Two injections, two different jobs in the joint

Patients researching injection options for knee pain often encounter both ChondroFiller and Arthrosamid in the same breath — two injections, both intra-articular, both non-surgical. The natural assumption is that they are competing versions of the same idea. They are not.

ChondroFiller injection is a regenerative collagen scaffold. Delivered as a liquid that gels within the defect, it provides a three-dimensional framework that the body's own progenitor cells migrate into, progressively building new cartilage matrix over six to twelve months. It is aimed at a specific, focal area of damaged load-bearing cartilage surface.

Arthrosamid is a permanent polyacrylamide hydrogel that integrates into the synovial membrane — the lining of the joint — where it acts as a long-term mechanical cushion. It does not repair cartilage; it addresses the broader arthritic joint environment from an anatomically distinct layer.

The cartilage surface and the synovial membrane are separate structures; a treatment aimed at one cannot substitute for a treatment aimed at the other. This is why, for a patient who has both a focal cartilage defect and diffuse osteoarthritic changes, both injections may be clinically relevant — but for entirely different reasons.

What follows covers how each product works, which patients are most likely to benefit, and what the published evidence shows.

What the ChondroFiller injection does at the defect site

The injectable product arrives as two liquid components that combine at the point of injection. Within minutes the mixture polymerises inside the focal defect — forming a stable three-dimensional collagen matrix shaped to the contours of the damaged area — an event that takes place during an ultrasound-guided outpatient procedure, with no theatre or general anaesthetic involved.

What makes the scaffold distinctive is what it does not contain: donor cells. ChondroFiller injection is entirely acellular. It acts as a homing environment for the patient's own biology: chondrocytes and mesenchymal progenitor cells from the surrounding cartilage margins, subchondral bone, and synovial fluid migrate into the porous matrix and begin depositing hyaline-like tissue. The formal term for this process is acellular matrix-induced chondrogenesis — the scaffold creates the structural and biochemical conditions; the patient's own cells carry out the repair.

A 2025 ex vivo study using osteochondral explants provided direct experimental support for this migration signal, recording a 2.4-fold increase in DNA content within ChondroFiller-treated defects by day 14 compared with untreated controls.

Tissue maturation is not immediate. The process continues over six to twelve months, and controlled physical loading during rehabilitation is mechanically integral to how the developing matrix forms correctly oriented, load-bearing cartilage — not an optional precaution. A porcine in vitro loading study found that the gel could not protect opposing cartilage under immediate full cyclic loading, which underpins the clinical requirement for a phased, supervised return to weight-bearing.

In published series, the mechanism has translated to approximately 30-point IKDC gains, MOCART cartilage quality scores in the range of 70 to 87, and a reported complaint rate of approximately 0.06% — outcome anchors that distinguish acellular matrix-induced chondrogenesis from purely palliative injection strategies.

What Arthrosamid does at the synovial lining

Arthrosamid consists of 2.5% cross-linked polyacrylamide (iPAAG) suspended in 97.5% non-pyrogenic water and is administered as a single 6 mL bolus into the joint space. The hydrogel is not absorbed by the body after injection — it is non-biodegradable and designed to remain in the joint permanently, which sets it apart from both viscosupplementation products and anything in the cartilage-repair category.

What happens after injection is not an instantaneous effect. Synovial cells begin proliferating into the gel within 10–14 days. By roughly 30 days in horse models and 90 days in rabbit models, a stable sub-synovial layer has formed — traversed by thin strands of vascularised connective tissue and covered by a fresh synovial lining. The scientific literature characterises this process as a low-level macrophage-driven foreign body response; in practical terms it means the body's own tissue slowly incorporates the hydrogel into the joint wall, creating a structural cushioning layer beneath the synovium. This tissue integration persisted for up to two years in animal studies. Critically, none of this occurs at the cartilage surface: Arthrosamid does not coat, fill, or regenerate the articular cartilage layer.

Because it works through the synovial lining rather than the cartilage surface, Arthrosamid is licensed and evidenced for diffuse osteoarthritis across KL grades II–IV — including bone-on-bone grade 4 cases where discrete focal cartilage repair is not anatomically viable. A 239-patient RCT showed non-inferiority to hyaluronic acid at 26 weeks, approaching superiority at 52 weeks (p=0.0572), with statistically superior outcomes in patients under 70, those with normal BMI, and those at KL grade 2–3. A separate 3-year prospective multicentre study confirmed the effect was sustained, and a 2025 retrospective cohort of 150 patients found iPAAG was the only agent among iPAAG, HA, and corticosteroid to remain above baseline on pain scores at 12 months.

The permanent nature of the implant is a meaningful consideration for any patient. It also explains why Arthrosamid cannot stand in for ChondroFiller: modifying the synovial environment is a different task from rebuilding load-bearing cartilage matrix.

Which patients are suited to each approach

The two treatments divide clearly along one fundamental anatomical question: is the damage focal or diffuse?

