Who each injection is actually for

The decision between ChondroFiller and Arthrosamid comes down to a single clinical question: what type of knee damage is actually present?

ChondroFiller is designed for patients with a focal, contained, full-thickness cartilage defect — a discrete area of damage, typically less than 2 cm², where the surrounding joint remains largely intact. It is best suited to younger or more active patients where the wider knee has not yet developed diffuse osteoarthritic change. When pre-existing widespread OA is already present, published evidence suggests outcomes are significantly poorer.

Arthrosamid is suited to established knee osteoarthritis — Kellgren–Lawrence grade II to IV — where cartilage loss is more generalised and the primary goal is sustained pain relief and delaying surgery. It does not regenerate cartilage; it targets symptom management in a joint already showing diffuse degenerative change.

The clearest decision rule: a focal defect in an otherwise reasonable joint points toward ChondroFiller; diffuse osteoarthritis across the joint points toward Arthrosamid. Neither treatment is interchangeable with the other, and neither is currently NHS-funded — both are private-pay pathways requiring specialist delivery.

Determining which applies requires a thorough assessment, including weight-bearing X-rays and MRI to characterise both the lesion pattern and the condition of the surrounding joint. A clinician's judgement — not product preference — should drive that decision.

How each injection works inside the knee

Despite sharing the same delivery route — an ultrasound-guided outpatient injection — the two products work through entirely different mechanisms once inside the knee.

ChondroFiller is an acellular collagen type I hydrogel scaffold. Placed into a focal cartilage defect, it forms a matrix within the lesion that recruits the patient's own progenitor cells from the surrounding synovium and subchondral bone — a process known as acellular matrix-induced chondrogenesis. ChondroFiller contains no cells; it does not itself produce new cartilage. Instead, it supports the body's own repair processes, providing the structural environment through which tissue regeneration can occur over 6 to 24 months. Physiotherapy during this period is not optional — the scaffold needs appropriate mechanical stimulus to guide repair, and in-vitro data suggest that full weight-bearing before the defect is stably filled carries a risk of adjacent cartilage stress.

Arthrosamid is a 97.5% water, cross-linked polyacrylamide hydrogel (iPAAG). A single 6 mL injection integrates with the synovial membrane, where it acts as a permanent, non-biodegradable cushion that provides biomechanical cushioning and may modulate the intra-articular environment. It does not regenerate cartilage, and it is not designed to. Its durability — the hydrogel remains in the joint indefinitely — underpins its long-lasting benefit, and is also a factor worth discussing explicitly with a specialist before proceeding.

What the clinical evidence shows

The evidence bases for the two products are genuinely asymmetric — and that asymmetry matters when calibrating what either injection can realistically deliver.

ChondroFiller has a modest but meaningful dataset in the knee. A prospective series of 17 patients demonstrated statistically significant improvements in Lysholm and IKDC functional scores at 3, 6, and 12 months; functional gains appeared primarily in the first six months and plateaued thereafter. A 2025 arthroscopic study by Demmer et al. found significantly better cartilage quality at follow-up in treated joints compared with controls — median Outerbridge grade 1.5 versus 3 (P=0.006), with ICRS scores similarly superior; fibrous tissue formation occurred only in overfilled defects. Where the data turn unfavourable is in diffuse disease: a hip cohort including Tönnis grade 2–3 patients confirmed poor outcomes when ChondroFiller is applied in the presence of established widespread OA, reinforcing the focal-defect indication rather than undermining the product overall.

Arthrosamid carries a broader and longer evidence base. A 2022 systematic review across 463 patients found statistically significant pain and functional improvements at 52 weeks and 13 months, with positive results sustained at two years and no long-lasting adverse events reported. A 2025 retrospective cohort of 150 Kellgren–Lawrence grade II–IV patients showed VAS scores returning to baseline by 12 months in the hyaluronic acid and corticosteroid groups, while iPAAG improvements held — with iPAAG significantly outperforming corticosteroid at six months. Five-year efficacy and 10-year safety data from the extended IDA Study, first presented at WCO 2025, support the hydrogel's long-term durability; response rates of 60–80% are consistently reported across studies.

No head-to-head randomised controlled trial comparing ChondroFiller with Arthrosamid exists. In practice, however, that evidence gap matters less than it might appear: the indications are clinically distinct enough that a direct comparison is rarely the right clinical question.

Recovery, timelines, and what to realistically expect

Planning practically around either pathway starts with accepting that their recovery clocks run at very different speeds.

