Two injections, two completely different targets

The question most patients arrive with is straightforward: which of these injections should I have? The honest answer is that it may not be an either/or decision — because the two treatments act on entirely different parts of the joint.

ChondroFiller® is placed onto the cartilage surface: the load-bearing layer that coats the ends of the bones inside the joint. When that layer is worn or damaged, ChondroFiller provides a collagen scaffold into which the body's own cells can migrate and begin laying down new cartilage-like tissue.

Arthrosamid® works somewhere else altogether. As a polyacrylamide hydrogel, it integrates into the synovial lining — the soft inner membrane that encases the joint — where it acts as a cushion and may help reduce inflammation. It makes no claim to rebuild cartilage.

Because these two treatments address anatomically separate structures, they are not interchangeable alternatives. Whether one, the other, or both is appropriate depends entirely on where the damage lies.

How ChondroFiller works as a regenerative scaffold

The scaffold concept is distinct from how most joint injections work. Rather than lubricating the joint or filling the space around worn surfaces, ChondroFiller® introduces a type I collagen framework — the same structural protein found naturally in cartilage — directly onto the damaged bone-bearing surface under ultrasound guidance, as an outpatient procedure.

Once in place, the scaffold acts as a biological template. Chondrocytes and progenitor cells from surrounding tissue migrate into the collagen matrix and begin producing new cartilage-like material from within. Published series suggest this rebuilding process unfolds gradually, typically over six to twelve months, rather than providing immediate mechanical relief. Patients are generally advised not to judge the outcome too early in that window.

The treatment does, however, depend on there being some cartilage tissue still present. The scaffold supports and reinforces what remains — it cannot bridge a surface where cartilage has been entirely lost and bone meets bone directly. A joint at that stage may need a different pathway. Where cartilage persists, even if thinned or unevenly worn, the scaffold has a viable surface to anchor to and local cells to draw from.

This gradual, cell-led process is what distinguishes ChondroFiller from cushion-style or lubricant-style injections: it is not designed to mask symptoms temporarily, but to provide a framework the body can build on over time.

How Arthrosamid works as a hydrogel cushion

Polyacrylamide hydrogel is a stable, synthetic material — not a biological scaffold and not derived from any living tissue. Arthrosamid® is formulated at a 2.5% concentration and delivered as a single intra-articular injection under ultrasound guidance. Once inside the joint, it does not coat the cartilage surface; instead, it integrates within the synovial membrane — the tissue envelope lining the joint cavity — where it becomes physically incorporated rather than simply sitting in the joint fluid.

The primary action is mechanical. By embedding in the synovial lining, the hydrogel provides a cushioning layer at the joint envelope, which may reduce the mechanical load transmitted across the joint with each step or movement. There is also a reported anti-inflammatory effect on the synovial tissue itself, which may contribute to the pain reduction seen in clinical studies, though the precise mechanism for this response remains an area of ongoing research.

Arthrosamid makes no claim to restore or regenerate cartilage. Its role is symptom modification: addressing the inflamed, overloaded joint lining rather than the worn cartilage surface. This is a meaningful distinction — patients should not expect it to reverse structural cartilage loss.

The durability data from published trials is worth noting. A five-year RCT extension (NCT04045431; 58 completers from an initial 119 participants) reported a sustained WOMAC pain improvement of −16.2 points at year five (p<0.0001), with no device-related serious adverse events throughout. A separate five-year open-label cohort reported comparable findings, with significant improvements in pain, stiffness, and physical function maintained over the full follow-up period without repeat injection.

What the published evidence shows for each

The evidence bases for the two products are genuinely unequal in size, and being transparent about that helps patients calibrate expectations.

Arthrosamid in the knee

Beyond the five-year trial findings described above, a 2025 retrospective cohort study (150 patients; 50 per treatment group) put durability in sharper relief: at twelve months, both the hyaluronic acid and corticosteroid groups had returned to their pre-injection pain levels, while the iPAAG group remained stable. For a patient weighing long-term value against cost and inconvenience, that difference is practically significant. A separate 24-month PROMs cohort covering 314 knees added a patient-selection dimension: lower Kellgren–Lawrence grade, older age, absence of diabetes, and bilateral osteoarthritis each independently predicted reaching a meaningful clinical improvement threshold. In plain terms, the injection tends to work better in earlier-to-moderate disease than in very advanced arthritis.

