When a cartilage defect is too large for standard repair

Once a full-thickness cartilage defect in the knee exceeds what the available repair tools can safely cover, the clinical question shifts from which repair technique to whether donor tissue is needed. That threshold matters in practice.

Microfracture, the most widely used first-line option historically, is suited to defects of roughly 2 cm² or less; beyond that size, the fibrocartilage repair tissue it produces becomes increasingly inadequate. Osteochondral autograft transfer (OATS or mosaicplasty) extends the upper boundary somewhat — typically to around 1–4 cm² — but is constrained by how much graft the surgeon can safely harvest from the same knee without causing donor-site problems. Matrix-based cell therapies such as MACI can address larger areas, but their cell-seeding efficiency and integration capacity diminish as defect complexity and depth increase, particularly when subchondral bone has also been lost.

Fresh osteochondral allograft (OCA) transplantation occupies a different size territory entirely. Across published case series, the mean defect area treated with OCA is 6.3 cm² — well beyond the practical ceiling of autograft techniques. Crucially, the composite graft replaces both the cartilage surface and the underlying bone in a single procedure, something that cell-based or marrow-stimulation approaches cannot reliably achieve for large lesions.

Post-traumatic injury is the dominant reason surgeons reach for OCA, accounting for approximately 38% of cases in large series. A high-energy mechanism — a direct impact, a plateau fracture, or a significant ligamentous injury with associated chondral damage — typically destroys both the cartilage tidemark and the subchondral bone beneath it. Articular cartilage carries no blood supply and cannot mount a meaningful healing response; left unaddressed, large full-thickness lesions on the femoral condyle or tibial plateau progress rather than resolve.

Why fresh donor tissue is the biological standard

The biological rationale for using fresh rather than frozen donor tissue rests on one variable: chondrocyte viability.

Hyaline cartilage derives its mechanical resilience from living chondrocytes embedded in a collagen-rich extracellular matrix. These cells maintain the matrix and sustain the tissue's load-bearing function over time. Freezing during processing kills chondrocytes — the structural scaffold remains, but it is acellular. An acellular cartilage surface degrades under physiological load, which is why frozen allografts are not the clinical standard for knee restoration.

Fresh allografts preserve this cellular population. The graft — a matched plug of living bone and cartilage from a cadaveric donor — is stored in nutrient culture medium and transplanted within a defined processing window to maximise chondrocyte viability at the point of implantation. Donor tissue undergoes systematic infectious-disease screening before release, ensuring safety for the recipient.

Anatomical size-matching is a further practical advantage. Each graft is selected to correspond with the recipient condyle's surface curvature, reproducing the geometry that plug arrays from autograft techniques cannot replicate across a large defect area.

The composite structure of the graft matters too. Its osseous portion integrates with host bone through osseointegration, while the cartilage surface functions as genuine hyaline tissue — load-bearing, low-friction, and sustained by resident chondrocytes. That combination is not achievable through marrow stimulation, which produces mechanically inferior fibrocartilage.

One biological aspect warrants acknowledgement: fresh allografts carry donor-derived surface proteins that differ from the recipient's own cells. Long-term immune-response data in humans remain limited, and the clinical significance for graft longevity is not yet fully established — surgeons factor this into patient selection and long-term follow-up planning, rather than treating it as a contraindication in otherwise suitable candidates.

Post-traumatic knee injuries that lead to OCA

Several distinct injury mechanisms produce the kind of knee damage OCA is designed to address — not gradual wear, but sudden structural failure that the joint cannot recover from.

High-energy events are the most direct route: a tibial plateau fracture, a dashboard-impact osteochondral fracture, or a severe twisting injury that shears through the cartilage surface and into the bone beneath. Each mechanism creates a focal, full-thickness void — destroying both the cartilage tidemark and its subchondral support simultaneously. The medial and lateral femoral condyles are the most commonly affected sites; the tibial plateau and patellofemoral joint are additional anatomical targets, particularly after direct-impact trauma.

Osteochondritis dissecans (OCD) represents a related subgroup, accounting for approximately 30% of OCA indications in large case series. When an OCD lesion progresses to loose-body separation or condylar collapse rather than stable healing, OCA can resurface the resulting defect in the same way it addresses an acute post-traumatic void.

