Two different targets, two different treatments

Both names appear in the same online searches, on the same clinic websites, and sometimes in the same conversation with a specialist — so it is natural to assume ChondroFiller® and Arthrosamid® are competing versions of the same thing. They are not. Each product treats a different anatomical structure inside the knee, and understanding that difference is the most useful starting point for any patient weighing their options.

ChondroFiller® targets the cartilage surface — the smooth, load-bearing tissue that covers the bone ends inside the joint. Delivered as an ultrasound-guided injection, the liquid collagen scaffold (type I, acellular, made by Meidrix Biomedicals GmbH) gels in situ within three to five minutes, creating a three-dimensional framework directly within the defect. Over the following months, the patient's own progenitor cells migrate into the scaffold and begin remodelling it — a process known as acellular matrix-induced chondrogenesis. The goal is to support the body's own cartilage repair process, not to act as a permanent filler.

Arthrosamid® targets an entirely different structure: the synovial membrane, the soft tissue lining that encloses the joint. A single outpatient injection of this 2.5% cross-linked polyacrylamide hydrogel (97.5% water) integrates sub-synovially within roughly ten to fourteen days, where it acts as a permanent mechanical cushion for the joint lining. It does not regenerate cartilage and is not designed to.

The practical implication is that the two treatments are not interchangeable. Which one — or whether both together — is appropriate depends entirely on the pattern of damage identified on MRI. A consultant assessment is needed to determine which structure is primarily affected.

The right patient for each injection

Deciding which injection is relevant begins with one question: is there a discrete patch of cartilage damage, or is the wear more widespread across the joint?

ChondroFiller® is suited to patients with a focal cartilage lesion — typically a defined area of damage up to roughly 2.5 cm² — in a knee that has not yet progressed to advanced, bone-on-bone disease. Younger and more active patients tend to be the best candidates, because the scaffold's aim is structural repair rather than pain control, and the biological remodelling it supports takes months to mature. Diffuse, generalised wear is not an appropriate indication.

Arthrosamid® is indicated for mild-to-moderate osteoarthritis — graded in clinical practice on the Kellgren–Lawrence (KL) scale that patients may see referenced in their own MRI or clinic letters. Grades 2 and 3 on that scale represent the target range. Published subgroup data from a randomised trial of 239 participants suggest outcomes are particularly favourable in patients aged under 70 with a normal BMI. KL grade 4, the bone-on-bone end of the spectrum, is a relative contraindication: in that same trial, hyaluronic acid outperformed Arthrosamid in this subgroup.

Because these two conditions can coexist — a focal lesion sitting within a joint that also has early synovial-driven OA — MRI is the necessary step before any treatment decision. It is also the only way to confirm whether a patient is a candidate for both injections in a single session. The endpoints differ too: ChondroFiller is assessed on structural cartilage quality over time; Arthrosamid on pain and function within weeks. Comparing them on the same outcome scale is not clinically meaningful.

What happens at your appointment

Neither treatment involves a theatre booking, general anaesthetic, or surgical incision. Both ChondroFiller® and Arthrosamid® are delivered as ultrasound-guided outpatient injections, and patients leave the clinic the same day.

ChondroFiller® is placed as an injectable collagen scaffold under image guidance, directly into the cartilage defect. Because a biological product is being introduced into the joint, the appointment typically includes intravenous antibiotic cover alongside the injection itself, with a structured follow-up at around six weeks. Patients are generally advised to protect the joint from heavy loading in the days immediately after the procedure — the scaffold needs time to settle before the body's own repair process begins in earnest.

Arthrosamid® is a single intra-articular injection. Because the hydrogel settles into the joint lining rather than within a discrete tissue defect, post-injection requirements are lighter: patients can typically bear weight and resume gentle daily activity promptly, though a short period of reduced exertion is standard practice.

Where MRI confirms both a focal cartilage defect and synovial-driven OA in the same knee, both injections can be delivered in a single session by the treating consultant — ChondroFiller addressing the scaffold role, Arthrosamid the synovial lining. This is not routine for every patient; the imaging must first establish that both structures are affected. The MRI is therefore the necessary starting point, not the appointment itself.

How long results last — and when both make sense

The timelines for these two treatments diverge sharply — and understanding why matters more than trying to decide which is "faster".

Arthrosamid® tends to provide symptom relief within days to weeks of the injection. Available data show durability out to two to three years, and a prospective multicentre Danish study presented at EULAR 2023 is tracking outcomes to five years, with 35 participants already in the extension phase at the time of reporting.

ChondroFiller®'s timeline is different in kind, not just in length. The collagen scaffold recruits the body's own progenitor cells over six to twelve months, with full consolidation of the repaired tissue possible up to 24 months. Patients should not expect any change in pain levels in the first weeks — that is not the treatment's purpose. Its goal is structural: measurable improvement in cartilage quality, assessed using imaging scores such as MOCART, rather than a reduction in pain scores at six weeks.

