Why post-traumatic knee defects need more than a cartilage repair

A traumatic knee injury — a heavy fall, a road collision, a sporting impact — does not always stop at the cartilage surface. The force that cracks or crushes the cartilage often damages the layer of bone directly beneath it too. That bone layer, the subchondral plate, is not merely structural background: it is the foundation that any cartilage repair has to build on.

Techniques such as ACI and MACI work by placing living chondrocytes — either under a periosteal patch or seeded onto a collagen membrane — and waiting for them to integrate and mature within the joint. For that to succeed, the subchondral bone must be intact and well-vascularised. When a post-traumatic injury has already compromised that foundation, the graft has nothing solid to anchor to. In these circumstances, ACI and MACI are either formally contraindicated or carry a substantially higher risk of failure.

Fresh osteochondral allograft (OCA) transplantation was developed for exactly this scenario. Rather than regenerating the cartilage layer alone, it provides mature hyaline cartilage together with its supporting subchondral bone scaffold as a single biological unit — restoring both injured layers in one operation. Clinical guidelines formally recognise OCA as the indicated approach for lesions larger than 2 cm² where subchondral bone injury prevents cell-based repair. Defect size and subchondral status, considered together, are what determine whether OCA is the appropriate route — and the sections below explain how those factors play out in practice.

Where OCA fits in the cartilage repair ladder

Cartilage repair does not follow a single pathway — the right procedure depends on how large the defect is and, critically, whether the subchondral bone beneath it remains structurally sound.

At the smaller end of the scale, microfracture was historically the first procedure tried for focal cartilage lesions. Its use has declined significantly: controlled studies show fibrocartilage breakdown by two to three years, and the drilling technique can damage the subchondral bone plate, narrowing the options for any subsequent repair. For lesions up to around 4 cm² with intact underlying bone, OATS or mosaicplasty — transferring small osteochondral cylinders from a lower-load region of the same knee — offers a single-stage autograft solution.

Above roughly 3 cm², cell-based repair takes over. The SUMMIT trial demonstrated that MACI produced better pain and function scores than microfracture at two and five years for defects of this size. However, MACI's evidence base thins as defects grow larger, and it cannot address bone loss independently of the cartilage layer.

The ladder's upper rungs belong to fresh osteochondral allograft. A 2025 case series examining 89 knees with bipolar defects averaging 16.7 cm² — a scale far beyond the reach of cell-based repair — reported 5-year survivorship of 73.8% and 15-year survivorship of 58.9%, even though 91% of those patients had undergone a mean of 3.2 prior procedures. Subchondral bone integrity, rather than defect area alone, is the true branching point: where bone loss is significant, OCA is the tool the evidence supports.

OCA as a rescue option after cell-based repair fails

For patients who have already undergone cartilage surgery without lasting benefit, the question shifts: is a second operation worth considering? The evidence suggests it often is — provided the right procedure is chosen.

A 2025 PRISMA-compliant systematic review pooling data from 349 patients with secondary OCA after failed index cartilage surgery found an overall failure rate of 16.6% and 5-year graft survival of between 79% and 87.8%. The most common entry point was failed marrow stimulation, which accounted for 73.8% of prior procedures — a reminder that microfracture, despite its declining reputation, remains a frequent first step in many patients' treatment journeys.

One finding from a separate high-chondrocyte-viability (HCV) graft study stands out, though it warrants careful interpretation given the patient numbers involved: when revision OCA followed a previously failed cell-matrix or ACI-type procedure, graft survival reached 100%, compared with 58.6% in cases where the failed index operation was itself an osteochondral procedure (p=0.049). In plain terms, the type of surgery that failed matters. A joint that has not had its bony architecture significantly altered by a prior osteochondral intervention appears to be a more receptive host for a salvage allograft.

The picture is not uniformly reassuring. Within the systematic review, defects of 9–10 cm² carried reoperation rates of 67% and failure rates of 39%, underscoring that defect size remains a real constraint even at the revision stage.

