Which procedure fits your defect?

The most useful question to settle first is not which procedure has the better evidence, but whether your cartilage damage reaches the bone beneath.

Both ChondroFiller injection and OATS mosaicplasty are designed for focal, contained, full-thickness defects — ICRS grade III or IV — in a joint whose surrounding cartilage is broadly intact. Neither is appropriate for diffuse or multi-compartmental disease, and neither should be mistaken for a remedy for established osteoarthritis.

Beyond that shared ground, a single structural fact divides them. OATS harvests cylindrical bone-and-cartilage plugs from a low-load zone of the same knee and press-fits them into the defect, restoring both the cartilage surface and the subchondral bone column beneath it. If your lesion extends into bone, that structural restoration matters and OATS has a meaningful advantage. If the damage is purely chondral — surface layer only, bone intact — the picture changes, and patient-specific factors carry more weight.

For surface-only defects under approximately 2.5 cm², a ChondroFiller injection is a realistic alternative to theatre-based surgery: an ultrasound-guided outpatient procedure that avoids a formal operation entirely. Whether that trade-off makes sense depends on the depth of the lesion, your capacity for post-surgical rehabilitation, and how much the prospect of a harvest site elsewhere in your knee concerns you.

How ChondroFiller injection works

Delivered in a clinic room under ultrasound guidance, the ChondroFiller injection requires no general anaesthesia, no incision, and no harvest site anywhere in the joint. The patient attends as an outpatient and leaves the same day.

The product itself is a CE-marked Class III acellular scaffold made from native type I collagen, supplied in a two-chamber syringe. As the two components meet on delivery, they begin to gel; within 3–5 minutes inside the defect the scaffold has set into a dimensionally stable hydrogel that conforms precisely to the defect's contour. No bone drilling, no fibrin glue, and no external cells are needed.

What the body does next

Once the scaffold is in place, the biological work begins. The collagen matrix acts as a chemotactic signal, drawing the patient's own progenitor cells — stem cells and chondrocytes — into the scaffold from the surrounding tissue and synovial fluid. Those recruited cells progressively deposit collagen and glycosaminoglycans, gradually building new cartilage tissue. This process is known as matrix-induced chondrogenesis. An ex vivo model confirmed a 2.4-fold increase in DNA content by day 14, reflecting active and measurable cellular recruitment into the scaffold.

One practical caveat follows directly from the mechanism: immediately after injection the hydrogel is soft and does not protect the opposing cartilage surface. Delayed weight-bearing in the days after treatment is therefore advisable, allowing the scaffold to stabilise before load is applied.

ChondroFiller injection is used across multiple joints — knee, hip, ankle, shoulder, elbow, wrist, and smaller hand joints — and no formal upper age limit is documented.

How OATS mosaicplasty works

OATS is a theatre procedure performed under general or regional anaesthesia. Using a coring instrument, the surgeon harvests one or more cylindrical plugs — typically 8–10 mm in diameter — from a low-load region of the patient's own knee, most commonly the peripheral femoral condyle. Each plug carries native hyaline cartilage bonded to a column of subchondral bone. The plugs are then press-fitted into the prepared defect within the same operative session, filling it with the full osteochondral unit and anchoring the cartilage surface via the bone component integrating into the host bed.

That bone column is the procedure's defining structural advantage. Because the plug restores both the surface cartilage and the subchondral architecture beneath it, OATS addresses lesion depth in a way that a surface-only scaffold cannot. This matters particularly when bone loss is part of the defect.

Mosaicplasty and fibrocartilage gaps

When the defect is wider than a single plug can cover, the surgeon tiles several smaller plugs in a mosaic pattern — extending coverage to roughly 4 cm². The gaps between plugs fill with fibrocartilage rather than hyaline cartilage, a recognised and documented limitation of the approach.

The harvest site is a separate consideration: it heals with fibrocartilage, and some patients notice discomfort or restricted movement at that location during recovery. Rehabilitation after OATS follows a structured programme and takes longer than the recovery expected after an outpatient injection pathway.

