Defect size is the primary fork in the road

Scan results showing a cartilage defect often prompt one immediate question: do I need OATS or MACI? The answer turns on a single measurement — the surface area of the lesion — not age, fitness level, or how long symptoms have been present.

For defects under 2 cm², OATS is the standard of care. One or two cylindrical plugs, each containing intact hyaline cartilage and the underlying bone, are harvested from a low-load zone of the patient's own knee and press-fitted into the defect in a single operation. The result is a structural repair using the body's own tissue.

For defects above 4 cm² — and the SUMMIT randomised trial confirmed the threshold is meaningful from ≥3 cm² — MACI becomes the preferred option. Its collagen membrane can be shaped to fit the defect precisely, scaling to sizes that would demand so many harvested plugs that donor-site damage becomes a significant problem in its own right.

The 2–4 cm² range sits in genuine clinical equipoise: published evidence finds no clear outcome advantage for either technique at this size, and the choice depends on individual anatomy and surgeon experience. That grey zone is addressed in detail in the next section.

One criterion applies equally to both procedures: the defect must be focal and contained, within a joint that is otherwise reasonably healthy. Diffuse cartilage loss from osteoarthritis places a patient outside the scope of either OATS or MACI.

Before any decision, patients should ask their surgeon for the exact lesion dimensions — measured on MRI or confirmed at arthroscopy using ICRS grading. The difference between 1.8 cm² and 2.5 cm² can determine which pathway is most appropriate.

One operation or two — what the staging difference means in practice

The difference in staging is one of the most practical distinctions between the two procedures — and one patients often underweight when focused on size thresholds alone.

OATS is a single-stage operation. The surgeon harvests cylindrical plugs from a low-load area of the knee and transfers them directly into the defect under the same anaesthetic. One hospital admission, one recovery.

MACI works differently. A small biopsy is taken at a first procedure — usually arthroscopic and brief. The tissue is then sent to a laboratory, where chondrocytes are expanded and seeded onto a collagen membrane over approximately four to six weeks. A second, more involved operation then implants the shaped membrane into the defect. Two separate hospital admissions, with a laboratory interval in between.

For patients with a defined return-to-work or return-to-sport timeline, OATS's single-stage design carries a real practical advantage: the clock to active rehabilitation starts immediately after one procedure rather than after two. The total calendar time from first intervention to the start of structured rehab is longer with MACI — not because the rehabilitation phase itself takes longer, but because those laboratory weeks sit between the biopsy and the repair.

That waiting period is not a design flaw. It is what makes MACI the only workable option when a defect is too large to bridge with autograft plugs alone.

The 2–4 cm² grey zone — when outcomes are equivalent

Published evidence for the 2–4 cm² range does not clearly favour either technique — and that distinction matters for patients whose scan results land in this band.

The most authoritative synthesis remains Richter et al. (2015), a review cited more than 400 times, which found OAT and ACI/MACI produce clinically equivalent outcomes at this defect size. OAT tends to deliver faster early functional improvement; ACI/MACI produces a higher proportion of hyaline-like repair tissue on histological assessment. Neither difference has consistently translated into a superior long-term result.

The evidence base here is non-randomised comparative data, not a head-to-head randomised trial. That is a genuine limitation — but the finding itself is reassuring: both techniques can produce good results in this range, which means a well-chosen procedure remains a well-supported one.

Surgeon experience and individual anatomy therefore become the legitimate deciding factors. A clinician who has performed several hundred OATS procedures in the 2–4 cm² range is likely to achieve better results with that approach than with a technique used less frequently — and the same logic applies in reverse for MACI.

Patients whose defect falls in this zone have one practical question to put directly to their surgeon: 'Which technique do you perform more often for a defect this size, and what does your outcome data show?' That answer carries more weight than any size threshold alone.

Two factors that override the size rule

Size thresholds guide the initial decision, but two clinical findings can render them irrelevant before the conversation about technique goes any further.

Subchondral bone involvement

MRI that reveals bone loss — whether a shallow erosion or an established cyst beneath the cartilage surface — shifts the algorithm firmly toward OATS, regardless of lesion area. OATS transfers cartilage and the underlying bone as a single osteochondral unit; the plug reconstitutes both layers simultaneously. MACI addresses cartilage only. Implanting a cell-seeded membrane into a defect that lacks adequate bone support underneath risks poor graft integration and early failure — a second bone-grafting procedure would be needed first, substantially changing the risk profile and staging of MACI.

