Two injections, two different jobs inside the knee

Most patients searching for a comparison between these two injections expect a clear winner. The more useful answer is that ChondroFiller® and Arthrosamid® are not competing for the same job — they act on entirely different structures inside the knee, which reframes the question from 'which is better?' to 'which matches your joint's problem, or do you need both?'

ChondroFiller® is an injectable collagen scaffold placed under ultrasound guidance at the load-bearing cartilage surface. Rather than filling the defect with a synthetic material, it provides a chemotactic matrix that recruits the patient's own progenitor cells, supporting endogenous repair through a process called acellular matrix-induced chondrogenesis. Think of it as resurfacing a worn road: the scaffold creates the conditions for the body to lay down new material where the surface has broken down.

Arthrosamid® is a 2.5% polyacrylamide hydrogel — 97.5% water — injected into and around the synovial lining. It integrates permanently into that tissue, reducing the mechanical load and modulating the inflammatory environment that synovial disease produces. It does not repair cartilage. The analogy holds: Arthrosamid® is closer to fixing the drainage beneath the road than resurfacing the tarmac itself.

Both are CE-marked, outpatient injections delivered under ultrasound guidance and available as self-funded private treatments in the UK. Neither is currently funded by the NHS. The sections that follow work through which patients suit each option — and when both are appropriate together.

When ChondroFiller is the right starting point

Candidacy for ChondroFiller® rests on one central question that an MRI can answer: is the cartilage damage focal and bounded, or has it spread across the whole joint surface?

The scaffold is designed for discrete defects — areas of cartilage loss typically up to around 6 cm² — in a knee that is otherwise mechanically stable. Diffuse, bone-on-bone degeneration affecting the whole joint surface does not fit that profile. The typical candidate is a younger or more active patient with a localised lesion visible on imaging, adequate bone stock beneath the defect, and no significant ligament laxity or malalignment that would continue to stress the repair site. Where those conditions are met, the structural environment is permissive enough for biological repair to take hold.

That suitability matters because of precisely how the treatment works. After ultrasound-guided placement, the injectable collagen scaffold gels in situ and begins recruiting the patient's own synovial and subchondral progenitor cells — a process called acellular matrix-induced chondrogenesis. The scaffold supports the body's own repair processes rather than substituting for them with a permanent synthetic implant; the distinction from a mechanical filler such as a hydrogel is meaningful in both biology and long-term expectation.

Clinical data give a sense of what that process produces. Across knee studies, International Knee Documentation Committee scores improve by approximately 30 points at 12 months, with a reoperation rate of 3–8% — figures that compare favourably with microfracture (up to 41% reoperation) and ACI or MACI procedures (up to 37%). A 2025 prospective study (PMC12498443, n=59) recorded significantly better cartilage quality at follow-up arthroscopy in treated versus control cases: median Outerbridge score 1.5 versus 3.0 (p=0.006). That study was conducted in the wrist, and the knee literature mirrors rather than directly replicates those findings. Safety data from more than 19,000 treated cases since 2013 show zero serious adverse events and a complaint rate of approximately 0.06%.

When Arthrosamid makes more sense

The patient profile that suits Arthrosamid® is shaped by a different diagnostic picture from the focal-defect candidate described above. Where pain arises primarily from diffuse synovial inflammation — the pattern typical of mild-to-moderate knee osteoarthritis — Arthrosamid®'s site of action matches the source of symptoms. Patients in this group have usually already tried anti-inflammatory medication and structured exercise without adequate relief; Arthrosamid® is a logical next step when those conservative measures fall short.

For this cohort, several practical features are clinically relevant. The treatment is a single injection, and the hydrogel's permanence in the joint provides mechanical durability without the need for repeat procedures. Registry evidence shows sustained pain reduction at two to three years on average from a single administration, with some cohorts followed to five years — a longer continuous follow-up horizon than is currently available for ChondroFiller® in the knee, where published data centres on 12-month outcomes. That asymmetry in evidence depth is worth acknowledging honestly when patients weigh their options.

Recovery is appreciably lighter. Approximately 10 to 14 days of relative rest allows the hydrogel to integrate before the joint is loaded normally; no structured physiotherapy programme is required afterwards. For patients who cannot commit to a rehabilitation schedule, or for whom the biological regeneration timeline of ChondroFiller® is not a practical match, this matters.

