What OCA transplantation is — and where it sits on the repair ladder

Being told you need a cartilage graft — particularly after a knee injury — raises an immediate and reasonable question: what exactly is going in, and where does it come from?

An osteochondral allograft, or OCA, is a plug of fresh donor tissue that includes both the articular cartilage surface and the underlying subchondral bone. That plug is shaped to match the damaged area and press-fitted directly into the prepared defect in a single operation. There is no laboratory stage, no cells grown in culture — the graft arrives from a donor, is matched for size, and is implanted while the chondrocytes within it are still alive.

To understand why OCA exists as an option, it helps to think of cartilage repair as a ladder. At the lower rungs sit marrow-stimulation techniques such as microfracture, suited to smaller defects of roughly 2 cm² or less. A step up, OATS (osteochondral autograft transfer) or mosaicplasty can address lesions of 1–4 cm² using bone-and-cartilage plugs taken from elsewhere in the patient's own knee. Cell-based procedures — ACI and MACI — extend coverage to mid-size defects of 2–10 cm². OCA occupies the top of this ladder: it is used when a defect is too large or too deep for any of those approaches, or when prior repair has failed.

Critically, OCA preserves the native joint. The patient's own knee is retained and the graft integrates biologically — distinguishing it clearly from knee replacement. For younger, active patients whose injury resulted from trauma rather than gradual degeneration, that distinction matters enormously, and post-traumatic aetiology is itself one of the strongest prognostic markers for a successful outcome.

Who is a strong candidate

Candidacy rests on three interlocking questions: how large and deep is the defect, how healthy is the surrounding joint, and where does this patient sit on the age and activity spectrum?

Defect size and depth. OCA is indicated for symptomatic focal lesions that are typically larger than 2–3 cm² and extend fully through the cartilage into the subchondral bone (ICRS grade 3–4). Once a defect exceeds roughly 4 cm², OCA tends to be preferred over cell-based approaches such as MACI, because it restores both the cartilage surface and the underlying bone in a single procedure rather than addressing cartilage alone. The graft also requires a stable structural rim: at least 50–75% of the circumferential containment wall must be intact to hold the plug securely and allow it to integrate.

Patient profile. The strongest candidates are active individuals usually under 40–50 who are not yet at the point where knee replacement would be considered. Post-traumatic lesions — those arising from an acute injury rather than gradual wear — carry a meaningfully better prognosis than defects associated with degenerative joint disease or avascular necrosis. The likely reason is biological: in a post-traumatic knee the surrounding cartilage and bone typically remain healthier, providing the graft with a more receptive environment in which to anchor and mature.

When OCA follows an earlier repair. Many patients arrive at OCA via a procedure that has not held. After failed microfracture, ACI, or mosaicplasty, OCA is the recognised next-line option — particularly because repeated marrow stimulation can damage the subchondral bone plate, making further cell-based repair less viable.

Contraindications. Several factors suggest that a graft is unlikely to integrate reliably: BMI above 35, current tobacco use, inflammatory arthritis, active infection, and diffuse degenerative change across the knee compartments. These are not administrative barriers but biological conditions under which donor tissue is less likely to survive and incorporate successfully.

Why alignment and joint stability must be addressed first

Even the best-matched graft will fail if the forces running through the knee are wrong. Cartilage tissue — donor or native — cannot survive chronic mechanical overloading, and OCA is no exception to that rule.

The clearest evidence comes from the Bone & Joint 2019 cohort of 60 patients undergoing fresh OCA with concomitant realignment. Those with persistent postoperative malalignment were more than six times more likely to lose their graft (hazard ratio 6.55; 95% CI 1.61–27.71), making uncorrected alignment the single strongest modifiable predictor of failure in the dataset. Corrective osteotomy — high tibial (HTO) for varus deformity, distal femoral (DFO) for valgus — is therefore not an additional procedure to consider later; it is planned alongside the graft from the outset.

Meniscal deficiency and ligamentous instability carry equivalent logic. An absent or significantly damaged meniscus alters contact pressures across the compartment; an unstable knee transmits shear forces the graft is not built to absorb. Both must be reconstructed or stabilised at or before the time of OCA implantation.

This is why the mechanical assessment that precedes surgery carries as much weight as the graft selection itself. Objective evaluation of limb alignment, compartment loading, and dynamic stability — using tools such as MAI Motion® gait analysis or onMRI™ cartilage mapping where appropriate — shapes whether, and in what sequence, realignment and graft work are planned. Identifying and correcting these factors before the graft is placed is, in practical terms, what determines whether it lasts.

