Why this combination is being tested in severe knee OA

For many patients with severe knee osteoarthritis, the choice can feel binary: live with the pain or proceed to a total knee replacement. Kellgren-Lawrence Grade IV — the most structurally severe stage — means bone-on-bone contact, with near-complete or full loss of cartilage across the joint surface. Conventional cartilage repair procedures such as MACI or OATS are designed for focal defects in younger, less-damaged joints; they were not developed for this level of diffuse, end-stage wear.

ChondroFiller injection is an acellular type I collagen scaffold. Delivered into the knee under ultrasound guidance as an outpatient procedure, it gels in situ and forms a structural matrix to which the patient's own progenitor cells — including chondrocytes and stem cells circulating in the synovial environment — can migrate and adhere. The scaffold itself contains no live cells; repair depends on what the joint can recruit.

Mesenchymal stem cells (MSCs), sourced here from the patient's own blood-derived concentrate, add what the scaffold cannot provide alone: cells capable of differentiating toward cartilage-forming chondrocytes and releasing anti-inflammatory paracrine signals that may slow ongoing joint breakdown.

The combination addresses two distinct biological problems simultaneously. The collagen matrix provides structural retention and a chemotactic cue for cell recruitment; the MSCs supply both reparative cellular material and trophic signalling to dampen the inflammatory environment. ChondroFiller injection holds CE-mark approval for focal cartilage defects, so its application in diffuse Grade IV OA is an extension of that established indication — one this article's evidence comes from an early prospective trial designed to begin testing.

What the Weninger 2025 trial actually tested

The trial at Avancell Medical in Vienna, published in the Journal of Surgery on 27 June 2025 and attributed to Weninger et al., enrolled 25 patients with Kellgren-Lawrence Grade IV knee osteoarthritis — the structural threshold at which total knee replacement is ordinarily the standard recommendation. Allocation between the two arms followed individual clinical decisions rather than a randomised protocol: 12 patients received intra-articular MSC concentrate alone, and 13 received ChondroFiller injection combined with MSC concentrate. In both cases the MSCs were autologous — drawn from the patient's own blood-derived concentrate rather than a donor source.

Outcomes were assessed at two months using a structured set of measures: all five subscales of the Knee Injury and Osteoarthritis Outcome Score (KOOS — a validated patient-reported instrument covering pain, symptoms, activities of daily living, sport and recreation, and quality of life), a visual analogue scale for pain, active range of motion, MRI findings including bone marrow oedema, joint effusion and synovitis, and serum MMP-13 as a biomarker of active cartilage matrix breakdown.

Two months is a short observation window for a chronic structural disease. The endpoint can capture early functional response and inflammatory signals, but it cannot address durability, structural repair quality, or whether either treatment meaningfully alters the trajectory of Grade IV joint deterioration over time.

What the results showed — KOOS, MRI, and cartilage markers

Across all five KOOS subscales — pain, symptoms, activities of daily living, sport and recreation, and quality of life — both treatment groups recorded statistically significant improvements from baseline at two months (p<0.01). It suggests the MSC component alone produces a measurable early functional effect in Grade IV knee OA, a population not typically considered a target for regenerative injection. The combination arm, however, consistently achieved higher mean KOOS scores than the MSC-alone group across every domain, as well as lower VAS pain ratings.

On MRI, the combination group showed less bone marrow oedema and less synovitis than the MSC-alone group at two months. These are reduced inflammatory markers on imaging — not evidence of structural cartilage regeneration at this early timepoint, but a signal that the collagen scaffold may be modifying the joint's inflammatory environment rather than simply cushioning it.

MMP-13 levels were lower in the combination group. MMP-13 is an enzyme the body releases when cartilage matrix is actively breaking down; its reduction in the combination arm may indicate that the collagen scaffold attenuates ongoing degradation, adding a biological mechanism beyond the symptomatic relief both groups experienced. The word 'may' is deliberate: biomarker changes at two months do not confirm long-term cartilage protection.

Neither group experienced a serious adverse event over the observation period. For context, the broader intra-articular MSC injection literature — across nearly 1,900 patients in a 2024 systematic review — records a 12.3% rate of transient, self-resolving reactions such as injection-site swelling, with no cases of infection or serious systemic complications.

The biological case — why scaffold plus stem cells may work together

Laboratory data from a 2025 ex vivo model published in the International Journal of Molecular Sciences provides the biological plausibility for what the Weninger trial measured clinically. The model used 61 osteochondral explants harvested from femoral condyles of patients undergoing knee arthroplasty — human tissue, tested under controlled conditions — with 4 mm full-thickness chondral defects created and assigned to one of six treatment groups, including ChondroFiller alone and ChondroFiller combined with MSCs.

