Two injections, two very different mechanisms

Both injections go into the same part of the knee — the joint cavity — but what happens next is quite different, and that difference shapes everything about how they are used.

Hyaluronic acid (HA) works on the joint fluid. In a healthy knee, synovial fluid contains naturally occurring HA molecules that give it a thick, gel-like consistency, allowing the joint surfaces to glide smoothly and absorb load. In osteoarthritis, HA concentration and quality decline, leaving the fluid thinner and less protective. An HA injection replenishes this depleted fluid, temporarily restoring lubrication and cushioning. Because HA is biodegradable, the body gradually breaks it down and clears it from the joint over weeks to months — which is why courses are typically repeated every six to twelve months.

Arthrosamid® takes a different route. Composed of 2.5% cross-linked polyacrylamide and 97.5% water, it is not designed to replace joint fluid. Instead, after a single injection, the hydrogel migrates to the synovial lining — the soft tissue layer that lines the inside of the joint capsule. Within ten to fourteen days the body incorporates the gel into this lining, and by around thirty to ninety days a stable, water-rich cushioning layer has formed beneath the synovial surface. Animal model data show this integrated layer persisting for at least two years. Unlike HA, Arthrosamid® is non-biodegradable; it is not cleared from the joint.

Put plainly: HA acts on the fluid inside the joint; Arthrosamid® acts on the lining of the joint. That single distinction — temporary fluid supplement versus permanent structural integration — drives every downstream difference in injection frequency, expected duration of benefit, and the types of patient each treatment suits.

What the clinical evidence shows for each

On the question of evidence, the two treatments stand at different points on the maturity curve, and the gap is significant.

HA viscosupplementation has been in clinical use since Peyron and Balazs first assessed it in 1974. The evidence accumulated since then includes the Bellamy et al Cochrane review (2006), the Bannuru et al network meta-analysis published in the Annals of Internal Medicine (2015), and the Vincent 2019 single-injection meta-analysis. Across these, the picture is consistent: a modest but meaningful reduction in pain and improvement in function for patients with mild-to-moderate knee osteoarthritis. That benefit is real, if not dramatic. In advanced disease the picture changes: Nicholls et al (2019) noted that clinical trials routinely exclude end-stage knee OA patients, and the evidence of benefit in that group is correspondingly weaker. HA is not an alternative to arthroplasty once OA is severe.

Arthrosamid® has a smaller, still-accumulating evidence base. Bliddal et al confirmed both effectiveness and safety at twelve months in an open-label study published in the Journal of Orthopaedic Surgery and Research (2024;19:274), and symptomatic benefit up to three years from a single injection has been documented in published series. An NHS-based study at the Robert Jones and Agnes Hunt Orthopaedic Hospital (RJAH), supported by a Contura Orthopaedics research grant of over £150,000, is currently under way to validate real-world outcomes in a UK clinical setting and refine the criteria for patient selection.

No head-to-head randomised controlled trial directly comparing Arthrosamid® with HA has yet been published — a gap patients should factor into their decision-making.

Where each injection sits on the treatment pathway

For most patients with knee osteoarthritis, the management journey follows a recognisable sequence before any injection is considered: physiotherapy, weight management where relevant, and oral analgesics or NSAIDs. When those measures no longer provide sufficient relief, an intra-articular injection becomes a reasonable next step — and for the majority of patients at that stage, HA is the first option to consider. Its evidence base is concentrated in mild-to-moderate disease, it is straightforward to administer, and courses can be repeated if an initial one proves helpful.

Arthrosamid® occupies a different position on that pathway. It is typically considered when one or more HA courses have been tried but have not delivered lasting benefit, or when OA has progressed further and joint replacement is not yet appropriate — or is simply not what the patient wishes to pursue at this point. That positioning is not a judgement about quality; it reflects a difference in clinical indication and the stage of disease each treatment is designed to address.

There is also a procedural difference worth noting before any decision is made. HA injections can be given in a standard outpatient clinic setting. Arthrosamid® requires ultrasound guidance and intravenous antibiotic cover as part of the clinical protocol — a more involved procedure that needs to be factored in practically.

