them matter clinically. Cross-linked hyaluronic acid viscosupplements restore lubrication to the joint. Injectable polyacrylamide hydrogel takes a structural approach, integrating with the synovial tissue rather than simply supplementing it. Platelet-rich plasma (PRP) draws on the patient's own blood to deliver growth factors aimed at tissue-level repair. Same visit, very different mechanisms.

None of these options is appropriate for advanced, bone-on-bone disease — a point worth stating plainly before any comparison. But for patients at an earlier stage, the choice between categories is not simply a matter of preference. OA severity, activity level, and prior treatment history all bear on which approach is most likely to help. The sections below map each category in turn.

Hyaluronic acid: the most established single-dose option

Cross-linked hyaluronic acid viscosupplements have the longest clinical track record of any single-injection category, with three products dominating the UK and US markets: Durolane®, Monovisc®, and Synvisc-One®. All are FDA-cleared and CE-marked; all work by replenishing the synovial fluid's natural lubricating and shock-absorbing properties, which decline as OA progresses. Peak effect typically arrives at four to six weeks post-injection, with pain and functional benefits generally persisting for up to six months.

Product-level differences are modest but worth noting for certain patients. Synvisc-One is derived from avian cartilage and carries a marginally higher risk of local allergic reactions; Durolane and Monovisc are both manufactured via bacterial fermentation, removing that concern. Durolane uses NASHA technology to achieve a high HA concentration; Monovisc is distinguished by its high molecular weight. Neither difference translates into a consistent head-to-head outcome advantage — direct comparative RCTs between individual branded products remain sparse.

The broader evidence picture is genuinely mixed, and patients deserve an honest account of it. A 2019 systematic review by McElheny et al. (11 trials) found no consistent difference in patient-reported outcomes between single-dose and multi-dose HA regimens, suggesting that multiple clinic visits do not reliably add benefit. An earlier meta-analysis, however, found that 2–4 and ≥5 injection courses did outperform intra-articular saline, while the single-injection arm did not reach statistical significance — a gap that remains unresolved in the literature. These findings are not cause to dismiss the category; they are context for realistic expectations.

More recently, a 2024 multicentre retrospective study (Tarantino et al.) confirmed that a single HA injection significantly reduces pain and improves joint function at four weeks. Improvements in isokinetic muscle strength were not statistically significant at that timepoint. Both the AAOS and ACR characterise the HA evidence base as modest overall — a position that reflects the heterogeneity of available trials rather than proof of inefficacy.

Arthrosamid: a synthetic hydrogel that works differently

Arthrosamid® belongs to a different product class altogether — not a lubricant, but a structural implant. Delivered as a single 6 ml injection under ultrasound guidance, it consists of 2.5% cross-linked polyacrylamide suspended in 97.5% water. Once inside the joint, this non-biodegradable gel integrates with the synovial membrane rather than sitting within the joint fluid. The practical effect is a form of cushioning from within the joint lining itself — a mechanism that is distinct from anything HA viscosupplementation does.

Because it does not degrade, the duration of effect is potentially longer than the six-month window typically associated with HA. Observational data suggest benefits may extend to three to five years, and the product is currently widely available in the UK and Europe, though it does not hold FDA approval in the United States.

The strongest controlled evidence comes from a 52-week open-label study by Bliddal et al. (2024), which followed 49 patients with a mean age of 70. A single iPAAG injection produced a clinically meaningful WOMAC pain reduction of 17.7 points (95% CI −23.1 to −12.4; p<0.0001) at one year, with 62.2% of participants meeting the OMERACT-OARSI responder threshold. Stiffness, physical function, and patient global assessment all showed sustained improvement across the same period.

The evidence base, while promising, currently rests on open-label data without a placebo-controlled arm — a genuinely important limitation. The absence of a sham-injection comparator means the contribution of expectation or natural disease fluctuation cannot be fully separated from treatment effect. Larger randomised trials are needed before the full benefit profile can be confirmed. For patients in the UK where the product is accessible, it represents a distinct option with a different mechanism and a potentially longer time horizon than HA — not a like-for-like comparison, but a different clinical proposition.

