The size rule that guides the choice

When a consultant mentions both OATS and MACI in the same appointment, the first thing they are doing — often before discussing technique at all — is measuring the defect. The surface area of the damaged cartilage patch is the single strongest predictor of which procedure is appropriate, and the decision broadly follows a three-tier rule.

Defects under 2 cm² are best matched to OATS (osteochondral autograft transfer, also called mosaicplasty). One or two cylindrical plugs of healthy hyaline cartilage and underlying bone, taken from a low-load zone of the same knee, fill the lesion in a single operation. For smaller focal defects, published reviews and long-term follow-up data support OAT as the standard approach, with trends towards greater durability in physically active patients compared with simpler interventions.

The 2–4 cm² range is a genuine clinical grey zone. At this intermediate size, Richter et al. (2015) and Camp et al. (2014) both report that OAT and ACI/MACI produce clinically equivalent outcomes, though improvement after OAT tends to arrive more quickly.

Above 4 cm², MACI becomes the preferred option. The SUMMIT randomised controlled trial demonstrated superior KOOS pain and function scores for MACI over microfracture at both two and five years in defects of 3 cm² or larger, and a 2020 meta-analysis by Zamborsky et al. confirmed OAT's advantage over microfracture extends through the 2–6 cm² range — useful context for borderline decisions.

Defect size is where the decision starts, not where it ends. Bone involvement, patient activity level, and practical considerations around staging can all shift the recommendation, as the sections below set out.

What OATS involves and where it works best

OATS works by transplanting a composite unit of tissue — a cylindrical plug of intact hyaline cartilage sitting on its own subchondral bone base — from a low-load area of the same knee (typically the periphery of the medial or lateral femoral condyle) directly into the damaged site. Everything happens in a single operation under one anaesthetic: harvesting, sizing, and press-fitting are completed in sequence with no laboratory interval between them.

The composite nature of the graft is a genuine structural advantage. Because each plug includes both the cartilage layer and the bone beneath it, the procedure restores the full osteochondral unit in one step — something surface-only techniques cannot achieve. This dual-layer replacement matters most when any bone loss underlies the cartilage damage, a consideration picked up in more detail in the section on secondary selection factors.

A single plug fills approximately 1–2 cm². For slightly larger defects, surgeons harvest several plugs and pack them together in a configuration known as mosaicplasty. Camp et al. (2014) place the technique's practical sweet spot at 1–4 cm². Beyond that, each additional plug draws from a progressively wider harvest zone, and while donor-site discomfort is generally well-tolerated, it becomes a proportionately larger concern as plug count increases — both in terms of local pain and the structural integrity of the harvest area.

Ten-year outcome data, including follow-up reported by Gudas et al. (2012), support sustained durability for smaller defects in younger and physically active patients. It is this evidence base that defines the practical ceiling: OATS delivers reliably within its range, but attempting to fill larger areas with multiple plugs introduces fibrocartilage in the gaps between plugs and escalates donor-site burden — two trade-offs that shift the balance towards alternative approaches.

What MACI involves and why it suits larger defects

MACI requires two separate operations, separated by several weeks of laboratory work — a structural difference from OATS that shapes the patient experience as much as it shapes clinical outcomes.

At the first stage, the surgeon takes a small arthroscopic biopsy of healthy cartilage from the patient's knee. The harvested cells are sent to a specialist laboratory, where chondrocytes are expanded and seeded onto a porcine Type I/III collagen membrane over several weeks. At the second operation, this cell-seeded membrane is trimmed to match the defect geometry precisely and secured in place — a process more accurately described as matrix-induced chondrogenesis than straightforward tissue transplantation.

That trimming step is clinically significant. Because the membrane can be cut to fit virtually any shape — accommodating defects up to approximately 20 cm² — MACI removes the size ceiling that multi-plug OATS configurations run into. There is no donor-site arithmetic: the membrane scales to the lesion.

The strongest outcome evidence for larger defects comes from the SUMMIT randomised controlled trial, which compared MACI against microfracture in cartilage injuries of 3 cm² or greater. MACI produced superior KOOS pain and function scores at both two and five years — a finding that anchors its clinical role at the larger end of the size range, where Richter et al. (2015) similarly identify ACI/MACI as the approach with the best published results.

