When a knee cartilage defect is too large for other repairs

Some knee cartilage defects simply outgrow the repairs designed for smaller injuries. Once a full-thickness lesion crosses a certain size, the standard options — microfracture, osteochondral autograft transfer (OATS), or matrix-induced autologous chondrocyte implantation (MACI) — either lack sufficient donor material or cannot reliably restore the joint surface. Fresh osteochondral allograft (OCA) transplantation exists specifically for that situation.

The reason surgical intervention is needed at all comes down to tissue biology. Articular cartilage is avascular and aneural — it carries no blood supply and no nerve fibres — which means it has negligible capacity to repair itself after full-thickness loss. Once the damage is through to bone, waiting or rehabilitating alone will not restore the joint surface.

Defect size is the primary decision driver. Microfracture is generally suited to lesions below 2 cm²; OATS can address defects up to approximately 4 cm² using the patient's own tissue, though donor-site availability limits its use at the larger end of that range; MACI extends the envelope to around 10 cm² but requires two surgical stages and several weeks of cell culture. OCA is typically indicated when defects exceed 2–4 cm² and autograft harvest is insufficient for the task.

In published OCA cohorts, mean defect size runs at 6.3–6.4 cm², and lesions are commonly classified as small (below 5 cm²), medium (5–8 cm²), or large (above 8 cm²). Post-traumatic injury is the single most frequent reason patients reach this threshold, accounting for 38% of cases — reflecting the high-energy impact forces that strip away both cartilage and the bone beneath it in one event.

OCA transplants a size-matched donor bone-and-cartilage unit to replace the damaged osteochondral segment, restoring the joint surface in a single operation.

Candidates most likely to benefit — and who may not qualify

Younger patients tend to do better — this is one of the clearest signals from long-term OCA data, and age above 50 is recognised as a relative contraindication. That does not mean older patients are automatically excluded, but it is a factor consultants weigh carefully against the overall condition of the joint.

Several other factors can complicate or preclude candidacy. Recognised relative contraindications include:

• BMI above 40
• Inflammatory arthritis (such as rheumatoid arthritis)
• Prior knee infection or a history of tumour in or around the joint
• Radiographic osteoarthritis rated above Kellgren-Lawrence grade 2 — meaning the joint surface beyond the defect must be reasonably well preserved

That last point is the most important framing principle: OCA is a focal restoration, not a whole-joint solution. Patients with diffuse or advanced knee arthritis are unlikely to benefit, and joint replacement is the more appropriate pathway in those cases.

Where the defect sits also influences what outcomes a patient can realistically expect. Lesions of the distal femoral condyle, the patellofemoral surface, and the tibial plateau each produce meaningfully different survivorship figures in published series, so counselling should draw on site-specific data rather than a single pooled number — patellofemoral OCA, for instance, carries its own systematic-review evidence base.

Many candidates arrive with more than one problem in the knee. Malalignment, ligament laxity, or meniscal deficiency does not automatically rule out OCA; in published cohorts, around 46% of patients underwent a concomitant procedure — most commonly osteotomy for malalignment or ligament reconstruction — either at the same sitting or as part of a staged plan. Addressing the mechanical environment is generally considered necessary for durable graft function, not an additional hurdle.

Why the graft must be fresh — and what that means for access

Unlike organ transplantation, OCA carries a very low risk of immune-mediated rejection — a misconception worth addressing early. Retrieval studies of grafts removed years after implantation show viable chondrocytes and well-preserved cartilage matrix with minimal histological evidence of immune response. The likely explanation is that articular cartilage is avascular: without a blood supply, the immune system has limited means of mounting an attack on donor cells embedded deep within the matrix.

That biological fortunacy does not, however, extend to frozen tissue. Freezing kills chondrocytes, which is why only fresh allograft is suitable for this procedure. Living donor chondrocytes are what differentiate OCA from a structural bone scaffold — they are what gives the transplanted surface the potential to function as hyaline-type cartilage rather than fibrous scar tissue.

Viability is time-sensitive. Chondrocyte survival in fresh-stored tissue falls below the clinically accepted threshold of approximately 70% by day 28 after procurement. The 28-day window is therefore a hard logistical constraint: the graft must be harvested, tested, size-matched, and implanted within that period. Size matching itself is a technical prerequisite — the shell-graft technique depends on precise donor-to-recipient anatomical correspondence to restore a congruent joint surface.