ChondroFiller alone produced a 2.4-fold increase in DNA content within the defect by day 14, compared with untreated controls. DNA content serves as a proxy for cell number: this result demonstrates that the collagen scaffold actively draws host cells into the repair site rather than simply filling it passively.

Adding MSCs to the scaffold further increased collagen deposition and glycosaminoglycan (GAG) production — GAGs are the structural molecules that give healthy articular cartilage its shock-absorbing capacity and resilience under load. Neither component produced both outcomes on its own: the scaffold recruited host cells; the MSCs enhanced the quality of matrix those cells then deposited.

These are ex vivo findings, not clinical outcomes, and the controlled laboratory environment does not replicate the mechanical and biochemical complexity of a living joint. What they offer is a mechanistic explanation for the MMP-13 and MRI signals described in the Weninger trial — biological plausibility for the combination, not confirmation of durable structural repair.

What the trial cannot yet answer — honest limits of the evidence

Taken on its own terms, the Weninger trial is genuinely interesting early-phase evidence. Taken as proof of a new standard of care, it is not.

The most fundamental limitation is design. There was no randomisation and no blinding: treatment allocation followed individual clinical judgement, which means the two groups may have differed in baseline characteristics, disease severity, or other factors that influenced outcomes independently of the treatment. Without randomisation, it is not possible to attribute the superior results in the combination arm to the intervention alone.

Sample size compounds this. Twenty-five patients across two groups is too small to draw reliable conclusions about efficacy, and two months is wholly inadequate to assess structural durability in a chronic, progressive disease. Grade IV knee OA does not declare itself stable or unstable within eight weeks. Whether any functional gains seen at two months persist, consolidate, or reverse at one, two, or five years remains entirely unknown from this study.

The patient population also sits well outside the typical indication for a scaffold designed to fill focal chondral defects. Grade IV OA involves near-complete or full cartilage loss across one or more compartments — not a contained lesion amenable to targeted repair. Applying findings from this group directly to younger patients with discrete focal defects, where most cartilage-repair evidence is generated, is not straightforward.

No data are available on cartilage volume change, total knee replacement conversion rates, or MRI-based structural repair beyond the two-month mark. These are the outcomes that would matter to a patient weighing whether an injection can meaningfully delay surgery.

Finally, ChondroFiller holds CE marking as a Class III medical device and is available across European clinics. It lacks FDA approval, limiting its direct relevance to patients in the United States. UK patients considering this pathway should confirm current device status with their clinical team before proceeding.

Larger, randomised, blinded trials with follow-up of at least two to three years are the necessary next step before this combination can be considered established evidence.

What this means if you have advanced knee OA and want options now

For patients at Kellgren-Lawrence Grade III or IV who are not yet ready to commit to a knee replacement — or who are not currently considered suitable for one — the Weninger trial offers a plausible but provisional signal. The combination described there may be worth raising with a specialist as one option to explore, not as an alternative to conventional pathways that should be dismissed, but as an area where the evidence is developing.

Patients with less advanced OA or a contained focal cartilage defect stand on firmer ground: the evidence base for ChondroFiller injection as a standalone scaffold treatment is more established within its intended indication, and the treatment is more clearly suited to that structural picture.

In either case, a thorough structural assessment must come before any treatment decision. MRI with cartilage-specific imaging establishes how much cartilage remains, where the damage is concentrated, and whether a patient's joint architecture is compatible with a regenerative approach or better served by a different pathway. Without that picture, any treatment discussion is premature.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. Consultant-led assessment at the Sleaford or Grantham clinics includes imaging that can clarify the structural situation before any decision is made. Book at lincolnshireknee.co.uk.

The honest clinical position at this stage of the evidence is measured: two months of outcomes data in 25 patients with end-stage OA is an encouraging start, not a conclusion. Patients and clinicians alike should weigh it accordingly — with genuine interest and appropriate caution.

1. [1] 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
2. [2] Intra-Articular Injection of Human Bone Marrow–Derived Mesenchymal Stem Cells in Knee Osteoarthritis: A Randomized, Double-Blind, Controlled Trial. (2025). https://doi.org/10.1177/09636897241303275 https://doi.org/10.1177/09636897241303275
3. [3] IMPLANTATION OF CHONDROFILLER LIQUID® AS A SCAFFOLD MATERIAL FOR THE TREATMENT OF CHONDRAL LESIONS OF THE KNEE JOINT. (2024). https://doi.org/10.5272/jimab.2024304.5936 https://doi.org/10.5272/jimab.2024304.5936
4. [4] Controlled, randomized multicenter study to compare compatibility and safety of ChondroFiller liquid with microfracturing of patients with focal cartilage defects of the knee joint. (2016). https://doi.org/10.5348/VNP05-2016-1-OA-1 https://doi.org/10.5348/VNP05-2016-1-OA-1