This is why MRI assessment precedes any technique discussion. A lesion that measures 2.5 cm² on plain axial imaging may harbour subchondral cyst formation that makes OATS the only structurally appropriate option — and that finding would not be apparent without cross-sectional imaging.

Global joint health and age

The focal-defect prerequisite — already noted as a shared requirement for both procedures — does most of the work here. Where that criterion holds, age is not an absolute barrier, though the published evidence for durable cartilage preservation is strongest in patients typically under 50 with post-traumatic or early focal lesions rather than generative joint-wide disease.

A clear MRI read and a frank clinical assessment of overall joint health are therefore prerequisites before either procedure is confirmed.

How long each repair holds — the durability evidence

Both procedures carry meaningful durability data — and both outperform microfracture convincingly enough to make that the starting point for realistic expectations.

OATS graft survival exceeds 80% at seven years and remains above 60% at fifteen years, based on long-term follow-up data published in 2018. The honest qualification is that the most comprehensive long-term cohorts include talus and mixed-joint populations rather than pure knee femoral condyle series — so the fifteen-year knee-specific figure is a reasonable extrapolation rather than a direct read from that population alone.

MACI's long-term record has sharpened considerably. A systematic review covering 188 treated defects at a minimum of ten years — mean patient age 37 at operation — recorded an all-cause reoperation rate of 9.0% and a progression rate to total knee arthroplasty of 7.4%. Both figures are acceptably low for a cartilage-preserving procedure in that age group, and patient-reported outcomes were significantly and durably improved across the cohort.

A 2020 meta-analysis by Zamborsky confirmed that both OAT and MACI produce significantly more good or excellent results than microfracture, which showed correspondingly more poor outcomes — useful context when patients ask why marrow stimulation is no longer the standard approach.

The direct comparison between OATS and MACI is where the evidence thins. Head-to-head randomised trials comparing the two techniques specifically are scarce; most of what is known comes from each measured against microfracture in separate cohorts. For high-load athletes, OATS carries stronger RCT support — Gudas (2012) is the principal anchor — while MACI comparisons in that subgroup remain limited.

Where other options sit in the cartilage repair pathway

OATS and MACI are not the only choices on the repair pathway — and for some patients, neither is the right starting point.

For smaller focal defects of approximately 2–3 cm² where the clinical picture steers away from surgery, injectable collagen scaffold treatment is an outpatient alternative. The ChondroFiller injection places an acellular matrix into the defect under ultrasound guidance; the scaffold recruits the patient's own progenitor cells to generate new cartilage tissue, bypassing the single-stage versus two-stage question entirely.

At the opposite end of the size spectrum, fresh osteochondral allograft (OCA) extends the options beyond what either OATS or MACI can address — large or post-traumatic defects where autograft supply is insufficient. OCA transfers donor cartilage and bone as a composite unit, with long-term follow-up data supporting its use as the step before joint replacement.

Microfracture, once first-line for smaller lesions, now has declining use. The fibrocartilage it produces breaks down within two to three years, and marrow stimulation can damage the subchondral bone plate — narrowing the options for any future OATS or MACI repair.

The prerequisite for any of these pathways is accurate lesion assessment: MRI dimensions, ICRS grade, and subchondral bone status. The single finding most likely to redirect a decision is the condition of the bone beneath the defect, not defect size alone — which is why that structural read, not symptom pattern, is the practical gateway to the right technique. In London, the injectable scaffold pathway is available at the London Cartilage Clinic on Harley Street, where patients with suitable focal defects can be assessed across the full range of restorative options.

1. [1] The Osteochondral Autograft Transfer System: A Comprehensive Evidence-Based Guide. (2025). https://doi.org/10.1097/SAP.0000000000004480 https://doi.org/10.1097/SAP.0000000000004480
2. [2] Minimum 10-Year Outcomes of Matrix-Induced Autologous Chondrocyte Implantation in the Knee. (2024). https://doi.org/10.1177/03635465231205309 https://doi.org/10.1177/03635465231205309