ChondroFiller injection is built for focal, full-thickness chondral defects — roughly up to 6 cm² — graded Outerbridge III or IV, in a joint whose surrounding cartilage and mechanics are otherwise sound. This profile most often describes a younger, active patient whose damage follows trauma, osteochondritis dissecans, or post-meniscal injury rather than generalised wear. The scaffold relies on healthy neighbouring tissue to supply the progenitor cells that drive repair; a joint already affected by diffuse osteoarthritis provides a weaker cellular environment for that process. In the published hip cohort, patients with Tönnis grade 2–3 pre-existing OA had poor outcomes — a finding that reflects this biological dependency directly. Stable joint mechanics matter equally: significant ligament instability or malalignment should be assessed and addressed before the ChondroFiller injection pathway is considered, as the developing scaffold cannot compensate for abnormal load distribution during the months it takes to mature.

Arthrosamid suits a different picture: diffuse osteoarthritis across the joint, classified KL grade II to IV. Because it acts through the synovial lining rather than the cartilage surface, it does not require an intact surrounding cartilage bed. The 239-patient RCT data points to the strongest benefit in patients under 70 with a normal BMI and KL grade 2–3, though the trial included grade 4 bone-on-bone cases where focal cartilage repair is no longer an option.

The more complex scenario arises when both pathologies coexist — a focal Outerbridge III–IV defect alongside background KL grade II–III osteoarthritis in the same joint. Neither product alone covers both problems in that situation: the ChondroFiller injection addresses the discrete defect, while Arthrosamid addresses the wider synovial environment.

Working out which profile applies — or whether both do — rests on specific imaging criteria: MRI for Outerbridge or ICRS grading of the focal lesion, and plain X-ray for KL classification of the surrounding joint space.

What the clinical evidence shows for each

Published evidence for the two treatments differs considerably in scale — a distinction worth stating clearly before drawing any conclusions from it.

For ChondroFiller, the dataset includes a randomised multicentre trial comparing the injectable scaffold against microfracture in 23 patients (13 in the ChondroFiller group), which recorded statistically significant IKDC improvements at 3, 6, and 12 months with no adverse events reported. A 26-patient hip cohort followed patients over 3–5 years, with 17 of 21 evaluable cases achieving good or excellent MRI-confirmed outcomes. A 17-patient knee series showed Lysholm and IKDC gains that plateaued between 6 and 12 months. A 2025 ex vivo osteochondral model provided mechanistic confirmation that cells actively migrate into the scaffold — the biological basis described in the earlier mechanism section, not restated in full here. A separate 2025 study extended documented use to the wrist. Longer-term knee RCT data beyond 12 months is still emerging.

Arthrosamid has a substantially larger trial record. A 239-patient RCT against Synvisc-One (hyaluronic acid) demonstrated non-inferiority at 26 weeks and approached superiority at 52 weeks (p=0.0572), with statistically significant advantages in patients under 70 (p=0.019), those with normal BMI (p=0.011), and KL grade 2–3 (p=0.033). A 3-year prospective multicentre study confirmed that the single-injection effect was sustained across WOMAC pain, stiffness, and function subscales and patient global assessment. A 2025 retrospective cohort of 150 patients found iPAAG remained above baseline at 12 months while hyaluronic acid and corticosteroid groups returned to baseline values. A 2025 in vitro study confirmed no cytotoxic or neurotoxic effects from the cross-linked gel — relevant context given that the uncrosslinked acrylamide monomer is known to be toxic. No head-to-head RCT between ChondroFiller and Arthrosamid has been conducted, so any direct comparison of outcomes is statistically unsupported.

The asymmetry in trial volume should not be read as a verdict. Because the two products address anatomically distinct structures and different patient presentations, the Arthrosamid RCT base does not speak to whether ChondroFiller works for focal defects — and vice versa. Each product's evidence answers a different clinical question.

When both injections make sense in the same session

The practical case for same-session delivery is straightforward: if imaging confirms both a focal Outerbridge III–IV defect and background KL grade II–III osteoarthritic change in the same joint, treating them at separate appointments simply means the patient waits longer to address both problems. Because the ChondroFiller injection occupies the cartilage surface and Arthrosamid occupies the synovial membrane, the two products do not compete for the same anatomical space — neither displaces nor mechanically interferes with the other.

In a combined session, the role of each product remains distinct. ChondroFiller is the regenerative scaffold component: the collagen matrix that invites the patient's own progenitor cells to begin repairing the focal lesion over the months that follow. Arthrosamid is the cushioning component: the permanent hydrogel that integrates into the synovial lining and attenuates the mechanical environment of a joint affected by diffuse wear. Treating them as variants of the same intervention would misrepresent both mechanisms.

One honest caveat is worth stating plainly: the combination protocol is mechanistically coherent, but no prospective trial has evaluated the two products delivered together as a defined intervention. The rationale rests on the independent evidence bases for each agent and on the anatomical logic described above — not on dedicated combination-trial data. Patients should be told this at assessment.

Assessment at the London Cartilage Clinic on Harley Street begins with imaging review and clinical history; not every joint with cartilage damage will suit one product, the other, or both. The cleaner frame is this: a discrete lesion in an otherwise sound joint points toward ChondroFiller injection; diffuse synovial wear points toward Arthrosamid; when both pathologies are present and confirmed on MRI and X-ray, a single appointment may address both — and the conversation about which combination, if any, is appropriate starts there, at londoncartilage.com.

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