When a single injection makes clinical sense

For many patients managing knee osteoarthritis, the appeal of a single injection goes beyond convenience. Clinicians may recommend a once-only formulation when the goal is to reduce procedural burden, limit cumulative tissue exposure, or when the agent itself is specifically engineered to deliver sustained effect from a single dose — rather than relying on repeated administration to build efficacy.

Four distinct categories now sit within this 'once-only' landscape, each with a different mechanism and evidence base:

• Single-dose hyaluronic acid (HA) — a viscosupplementation option, with cross-linked high-molecular-weight formulations (such as Monovisc) designed for longer joint residence than standard HA courses.
• Polyacrylamide hydrogel (iPAAG / Arthrosamid) — a permanently integrated scaffold injected once to cushion and support the joint lining.
• Extended-release corticosteroid (Zilretta) — a microsphere-based triamcinolone formulation providing structured, time-limited pain relief from a single administration.
• Single-dose PRP — a biologically derived option drawing on the patient's own platelets to support joint environment over months.

No randomised controlled trial has yet directly compared all four head-to-head, so what follows is a landscape of separate evidence streams rather than a clear league table. Which option is realistic depends on OA severity, individual health factors, and whether NHS or private access is in play.

Single-dose hyaluronic acid: where the debate sits

The core clinical question with single-dose HA is straightforward: does one injection work as well as three? The short answer, based on current evidence, is that it depends on the formulation — and the evidence base is genuinely mixed.

On efficacy versus placebo, the picture is reasonably clear. A network meta-analysis of 43 RCTs involving 5,554 patients confirmed that intra-articular HA significantly outperforms placebo for pain reduction (SMD 0.94), and a systematic review of 38 RCTs found adequate pain relief and functional improvement maintained for up to six months in most patients.

The once-only question is more contested. A direct comparison found that a single injection of cross-linked, high-molecular-weight (HMW) HA was non-inferior to three weekly low-molecular-weight (LMW) injections for pain and function at two and six months — though LMW formulations showed better stiffness outcomes at two months (Bahrami et al.). Against that, an earlier meta-analysis found that two-to-four and five-or-more injection regimens achieved significant pain relief over saline, while single-injection HA did not reach statistical significance at three to six months. A separate systematic review of 11 studies found no consistent difference in patient-reported outcomes between single and multiple regimens — illustrating how unsettled this area remains.

Regardless of formulation, two 2025 systematic reviews agree that HA's effects rarely extend meaningfully beyond six months. In the UK, Monovisc — a cross-linked, single-injection HMW product — is the most widely available once-only HA option, and is typically considered after first-line conservative management has not provided sufficient relief. Cross-linked formulations are designed for longer joint residence, which is a plausible mechanism for narrowing the gap with multi-dose courses, but guideline bodies have not yet reached consensus on whether this translates into a clinically significant difference.

Polyacrylamide hydrogel: the durability case

Unlike lubricants or biologics, polyacrylamide hydrogel (iPAAG, marketed as Arthrosamid) is a structural scaffold. Once injected, it integrates into the synovial tissue rather than dispersing within the joint fluid — a mechanism that may explain why its durability data stand apart from other once-only options.

The most detailed published evidence comes from Bliddal et al. (2024), a 12-month open-label study of 49 patients who received a single ultrasound-guided 6 ml injection. At 52 weeks, participants showed a mean WOMAC pain reduction of −17.7 points (p<0.0001), with 62.2% meeting OMERACT-OARSI responder criteria. An independent UK cohort study — not manufacturer-funded — followed 269 patients across 314 knees and found sustained improvements in pain and function for up to two years in appropriately selected patients. A 2025 systematic review (Gao et al.) supports this durability profile, and a retrospective safety cohort of over 6,000 patients confirmed a favourable long-term safety record.

The evidence limitation is important to state plainly: all major outcome data are open-label. No large placebo-controlled RCT has yet been published, which is the primary reason iPAAG is not routinely available on the NHS in the UK. Patients accessing it currently do so privately.

Patient selection appears to matter. The independent UK cohort found better outcomes in those with mild-to-moderate OA, older age, and no diabetes — criteria worth discussing with a specialist before considering this route. Patients with advanced OA were more likely to progress to knee replacement despite treatment.

