The size threshold that separates autograft from allograft

Being told a defect is 'too large for a standard graft' usually means one thing: the lesion has crossed the zone where your own tissue can realistically fill it.

The clinical literature consistently places that zone at roughly 2–4 cm². Below it, osteochondral autograft transfer — OATS or mosaicplasty — is a practical option: surgeons harvest small cartilage-and-bone plugs from low-load areas of the patient's own knee and press them into the damaged site. Above it, the same approach runs into a hard supply limit. The non-weight-bearing zones of a single knee simply cannot donate enough plugs to cover a large lesion without causing meaningful harm at the harvest site itself. The constraint is logistical, not one of graft quality.

Osteochondral allograft transplantation (OCA) exists precisely to remove that ceiling. A size-matched graft from a cadaveric donor replaces the damaged cartilage and underlying bone in one procedure, with no volume restriction imposed by the patient's own anatomy. This is why defect area — rather than symptoms, age, or activity level alone — is the first question surgeons ask when triaging a large focal knee defect. The 2–4 cm² boundary is not a rigid cut-off; it is the range where autograft supply starts to fail and allograft planning begins in earnest.

Why autograft runs out of road beyond 4 cm²

The harvest zones available for OATS — the trochlear margins and periphery of the femoral condyles, where contact stress is low — represent a small and non-renewable resource. Each cylindrical plug removed leaves its own defect, and the surrounding cartilage must absorb the redistributed load. For a single small lesion, this trade-off is manageable. Once a defect exceeds roughly 4 cm², filling it requires so many plugs that cumulative harvest-site damage becomes a clinical problem in its own right: the surgeon risks replacing one area of cartilage loss with another.

Microfracture — an older marrow-stimulation technique — avoids the donor-site issue but is not a solution for large defects. It produces fibrocartilage rather than hyaline-like tissue, and published evidence indicates this repair tissue tends to break down within approximately two to three years, with concurrent damage to the subchondral bone plate that can limit future repair options. Its clinical role is widely regarded as declining, and it does not constitute a like-for-like alternative for substantial lesions.

OCA sidesteps both constraints. The graft — cartilage and underlying bone together — comes from a size-matched cadaveric donor, so the patient's knee contributes nothing and sustains no harvest damage. The cost differential reflects this directly: autograft OATS runs to around £14,000, while OCA is approximately £28,000. That gap is not simply a complexity premium; it represents donor procurement, tissue-banking logistics, and the regulatory framework governing fresh cadaveric grafts — overheads that autograft avoids entirely because the tissue never leaves the patient's body.

How well OCA holds up over time

Survivorship figures for OCA in large-defect cases are, by any measure, encouraging for a procedure used when other options have run out. A 2025 systematic review of secondary OCA — transplantation after a failed first cartilage repair — reported 5-year survival of 79–87.8% and 10-year survival of 61–82%, across cohorts with a mean defect size of approximately 5.8 cm². Long-term data for distal femoral allografts, followed at a mean of 22 years, extend that picture of durability into the decades beyond the mid-term window.

That durability, however, is defect-size sensitive. Lesions reaching 9–10 cm² carry a failure rate of approximately 39% and a reoperation rate of around 67% — figures drawn from the same systematic review and regularly used in pre-operative counselling. Defect area, it turns out, does not stop mattering once the decision to proceed with OCA has been made; it continues to shape the risk profile all the way through.

At the far end of the spectrum sit bipolar lesions, where cartilage loss affects both opposing joint surfaces simultaneously. In published series, these patients present with a mean defect area of 16.7 cm² and have typically undergone an average of 3.2 prior surgeries. For this group, OCA is not a first-line repair but a joint-salvage procedure — the substantive alternative to early arthroplasty in a relatively young cohort. Survivorship reaches 73.8% at 5 years, declining to 58.9% at 15 years, outcomes that remain clinically meaningful given the severity of the starting point. It should be noted that the great majority of this evidence derives from cohort studies and case series rather than randomised controlled trials — a limitation that reinforces the importance of careful patient selection rather than undermining the overall picture, which spans multiple decades of follow-up.

Candidacy factors beyond defect size

A defect measurement qualifies a patient for OCA; it does not complete the selection. Surgeons weigh several patient-level factors that can either shift the preferred approach or prompt optimisation work before proceeding.

Age is perhaps the most telling variable — not because older patients are excluded, but because the typical OCA recipient in published series is a relatively young adult, with mean ages of 31–38 years across cohorts. The procedure exists to preserve cartilage and defer joint replacement, a calculation that carries greatest weight when decades of active life remain. Beyond age, factors associated with higher failure risk include elevated BMI, active nicotine use, significant OCA volume, bipolar joint involvement, and planned concurrent procedures such as ligament reconstruction or osteotomy. Rehabilitation adherence also features in pre-operative discussions: OCA recovery extends to nine to twelve months before return to higher-impact activity, and graft integration depends partly on careful load management throughout that period.

Male patients, on average, present with larger defects and are more likely to require a realignment osteotomy — an HTO or DFO — to correct abnormal joint loading before or alongside the graft. Whether to combine OCA with osteotomy in a single setting or stage the two procedures is a surgeon-led decision, determined by the degree of malalignment and the patient's overall condition at the time.

For defects broadly in the 2–10 cm² range where bone involvement is limited and chondrocyte supply is adequate, ACI or MACI may represent a more appropriate path than OCA. Once bone loss becomes significant, or when a prior cell-based repair has already failed, OCA becomes the more directly indicated option.

Donor matching, recovery, and returning to activity

Fresh osteochondral allografts are sourced from cadaveric donors through regulated tissue banks, with each graft size-matched to the recipient's joint surface — a step that adds logistical complexity absent from autograft procedures, but one that modern tissue banking has substantially resolved.

One question that regularly arises in pre-operative counselling is whether donor sex affects outcomes. A study of 285 patients found no significant difference in graft survivorship between same-sex and different-sex pairings (p=0.70), confirming that size-matched grafts drawn from the available donor pool are not meaningfully compromised by sex mismatch. A nuance worth noting, however, is that when failures did occur in the mismatched group they tended to happen sooner — a median of 353 days compared with 864 days. This is a counselling consideration rather than a reason to decline an otherwise suitable graft.

Functional recovery post-OCA is broadly comparable to that seen after ACI: published series report post-operative knee range of motion typically exceeding 120°, with no statistically significant difference between the two approaches in functional outcome measures. Return to low-impact activity follows at around four to six months; return to higher-impact sport takes nine to twelve months. That timeline is not compressible — graft integration is load-sensitive, and a structured, phase-by-phase rehabilitation programme is a meaningful determinant of how well the graft beds in. Patients who approach recovery expecting a rapid return to full load are likely to be disappointed; those who engage consistently with the rehabilitation process give the graft the best conditions to consolidate.

How to find an OCA specialist in the UK

OCA is performed at a relatively small number of specialist knee centres in the UK — not every orthopaedic unit maintains the tissue-bank relationships and surgical caseload the procedure requires. When evaluating a surgeon, it is worth asking directly whether they run an active OCA programme, how frequently they implant fresh allografts, and which regulated tissue bank supplies their grafts. A surgeon whose practice spans both cartilage preservation and joint replacement is better placed to counsel honestly about where OCA sits in that continuum — and when it is, or is not, the right next step.

Search MSK lists knee and cartilage-preservation specialists across the UK — filter by region and specialty to find a surgeon offering OCA near you.