PRP: the single-injection orthobiologic

Platelet-rich plasma takes a fundamentally different biological approach to the two options covered above. A small volume of the patient's own blood is drawn, centrifuged to concentrate the platelets, and the resulting preparation injected directly into the knee — making PRP the only autologous option in this comparison. The aim is not lubrication or structural cushioning but biological: concentrated platelets release growth factors that may support tissue repair and help modulate the inflammatory environment within an arthritic joint.

Published reports suggest duration of benefit ranging from six to twelve months or beyond in suitable patients, which — when effective — positions PRP as the longest-acting of the three single-injection categories. Evidence tends to be strongest in early-to-moderate disease; in advanced OA, results are weaker and less consistent across studies.

The most clinically important caveat with PRP is that it is not a standardised product. Preparation protocols differ substantially between centres — in platelet concentration, in leukocyte content (leukocyte-rich versus leukocyte-poor preparations behave differently in an inflamed joint), in activation method, and in injection volume. These variables affect what is actually delivered, which means individual PRP studies are difficult to pool and harder still to compare against HA or iPAAG data. A patient asking whether a single PRP injection outperforms a single HA injection cannot receive a definitive evidence-based answer at present — not because the evidence is uniformly negative, but because trials frequently involve materially different preparations. When choosing a centre, it is worth asking how the PRP is prepared and what protocol the clinician follows — the answer matters clinically, not just administratively.

How the three categories compare on duration, evidence, and fit

No head-to-head randomised trial has yet compared a single HA injection against iPAAG or PRP in the same cohort, which means any comparison across the three categories rests on indirect evidence. What the side-by-side does clarify, however, is that the trade-offs differ in ways that are clinically meaningful — and the 'best' option is less a question of which product scores highest than which set of trade-offs fits a particular patient's situation.

Duration and reversibility are the sharpest differentiators. HA provides symptom relief on a roughly six-month cycle, which means repeat injections are feasible if the first works well. PRP may extend that window, sometimes beyond a year, though this varies by preparation and individual response. Arthrosamid sits apart: the gel does not degrade, and observational data suggest effects lasting three to five years. The flip side is that permanence is a one-way door — if a patient tolerates the implant poorly, it cannot be removed. For HA or PRP, ceasing treatment simply means waiting for the effect to wear off.

Evidence architecture differs as much as mechanism. HA carries the largest randomised trial base of the three, though — as the 2019 systematic review of 11 trials showed — single-injection data specifically remain contested. iPAAG evidence, while consistent in direction, is currently confined to uncontrolled studies; the absence of a placebo arm is a structural limit, not a minor footnote. PRP randomised data exist but cannot easily be pooled across studies because preparation protocols vary so substantially between centres.

Access and cost introduce a practical filter before clinical fit is even considered. Single-dose HA products are widely available and carried by some insurers; Arthrosamid is a specialist-only procedure in the UK; PRP pricing and availability vary by centre.

For patients in the mild-to-moderate range — where all three options apply — the most useful questions are: how long do you want a single treatment to last, how much does the uncertainty in the evidence base matter to you, and what is practically accessible? A specialist assessment remains the appropriate route for weighing those factors against individual joint status.

Getting the right assessment before committing to an injection

Assessment starts with joint status — specifically, how far OA has progressed. X-ray remains the primary grading tool: bone-on-bone contact significantly reduces the likelihood of benefit from any of the three injection categories discussed here, and that finding shifts the conversation towards joint replacement planning rather than symptom management.

Where the X-ray picture is ambiguous, or where a concurrent structural problem (such as a contributing meniscal issue) may be involved, MRI adds the soft-tissue detail needed to complete the picture. Neither image alone constitutes a treatment decision — that requires clinical context: symptom duration, functional limitations, prior treatments tried, and any contraindications such as active joint infection, coagulopathy, or a known sensitivity to the intended product.

Technique matters too. Arthrosamid requires ultrasound guidance as standard; accurate intra-articular placement of a non-degradable implant is not optional. The same principle applies to HA and PRP — ultrasound guidance is considered best practice because product that misses the joint space cannot work, regardless of which category is chosen.

The AMSK suitability assessment is built around this sequence: joint status and imaging first, clinical fit second, product selection third. It is a logical starting point for anyone who has not yet had a structured specialist review — and a useful checkpoint for those who have had one but are uncertain which option, if any, is appropriate for their stage of disease. What should drive that decision is what is actually happening in the joint.