Two injections, two different parts of the knee

Patients researching these two injections often arrive expecting a straightforward comparison — which one works better for a damaged knee? The more useful question is what each injection is actually treating, because ChondroFiller® and Arthrosamid® address different structures inside the joint and are not interchangeable.

ChondroFiller® is an injectable collagen scaffold. Placed under ultrasound guidance as an outpatient procedure, it coats the worn cartilage surface at the load-bearing bone ends and recruits the body's own cells to support repair — a regenerative process that unfolds over months. Arthrosamid® is a permanent polyacrylamide hydrogel that integrates into the synovial membrane lining the joint cavity, providing mechanical cushioning for the symptoms of knee osteoarthritis. It does not act on the cartilage surface at all.

That distinction — regenerative scaffold at the bone surface versus lasting cushion within the synovial lining — is the practical frame for everything that follows. Neither injection is universally superior; the more relevant question is which structure is driving a patient's symptoms and needs attention. In some knees, there is a separate clinical rationale for both.

What ChondroFiller does inside the joint

Collagen is the structural protein the body uses to build cartilage, and ChondroFiller® delivers it in a form the joint can engage with directly. Manufactured by meidrix biomedicals GmbH using a weak-acid extraction from rat-tail tendon, it preserves the telopeptide regions responsible for the protein's natural self-assembly — a meaningful distinction from enzymatically processed collagen, which loses those cross-linking sites and cannot organise in the same way.

The process the scaffold drives is called acellular matrix-induced chondrogenesis. Rather than introducing cells, ChondroFiller® creates a chemotactic environment that draws the patient's own progenitor cells — from the synovium and subchondral bone — into the defect, where they deposit new matrix and support the body's own repair processes over six to twelve months. A 2025 ex vivo model confirmed this directly, showing a 2.4-fold increase in DNA content by day 14, validating that cells do migrate into the material.

Published clinical outcomes give a concrete sense of what this translates to. In knee series, IKDC function scores have improved by approximately 30 points at twelve months, with MOCART MRI regeneration scores in the range of 70 to 87. More than 19,000 implantations have been reported globally over more than a decade.

Patient selection shapes the result. ChondroFiller® performs best in focal defects within a joint retaining reasonable cartilage integrity; in patients with pre-existing Tönnis grade 2–3 osteoarthritis, published cohort data show weaker outcomes. Because the scaffold requires time to stabilise after placement, the treating consultant will typically advise a graduated return to loading — full weight-bearing is usually deferred in the early post-injection period, not as an unusual restriction, but as part of allowing the repair process to take hold.

What Arthrosamid does inside the knee

Administered as a single outpatient injection under local anaesthetic, Arthrosamid® delivers a hydrogel composed of 97.5% water and 2.5% cross-linked polyacrylamide (iPAAG) into the knee joint. It integrates into the synovial membrane and remains there permanently — the material is non-resorbable and does not degrade or remodel over time. This is a material point for any consent conversation: what is placed stays in the joint indefinitely, and that should be clearly understood before proceeding, though it is not in itself a contraindication.

Its licensed indication is symptomatic relief in adult knee osteoarthritis, not cartilage repair or disease modification. Observational cohorts have reported meaningful reductions in pain and improvements in quality of life for up to three years following a single injection. How the hydrogel achieves this is still being studied, though researchers are not starting from nothing: synovial inflammation is closely linked to pain in knee OA, and NHS-funded research at the Robert Jones and Agnes Hunt Orthopaedic Hospital is currently mapping synovial fluid biomarker changes after injection to define the pathway through which synovial integration produces symptom relief.

Because it acts on the joint lining rather than the cartilage surface, and carries a knee-specific licence, Arthrosamid® does not substitute for a scaffold-directed treatment where focal surface damage is the primary concern. The two injections fill structurally distinct roles — a distinction the next section addresses directly.

Which patients suit each treatment

The starting point for deciding between these two injections is understanding which part of the knee is primarily driving the problem.