Long-term survivorship and functional outcomes

Survivorship, in this context, means the graft is still functioning — that the patient has not gone on to need a knee replacement. It is the most clinically meaningful long-term measure, and the figures are encouraging, particularly for the patient group this procedure targets.

The landmark cohort

The Bone & Joint 2019 study of 60 patients (mean age 28.9 years, concomitant realignment) provides the most granular long-term picture. Kaplan-Meier graft survivorship ran 87.3% at five years, 85.0% at ten, 74.8% at fifteen, and 59.8% at 25 years. That final figure deserves context: more than half of those patients still carried their original graft a quarter-century after surgery — a group who, without intervention, would have needed knee replacement considerably earlier. Mean modified Hospital for Special Surgery (mHSS) score improved significantly alongside those survival rates, rising from 74.1 before surgery to 89.0 at final follow-up.

Zooming out to the broader literature

Across wider meta-analytic summaries, graft survival sits at roughly 82–87% at five to ten years and 73–75% at fifteen years. In the strongest patient subgroups — those under 30 with unipolar femoral condyle lesions, a post-traumatic aetiology, and grafts under 8 cm² — ten-year survivorship can exceed 90%. These figures are observational rather than trial-derived, but they are consistent across multiple independent cohorts.

Return to sport

For active patients, the functional picture matters as much as graft longevity. At a mean six-year follow-up, 75.2% of patients (112 of 149 knees) returned to sport or recreational activity, with 71% rated as very good to excellent on IKDC evaluation. A 2025 review confirmed a 72% overall return-to-sport rate, with 84% of those returning reaching the same or higher competitive level as before their injury.

Reoperation rates versus true graft failure

The headline reoperation figure deserves unpacking before it becomes a source of unnecessary alarm. Roughly 4 in 10 patients (39–43%) undergo a further knee procedure after OCA — a number that sounds substantial until the composition of those reoperations is examined. The large majority involve hardware removal, arthroscopic debridement, or synovitis management: secondary procedures that address the surgical environment rather than graft failure itself. Conversion to total knee arthroplasty occurs in approximately 16% of cases overall — a meaningfully different outcome than a reoperation, and one worth distinguishing clearly during pre-operative discussion.

Who is at higher risk of true graft failure

Several predictors of actual failure are well-established, and most cannot be modified once surgery has been planned. In the Bugbee ISAKOS cohort of 673 knees (1997–2016), patients aged 30 or older were 2.4 times more likely to experience treatment failure than younger patients. Grafts exceeding 8 cm² independently increased failure risk. Across the Kunze 2023 meta-analysis of 15 studies, bipolar (two-surface) defects, male sex, and greater BMI were each significantly associated with higher failure rates.

Diagnosis matters here in a way that carries direct clinical relevance. In that same 673-knee dataset, degenerative joint disease and avascular necrosis diagnoses were associated with substantially higher failure rates than osteochondritis dissecans or traumatic chondral injury — providing a specific, data-grounded basis for the more favourable prognosis in post-traumatic patients, rather than simply a categorical preference.

Evidence quality, honest gaps, and getting assessed

No randomised controlled trials exist in OCA transplantation. Understanding why matters more than simply noting the fact. Defects large enough to warrant OCA — full-thickness lesions typically exceeding 3–4 cm² in a young, high-demand knee — present genuine barriers to randomisation: withholding a procedure backed by consistent multi-centre survivorship data extending to 25 years, or assigning patients who are already beyond the effective ceiling of marrow stimulation to an inferior comparison arm, would not pass ethical review in most institutions. The evidence base instead comprises multiple large independent cohorts, systematic reviews including Haikal et al. (2023), and registry-scale datasets — Bugbee's 673-knee series alone spans nearly two decades — that collectively produce consistent signals across survivorship, function, and failure risk. That consistency, rather than trial architecture, is what makes clinical guidance reliable here.

Two gaps remain worth naming honestly. No head-to-head study directly compares OCA with MACI for the post-traumatic knee specifically; the preference for OCA in larger, bone-involving defects rests on indirect evidence and structural reasoning — restoring both tissue layers in a single stage — rather than a comparative trial. And the optimal timing of OCA after acute trauma, early versus delayed surgery, is not well characterised in the published literature; timing decisions reflect clinical judgement and defect maturity rather than protocol.

A first assessment for OCA involves more than imaging. It requires MRI characterisation of defect size, depth, and circumferential containment; a full mechanical alignment and stability review; and a staged discussion of every option, including concurrent corrections, that fits the individual knee. That conversation — with the complete clinical picture available — is what translates consistent population-level evidence into a plan for one patient.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without a GP referral. Book an assessment at lincolnshireknee.co.uk.