Three injections, three different jobs

All three arrive via a single needle into the knee joint. The needle is where the similarity ends.

Hyaluronic acid (HA) mimics and tops up a component already present in healthy synovial fluid. Over time, the joint fluid in an arthritic knee loses its natural viscosity and cushioning quality; HA restores it, temporarily reducing friction and pain. The effect is real but resorbable — the body gradually clears the material over months.

Platelet-rich plasma (PRP) works differently. A small sample of the patient's own blood is centrifuged to concentrate platelets and the growth factors they carry. When injected, this concentrate is thought to modulate the inflammatory environment within the joint — biological signalling rather than mechanical top-up. How long that signal persists, and how reliably, varies with the preparation used.

Polyacrylamide hydrogel (Arthrosamid®) belongs to neither of the above categories. Classified as a medical device rather than a drug or biologic, the gel is non-biodegradable and is not metabolised; once injected, it integrates into the synovial membrane and remains, providing mechanical cushioning from within.

What the evidence shows for each option

The evidence behind each option sits at a different stage of maturity — and the gaps are as informative as the findings.

Hyaluronic acid — established but contested

HA has the longest clinical record of the three, though the single-injection question remains genuinely unresolved. A 2017 systematic review of 11 head-to-head studies found no consistent difference in patient-reported outcomes between single and multi-injection formulations; earlier work had suggested that two-or-more injection regimens outperformed saline while the single dose did not. A 2019 meta-analysis by Vincent P addressed single-injection products specifically and found evidence of benefit, though the debate has not settled. Newer cross-linked (reticulated) formulations are engineered for longer joint residence to offset the reduced total dose; Cingal pairs HA with a corticosteroid to accelerate early relief where inflammation is the primary concern.

Confidence level: established but internally contested.

PRP — promising but inconsistent

As of 2020, the wider PRP evidence base showed mixed results — effective in some conditions, inconsistent in others. In knee OA specifically, preparation method appears decisive: Bansal et al. (Scientific Reports, 2021) identified correct platelet dosing as critical for long-term efficacy, and whether a preparation is leukocyte-rich or leukocyte-poor adds a further layer of variability. The literature can look both encouraging and underwhelming depending on which trials are counted.

Confidence level: promising but inconsistent.

Arthrosamid® — early but growing

The controlled evidence base is smaller but developing. Bliddal and colleagues published six-month prospective data in 2021 and twelve-month open-label follow-up in 2024, both supporting effectiveness and safety. A 2022 case series noted a reduction in patellofemoral bone marrow lesions following a single injection — a potential structural signal that awaits confirmation in randomised trials.

Confidence level: early but growing.

No randomised controlled trial has yet compared all three approaches head-to-head in a single-injection protocol. That is a gap in the clinical evidence — one that makes any direct comparison between them a matter of indirect inference rather than trial data.

How long relief typically lasts

Duration is often the deciding factor in how patients and clinicians approach the repeat-injection question.

Hyaluronic acid typically provides symptom relief for six to twelve months from a single injection. Repeat courses are routine and well-established in clinical practice, which for many patients makes the predictable annual or biannual cycle a reassuring framework rather than a drawback.

PRP offers a less predictable window. Clinical guidance characterises its benefit as lasting 'typically months' — a range that reflects genuine variability linked to preparation method and platelet dose. Some uncertainty in how long any given course holds is a practical consideration for any patient weighing this option.

Arthrosamid® has the longest published duration profile: an average of two to three years of symptom relief from a single injection. This longer profile stems from the hydrogel's non-biodegradable nature — the material is not cleared by the body, so the mechanical effect is sustained rather than gradually resorbed. Open-label data at six and twelve months support the safety and effectiveness of the injection across this period.

A longer-lasting option is not automatically the right choice. Some patients prefer the predictable rhythm of an annual HA injection they know well over a longer-acting alternative with a shorter evidence history. Duration also affects total treatment cost and the number of clinic visits, both of which are practical factors when planning around a joint-preservation strategy.

None of these three injections modifies the underlying structural progression of knee OA. They address symptoms; their benefit, however sustained, does not reverse or halt joint degeneration.

Matching the right injection to your situation

Several practical factors help narrow which option is most likely to suit a given patient — though none of them replaces a specialist assessment.

OA severity is the most useful starting point. HA and PRP are best supported for mild-to-moderate disease, where the joint retains enough structural integrity for viscosupplementation or biologic signalling to work on. At moderate-to-severe grades, the case for Arthrosamid's mechanical cushioning grows stronger: when the biological substrate is degraded, a non-resorbable device that provides direct physical support is less dependent on residual tissue to deliver its effect. HA evidence weakens further at end-stage (grade IV) disease, where the structural deficit is likely beyond what fluid-phase lubrication can meaningfully address.

Inflammation profile adds a secondary filter. When acute inflammatory pain accompanies the chronic OA picture — swelling, warmth, marked stiffness — Cingal's combined HA-and-corticosteroid formula may offer faster early relief than plain HA alone. That earlier onset of pain reduction is clinically useful when function needs to be restored quickly.

Patient preference carries weight too. Those who want a treatment drawn from their own biology often consider PRP; it is particularly well-suited to earlier-stage disease or as part of a staged plan, rather than as a standalone long-term choice for established moderate-to-severe OA. Patients seeking a longer-duration, device-based option with a single stable intervention may lean towards Arthrosamid.

Arriving at a consultation with a sense of where priorities lie — OA grade, speed of relief, preferred duration, biological versus mechanical approach — helps focus the clinical conversation. Suitability for any of these injections depends on individual imaging, symptom pattern, and clinical history, all of which a specialist assessment is needed to properly weigh.

Cost and the repeat-injection calculation

Headline price is only part of the calculation — how long a single injection lasts changes the picture considerably when costs are spread across a multi-year planning horizon.

In UK private practice, single-injection HA products (including Cingal variants) are typically priced at around £1,200, though exact figures vary by provider and any associated consultation fees. PRP sits in a broadly similar range, with variation reflecting differences in preparation protocol and clinic.

Arthrosamid® is priced from approximately £3,000 per injection — a higher upfront cost that reflects both the device itself and the longer expected duration of effect. When that two-to-three-year average is set against an HA course repeated annually at roughly £1,200 a time, the per-year figures become more comparable over the same period.

Indirect costs also deserve consideration. Each repeat injection involves a clinic visit, possible time off work, and a short recovery window. For patients with demanding schedules or limited mobility, reducing injection frequency has a practical value that does not appear in the headline price.

None of these figures should be read as guarantees. Individual response varies, and a longer expected duration does not mean every patient achieves the average. Suitability — not price — should lead the decision.

Questions to bring to your consultation

Arriving prepared makes a specialist appointment more productive — not because the questions lead to a predetermined answer, but because they push the conversation towards your specific picture rather than a generic recommendation.

Useful questions to raise:

• 'What grade of OA does my imaging show, and does that steer the evidence towards one option over the others?'
• 'Is there active inflammation in the joint right now, or is this settled chronic pain — and does that change what you would recommend?'
• 'If PRP is on the table, what preparation protocol would be used, and how does the leukocyte content of that formula affect what I should expect?'
• 'Looking five years ahead, how many injections am I likely to need under each option, and what does that cumulative plan look like?'
• 'Are there anything in my history — anticoagulant use, prior joint infections, known allergies — that rules any of these out for me?'

These questions shift the focus from product names to clinical reasoning. A good consultation should end with you understanding why a particular option fits your situation — not simply which one was chosen.

1. [1] Platelet-rich plasma — Wikipedia. https://en.wikipedia.org/?curid=22050188 https://en.wikipedia.org/?curid=22050188