Why the 2–4 cm² range is a genuine decision point

Defect size matters — but not in a simple 'bigger means one surgery, smaller means another' way. For very small focal chondral defects, roughly under 2 cm² (about the area of a small fingernail), a single bone-and-cartilage plug transferred from a low-load part of the knee tends to provide reliable, single-stage repair. At the other end, lesions beyond 4 cm² generally exceed what autograft tissue can comfortably cover, and cell-based repair becomes the more practical route.

The 2–4 cm² band sits between those two cleaner decisions. A defect in this range — say, 3 cm², closer to a thumbnail — is large enough that a single plug is insufficient, yet not so large that one technique clearly dominates on evidence or logistics. Both osteochondral autograft transfer and matrix-induced autologous chondrocyte implantation are defensible choices within this range, and current clinical guidelines reflect that ambiguity rather than resolving it.

Adding to the complexity, no large randomised trial has directly compared the two approaches specifically within this 2–4 cm² window. Much of the available evidence is extrapolated from studies of smaller or larger defects, which means specialist clinical judgement carries more weight here than a straightforward size-based algorithm.

What OATS offers — and where its limits appear

OATS works by moving one or more cylindrical plugs — each comprising living cartilage bonded to a core of bone — from a peripheral, low-load zone of the same knee into the damaged area. Because the transplanted tissue is native hyaline cartilage rather than scar-like fibrocartilage, it provides the closest biological match to the original joint surface available in a single operation.

That immediate, single-stage quality is OATS's most practical advantage: the patient undergoes one procedure and leaves with repaired tissue already in place, with no waiting period, no laboratory stage, and no return to theatre. For lesions where the damage extends into the subchondral bone beneath the cartilage surface, OATS holds an additional structural benefit — the bone component of the plug integrates with the host bone, restoring both layers simultaneously rather than addressing the cartilage surface alone.

The constraints become visible as defect area grows. At around 2–2.5 cm², one or two plugs can typically achieve adequate coverage. Above that, surgeons use a mosaicplasty technique — multiple plugs arranged to tile the defect — which extends coverage but introduces small gaps between plugs. Those gaps fill with fibrocartilage rather than hyaline tissue, partially diluting the biological quality advantage that makes OATS attractive in the first place. Harvesting more plugs also places greater demands on donor sites at the knee's periphery, which hold a finite volume of usable tissue; morbidity at those harvest sites is a recognised, if individually variable, consideration that warrants discussion before surgery.

What MACI offers — and what it demands from the patient

Unlike OATS, MACI begins weeks before any repair tissue reaches the knee. In the first stage — a short arthroscopic procedure — a small sample of the patient's own cartilage cells is removed. Those cells are then sent to a laboratory, where they are cultured and multiplied over roughly three to six weeks. In the second operation, they are returned to the knee seeded onto a thin collagen membrane, which is trimmed to fit the exact contour of the defect and secured in place.

That membrane-trimming step is where MACI holds a genuine advantage over OATS in this size band. Regardless of whether a lesion is round, elongated, or irregularly shaped, the membrane conforms to it without the coverage gaps that arise when fitting multiple cylindrical plugs into an uneven space. There is no fixed upper-size ceiling within the 2–4 cm² range — a meaningful contrast with the donor-site constraints that limit mosaicplasty at the higher end.

The clearest evidence anchor is the SUMMIT RCT, which showed significantly improved KOOS pain and function scores versus microfracture at both two and five years for lesions of 3 cm² or greater. Longer follow-up series — including a minimum ten-year outcome study (Minas, 2014) and five-year MRI-based data from Ebert et al. — support durable functional improvement well beyond the trial period.

The procedural burden is real: two operations, a laboratory waiting period of several weeks, and higher resource costs are features patients weigh differently depending on their circumstances. It is also worth noting that where bone loss underlies the chondral defect, the collagen membrane addresses the cartilage surface only and does not restore the bone beneath it.

