Two injections, two different jobs in the knee

Patients comparing these two injections often assume one must be superior to the other. The more useful question is different: which matches what is actually wrong with your knee?

ChondroFiller injection is a regenerative collagen scaffold. Delivered under ultrasound guidance, it coats the articular cartilage — the load-bearing surface on the ends of the bones — and recruits the body's own progenitor cells to rebuild cartilage matrix over the following months. It is targeting a structural problem: focal damage to the cartilage itself.

Arthrosamid is a synthetic polyacrylamide hydrogel that, once injected, permanently integrates into the synovial lining — the thin membrane that encases the knee joint and produces its lubricating fluid. Inflammation of that lining is a principal driver of pain in knee osteoarthritis, and Arthrosamid works by cushioning the joint mechanically and settling that inflammatory environment. It does not repair cartilage.

Because the two products act on anatomically separate structures — articular cartilage versus synovial membrane — they are not alternatives in the usual sense. A patient with a discrete focal cartilage lesion and a patient with diffuse osteoarthritis pain may both present asking about a knee injection, but they need different things. That distinction, not a head-to-head comparison, is what should guide the decision.

How each injection works in the knee

The mechanics of each injection differ at the molecular level, which is what makes them suited to different diagnoses.

ChondroFiller injection delivers a cell-free type I collagen gel into the articular cartilage defect under ultrasound guidance. The gel self-sets within three to five minutes of placement, bonding to the surrounding cartilage surface and forming a temporary, biodegradable scaffold. Over the following six to twenty-four months, the patient's own progenitor cells migrate into that scaffold, mature into chondrocytes, and deposit new cartilage matrix — a process known as matrix-induced chondrogenesis. As the regenerated tissue forms, the scaffold is gradually resorbed. The aim is durable biological repair rather than symptom suppression alone. ChondroFiller carries CE-marking as a Class III medical device and has been used across Europe for close to two decades.

Arthrosamid is a permanent, non-biodegradable hydrogel composed of 97.5% water and 2.5% polyacrylamide. A single injection introduces it into the joint space, where it integrates with the synovial lining — the membrane that encases the knee cavity and produces its lubricating fluid. Inflammation of this lining is closely linked to the pain experienced in knee osteoarthritis; Arthrosamid cushions the joint mechanically and settles that synovial environment, providing sustained relief without being resorbed or requiring repeat injections in the near term. It is CE-marked specifically for knee osteoarthritis.

The longevity models are therefore distinct: ChondroFiller targets lasting biological repair of the cartilage surface; Arthrosamid delivers lasting structural cushioning of the joint lining. Neither is currently available on the NHS.

Which patients are right for ChondroFiller injection

The clearest candidate for ChondroFiller injection has a focal cartilage defect — a discrete, localised area of Grade III or IV damage (partial-to-full thickness cartilage loss), most often arising from a sports injury, a twisting accident, or an osteochondral dissecans (OCD) lesion. The surrounding cartilage in the rest of the joint is broadly intact. Pain tends to be compartment-specific rather than a diffuse, whole-knee ache, and patients frequently report mechanical symptoms — catching, swelling after activity, or a familiar point of tenderness — tracing back to an identifiable incident or injury.

There is no upper age limit for the treatment. Active adults in their 60s and 70s who want to defer or avoid knee replacement are a recognised group, provided the damage pattern remains focal rather than end-stage. Similarly, there is no defect-size ceiling when ChondroFiller is delivered as an ultrasound-guided injection, making it applicable to a wider range of lesion extents than some earlier cartilage repair techniques.

The injection is less appropriate where cartilage loss is diffuse across the whole joint surface — the territory of advanced bone-on-bone osteoarthritis — or where a significant structural alignment problem is the underlying driver of wear. In that latter scenario, a joint-preservation approach such as osteotomy to correct the mechanical axis may need to accompany any cartilage treatment.

Confirming a focal lesion requires imaging before any injection is planned. MRI with cartilage-specific sequencing identifies lesion depth, boundaries, and the condition of the surrounding tissue — the detail needed to judge whether the defect is genuinely contained. A consultant assessment interprets that imaging alongside the patient's history to confirm suitability.