What the evidence shows

Across four independent knee studies, ChondroFiller injection consistently produces IKDC score improvements of approximately 30 points — well above the recognised minimum clinically important difference of 16.7 points. MOCART imaging scores, which assess defect filling and tissue integration on MRI, range from 70 to 87, indicating reliable structural repair. In the Jerosch post-market follow-up, patients reached a mean IKDC of 80 at three years, with scores stable or marginally improving between one and three years. Hip data are more limited in size: in a 26-patient cohort followed for 3–5 years, 17 of 21 evaluable patients achieved good or excellent results, though patients with established osteoarthritis (Tönnis grade 2–3) fared consistently poorly — a finding that reinforces the importance of patient selection.

The global safety record sits above 19,000 cases with zero serious complications reported, which is a meaningful signal even in the absence of decade-long follow-up. That durability gap is the principal caveat: the longest published follow-up in focal cohorts reaches five years, and sustained outcomes beyond that point are not yet confirmed in the literature.

For OATS, the anchor is a 63-patient institutional registry with data out to 10 years — the most mature independent dataset available for either procedure. IKDC improved from 46.4 at baseline to 70.4 at 10 years; 60% of patients still exceeded the MCID at a decade, and only 2 of 63 ultimately required arthroplasty. The 28.6% reoperation rate sounds substantial but predominantly reflects minor secondary procedures such as meniscectomy, chondroplasty, and loose body removal rather than implant failure.

No head-to-head randomised controlled trial comparing ChondroFiller injection directly with OATS exists. All cross-procedure comparisons draw on separate study populations, different follow-up durations, and varying outcome tools. This is a limitation of the field as a whole, not a flaw specific to either treatment.

Five factors that shift the decision

Five considerations tend to do most of the sorting work when a patient and clinician sit down to compare these two pathways.

1. Bone involvement. This is the closest the decision comes to a hard rule. ChondroFiller injection addresses the cartilage surface; it does not structurally rebuild subchondral bone that has been lost. Where imaging confirms the lesion extends into the bone column beneath the cartilage, OATS — which restores the full osteochondral unit — is the structurally appropriate choice.

2. Defect size. ChondroFiller injection is supported for defects up to approximately 3 cm², with extension to around 6 cm² in some cases. A single OATS plug typically covers 1–2 cm²; mosaicplasty extends that to roughly 4 cm² at the cost of fibrocartilage forming between plugs. For surface-only defects under 2.5 cm², both procedures fall within their respective working ranges.

3. Surgical tolerance and recovery capacity. ChondroFiller injection is an outpatient procedure — no general anaesthesia, no incision, no theatre session. OATS requires anaesthesia and a structured rehabilitation programme. For patients with medical contraindications to surgery, or those who cannot commit to a longer recovery, the injection pathway carries an appreciable practical advantage.

4. Donor-site concern. Harvesting plugs from the low-load zone of the knee creates a second wound. Most patients tolerate it well, but some experience discomfort or stiffness at the harvest site during recovery. ChondroFiller injection involves no harvest and carries no donor-site risk.

5. Evidence maturity versus invasiveness. OATS has 10-year registry data demonstrating that 60% of patients still exceed the minimum clinically important difference at a decade, and it restores native hyaline cartilage. ChondroFiller injection offers a less invasive route with consistent short-to-mid-term signals across more than 19,000 cases, but published follow-up beyond five years remains limited. Patients who weight long-term certainty over procedural burden may lean toward OATS; those prioritising minimal intervention with strong near-term evidence may weigh the injection pathway differently.

In practice, MRI characterisation of the lesion — its depth, area, and subchondral status — resolves the most critical of these factors objectively, which is why imaging review sits at the centre of any specialist assessment.

Getting an assessment at the London Cartilage Clinic

For patients who want to progress from research to a clinical decision, the next step is an assessment at the London Cartilage Clinic on Harley Street — the UK's certified centre for ChondroFiller injection, where the procedure is led by Professor Paul Y. F. Lee.

The assessment centres on MRI review: the same factors that determine which procedure is appropriate — defect depth, surface area, and subchondral bone status — are confirmed through imaging rather than left to self-diagnosis. Patients who arrive uncertain whether they need ChondroFiller injection or a surgical procedure such as OATS will leave with a clear explanation of what the scan shows and which pathway fits it. Where imaging indicates bone loss or a defect outside the suitable range for the injection, that will be stated plainly at assessment, and onward referral toward OATS or another restorative option will be discussed.

Assessments can be booked at londoncartilage.com.

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2. [2] Arthroscopic utilization of ChondroFiller gel for hip articular cartilage defects: 12–60-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
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6. [6] 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
7. [7] 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