Neither injection is a substitute for joint replacement when osteoarthritis has reached end stage. Both are non-surgical options whose role is symptom management; when and whether that threshold has been crossed is a question for a consultant assessment, not the choice of injection alone.

Safety profile: what to know about each

Both injections share a similar short-term local side-effect profile: temporary pain, mild swelling, warmth, and joint stiffness in the first few days following the procedure. These typically settle without treatment.

Arthrosamid®

The most discussed safety consideration specific to Arthrosamid® is the theoretical presence of unreacted acrylamide monomer in the polymer matrix. Acrylamide monomer is a known neurotoxin and suspected human carcinogen; manufacturers state that residuals are removed through washing processes during production, and no confirmed clinical cases of monomer-related harm from Arthrosamid® have been reported. Regulatory commentary nonetheless regards this as an open concern for a permanent implant rather than a resolved one, and patients should raise it directly with their clinician before proceeding. Separately, the macrophage-driven foreign body response involved in synovial integration is documented in histological studies and is generally characterised as low-level; joint effusion has been recorded as part of this response in some patients.

Because Arthrosamid® is permanent, patients should inform any surgeon performing a subsequent knee procedure — including joint replacement — about the prior injection, so that the integrated hydrogel layer can be accounted for in the surgical plan.

Hyaluronic acid

HA viscosupplementation carries a well-characterised safety record built over roughly five decades of clinical use. Serious adverse events are uncommon; they include allergic reactions and, rarely, pseudoseptic (acute inflammatory) reactions. Both are managed in a clinical setting and do not represent a common risk for most patients.

Cost and access in the UK

Hyaluronic acid injections are available on the NHS in some areas, subject to local Integrated Care Board policy and eligibility criteria, and privately at comparatively modest cost. No specialist protocol or referral is required. Arthrosamid® is a different matter: it is self-pay private only in the UK — not funded by the NHS and not covered by major health insurers including Bupa and AXA. Private fees range from approximately £3,000 for a single box to £8,000 for three boxes, inclusive of consultation, ultrasound guidance, the product itself, and follow-up.

For patients weighing long-term management costs, that comparison is worth thinking through carefully. A one-off Arthrosamid® fee, if the treatment provides sustained relief over several years, may over time be comparable with the cumulative cost of repeated HA courses. That calculation depends on how often HA courses are needed and whether they continue to work — both of which vary between patients.

Patients who want to explore either option without waiting for a GP referral can do so at consultant-led MSK clinics. Lincolnshire Knee, part of the MSK Doctors group, offers assessment and injection therapies at Sleaford (NG34) and Grantham (NG31); appointments can be booked directly at lincolnshireknee.co.uk.

Which injection is more likely to suit you

The most useful framing here is not 'which injection is better' but 'which patient is each treatment suited to — and at what point on the pathway?'

For patients with mild-to-moderate knee OA who have not yet tried intra-articular therapy, HA is the natural starting point: the largest evidence base, the simplest administration, and the lowest financial commitment. If it provides sustained relief, repeated courses are a reasonable long-term strategy.

Arthrosamid® occupies a different position. Its permanence — the feature that separates it mechanically from HA — also changes the nature of the decision. Patients who have completed one or more HA courses without lasting benefit, or those with moderate-to-advanced disease who are not yet surgical candidates, may find it a meaningful next step. Because it does not degrade, there is no retreatment cycle; but by the same token, the decision cannot be reversed.

For patients whose imaging already shows Kellgren–Lawrence grade 4 changes across the joint, joint replacement suitability is the more pressing question first. Both injections carry weaker evidence at this stage of disease, and neither substitutes for a surgical conversation.

Whichever option is under consideration, physiotherapy and weight management where relevant remain part of the picture — injections address pain, not the underlying biomechanical load on the joint.

What the evidence reviewed across this article ultimately makes clear is that the two treatments are not in competition: they serve different points on a pathway. Identifying where a given patient sits on that pathway — through imaging review, severity grading, and a full treatment history — is what makes the choice clinically meaningful rather than a matter of personal preference.