How defect size and bone involvement guide the choice

Two questions do most of the clinical sorting within the 2–4 cm² band: where exactly does the defect fall, and does it extend into the underlying bone?

At the lower end — roughly 2–2.5 cm² — OATS/mosaicplasty is generally the preferred starting point, provided the defect geometry suits plug coverage. The technique restores both cartilage and bone in a single procedure, and that dual-layer correction is precisely the right match when bone involvement is present. If subchondral bone loss accompanies the chondral defect at any point within the range, that preference strengthens: the MACI membrane addresses the cartilage surface only, so meaningful bone involvement calls for the structural correction an osteochondral plug can deliver — or, where bone loss is extensive, a fresh osteochondral allograft rather than either technique.

At the upper end — 3 cm² and above — the decision typically shifts toward MACI or an equivalent cell-based repair. The SUMMIT trial's ≥3 cm² superiority threshold lands squarely here, donor-site constraints become a practical ceiling for mosaicplasty, and the collagen membrane's ability to conform to irregular or elongated defect margins outweighs the staging burden for most patients. A confluent or awkwardly shaped lesion tends to reinforce this shift regardless of whether it sits at 2.8 or 3.5 cm².

The part of the range the two-filter framework handles least cleanly is the middle — around 2.5–3 cm² without clear bone involvement — where defect shape, prior surgical history, and individual tolerance for a two-stage procedure all carry weight. There, size alone does not settle the matter; but the framework still narrows the starting position considerably, and knowing which way those factors point is the most useful preparation for a specialist assessment.

Other factors that shift the balance

Defect size and bone status narrow the field considerably, but a specialist assessment typically weighs at least three further variables that can shift the balance independently of those two filters.

Prior microfracture to the same site is arguably the most consequential history modifier. Marrow-stimulation procedures alter the subchondral bone plate and the local biological environment in ways that may reduce the reliability of subsequent OATS. Where the same area has previously been microfractured, a cell-based approach such as MACI generally becomes the preferred route — targeting the cartilage layer without further disrupting the bone environment beneath.

Limb malalignment — a varus or valgus deformity that places disproportionate load on the repaired compartment — is a mechanical threat to whichever procedure is chosen. Corrective osteotomy (HTO for varus, DFO for valgus) may need to be planned as a concurrent or staged adjunct; evidence suggests that neither OATS nor MACI performs as reliably when load is not redistributed away from the repair site.

Staging tolerance is the third variable — and one that introduces a further option worth knowing about. For patients who find MACI's two-operation requirement impractical due to work commitments, travel, or a low tolerance for staged procedures, that logistical barrier can itself tip the decision. STACi, a next-generation cell-based technique available at some specialist UK centres, addresses this directly: autologous chondrocytes are delivered via a three-dimensional scaffold in a single operation, removing the laboratory waiting period. The underlying principle is the same as MACI — the patient's own cells, cultured and returned to the joint — but the scaffold architecture also accommodates larger or more complex defect geometries. Long-term evidence for STACi is more limited than for established MACI, and it should be treated as an emerging option rather than a proven equivalent.

Finding a cartilage specialist for this decision

The complexity mapped in this article is precisely why no online resource substitutes for a face-to-face assessment with a surgeon experienced in both techniques. A consultant who works regularly across OATS and MACI can select on clinical grounds rather than availability — which matters when the two approaches are genuinely competitive at the same defect size.

Arriving prepared makes that consultation more productive. Bring any existing MRI report — ideally with a defect size estimate from the reporting radiologist — alongside a clear account of symptom onset and any previous knee procedures. Three questions worth raising: What repair tissue will this produce, and how durable is it at my defect size? Does my bone status affect which approach is better? And, if MACI is recommended, is a single-stage alternative worth considering in my case?

Find a specialist

Search MSK lists cartilage repair specialists across the UK — filter by region and treatment type to find a consultant who offers both OATS and MACI and can advise which fits your situation.