Taken together, this evidence positions OCA as a considered, evidence-supported option after cell-based failure — not merely a holding measure before joint replacement.

How graft quality shapes what patients can expect

Surgery technique is only part of what determines how well an OCA graft performs. The viability of the donor chondrocytes — the living cartilage cells within the transplanted tissue — is an equally important variable, and one that is partly under the control of whoever prepares and stores the graft.

Chondrocytes need to be alive at the point of implantation to integrate properly and sustain the hyaline cartilage layer over time. Studies using high-chondrocyte-viability (HCV) grafts report functional survival above 79% for primary OCA at a mean follow-up of around 43 months, with revision OCA using the same preparation achieving 71.4% functional survival — figures that hold up into the midterm range for well-prepared allografts.

Perhaps the most counterintuitive finding in the outcomes data is where failures actually occur. In a mid-term cohort study, cartilage-specific failure accounted for only 4.7% of graft failures. The dominant causes were bone integration failure (34.9%) and meniscal allograft failure (30.2%), with joint disease progression making up most of the remainder. The cartilage component of a carefully prepared OCA, in other words, tends to be durable; it is bone healing and the condition of surrounding structures — particularly the meniscus — that carry the greater clinical risk.

For patients, this means that the period after surgery matters considerably. Bone integration depends on controlled loading and rehabilitation adherence; meniscal health is a pre-existing factor that surgeons will assess beforehand. Asking about graft preparation standards is therefore a reasonable question at any pre-operative consultation.

Who is a realistic candidate and where the risks lie

Deciding whether OCA is appropriate involves weighing the surgical complexity of the lesion against patient-level factors that influence how well a graft heals and how consistently a patient can follow rehabilitation.

The single strongest surgical risk factor is bipolar involvement — where opposing joint surfaces on both the femur and tibia are affected. Compared with a unipolar lesion, bipolar tibiofemoral OCA carries an odds ratio of 3.86 for graft failure. That figure warrants an honest conversation: survivorship figures at 15 years for large bipolar lesions fall to around 58.9%, and most patients in those cohorts had already undergone an average of 3.2 prior procedures. OCA can still be the right choice in this context, but realistic expectations matter.

Concurrent ligament reconstruction and ipsilateral osteotomy add procedural complexity and are also recognised risk factors. Alignment problems — varus or valgus deformity shifting load onto the repaired compartment — will often require adjunctive high tibial or distal femoral osteotomy. This is best understood as careful surgical planning rather than a complication; correcting the mechanical environment gives the graft its best chance.

At the patient level, older age, higher BMI, and difficulty adhering to post-operative rehabilitation all reduce the likelihood of a durable outcome. For patients with more advanced or diffuse osteoarthritis across multiple compartments, joint replacement is generally the more appropriate pathway — OCA's role in joint preservation is strongest in younger patients with discrete post-traumatic defects and structurally sound surrounding tissue.

What the evidence still does not tell us

No randomised controlled trial has yet placed OCA directly against MACI for large post-traumatic defects with subchondral involvement — a comparison group that is genuinely difficult to assemble and ethically to randomise. The comparative case for OCA in this setting therefore rests on biological rationale and cohort-level outcomes rather than trial data.

Two further gaps are worth naming. Biomechanical evidence remains thin: a systematic review identified only 8 studies covering 54 participants, meaning durability under real-world joint loading is less well characterised than functional outcome scores. Prospective long-term data on whether OCA slows or prevents osteoarthritis progression in post-traumatic knees are also absent — a question that matters considerably for younger patients hoping to postpone joint replacement.

Neither gap changes the underlying clinical logic. When a large defect involves both cartilage and subchondral bone, the dual-layer restoration that fresh OCA provides addresses a structural problem that cell-based repair cannot, regardless of trial design. OCA carries the strongest available evidence for this clinical scenario; the gaps reflect the practical difficulty of conducting trials in this patient group, not a shortcoming of the technique itself.

Patients can book an assessment without a GP referral at mskdoctors.com.

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