ChondroFiller in the knee

The published peer-reviewed evidence for ChondroFiller as an injectable treatment specifically in the knee is more limited at present. The most robust data comes from other joints: a hip cohort of 26 patients with acetabular cartilage lesions reported good or excellent outcomes in 17 of 21 followed up at three to five years, though those with moderate-to-severe pre-existing osteoarthritis fared poorly. A 2025 wrist study demonstrated significantly better cartilage quality at follow-up assessment compared with a control group. The biological rationale — a collagen scaffold enabling cell-led repair — is consistent across these joint types, making the case for knee use clinically coherent; it is not, however, equivalent to knee-specific randomised trial data, and knee injection evidence is still accumulating.

A gap worth naming

No published head-to-head randomised controlled trial has compared ChondroFiller directly with Arthrosamid. That absence is common with newer devices entering clinical use, but it is a limitation patients deserve to know about when weighing the options.

Which patients tend to suit which injection

Orienting yourself starts with a simple question: where is the primary problem in your joint?

A dominant cartilage surface problem — focal wear or damage to the load-bearing layer — is the territory where ChondroFiller is positioned. The scaffold requires meaningful cartilage remaining for the repair process to take hold; published cohort data from hip studies found that patients with moderate-to-severe pre-existing osteoarthritis fared poorly, a finding that carries clinical weight when considering any cartilage-surface scaffold. This is not a treatment designed for bone-on-bone end-stage arthritis.

A dominant synovial picture — active swelling, pain at rest, or regular inflammatory flares — points more towards Arthrosamid's mechanism. Published data suggests that outcomes tend to be stronger in patients with lower Kellgren–Lawrence grades, meaning earlier-to-moderate disease rather than very advanced structural change. The same 24-month cohort study (314 knees) found that non-diabetic status and a bilateral osteoarthritis pattern were each independently associated with reaching a meaningful improvement threshold. Neither finding is a hard rule, but both are worth raising during a clinical assessment.

Age alone is not a disqualifying factor for either product; the structural picture inside the joint matters more than the patient's age at presentation.

Where the joint shows evidence of both surface cartilage damage and an active synovial component, a combination approach may be appropriate — that pathway is discussed in the next section.

When both injections are used together

The most important transparency point about combining ChondroFiller and Arthrosamid in a single appointment is also the most straightforward: no published randomised controlled trial has tested the combination protocol as a unit. The rationale for using both rests on mechanism and anatomy — not on a head-to-head combination study — and patients are better served knowing that upfront.

What the rationale does rest on is sound. Because the two products act on separate structures that are not already addressed in a single-product trial (ChondroFiller at the cartilage surface; Arthrosamid at the synovial lining), treating both simultaneously is clinically coherent when a joint shows damage at both sites. This is not doubling up on the same treatment — it is addressing two distinct biological problems in one outpatient appointment, each injection delivered under ultrasound guidance without any surgical procedure involved.

The candidate for this approach is a patient whose imaging and clinical picture point to both focal surface wear and an active synovial component — persistent swelling, rest pain, or inflammatory flares — where a single-product injection would leave one problem untreated. Whether that profile fits is a question for individual clinical assessment; the published evidence for each product remains from separate trials rather than a combined-protocol study.

If your scans show cartilage surface damage alongside signs of joint-lining inflammation, that is a concrete reason to ask whether both targets warrant treatment — and to explore the evidence for each independently before any decision is made.

1. [1] 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
2. [2] Arthroscopic utilization of ChondroFiller gel for the treatment of hip articular cartilage defects: a cohort study with 12- to 60-month follow-up. (2021). https://doi.org/10.1093/jhps/hnab002 https://doi.org/10.1093/jhps/hnab002
3. [3] Cartilage reconstruction using Chondrofiller in intra-articular distal radius fractures. (2025). https://doi.org/10.1186/s42836-025-00333-y https://doi.org/10.1186/s42836-025-00333-y
4. [4] 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
5. [5] 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