In practical terms, this means OCA can only be offered at centres with established tissue-bank partnerships and the infrastructure to coordinate rapid graft delivery and scheduling. Outside major cities, access can be meaningfully limited — a factor worth exploring early in the referral process.

What the outcome data show — survivorship, function, and failure

Survivorship figures across published cohorts consistently land in the same range. Using a composite endpoint — graft revision, conversion to total knee replacement, or an HSS functional score falling below 70 — one large series places survival at 82.6% at five years and 69.6% at ten years. A separate cohort tells a broadly similar story: 95% early survivorship, with 68% of grafts still functioning in situ at a mean of 12.9 years.

Perhaps the most striking single data point is the long-term follow-up of distal femoral OCA: one series tracked patients at a mean of 22 years after surgery — an interval that rivals the evidence base for total knee replacement and that very few cartilage procedures of any kind can match.

For the patellofemoral compartment specifically, a systematic review found 87.9% survival at five years and 77.2% at ten years. IKDC scores rose from 41.8 before surgery to 68.1 afterwards — in plain terms, patients rated their knee function substantially higher, with the improvement reaching statistical significance (P<0.001).

Across all treated sites, around 86% of patients reported satisfaction with the procedure, and 65% showed little or no radiographic arthritis at final follow-up. The short-term complication rate is low at 2.4%.

The 18% overall failure rate deserves context rather than alarm. These are large, post-traumatic defects in knees where the practical alternative, short of joint replacement, is limited. It is also worth acknowledging honestly that no randomised controlled trials directly compare OCA to arthroplasty or ACI for post-traumatic lesions — that remains a genuine evidence gap. Even so, the 22-year follow-up data offer a level of long-term confidence that is unusual in cartilage restoration surgery.

Surgery, concomitant procedures, and what the operation involves

On the day of surgery, OCA is performed as a single-stage open procedure. The damaged segment of bone and cartilage is excised back to a clean margin, and the prepared donor plug or shell — trimmed to match the recipient site precisely — is press-fitted to restore the articular surface contour. No second operation is needed to harvest cells or re-implant them; the restoration is complete in one theatre visit.

For nearly half of patients, that visit involves more than the graft alone. In published series, 46% undergo a concomitant procedure — most commonly a corrective osteotomy or ligament reconstruction. This is not incidental. Large post-traumatic defects rarely occur in mechanically neutral knees: the same high-energy impact that damages cartilage often destabilises ligaments or shifts load distribution onto the repair site. Addressing malalignment or instability at the same time protects the graft; leaving either uncorrected risks overloading the new surface before it integrates.

Defect location also shapes what the operation involves. Femoral condyle reconstruction carries the longest follow-up evidence; patellofemoral grafts require a different approach that accounts for patella tracking; tibial plateau lesions present distinct access challenges. A patient's surgeon will explain which approach applies to their anatomy.

For patients outside specialist centres, the most practical question to raise early is tissue-bank access. If a local unit cannot guarantee graft delivery within the 28-day procurement window, referral to a centre with an established supply chain — rather than waiting — is the step worth asking about at the initial consultation.

Recovery timeline and return to sport

Recovery unfolds in well-defined stages, and progression through each one depends on objective evidence of healing — not symptom resolution alone.

For the first two to three months, the priority is protecting the graft while bone integration begins. Weight-bearing is introduced gradually, with most patients tolerating full load by months three to four. Low-impact activity — swimming, cycling, walking on even ground — becomes realistic from around four to six months, provided imaging confirms the graft is tracking as expected.

Return to impact sport is not governed by how the knee feels. Radiographic confirmation of integration is the standard clinical gate, and high-impact activity is generally withheld until nine to twelve months postoperatively. In a cohort of 149 knees followed over a mean of six years — the majority competitive or recreational athletes — 75.2% returned to sport or physical recreation, and 79% were classified as functioning at a high-activity level on the IKDC scale.

Where the original operation included concomitant procedures such as osteotomy or ligament reconstruction, recovery extends beyond these benchmarks. Alignment correction and soft-tissue healing impose their own timetable, and patients should factor this into their planning from the outset rather than treating the OCA milestones as the whole picture.

The longer-term outlook for successfully integrated grafts is broadly reassuring. A 22-year follow-up series for distal femoral OCA represents one of the longest documented outcome records in cartilage restoration surgery — a meaningful finding for patients whose index defects, had surgical restoration not been an option, would have placed them on an earlier trajectory towards total knee replacement.