Extended-release corticosteroid: structured, time-limited relief

Standard corticosteroid injections are familiar to most patients with knee OA — rapid, reliable pain relief that typically fades within four to eight weeks. Zilretta (triamcinolone acetonide extended-release, 32 mg) uses the same active drug but delivers it via a biodegradable microsphere system, releasing it gradually into the joint over weeks rather than days. It is the only corticosteroid formulation to have received FDA approval specifically for this once-only, sustained-release approach to knee OA pain.

The clinical benefit is a meaningful extension of the relief window: evidence supports 3–6 months of pain reduction from a single injection — roughly double the duration of conventional intra-articular corticosteroids. That difference matters in practice, because the established concern with repeated conventional steroid injections is cumulative cartilage exposure; frequent courses have been associated with accelerated joint tissue changes. Zilretta's once-per-cycle positioning sidesteps that risk not by reformulating the steroid, but by removing the clinical driver to repeat it. Crucially, the safety and benefit of repeat Zilretta administration have not been demonstrated, so it is best understood as a structured, time-limited option rather than part of an ongoing injection programme.

In the UK, Zilretta is not routinely available on the NHS and is currently accessed privately.

Single-dose PRP: where the evidence is heading

Platelet-rich plasma works differently from the other options in this comparison. Prepared from a small sample of the patient's own blood, it delivers a concentrated mixture of growth factors directly into the joint — targeting inflammatory pathways and, in some studies, supporting tissue repair rather than simply lubricating or cushioning the joint surface.

That biological mechanism appears to translate into meaningful durability. Mayo Clinic data from 2025 report a 60–70% success rate, with many patients experiencing 6–12 months of pain relief from a single PRP treatment. One practical caveat matters early in treatment: PRP typically takes four to six weeks to show meaningful effect, which is considerably longer than corticosteroids and requires realistic expectations in the initial weeks after injection.

Where head-to-head evidence exists, single-dose PRP performs well. Kesiktas et al. (2020) found greater short-term pain reduction with a single PRP injection than with single-dose HA or peptide injections. The dosing question most directly relevant to a once-only pathway — whether one injection is sufficient, or whether a course of three is materially better — is addressed by a 2025 RCT (Dönmez et al.) comparing single versus triple PRP injections on pain and quality-of-life outcomes; that evidence will be important in shaping future guidance on once-only protocols.

The main constraint on reading the PRP literature collectively is preparation heterogeneity. Leukocyte content, cell concentration, and activation method vary substantially between centres and commercial systems, making cross-trial comparisons difficult and limiting the confidence guideline bodies can place in pooled results.

One further agent sometimes cited in this space is nSTRIDE (autologous protein solution), a single-injection biological therapy with a claimed durability of up to three years. That claim rests on fewer independent trials than the other agents discussed here, and nSTRIDE should currently be regarded as the least evidence-mature option in this category.

What the evidence gaps mean for your decision

Three honest barriers stand between the evidence reviewed here and a clean answer to 'which is best.'

The first is the absence of a direct trial. Because no study has placed iPAAG, Zilretta, single-dose HA, and single-dose PRP in the same patient population, comparisons across separate trials remain indirect — differences in observed outcomes may reflect study populations as much as genuine treatment effects.

The second is measurement inconsistency. Duration-of-effect claims are expressed in different currencies: WOMAC pain scores at 52 weeks for iPAAG, VAS or NRS at 3–6 months for Zilretta, patient-reported success rates for PRP. Like-for-like comparison across those metrics is unreliable, regardless of how the numbers look in isolation.

The third is access. Conventional corticosteroids and some HA formulations are available on the NHS; iPAAG (Arthrosamid), Zilretta, and single-dose PRP are currently private-pay in the UK. For many patients, that shapes the realistic shortlist before clinical factors are considered at all.

Patient selection is, consistently, the strongest predictor of outcome across all four categories — OA severity, age, comorbidities such as diabetes, and realistic expectations all influence which option is likely to suit a given individual. That is not a weakness in the evidence; it is the clinical argument for a structured individual assessment rather than a generic recommendation.

Among agents still under investigation — including FGF-18 analogues, gene therapy, and LEVI-04, which targets neurotrophin-3 and represents a new class of OA pain therapy — none has yet reached routine clinical practice, and they do not alter today's decision landscape.

For patients working through this decision now, the most productive step is a clinical assessment that maps personal health factors, OA stage, and access options to the approach most likely to be appropriate.