This is why comparing onset times directly is misleading. These treatments are measured on different outcomes because they work on different parts of the knee. Arthrosamid® is evaluated on pain and function questionnaires such as WOMAC. ChondroFiller® success is partly assessed on cartilage quality at follow-up; a 2025 prospective study by Demmer et al. recorded median Outerbridge scores of 1.5 in treated joints versus 3 in controls (p=0.006) — a structural finding, not a symptom score.

One further asymmetry deserves attention before any treatment decision. The iPAAG hydrogel in Arthrosamid® is non-biodegradable: it integrates sub-synvially within 10–14 days and remains in the joint indefinitely. The ChondroFiller collagen scaffold, by contrast, degrades biologically as repair matures — it is a temporary framework, not a permanent implant. For patients who may require further joint procedures at some point in future, the permanence of iPAAG is a practical consideration worth raising explicitly during the consultant assessment.

What the clinical evidence shows

Taken together, the published data for these two treatments sit at different points of maturity — and on different outcome scales.

Arthrosamid® has the larger evidence base for knee OA. A 2022 systematic review covering 463 patients confirmed statistically significant improvements at 52 weeks and 13 months, with maintained benefit at two years in one included study. In an RCT of 239 participants, iPAAG was non-inferior to hyaluronic acid at 26 weeks and approached superiority at 52 weeks (WOMAC pain, p=0.0572). One nuance is worth stating plainly: on aggregate at 12 months, iPAAG did not significantly outperform hyaluronic acid. Advantages did emerge in defined subgroups — patients aged under 70, those with a normal BMI, and those with KL grade 2–3 disease — while KL grade 4 patients actually fared better with hyaluronic acid. A 2025 retrospective cohort of 150 patients confirmed superiority over corticosteroid at six months (p<0.001), without a statistically significant difference from hyaluronic acid at 12 months.

ChondroFiller® evidence is anchored to structural metrics: published series report IKDC improvements of approximately 30 points and MOCART cartilage-quality scores in the 70–87 range. Longer-term knee-specific data are still emerging, which reflects the different disease stage and endpoint rather than an absence of clinical support.

No direct head-to-head RCT between the two products exists. Because they address different anatomical structures and are measured on different endpoints, a conventional comparison trial would not be scientifically meaningful — patients should not read this absence as a signal of uncertainty about either treatment.

Two separate evidence notes on Arthrosamid® merit flagging. First, a 2025 in vitro study found that 2.5% iPAAG caused no statistically significant effects on human neuron survival, apoptosis, or neurite network area, directly addressing the theoretical concern that arises from its acrylamide monomer origin. Second, and on a distinct point, some researchers have raised a methodological caution: patient-reported 'sensation of distension' used in some Arthrosamid trials may not capture all clinically detectable inflammatory effusions, meaning published adverse-event rates could modestly underestimate the true frequency of post-injection inflammation.

Cost, access, and getting started

Neither ChondroFiller® nor Arthrosamid® is currently available on the NHS — both are self-funded private treatments. UK guide costs start from approximately £3,000 for a single Arthrosamid® injection and from approximately £3,000 for a single-box ChondroFiller® treatment; where the extent of cartilage involvement requires more than one box, costs may be higher. These figures should be confirmed directly with the treating clinic, as individual pricing reflects the full package of consultation, imaging guidance, product, and follow-up rather than the injection in isolation.

Before any treatment decision, MRI is needed to map the joint accurately — to distinguish a focal cartilage defect from more generalised synovial disease, and to determine whether a combined approach is clinically appropriate. For patients in Lincolnshire and the East Midlands, the Sleaford clinic (NG34) has an on-site Open MRI scanner, which can simplify this step if local imaging access is limited.

A GP referral is not required to make an enquiry or book a consultation. For patients based in London, equivalent clinical access is available through the London Cartilage Clinic. To arrange a no-referral assessment, visit mskdoctors.com.

1. [1] A Systematic Review of Arthrosamid Polyacrylamide (PAAG) Hydrogel for Treatment of Knee Osteoarthritis. (2022). https://doi.org/10.18103/mra.v10i8.2950 https://doi.org/10.18103/mra.v10i8.2950
2. [2] Polyacrylamide Hydrogel for Knee Osteoarthritis: 3-Year Follow-Up of a Prospective Clinical Study (EULAR 2023). (2023). https://doi.org/10.1136/annrheumdis-2023-eular.6038 https://doi.org/10.1136/annrheumdis-2023-eular.6038
3. [3] Injectable 2.5% Cross-Linked Polyacrylamide Hydrogel Demonstrates No Neurotoxicity in Human iPSC-Derived Neurons. (2025). https://doi.org/10.3389/ftox.2025.1585430 https://doi.org/10.3389/ftox.2025.1585430
4. [4] Polyacrylamide Hydrogel Versus Hyaluronic Acid in Knee Osteoarthritis: A Subgroup Analysis of a Randomized Study. (2023). https://doi.org/10.1302/1358-992x.2023.13.081 https://doi.org/10.1302/1358-992x.2023.13.081
5. [5] Comparative efficacy of polyacrylamide hydrogel versus hyaluronic acid and corticosteroids in knee osteoarthritis: A retrospective cohort study. (2025). https://doi.org/10.1097/MD.0000000000044655 https://doi.org/10.1097/MD.0000000000044655