ChondroFiller® is most appropriate where imaging shows a discrete, focal cartilage defect in a joint that still retains meaningful cartilage elsewhere. Younger or more active patients with contained surface damage — without widespread joint degeneration — tend to fall into this category. The importance of that context is not arbitrary: published hip cohort data show that patients with pre-existing osteoarthritis at Tönnis grade 2–3 consistently achieve weaker outcomes, because the scaffold depends on a viable joint environment to recruit repair cells and support progressive tissue maturation.

Arthrosamid® fits a different clinical picture — symptomatic knee osteoarthritis that has not responded adequately to conservative measures such as physiotherapy or standard injection therapy, where the primary goal is meaningful, longer-duration pain relief rather than surface repair. It is a knee-specific treatment and does not address the underlying cartilage defect.

The two presentations can overlap. If your MRI shows both focal surface loss and signs of OA-related synovial inflammation, a combined pathway may be worth discussing — but only where there is a separate, clearly defined rationale for each injection. That distinction matters, because using both products does not mean the same problem is being treated twice through different routes.

A consultant-led assessment, with review of current imaging to characterise defect size, location, and the condition of surrounding cartilage, is needed before any pathway decision can be confirmed. Symptom pattern alone is not sufficient to establish which structure is the dominant driver.

When both injections are considered together

Some knees present with both a focal cartilage surface defect and the synovial inflammation characteristic of osteoarthritis — two distinct problems in the same joint. Combining ChondroFiller® and Arthrosamid® in a single visit is clinically meaningful in this situation precisely because the two injections are directed at different anatomical targets: ChondroFiller® addresses the cartilage surface at the load-bearing bone ends, while Arthrosamid® acts on the synovial membrane lining the joint cavity. They are not two versions of the same thing; they work through different mechanisms on different structures.

No direct trial evidence yet exists for this combination as a paired protocol — the rationale is mechanistic rather than RCT-confirmed, and patients should understand that plainly. What is established is the individual evidence base for each component: ChondroFiller® as a regenerative scaffold that supports endogenous surface repair, and Arthrosamid® as a longer-acting hydrogel for symptomatic OA management. Where both problems are present and each warrants treatment in its own right, the case for a combined approach rests on that complementarity, not on the assumption that more product means more benefit.

At MSK Doctors, consultants discuss a combined pathway only where assessment identifies a separate clinical rationale for each injection. Whether that applies — given your defect characteristics, OA grade, and symptom pattern — is confirmed at consultation, not before it.

Getting a decision at MSK Doctors

The right treatment depends on whether imaging shows a focal cartilage surface defect, synovial OA disease, or both — and only a clinical assessment with up-to-date imaging can establish that for your joint. Both ChondroFiller® and Arthrosamid® are delivered as ultrasound-guided outpatient injections; neither requires theatre admission or general anaesthetic.

MSK Doctors is a CQC-registered, consultant-led group. Patients are seen without a GP referral and without the waiting associated with NHS pathways. For non-London patients, consultations and imaging — including an Open MRI scanner — are available from Sleaford, Lincolnshire (NG34) and Grantham (NG31). London-based readers can access equivalent consultant-led assessment through the London Cartilage Clinic.

To book an assessment and discuss which pathway fits your imaging and goals, visit mskdoctors.com.

1. [1] 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
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] Influence of cartilage defects and a collagen gel on integrity of corresponding intact cartilage: a biomechanical in-vitro study. (2024). https://doi.org/10.1007/s00402-024-05530-z https://doi.org/10.1007/s00402-024-05530-z
4. [4] Controlled randomized multicenter study comparing ChondroFiller liquid with microfracturing for focal cartilage defects of the knee. (2016). https://doi.org/10.5348/VNP05-2016-1-OA-1 https://doi.org/10.5348/VNP05-2016-1-OA-1
5. [5] Development of an Ex Vivo Osteochondral Biomimetic Platform for Mechanistic Investigation of Cartilage Regeneration. (2025). https://doi.org/10.3390/ijms262311759 https://doi.org/10.3390/ijms262311759
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