What the Weninger trial found

A prospective controlled trial published in the Journal of Surgery (27 June 2025) by Weninger, Reitböck, and colleagues at Avancell Medical in Vienna found that combining a ChondroFiller® injection with autologous mesenchymal stem cell (MSC) concentrate produced measurable improvements in patients with Kellgren-Lawrence Grade IV knee osteoarthritis — the most advanced stage of joint degeneration before replacement becomes the conventional recommendation. Both treatment arms improved significantly, but the combination outperformed MSC therapy alone across every measured outcome.

The study enrolled 25 patients and compared two approaches: MSC-rich concentrate injected alone (n=12) and the same MSC concentrate given alongside a ChondroFiller® liquid collagen injection (n=13). Outcomes were assessed at two months using the KOOS — a validated questionnaire scoring pain, symptoms, daily activities, sport and recreation, and knee-related quality of life — as well as a visual analogue pain scale, range of motion, and MRI. Both groups showed statistically significant improvements across all five KOOS subscales (p<0.01); the combination arm consistently reached higher mean scores.

The structural findings reinforced the patient-reported results. The combination group showed a greater reduction in MMP-13, a tissue marker reflecting active cartilage breakdown, suggesting the collagen scaffold may help slow degradation rather than simply mask symptoms. MRI also demonstrated less bone marrow oedema in the combination arm. No serious adverse events occurred in either group.

These results come from 25 patients assessed at two months — a short window for a condition as chronic and structurally complex as Grade IV knee osteoarthritis.

Why Grade IV knee OA makes this trial significant

Kellgren-Lawrence Grade IV represents the point at which imaging shows near-complete or full cartilage loss, bone-on-bone contact, and severe joint space narrowing across the affected knee compartment. For most patients who reach this stage, total knee replacement (TKR) is the clinically appropriate recommendation — a procedure with decades of evidence, predictable outcomes, and high satisfaction rates in suitable candidates.

The clinical gap arises in two specific groups: patients who are too young to accept the functional constraints and expected revision timeline of a primary implant, and those whose general health or comorbidities make major surgery a meaningful risk. For these patients, joint-preservation options at Grade IV are genuinely scarce. Most injection therapies — corticosteroids, hyaluronic acid, and even biologics such as PRP — are not designed to address the structural deficit present at this severity. Existing meta-analyses of intra-articular MSC injections, including a 2025 analysis of eight randomised controlled trials (502 patients) confirming significant WOMAC and KOOS improvements, largely draw on populations with milder disease; robust data specific to Grade IV remains thin.

That gap is precisely what gives the Weninger 2025 trial its clinical relevance: by its authors' account, it is the first formal evaluation of combining an injectable collagen scaffold with autologous MSC concentrate specifically in Kellgren-Lawrence Grade IV disease.

How ChondroFiller injection works as a collagen scaffold

ChondroFiller injection is an acellular Type I collagen hydrogel — a CE-marked Class III medical device delivered as an ultrasound-guided outpatient injection rather than an arthroscopic or theatre-based procedure. Once placed inside the knee joint, the liquid collagen polymerises in situ: it sets in place within minutes to form a viscoelastic three-dimensional matrix that conforms to and cushions the worn articular surface.

The product contains no cells of its own. Its mechanism is matrix-induced chondrogenesis — the scaffold creates a structured environment that the patient's own progenitor cells (resident mesenchymal stem cells and chondrocytes) can migrate into and populate. Over time, this recruitment aims to support localised tissue repair at the cartilage defect, without requiring a cell biopsy or second-stage procedure.

The device has been used in over 20,000 European patients. Earlier knee studies — including a 2016 randomised trial and a 2024 series of 17 patients — reported significant IKDC and Lysholm score improvements at 3, 6, and 12 months, with MRI MOCART scoring confirming progressive defect filling and cartilage maturation.

When ChondroFiller injection is used alongside exogenous MSCs, the mechanism becomes a two-pronged approach: the collagen scaffold provides a structural anchor and a chemotactic environment, while the MSC concentrate delivers an amplified source of progenitor cells beyond what resident tissue alone would supply. A 2025 ex vivo osteochondral model using femoral condyle tissue from knee arthroplasty patients confirmed this interaction directly — ChondroFiller alone produced a 2.4-fold increase in DNA content by day 14, while MSC-containing groups showed synergistic enhancement of collagen deposition and glycosaminoglycan (GAG) synthesis, supporting the mechanistic rationale for combining the two therapies.

