Why focal knee cartilage defects rarely heal on their own

Articular cartilage — the smooth, white tissue lining the ends of the femur and tibia — has no blood supply of its own. That single biological fact explains why a cartilage injury in the knee does not mend the way a muscle tear or a bone fracture can: without blood vessels, the body cannot deliver the repair cells needed to close the gap.

A focal chondral defect is a contained, full-thickness or deep-partial loss of that cartilage surface. It is a different condition from the widespread thinning seen across the joint in knee osteoarthritis. These lesions typically arise from a single traumatic impact, from osteochondritis dissecans — a subchondral bone disorder that destabilises the overlying cartilage, causing pain during sport, swelling, and sometimes catching or locking — or from repetitive overload that the cartilage eventually cannot absorb. Without intervention, the exposed bone and the edges of surrounding cartilage tend to deteriorate further, and symptoms commonly worsen over time rather than plateau.

Two variables shape every treatment decision that follows: the size of the defect, measured in square centimetres (cm²), and whether the underlying subchondral bone layer is also involved. Both dimensions will be returned to throughout this comparison. Improvement across all cartilage repair strategies is consistently tracked using the International Knee Documentation Committee (IKDC) score, a validated patient-reported measure of knee symptoms and function, and it serves as the benchmark reference in the evidence discussed here.

ChondroFiller: how an injectable collagen scaffold works

On the day of treatment, the patient attends a clinic appointment — no theatre admission, no general anaesthetic, no overnight stay. Under ultrasound guidance, ChondroFiller (marketed as Liquid Cartilage™) is injected directly into the focal defect, where it self-gels within minutes to form a stable collagen matrix that bonds to the surrounding cartilage edges.

The mechanism is acellular matrix-induced chondrogenesis: the scaffold contains no cells of its own, but its Type I collagen structure acts as a chemotactic signal, drawing the patient's own progenitor cells — from the synovium and adjacent tissue — into the matrix. Those cells differentiate into chondrocytes and begin producing new tissue; as repair progresses over 12 months or more, the scaffold gradually resorbs and is replaced by the body's own cartilage-like material. The device does not inject cartilage; it supports the body's own repair processes by providing a biological framework — and because no bone is harvested or drilled, nothing is removed from the joint in the process.

ChondroFiller is a CE-marked Class III medical device composed of murine-derived, acid-extracted Type I collagen, with over 20,000 implantations recorded across more than 10 years of clinical use and post-market follow-up data available. The treatment is suited to focal defects measuring up to 6 cm² where the subchondral bone beneath the lesion remains structurally intact — a point that becomes directly relevant when comparing it with surgical options designed around bone-level involvement.

OATS and mosaicplasty: the surgical osteochondral transfer approach

OATS and mosaicplasty are surgical procedures carried out under general or regional anaesthesia, typically in a day-surgery theatre. The core principle of OATS is straightforward: a cylindrical plug of bone and cartilage — roughly 4–10 mm in diameter — is harvested from a low-load area of the same knee, then press-fitted into the prepared defect site. Because the plug includes both the cartilage surface and its underlying bone, the repair addresses both tissue layers in a single stage.

Mosaicplasty follows the same logic but uses several smaller-diameter plugs arranged in a mosaic pattern to cover defects typically up to 4 cm², most commonly on the medial femoral condyle. A recognised limitation of this approach is that the gaps between plugs tend to fill with fibrocartilage rather than true hyaline tissue — a structural compromise compared with plug centres, though not a reason to dismiss the technique outright.

Smaller defects can often be addressed arthroscopically; larger lesions may require a more open approach. Donor-site morbidity — pain, swelling, and sometimes persistent discomfort at the harvest zone — is an acknowledged risk, and recovery typically involves several weeks of protected weight-bearing before full loading is permitted.

OATS is an established, evidence-backed pathway that has shown sustained IKDC and Lysholm gains at 10-year follow-up, and notably higher return-to-sport rates compared with alternative techniques in a systematic review of more than 1,100 patients.

Five factors that point toward one approach over the other

Five practical considerations tend to orient this decision before a clinical consultation is even complete.

