Which injection fits your knee?

Choosing between ChondroFiller and Arthrosamid is not a matter of picking the newer product or the lower price — these two injections are designed for different problems inside the same joint. ChondroFiller is a collagen scaffold placed under ultrasound guidance to address focal, discrete cartilage defects: isolated damage at a specific load-bearing surface. Arthrosamid is a synthetic hydrogel injected into the synovial cavity to reduce pain from diffuse knee osteoarthritis, where cartilage loss is widespread rather than localised.

The right starting point is therefore diagnosis, not brand preference. An accurate assessment — typically combining clinical examination and MRI — determines which pathology is present, and that finding drives the product choice. A patient with a contained post-traumatic lesion and a patient with diffuse OA grade III changes are unlikely to be candidates for the same injection.

Some patients present with both a focal defect and background osteoarthritis simultaneously. In those cases a combined plan may be clinically discussable, but each product requires its own separate justification — they are not interchangeable parts of a single treatment.

Both injections are available as outpatient, ultrasound-guided appointments at MSK Doctors with no GP referral and no NHS waiting list.

How ChondroFiller works as a collagen scaffold

ChondroFiller is made from type I and III collagen — the structural proteins the body already uses to build connective tissue — supplied as an acellular scaffold containing no donor cells of any kind. That biological origin matters: the material is not a synthetic implant, and because it is fully resorbable, it does not remain in the joint permanently in the way that non-biodegradable gels do.

During an ultrasound-guided outpatient appointment, the collagen matrix is mixed in a dual-chamber syringe and injected into the cartilage defect, where it hardens into a dimensionally stable gel within approximately 3–5 minutes. The clinical term for what follows is acellular matrix-induced chondrogenesis: the scaffold holds the defect space open and acts as a template, drawing the patient's own progenitor cells in from the surrounding synovium and subchondral bone. Those cells then lay down repair tissue over the following 6–12 months.

This is not instant cartilage regrowth. ChondroFiller supports the body's own repair processes on a biological timescale, and patients are counselled accordingly — measurable improvement on MRI and in symptoms develops over months rather than weeks.

Published multi-centre cohort data, drawn from more than 19,000 cases globally, show IKDC scores improving by approximately 30 points in the knee — a shift that translates into reduced pain and better day-to-day functional capacity. MOCART MRI scores, which grade cartilage fill quality on imaging, sit in the 70–87 range, indicating good defect coverage. The documented complaint rate is approximately 0.06%. ChondroFiller holds CE marking as a Class III medical device.

How Arthrosamid cushions an arthritic joint

Arthrosamid is 97.5% sterile water bound in a 2.5% polyacrylamide matrix — a transparent synthetic gel that, once injected into the synovial cavity, integrates into the joint's inner lining rather than sitting loosely inside it. From that position it acts as a long-term mechanical cushion, redistributing load across the joint space and reducing the friction and pressure that drive OA pain. No tissue repair or cartilage rebuilding takes place; the hydrogel's role is purely mechanical.

Because it addresses the global mechanical environment of the joint rather than a discrete defect, Arthrosamid is licensed for symptomatic adult knee osteoarthritis across Kellgren–Lawrence grades II–IV — precisely the diffuse OA picture for which ChondroFiller, as a scaffold targeting focal surface lesions, is not designed.

A single 6 mL dose is the standard treatment. Once in place, the hydrogel remains indefinitely — it is non-biodegradable and the body does not resorb it. That permanence is the source of its durability and also something patients should understand clearly before proceeding, since it distinguishes Arthrosamid categorically from resorbable therapies such as ChondroFiller.

The clinical evidence supports meaningful, sustained pain relief. A 5-year extension of the original RCT (n=119) found WOMAC pain improved by −16.2 points (95% CI −20.0 to −12.4; p<0.0001) versus baseline, with no device-related serious adverse events reported across years one to five — a durability profile not matched by hyaluronic acid or corticosteroid, both of which return patients to baseline by twelve months in comparative data.

Focal defect vs diffuse OA: which pathology points where

Three features of the damage pattern tend to point toward ChondroFiller: the lesion is discrete rather than diffuse, it affects a load-bearing cartilage surface, and the surrounding cartilage retains enough quality to anchor the scaffold and support cell recruitment. Post-traumatic lesions, osteochondritis dissecans, and focal grade III–IV chondral damage in younger patients with an otherwise intact joint fit this profile well. Where damage is widespread across the compartment rather than localised, the biological conditions the scaffold depends upon are unlikely to be present.

Arthrosamid's indicators run the other way. The target is diffuse, symptomatic knee osteoarthritis — Kellgren–Lawrence grade II, III, or IV — where pain relief is the primary goal and tissue restoration is not. Real-world cohort data from 314 knees identified older age, non-diabetic status, lower KL-grade OA, and bilateral joint involvement as the factors most consistently associated with reaching a clinically meaningful improvement at two years. That bilateral finding is counter-intuitive but appears reliable across the published data. Equally worth knowing: higher-grade OA predicted progression to total knee replacement within those same two years, which is information that should be part of any pre-treatment conversation.