Which patients are right for Arthrosamid

Arthrosamid suits a different presentation altogether: the patient with gradual-onset, diffuse knee pain rather than a single identifiable injury. The typical profile is mild-to-moderate osteoarthritis — morning stiffness that eases within half an hour, an aching discomfort after walking or stairs, and symptoms spread across the joint rather than concentrated at one compartment. Imaging tends to show generalised joint-space narrowing and signs of synovial thickening rather than a contained cartilage lesion.

The treatment is not appropriate for everyone with knee pain. Inflammatory arthritis — rheumatoid or psoriatic — is a contraindication, as is end-stage bone-on-bone disease where joint replacement is the more appropriate next step. Both exclusions carry real clinical weight; using Arthrosamid outside these boundaries is unlikely to deliver meaningful benefit.

For patients with patellofemoral osteoarthritis specifically, the 2022 case series by Maulana, Cole, and Lee — referenced in the mechanism discussion above — observed a reduction in patellofemoral bone marrow lesions following a single iPAAG injection, suggesting the effect may extend modestly beyond mechanical cushioning alone. This remains an additional clinical observation rather than the primary indication.

Clinical studies report a success rate above 70%, with pain relief lasting two to five years from a single injection — substantially longer than most corticosteroid or hyaluronic acid options.

Where a patient presents with both focal cartilage wear and diffuse synovial inflammation, both treatments may be clinically relevant; the combination scenario is examined in the following section.

Evidence, outcomes, and what to realistically expect

ChondroFiller injection — clinical evidence

Independent cohort data report 70–85% sustained symptom relief at five years, and patient-reported IKDC scores improve by approximately 30 points over 12 months. A 2025 study published in PMC found significantly better cartilage quality in treated patients versus controls at follow-up arthroscopy — median Outerbridge grade 1.5 versus 3.0 (p=0.006). The arthroscopic measurement reflects standard research methodology for assessing cartilage; it is separate from the current outpatient injection pathway but the quality findings are directly relevant to understanding the treatment's regenerative effect. ChondroFiller has been CE-marked and in clinical use across Europe for approximately two decades; most outcome data in injection mode come from independent cohorts rather than large randomised controlled trials.

Arthrosamid — clinical evidence

Bliddal and colleagues published a six-month prospective study in 2021 and a 12-month open-label follow-up in 2024 in the Journal of Orthopaedic Surgery and Research, both confirming effectiveness and safety in knee OA. Across clinical studies, more than 70% of patients report significant pain reduction; relief from a single injection typically lasts two to five years — substantially longer than most corticosteroid or hyaluronic acid options. Long-term randomised data beyond five years are limited, and the published evidence is largely open-label or prospective cohort in design.

No head-to-head trial comparing ChondroFiller with Arthrosamid has been conducted, which is unsurprising given that the two products treat different diagnoses. Aggregate success rates cannot meaningfully be set against each other across such different indications. Both carry established safety profiles in the published literature, with low rates of serious adverse events.

Cost, procedure, and making the right choice

Both injections are delivered in an outpatient clinic setting — ultrasound-guided, under local anaesthetic, with patients going home the same day. Neither requires general anaesthesia or a hospital admission, which is one practical reason patients explore these routes before committing to surgery.

On cost, approximate UK private fees: Arthrosamid starts at around £3,000 per injection; ChondroFiller injection ranges from approximately £3,000 to £8,000 depending on the volume of collagen scaffold required. Neither treatment is currently available on the NHS.

Choosing between the two — or determining whether both are relevant — is not reliably possible from symptoms alone. Focal cartilage damage and diffuse osteoarthritis can produce very similar discomfort, and the distinction turns on lesion type, grade, size, and location. Clinical examination combined with MRI imaging provides the information needed to differentiate them. Where imaging confirms both a focal cartilage defect and broader synovial pathology, a consultant may recommend delivering ChondroFiller and Arthrosamid at the same appointment — addressing both anatomical sources of pain in a single session. That is a clinical decision reached after assessment, not a patient-driven add-on.

Lincolnshire Knee, part of the MSK Doctors group, offers consultant-led assessment at clinics in Sleaford (NG34) and Grantham (NG31) without the need for a GP referral. Cartilage MRI — including onMRI analysis where indicated — is part of the diagnostic work-up that informs treatment planning. Further information is available at lincolnshireknee.co.uk.