Defect size

ChondroFiller is indicated for focal defects up to 6 cm². Single-plug OAT typically suits lesions below 2 cm²; mosaicplasty extends coverage to around 4 cm². Where a lesion falls between 4 and 6 cm², ChondroFiller's wider size ceiling tends to make it the more applicable option.

Subchondral bone

Where the bone layer beneath a defect is structurally intact, ChondroFiller addresses the cartilage surface without disturbing anything below it. Where subchondral bone is significantly compromised — as in some osteochondral lesions — OATS's capacity to restore both tissue layers simultaneously tends to be more appropriate.

Invasiveness and recovery

ChondroFiller involves no bone harvest, no theatre admission, and no post-operative wound care. OATS carries an acknowledged risk of donor-site discomfort at the harvest zone, alongside a rehabilitation period of several weeks of protected weight-bearing. For patients for whom surgical recovery is impractical, the injectable pathway tends to suit.

Return to sport

OATS carries the stronger published dataset here: Campbell et al.'s systematic review of 1,117 patients demonstrated significantly higher return-to-sport rates for OAT compared with alternative techniques at a mean 3.6-year follow-up. ChondroFiller's sports-return evidence is present but less extensive.

Patient eligibility

OATS is typically contraindicated at BMI above 40, age above 50, or osteoarthritis graded beyond Kellgren-Lawrence Grade 2. ChondroFiller's eligibility criteria are less age-restrictive — though a consultant assessment determines individual suitability in both cases.

What the clinical evidence shows — and where the gaps are

Four clinical studies of knee cartilage defects have evaluated ChondroFiller, each showing IKDC improvement of approximately 30 points — exceeding the established minimal clinically important difference of 16.7 points. The most robust is the Jerosch et al. prospective post-market clinical follow-up study, which recorded a mean gain of 32.4 IKDC points sustained at three-year follow-up, with patients reaching a mean score of 80. MRI-based MOCART scores across European studies range from 81.6 to 84.3, indicating more than 80% defect filling and good structural integration with surrounding native cartilage; scores progress from a mean of 65.3 at four weeks to 81.6 at twelve months, reflecting gradual tissue maturation rather than an abrupt repair event. Complication rates across the evidence base are approximately 0%, with reoperation rates of 3–8%. It is worth noting that these clinical studies are manufacturer-sponsored, that no head-to-head randomised controlled trial has yet compared ChondroFiller directly with OATS or mosaicplasty, and that long-term data beyond three years for large defects remain limited to single prospective series — important context when interpreting these figures, though not a reason to dismiss them.

For OATS, Pareek et al.'s 10-year systematic review confirmed that IKDC and Lysholm scores remain significantly improved at long-term follow-up — though Tegner activity scores did not change significantly over the same period, a nuance that rarely appears in summary comparisons. Lim et al.'s level 2 study adds further complexity: comparing microfracture, ACI, and OAT directly, no statistically significant difference emerged on either Lysholm or Tegner scores between the three techniques. That finding does not make technique selection irrelevant — it reinforces that defect size, bone involvement, activity demand, and individual patient profile drive outcomes as much as any single procedure. A consultant assessment remains the appropriate route to weigh these variables for a given individual.

Getting the right assessment for your knee

Making the decision hinges on information that symptoms alone cannot supply. MRI — cartilage-sensitive sequences that map defect depth, size, and the condition of the subchondral bone — is the diagnostic step that converts the comparative factors in the preceding section into an individual recommendation. Without imaging, treatment selection rests on incomplete ground.

The clearest clinical dividing line is subchondral bone status combined with defect size. An isolated cartilage lesion over structurally sound bone, in a patient who wants to avoid theatre admission, points toward the injectable ChondroFiller pathway. Osteochondral involvement, a strong return-to-sport requirement, and eligibility within OATS's age and BMI thresholds point toward a surgical consultation instead. Many patients sit between these two profiles — which is precisely why a consultant-led assessment, rather than self-triage, is the appropriate next step.

Lincolnshire Knee, part of the MSK Doctors group, accepts patients without a GP referral at clinics in Sleaford and Grantham. An assessment combining MRI review with clinical evaluation will clarify which pathway — injectable scaffold or surgical transfer — fits both the knee and the patient's circumstances. Book at lincolnshireknee.co.uk.