Two injections, two different jobs inside the same joint

Most patients arrive at this comparison expecting to choose between two cartilage injections — as though ChondroFiller and Arthrosamid are doing the same job and the question is simply which does it better. That framing is understandable, but it is not quite right, and getting it straight early makes every subsequent decision simpler.

The two injections work in different parts of the joint. ChondroFiller is placed directly onto the cartilage surface where a focal area of damage — often described as a pothole — has worn the protective covering down to bone. Arthrosamid, by contrast, does not touch the cartilage surface at all; it integrates into the joint lining, the soft tissue layer that surrounds the interior of the knee, where it acts as a long-term cushioning agent.

Because they address different structures, the clinical literature generally frames them as complementary rather than competing. Asking which is 'better' is a little like asking whether a road repair or a new suspension system is the better solution — the answer depends entirely on what is actually wrong.

What determines that? The pattern of damage visible on imaging. A discrete, localisable defect on an otherwise stable cartilage surface points towards one conversation; diffuse wear across the whole joint surface points towards another. That imaging-driven distinction is the gateway decision — not personal preference, and not price.

Which damage pattern points to ChondroFiller

Focal Grade III or IV cartilage loss describes a bounded, localisable defect — a defined area, typically measuring centimetres rather than spanning the whole joint, where the cartilage layer has severely thinned or worn through to subchondral bone. The surrounding architecture remains structurally intact enough to contain a regenerative scaffold and support the cellular repair process that follows. This is the documented indication for ChondroFiller, and the specificity matters: the scaffold is placed at the defect site, not distributed across a broadly degraded surface.

Grade IV is significant because it denotes full-thickness cartilage loss — a severity at which surgical pathways such as microfracture or cell-based transplantation are often first considered. Weninger et al. (2025) provided specific clinical support for ChondroFiller at this grade in the knee, establishing that an ultrasound-guided injectable collagen scaffold can represent a non-surgical option within this range for suitably focal lesions.

Diffuse osteoarthritis sits in a genuinely different clinical category. Under Kellgren-Lawrence (KL) grading, Grade II to IV OA reflects widespread joint-surface degradation rather than a single repairable site — there is no discrete lesion that a scaffold could address. Arthrosamid is documented for this picture, cushioning the synovial lining across the joint rather than targeting a focal repair zone. Referencing Arthrosamid here is not a ranking; it is a factual boundary that applies on biomechanical grounds.

Assessing which category applies requires imaging — MRI or ultrasound — to distinguish a bounded lesion from diffuse wear. Symptom severity alone cannot reliably make that distinction.

How ChondroFiller works as an injectable scaffold

The mechanism behind ChondroFiller is worth understanding in some detail, because it explains both what makes the scaffold suitable for focal defects and why it is not simply another cushioning agent.

ChondroFiller is a Type I/III collagen scaffold — CE-marked as a Class III medical device — delivered as a liquid that polymerises into a stable gel within approximately 3–5 minutes of injection at the defect site. The scaffold itself contains no cells. Instead, it acts as a temporary biological framework that signals the patient's own progenitor cells — drawn from the surrounding synovium and subchondral bone — to migrate in and lay down repair tissue. This is acellular matrix-induced chondrogenesis: the material promotes endogenous repair rather than supplying pre-formed cartilage.

The approach is additive rather than subtractive. Nothing is removed or disturbed during placement; the collagen layer is applied directly over the defect site without any requirement for debridement or alteration of surrounding tissue. Surgical alternatives typically clear the defect first — a subtractive step that this injectable pathway avoids entirely.

Structural remodelling progresses over 6–24 months as the biodegradable scaffold is gradually replaced by native-like repair tissue. That biodegradable quality is worth noting alongside the broader comparison in this article: Arthrosamid's polyacrylamide hydrogel, by contrast, is a permanent implant that remains in the joint lining indefinitely.

The current service pathway at London Cartilage Clinic is an ultrasound-guided outpatient injection — no general anaesthetic, no theatre admission, and no incisions. The scaffold is placed under real-time imaging guidance at the defect site within a single clinic appointment.