The permanence of the hydrogel implant is both its durability advantage and the reason careful patient selection is essential. Because the material remains in the joint indefinitely, pre-procedure imaging review and a thorough discussion of individual anatomy and expectations are a prerequisite — not a formality. It is not grounds for avoiding the treatment, but it does make assessment more than a routine step.

How the evidence compares — and where gaps remain

No randomised controlled trial has yet compared ChondroFiller® and Arthrosamid® directly — the evidence for each product comes from separate clinical streams, and those streams are not straightforwardly comparable.

ChondroFiller®'s evidence base draws on a European Clinical Evaluation Report consolidating post-market experience and on prospective knee studies with validated functional and structural endpoints measured to 12 months. The primary outcome in that body of work is cartilage quality — objective repair tissue assessed at follow-up imaging or arthroscopy. Arthrosamid®'s data come from a larger registry specifically in knee osteoarthritis, with pain and function scores tracked to five years from a single injection. The difference in follow-up horizon is real, but so is the difference in what is being measured: one literature asks whether joint structure has improved; the other asks whether pain has subsided.

Treating those two questions as interchangeable produces a false equivalence. A five-year pain registry and a 12-month cartilage quality study are not the same kind of evidence, and neither makes the other irrelevant — each speaks to what the treatment in question was designed to do. Where ChondroFiller®'s structural outcome data are compelling at available time points, longer-horizon knee-specific follow-up remains an area still being built out. Where Arthrosamid®'s symptom data are extensive, the literature does not address tissue regeneration, because tissue regeneration is not the mechanism.

For UK patients, both carry CE marking as Class III devices and are available through licensed practitioners. ChondroFiller® does not hold FDA approval, which is relevant only to patients considering treatment in the United States.

The combination approach: when both injections work together

Some patients, after reviewing their MRI, face a picture that fits neither profile described above: a discrete cartilage defect on the load-bearing surface alongside synovial thickening consistent with diffuse osteoarthritic inflammation. For that patient, choosing between ChondroFiller® and Arthrosamid® sets up a false dilemma — both structures are damaged, and each injection addresses a different one.

In this scenario, the two products can be delivered during the same outpatient appointment. ChondroFiller®'s collagen scaffold is placed at the cartilage defect site, where it supports endogenous repair at the bone surface. Arthrosamid®'s polyacrylamide hydrogel is injected into the synovial lining, providing mechanical cushioning and modulating inflammation from within the joint membrane. The mechanisms are distinct; the anatomical targets are distinct. These are not components of a blended compound or a single combined procedure — they are two separate injectable therapies working through separate biological pathways at the same time.

The patient most likely to benefit from combination delivery typically presents with more advanced knee OA, where imaging confirms both a focal chondral defect and meaningful synovial disease. Addressing both in a single appointment rather than staging them across separate visits carries an obvious practical benefit for patients who have travelled for specialist assessment.

The evidence context is important to state clearly. Combination delivery is supported by clinical practice experience and registry-level outcome data — including published series reporting IKDC improvements of approximately 30 points sustained to three years — rather than by a prospective randomised controlled trial. It is an emerging, well-reasoned approach that warrants honest framing as such, not as an established standard of care.

What to expect from assessment and treatment at MSK Doctors

Deciding between these options in practice starts with imaging. A consultant review of current MRI findings — or, where available, AI-assisted onMRI™ analysis — establishes defect size, synovial condition, and joint stability before any treatment plan is finalised. That assessment shapes whether ChondroFiller®, Arthrosamid®, or a combination approach is appropriate; treatment selection follows the clinical picture rather than a fixed default.

Both treatments are delivered as ultrasound-guided outpatient injections — no theatre, no general anaesthetic, no surgical wound. The post-procedure period differs between them: ChondroFiller® requires structured physiotherapy to support the scaffold as repair tissue develops, whereas Arthrosamid® calls for a shorter period of relative rest while the hydrogel integrates into the synovial lining. The clinical team will set out a specific recovery plan based on whichever pathway applies.

Both are self-funded private treatments. Guide costs start from approximately £3,000 per treatment; the final figure depends on defect size and the number of boxes required, confirmed after imaging review at the time of booking. Neither treatment is currently available on the NHS or covered by most major UK private medical insurance policies.

MSK Doctors clinics in Sleaford (NG34) and Grantham (NG31) accept patients without a GP referral and without NHS-style waiting lists. To arrange a consultant assessment, visit mskdoctors.com.