ChondroFiller sets in motion a biological repair process. The scaffold recruits the patient's own cells, but tissue maturation takes anywhere from 6 to 24 months — and physiotherapy throughout that entire window is an active participation requirement, not an afterthought. In a published series of 17 knee patients, functional gains in Lysholm and IKDC scores emerged within the first three to six months and largely plateaued by 12 months; the IKDC improvement in that series approached 30 points. Most patients should not expect meaningful symptom relief in the first few weeks — the repair biology simply needs time. High-impact activities are typically deferred until the later phase of the recovery pathway, guided by specialist review and follow-up imaging.

Arthrosamid operates on a different timeline entirely. Many patients report some pain reduction within 24 hours of injection, though the degree of early response varies. Maximum patient-reported benefit is typically reached around 12 weeks, and a 2025 retrospective cohort of 150 Kellgren–Lawrence grade II–IV patients found that VAS improvements remained stable at 12 months — unlike the hyaluronic acid and corticosteroid groups, whose scores had returned to baseline by the same point. Because the hydrogel is permanent, there is no planned repeat-injection schedule; individual responses vary, however, and a meaningful improvement is not achieved in every case.

For both pathways, follow-up assessment — including imaging where indicated — helps a specialist judge how repair or integration is progressing and whether further management is needed.

Access, cost, and candidacy in the UK

Neither injection is routinely funded through the NHS in the UK, making private access the practical reality for most patients. Arthrosamid holds CE-mark status and is available at a small number of NHS centres, but coverage is not uniform across England. ChondroFiller is a CE-marked Class III medical device available through private clinics only, with a guide cost from approximately £3,000 — confirmed at the point of assessment with the treating clinic. Arthrosamid pricing should similarly be verified directly with the provider, as costs vary.

Pre-injection imaging is a genuine clinical requirement for both pathways, not an administrative formality. For ChondroFiller, MRI characterisation of defect size, depth, and containment is essential; diffuse joint disease or Outerbridge grade 4 lesions with surrounding osteoarthritis fall outside the suitable range. For Arthrosamid, imaging helps confirm OA grade — the product spans Kellgren–Lawrence grades II to IV — and exclude structural pathology better managed surgically. AI-assisted MRI analysis (onMRI™, used at specialist centres) can help map cartilage depth and grade as part of this candidacy workup.

Beyond imaging, additional candidacy factors include age, BMI, and inflammatory burden for Arthrosamid; defect containment and overall joint integrity are the parallel considerations for ChondroFiller. Both treatments require specialist imaging-guided delivery, and a knee specialist assessment draws these threads together. A GP referral is not required to book an appointment.

Getting the right assessment before deciding

Narrowing this decision to the right injection comes down to a single clinical distinction: is the damage focal and contained, or is it diffuse joint-wide disease? That question — not symptoms alone, not age, not how long the pain has lasted — determines which pathway, if either, is appropriate. ChondroFiller addresses the first scenario; Arthrosamid the second. Where the damage pattern is mixed, or where the clinical picture is ambiguous, a different approach altogether may serve the patient better.

Confirming that distinction requires specialist imaging and a structured clinical assessment. MRI findings, Kellgren–Lawrence grading, lesion size and containment, activity level, and individual treatment goals all feed into the recommendation — and a thorough review may reveal that PRP, microfragmented adipose tissue, or a combined injection strategy is the more appropriate starting point for that patient.

Lincolnshire Knee (Sleaford NG34 and Grantham NG31) provides consultant-led knee assessment, including cartilage pathway planning, without requiring a GP referral. Professor Paul Y. F. Lee, who has published in focal cartilage repair — including outcomes following injectable scaffold treatment — is available for direct consultation on candidacy and imaging review.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. Book an assessment at lincolnshireknee.co.uk.

1. [1] 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
2. [2] 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
3. [3] THE IMPACT OF PATIENT FACTORS ON THE MINIMAL CLINICALLY IMPORTANT DIFFERENCE OF ARTHROSAMID POLYACRYLAMIDE HYDROGEL INJECTION FOR KNEE OSTEOARTHRITIS: A COHORT STUDY. (2025). https://doi.org/10.1016/j.joca.2025.02.648 https://doi.org/10.1016/j.joca.2025.02.648
4. [4] A Systematic Review of the Novel Compound 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
5. [5] Inflammatory markers in the knee in patients with osteoarthritis assessed with ultrasound: a secondary analysis of a randomized controlled trial of Arthrosamid vs. Synvisc-One. (2024). https://doi.org/10.1016/j.joca.2024.03.100 https://doi.org/10.1016/j.joca.2024.03.100
6. [6] Intra-articular Arthrosamid® injection for knee osteoarthritis: A synovial fluid biomarker study. (2025). https://doi.org/10.1016/j.joca.2025.02.214 https://doi.org/10.1016/j.joca.2025.02.214