What Arthrosamid does — and what it does not

Arthrosamid sits in a different therapeutic category. The hydrogel — 97.5% water bound in a 2.5% polyacrylamide matrix — integrates into the sub-synovial lining within 10–14 days of injection, acting as a permanent mechanical buffer for the whole joint. It cushions load and may settle synovial inflammation, but it does not contact, repair, or regenerate the cartilage layer at the bone surface. This is not a shortcoming specific to Arthrosamid; it is the intended role. The therapeutic target is the joint lining, not the articular surface — which is why the two products are assessed by different outcome measures and should not be directly ranked against each other.

Because the hydrogel is not biodegradable, it is not resorbed or replaced over time. Once integrated into the synovium, it remains — a meaningful distinction from ChondroFiller's collagen scaffold, which is designed to be gradually replaced by the patient's own repair tissue.

The durability of pain relief is where iPAAG's evidence makes its case. Bliddal et al. documented safety and functional outcomes prospectively at six months (2021) and twelve months (2024). A 2025 retrospective cohort of 150 patients across KL Grade II–IV adds comparative context: iPAAG, hyaluronic acid, and corticosteroid all produced broadly similar VAS reductions at three months, but only iPAAG maintained modest improvement at twelve months while HA and steroid returned to near-baseline. Short-term pain reduction was similar across all three; the twelve-month durability was not.

At two years, roughly one in six patients in a 2025 PROMs cohort (314 knees, 269 patients) went on to require total knee replacement — a rate that concentrates in higher-grade OA. The same cohort found that older, non-diabetic patients at lower KL grades, particularly those with bilateral OA, were the most likely to achieve meaningful improvement at 24 months. Selection, in other words, shapes outcomes substantially. No head-to-head randomised trial comparing Arthrosamid with ChondroFiller has been conducted, and given their anatomically distinct targets, designing one would present fundamental methodological challenges.

When both injections may apply: the CFI+ combination pathway

Some knees present both pathology patterns simultaneously — a discrete focal defect at the articular surface alongside broader synovial inflammation or global joint degradation. Where independent clinical assessment confirms a separate rationale for each, the CFI+ combination pathway offers both components in a single clinic visit: ChondroFiller placed at the cartilage defect on the bone surface, and Arthrosamid integrated into the sub-synovial lining.

The mechanism distinction established in earlier sections matters here, too. These are not two versions of the same treatment amplifying a shared effect. ChondroFiller is the scaffold component — recruiting progenitor cells to support endogenous cartilage repair at a focal site. Arthrosamid is the cushioning component — providing permanent mechanical buffering at the joint lining. Combining them does not speed up cartilage repair, nor does it extend Arthrosamid's role beyond the synovium. The two injections work in parallel on separate anatomical targets.

Guide costs for the dual-injection protocol are approximately £6,000, rising to approximately £11,000 where autologous mesenchymal stem cell augmentation is added — both figures should be confirmed with the treating clinic at assessment.

The evidence gap for CFI+ as a combined protocol should be stated plainly: no prospective comparative cohort exists for the combination. Each component carries its own independent evidence base, and it is that independent evidence — alongside imaging that confirms both indications separately — that provides the clinical justification. A patient with a focal defect alone, or diffuse OA alone, does not qualify for the combined pathway by virtue of the other condition being absent. The addition of either product requires its own diagnostic rationale, not an assumption that more is better.

Getting the right assessment before any injection decision

The decision between these pathways ultimately rests on a single clinical question: what does the MRI show? A focal, bounded defect at the articular surface points in one direction; diffuse joint-space narrowing with synovial involvement points in another; some knees show both. No injection decision — for ChondroFiller, Arthrosamid, the CFI+ combination, or none of the above — should precede a thorough review of current imaging alongside clinical examination.

What that assessment should include: characterisation of defect grade and location, confirmation of joint stability, review of whether the damage pattern is focal or global, and an honest conversation about what each candidate pathway can and cannot achieve for that specific MRI finding.

For patients with Grade III–IV lesions or complex defect patterns where delivery route and defect grading require careful judgment, specialist-level assessment matters — Professor Paul Y. F. Lee's work in cartilage preservation and image-guided injection is particularly relevant in those cases.

Patients considering this evaluation can arrange an initial consultation at londoncartilage.com.

1. [1] 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
2. [2] 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