The two anaesthetic events and the cell-culture waiting period are a genuine patient-burden consideration. For anyone who medically requires or strongly prefers a single procedure, that staging is a meaningful constraint — one reason the choice at intermediate defect sizes remains a two-way conversation rather than a straightforward referral.

When bone loss overrides the size rule

Surface area is not the whole picture. How deep a defect goes — specifically, whether it has eroded into the subchondral bone plate beneath the cartilage — can override the size-based algorithm entirely.

MACIaddresses only the cartilage surface. The collagen membrane, however well-fitted, sits on top of the lesion; it cannot restore bone that is no longer there. OATS, by contrast, transfers a full osteochondral plug — cartilage and bone together as a single composite unit — making it the preferred approach whenever meaningful bone loss accompanies the cartilage damage, regardless of surface area.

In practical terms, a lesion that measures 3 cm² on surface imaging — technically within MACI territory by the size rule — can shift firmly into OATS territory the moment the pre-operative MRI reveals significant depth involvement. This is why assessing defect depth, not just surface dimensions, is an essential part of pre-operative planning. MRI allows the clinical team to map the full three-dimensional extent of the lesion before committing to either pathway.

Patients are rarely aware that this distinction exists. Knowing to ask about depth, not just size, leads to a more informed conversation at the planning stage.

What the evidence says about outcomes

Across the size spectrum, the strongest comparative signal runs through both procedures measured against microfracture rather than against each other. A 2020 meta-analysis by Zamborsky et al. found OAT produced significantly more excellent or good results than microfracture, with that advantage holding across lesions from 2 to 6 cm²; ACI and MACI also outperformed microfracture on poor-outcome rates. Microfracture matters here mainly as a backdrop: its fibrocartilage repair tends to deteriorate within two to three years, and the marrow-stimulation process can damage the subchondral bone plate, narrowing options for any subsequent repair. It is now a declining-use comparator, not a current first-line choice.

Within the 2–4 cm² grey zone, Richter et al. (2015) found OAT and ACI/MACI produced clinically equivalent outcomes — the procedures level out at intermediate sizes, though OAT showed a trend towards faster early improvement. When published outcomes converge like this, the deciding factors shift to procedural considerations: whether bone involvement is present (covered in the previous section), whether a single-stage operation is medically or personally preferred, and the specific geometry of the lesion.

What is currently absent is a direct head-to-head randomised trial comparing OATS against MACI at those intermediate defect sizes. The existing comparative evidence is predominantly level three and four — reviews and pooled heterogeneous cohorts. In practice, surgeons navigating the grey zone draw on pre-operative MRI mapping of defect depth and geometry, a patient's activity demands, and their preference regarding staged versus single-operation pathways — the factors that trial data at this size range has not yet been able to resolve.

How the decision is made at MSK Doctors

The decision pathway begins with MRI — not only to confirm cartilage damage but to extract the two measurements that drive everything: surface area and depth into the subchondral bone. Together, these tell the consultant which tier the defect sits in, and whether bone involvement overrides the surface-area calculation before any technique discussion begins. AI-driven analysis through onMRI™ can support objective lesion characterisation at this pre-operative stage, reducing the variability that has historically made defect grading assessor-dependent.

Where the grey zone applies — roughly 2–4 cm² with no dominant bone loss — the imaging findings open a conversation rather than close it. A consultant will typically explore staging preference, prior treatments that may have altered the subchondral plate, activity demands, and whether donor-site considerations carry particular weight for that patient. Coming prepared to discuss those factors — one operation or two, previous marrow-stimulation work, return-to-sport ambitions — tends to sharpen the initial consultation considerably.

MSK Doctors consultants manage the full range of cartilage restoration options, including both OATS and MACI, and can see patients without a GP referral at clinics in Sleaford (NG34) and Grantham (NG31). London-based patients can access equivalent expertise through the London Cartilage Clinic. Appointments can be booked directly, without a referral, at mskdoctors.com.