For neither product does imaging alone settle the question. MRI characterises the extent and grade of damage, but defect geometry, surrounding cartilage quality, joint alignment, and the patient's symptom burden all feed into whether either pathway is appropriate. At MSK Doctors, onMRI™ AI-driven MRI analysis adds quantitative precision to imaging interpretation during treatment planning — helping characterise pathology before the clinical conversation — though the decision rests on full consultant assessment rather than scan findings in isolation.

Where a patient presents with both a focal chondral defect and diffuse background OA, the two products can in principle address separate roles within the same joint. That requires individual clinical justification for each component; no randomised trial evidence currently exists for the combination.

What the clinical evidence shows for each

The two products sit on different rungs of the evidence ladder — not because one is clinically superior, but because they were developed for different indications and studied through different trial designs.

Arthrosamid has the more controlled evidence base: a randomised controlled trial supported by three- and five-year extension data, multiple open-label studies, and a 24-month real-world PROMs cohort covering 314 knees across 269 patients. That breadth of design — from RCT through to large-scale post-market follow-up — gives clinicians a reasonably clear picture of what the hydrogel does and how long the effect is sustained.

ChondroFiller's published evidence comes from multi-centre, multi-joint observational cohort studies spanning more than 19,000 treated cases globally. The real-world scale is large and the safety record notable — a documented complaint rate of approximately 0.06% across reported cases — but a placebo-controlled RCT does not yet exist. For patients familiar with appraising clinical data, that is a meaningful distinction; for those less so, it means outcomes are well-characterised across a large real-world population but without the formal control condition an RCT provides.

Neither product has been tested against the other in a clinical trial. Any comparison between them is indirect — and largely beside the point, given that they target different pathologies inside the joint.

On safety, one cohort reported complications in 155 of 314 Arthrosamid-treated knees. The published cohort data do not break down severity, but the documented complication categories include Baker's cyst swelling, transient injection-site pain, allergic reaction to anaesthesia or the hydrogel itself, and infection — a range that spans from self-limiting to clinically significant. What that distribution looked like in any individual patient's situation is a direct question worth raising with a consultant before proceeding.

Costs, what to expect at the appointment, and next steps

Neither injection is available on the NHS — both are delivered as private, self-pay outpatient treatments, typically in a single clinic appointment under ultrasound guidance. There is no theatre admission, no general anaesthetic, and no surgical wound.

As a guide, ChondroFiller costs broadly between £3,000 and £8,000 depending on the joint involved and the clinical complexity of the defect; Arthrosamid is typically priced between £2,000 and £3,000 across the UK market, with clinic-specific pricing starting from £3,000 at some specialist providers. Confirm exact costs at the time of consultation, as they reflect individual clinical factors.

The recovery expectations differ in line with each product's mechanism. Arthrosamid acts mechanically from the outset, and most patients return to normal daily activity within a short period after injection. ChondroFiller's biological process — recruiting the body's own cells into the collagen scaffold over the following months — means a period of activity modification is typically advised while integration progresses over a six-to-twelve month window.

At MSK Doctors, patients can be seen at the Sleaford or Grantham clinics without a GP referral and without an NHS-style waiting list. The initial consultation includes a full clinical assessment and imaging review; no injection is recommended before that picture is complete. To book, visit mskdoctors.com.

1. [1] Sustained symptom relief and safety over five years following a single intra-articular injection of 2.5% polyacrylamide hydrogel in patients with knee osteoarthritis. (2025). https://doi.org/10.55563/clinexprheumatol/bsper8 https://doi.org/10.55563/clinexprheumatol/bsper8
2. [2] A prospective, open-label, clinical investigation of a single intra-articular polyacrylamide hydrogel injection in participants with knee osteoarthritis: a 5-year extension study. (2025). https://doi.org/10.1186/s13018-025-06526-0 https://doi.org/10.1186/s13018-025-06526-0
3. [3] Polyacrylamide hydrogel injections in knee osteoarthritis: A PROMs-based 24 month cohort study. (2025). https://doi.org/10.1016/j.jcot.2025.103136 https://doi.org/10.1016/j.jcot.2025.103136
4. [4] Three-year follow-up from a randomised controlled trial of a single intra-articular polyacrylamide hydrogel injection in subjects with knee osteoarthritis. (2025). https://doi.org/10.55563/clinexprheumatol/5lofry https://doi.org/10.55563/clinexprheumatol/5lofry
5. [5] Effectiveness and safety of polyacrylamide hydrogel injection for knee osteoarthritis: results from a 12-month follow up of an open-label study. (2024). https://doi.org/10.1186/s13018-024-04756-2 https://doi.org/10.1186/s13018-024-04756-2
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