Two techniques, one shared goal

Both osteochondral autograft transfer (OATS) and fresh osteochondral allograft (OCA) address the same surgical problem: a focal defect that has breached the cartilage surface and damaged the underlying bone. Because both layers need to be replaced together, each technique transplants a composite plug of bone and overlying hyaline cartilage in a single operation — no staged cell culture, no second procedure.

The difference is the source. In OATS, the surgeon harvests one or more cylindrical plugs from low-load areas of the patient's own knee — typically the superolateral trochlea or intercondylar notch — and press-fits them into the defect. In OCA, a size-matched plug is machined from a fresh cadaveric donor graft supplied by a tissue bank, removing the donor-site equation for the patient entirely but introducing a logistical one.

Neither technique is designed for diffuse or end-stage osteoarthritis; both are reserved for contained, focal lesions. Which of the two is appropriate comes down, above all else, to the size of the defect.

Why defect size is the first filter

Defect area drives the first branch in the decision tree, and the 2 cm² threshold that appears across surgical guidelines is not arbitrary — it reflects a genuine anatomical constraint.

Below roughly 2 cm², a single OATS plug harvested from the superolateral trochlea or intercondylar notch can fill the lesion in one step. Both sites are low-load zones that tolerate harvest without meaningful functional loss, and the bone-cartilage plug integrates directly with the recipient site. For most patients in this size band, OATS is biologically sound and logistically straightforward.

Between approximately 2 cm² and 4 cm², mosaicplasty — tiling the defect with several smaller plugs rather than a single cylinder — can bring autograft within reach. The technique extends the practical ceiling, but each additional plug draws from the same finite harvest zone. Donor-site morbidity risk rises proportionally: more harvest means a greater chance of persistent pain or structural change at the collection site, so the benefit-to-risk calculation shifts with every plug added.

Above 4 cm², or wherever the defect involves significant subchondral bone loss, the donor supply is simply exhausted. OCA becomes the only viable biological option before arthroplasty is considered. Cystic lesions exceeding 3 cm³ in depth trigger the same escalation independent of surface area, because deep bone involvement cannot be addressed with surface plugs alone. Published series confirm the scale of lesions that arrive at this decision: in one series of 156 knees treated with fresh OCA, the mean defect area was 6.4 cm² — well beyond anything autograft can supply.

Size, then, is the starting filter. Patient age is the factor that refines the choice within each size band.

What patient age adds to the picture

Age operates differently from defect size: rather than opening or closing access to a technique outright, it shapes how well each option is likely to perform.

For OATS, the ideal candidate is typically under 40–50 years old. Younger native cartilage integrates more readily with a transplanted plug, and the surrounding tissue retains the regenerative capacity to consolidate the repair. Someone in their 20s or 30s who presents with a sub-2 cm² lesion sits in biologically favourable territory for autograft — the local environment is working with the procedure, not against it.

OCA does not carry the same age ceiling at its lower end. The procedure spans a wide range of patients, but outcomes consistently favour younger recipients — and the reason is cellular. Chondrocyte proliferative potential, sulphated glycosaminoglycan production, collagen deposition, and responsiveness to growth factors all decline with age. These differences translate into measurable results: younger individuals achieve better MRI cartilage fill scores and stronger clinical outcome scores across cartilage repair modalities. Across major published series, fresh OCA graft survivorship at 10 to 25 years runs between 59% and 91%, with those in their 20s and 30s consistently achieving the best long-term results.

At the other end of the spectrum, an individual over approximately 60 with a large defect and bone involvement faces a different calculation. The evidence in this group tilts towards partial or total knee arthroplasty — not as a concession, but because the restorative biology is less favourable and arthroplasty offers more predictable outcomes at this stage of joint deterioration.

The genuine decision zone: 2–4 cm² in middle-aged patients

The two poles make the logic clear: a patient in their late 20s with a 1.5 cm² condylar lesion is an obvious OATS candidate; a 52-year-old presenting with a 5 cm² defect and subchondral bone loss has, in practical terms, one biological option before arthroplasty enters the conversation. Most patients researching this question, however, sit somewhere between those markers.