What the MSC component adds to the combination

The MSC concentrate used in the trial comes from the patient's own blood — autologous, meaning no donor tissue, no cell bank, and no culture stage. These mesenchymal stem cells (MSCs) are multipotent progenitor cells capable of differentiating along cartilage-forming (chondrogenic) lines, and drawing on the patient's own biology removes the immunological complications associated with donor material. Among the main cell sources studied in knee OA, autologous bone marrow–derived MSCs rank highest for pain relief and range of motion improvement across published meta-analyses.

Deployed alone, MSC injection already produces meaningful clinical gains — the trial's MSC-alone arm (n=12) confirmed this at Grade IV severity, with statistically significant improvement across all five KOOS domains at two months (p<0.01). The combination arm, however, produced consistently higher mean KOOS scores, and the structural and biochemical markers the trial tracked followed the same pattern.

The proposed explanation for that gap centres on retention. Injected MSCs distributed freely through the joint tend to disperse away from the area of cartilage loss before they can establish, differentiate, and begin producing repair matrix. By polymerising at the worn articular surface, the ChondroFiller scaffold creates a physical anchor — the cell concentrate stays where it is needed. The 2025 ex vivo work using femoral condyle tissue corroborated this picture: when MSCs were combined with the collagen matrix, collagen deposition and glycosaminoglycan synthesis were enhanced beyond what either component achieved in isolation, pointing to an additive rather than simply parallel effect.

The trial's limitations and what the evidence can genuinely claim

Three features of the Weninger trial design set a ceiling on what its results can currently establish: the sample is small (n=25), treatment allocation followed individual clinical judgement rather than randomisation, and follow-up extended only to two months.

Two months is too short a window to determine whether cartilage tissue is regenerating in any durable sense. The improvements in KOOS scores, VAS, and range of motion are clinically meaningful, but remain patient-reported functional gains at an early time point. The MMP-13 reduction and reduced bone marrow oedema seen on MRI in the combination arm are encouraging biomarker signals — they suggest less active cartilage degradation and less subchondral stress — but neither confirms that new hyaline cartilage has formed. Histological verification and long-term structural MRI data are not yet available from this cohort.

The Weninger findings also sit within a broader evidence base that is still developing. A 2025 systematic review with Cochrane-level methodology concluded that stem cell injections for knee osteoarthritis may slightly improve pain and function compared with placebo — but rated the evidence as low-certainty, reflecting the scarcity of large, powered RCTs across cell-therapy approaches.

One biomechanical consideration bears directly on post-injection care. A 2024 in vitro study found that ChondroFiller did not reduce wear on the opposing cartilage surface under cyclic loading compared with an untreated defect, attributed to initial gel instability. That finding underlines why weight-bearing protocols in the days following injection require careful clinical management rather than early unrestricted loading.

A powered randomised controlled trial with at least twelve months of follow-up, supported by structural MRI and, where possible, biopsy data, would be the appropriate next evidence tier — confirming whether the biomarker and functional gains seen at two months translate into lasting joint preservation.

What to do if you have severe knee OA and want to explore this

For a patient at Grade IV — the stage where bone-on-bone contact is already visible on imaging — the Weninger trial is an early signal worth knowing about, not a prescriptive guide to a specific treatment pathway. Acting on a single small non-randomised study without proper clinical assessment would be premature; what the findings do establish is that the question merits a conversation with an appropriately experienced specialist.

The first clinical consideration is whether the degeneration is diffuse or focal. Kellgren-Lawrence grading reflects overall joint severity, but Grade IV changes concentrated in one compartment over a largely preserved joint represent a very different clinical picture from end-stage disease distributed throughout the knee. The cell and scaffold combination explored in the Weninger trial is more plausible where meaningful joint architecture remains than in a knee where virtually all compartments are exhausted.

A thorough assessment at this stage should cover clinical history, weight-bearing physical examination, and MRI to map the extent of cartilage loss, subchondral bone marrow changes, and joint alignment — the same range of information the Weninger team collected before and during the trial. ChondroFiller injection is CE-marked and available in the UK, but the combined approach — scaffold paired with autologous MSC concentrate — requires access to a clinic equipped to prepare and deliver both components. A consultant experienced in joint preservation can review the full picture to determine whether a non-replacement pathway is realistic for an individual patient, or whether the degree of whole-joint involvement makes arthroplasty the more appropriate next step.

1. [1] Joint Preservation in Patients with Grade IV Osteoarthritis of the Knee: Use of an Acellular Collagen Scaffold (ChondroFiller® Liquid) and Blood Derived Stem Cell Rich Graft - A Prospective Controlled Trial. (2025). https://doi.org/10.29011/2575-9760.011360 https://doi.org/10.29011/2575-9760.011360
2. [2] 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
3. [3] Controlled, randomized multicenter study to compare compatibility and safety of ChondroFiller liquid (cell free 2-component collagen gel) 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
4. [4] 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