The genuine decision zone spans roughly 2–4 cm² in patients aged approximately 35–55 — and within it, no single variable settles the choice. Subchondral bone involvement carries the most weight of the secondary factors. Even a 2.5 cm² defect with significant cystic change will typically favour OCA over mosaicplasty, because surface plugs cannot reliably reconstruct deep bone loss; the structural foundation must be sound before an overlying cartilage surface can function and endure load.

Lesion location introduces a separate layer of biomechanical complexity. Condylar defects and patellofemoral lesions operate under different contact pressures and loading patterns, and the suitability of each technique on the medial femoral condyle does not transfer automatically to the trochlea or patella. Surgeons weigh these site-specific mechanical demands when both options nominally remain available.

Mechanical malalignment adds a further consideration. Where varus or valgus deformity concentrates load on the repair site, either graft becomes more vulnerable to early failure. A corrective osteotomy — realigning the mechanical axis — is typically addressed before or alongside the cartilage procedure, because no graft survives reliably in an environment that continues to overload it.

These overlapping variables are precisely why decisions in this range belong in specialist assessment rather than in a fixed checklist.

How a previous procedure changes the equation

Defect size and age are not the only triggers that direct a surgeon toward allograft. When a previous cartilage procedure has failed — microfracture, ACI, or an earlier OATS — OCA becomes the standard biological next step, and this holds regardless of where the lesion sits on the size scale.

The reason lies in what repeated marrow stimulation does to the subchondral bone plate. Microfracture works by breaching that plate to release marrow-derived cells, but evidence shows the resulting fibrocartilage typically breaks down within two to three years. What remains is not a clean substrate: the bone bed is disrupted, and the scaffold that new cartilage would need to grow into has been structurally altered. Returning to a cell-based repair — ACI, MACI, or a second autograft — in this compromised environment is considerably less reliable than performing it on an undisturbed bed.

OCA sidesteps this problem. Rather than asking the damaged subchondral layer to support new growth, the allograft replaces both the cartilage surface and the underlying bone as a single unit, restoring the structural foundation from below.

The OATS outcome data provide a useful reference point here: at ten-year follow-up, IKDC and Lysholm scores improve significantly, though Tegner activity scores may not change — meaning functional recovery is meaningful but a full return to pre-injury sport is not guaranteed. That outcome window effectively closes once prior marrow stimulation has altered the bone bed, which is why salvage timing matters.

None of this reflects poorly on the original procedure. Microfracture was a reasonable first-line choice for smaller defects over many years, and current evidence about its limitations post-dates many of those decisions.

Choosing a specialist for OATS or OCA

Finding the right surgeon matters as much as understanding the decision itself. Both OATS and OCA are technically demanding, and not every orthopaedic unit offers both — centres that routinely perform autograft transfer may not have the tissue-bank relationships needed to source and schedule fresh allograft reliably.

The 28-day viability window for fresh OCA is a practical constraint worth understanding. Donor tissue must be implanted within approximately 28 days of harvest to preserve chondrocyte viability, which means the treating centre must be able to confirm a size-matched graft and complete the operation within that window. Units without established tissue-bank access may find this difficult to deliver consistently — and that can, in practice, influence which technique is offered.

For patients whose defect falls in the 2–4 cm² grey zone, a specialist who can genuinely offer both options provides the broadest clinical flexibility; being in a unit that performs both procedures avoids the situation where the available technique shapes the recommendation rather than the other way around.

A platform such as Search MSK lists knee cartilage specialists practising across the UK, with filters by region and specialty that make it practical to identify surgeons experienced in osteochondral reconstruction close to home.

One caveat applies regardless of technique: bipolar or 'kissing' lesions — cartilage damage on both opposing joint surfaces simultaneously — are a relative contraindication for OATS and OCA alike. Patients in this category may need a broader joint-preservation discussion before either procedure is appropriate.