What differentiates these lesions from generalised degeneration is their focality. The surrounding cartilage is typically intact, the patient tends to be younger, and the defect has a defined margin suited to graft seating. OCA is appropriate only when a lesion is symptomatic and mechanically significant — not an incidental imaging finding. Confirmation requires MRI; cartilage-specific sequences including T2 mapping and AI-assisted cartilage segmentation can characterise lesion depth and area, informing surgical planning before any decision about graft sizing.

What OCA surgery involves and why alignment often needs addressing

On the day of surgery, OCA is performed as a single-stage open procedure under regional or general anaesthesia. The damaged bone-cartilage segment is removed from the recipient site, and the prepared bed is shaped to accept the donor plug — sizing the graft to match the exact surface curvature of the recipient condyle is a critical intraoperative step. The plug is either press-fitted into place or secured with bioabsorbable fixation hardware; once seated correctly, the hyaline cartilage surface is flush with the surrounding joint.

What many patients do not anticipate is that surgery may address more than the cartilage defect alone. Across large published case series, 46% of OCA patients required concomitant procedures at the same index operation — most commonly a high tibial osteotomy (HTO) or distal femoral osteotomy (DFO), and in some cases meniscus repair. This is not incidental: uncorrected varus or valgus malalignment concentrates mechanical load directly onto the graft site. A fresh allograft placed into a malaligned knee faces concentrated stress that significantly reduces its chance of long-term survival. HTO and DFO are therefore not separate, unrelated interventions — they are load-redistribution procedures that protect the graft by restoring a more balanced mechanical environment across the joint.

Rehabilitation begins immediately after surgery, with range-of-motion exercises started in the first days to maintain joint health and support cartilage surface maturation. Weight-bearing, however, is protected — typically with crutches for six to eight weeks — to allow the osseous portion of the graft to integrate with host bone before progressive loading is introduced.

Survivorship, return to sport, and long-term outcomes

Published series now span more than two decades of follow-up, and the survival picture they paint is consistent: OCA grafts are durable, but they are not permanent, and patients deserve both halves of that sentence.

At five years, graft survivorship ranges from 82.6% to approximately 95% depending on cohort and how failure is defined — the stricter analyses count any knee scoring below the threshold on the Hospital for Special Surgery scale, any revision procedure, or any conversion to arthroplasty. By ten years the figure settles around 69–70%, and one extended cohort found that 44 of 65 grafts (68%) remained in situ and functioning at a mean of 12.9 years. The clearest long-term anchors in the literature are Gross et al. (Clin Orthop Relat Res 2008), which established OCA as a viable strategy specifically for post-traumatic knee defects over the long term, and Raz et al. (JBJS Am 2014), who reported outcomes for distal femoral allografts at a mean follow-up of 22 years — a timeframe that places OCA durability in a context no other cartilage restoration technique can yet match.

Functional and patient-reported results at a mean six-year review are encouraging: 75.2% of knees (112 of 149) returned to sport or recreational activity, 71% of patients rated their knee function as 'very good' to 'excellent', and 79% met criteria for high-level activity on the IKDC subjective evaluation. Overall satisfaction stands at 86%, and 65% showed little or no radiographic arthritis at final follow-up.

The overall failure rate is approximately 18%, and when grafts do fail — through collapse, fracture, or fragmentation — conversion to total knee arthroplasty is the most common end-stage outcome. Revision OCA is feasible in some cases, though the goals shift from sport return towards restoring activities of daily living and delaying arthroplasty.

One important caveat: the evidence base, though substantial, is almost entirely observational. No randomised controlled trial directly comparing OCA to MACI or ACI for large defects has been published. The conclusions drawn from these series are therefore based on cohort data rather than head-to-head comparisons, and individual outcomes depend on defect characteristics, alignment, and patient factors that only a consultant assessment can properly weigh.

How OCA fits the wider cartilage restoration decision

The clinical decision between OCA and cell-based repair does not always resolve cleanly. At the extremes the choice is fairly clear — very large defects with significant bone loss point towards allograft; smaller lesions with intact subchondral bone point elsewhere. The genuinely difficult territory sits roughly in the 3–6 cm² range, where two experienced surgeons may reasonably reach different conclusions.

One distinction that does carry consistent clinical support is the role of the subchondral bone layer. MACI and ACI restore the cartilage surface but cannot reconstitute the bone beneath it. Where the subchondral plate is compromised — by the original injury, by prior marrow-stimulation procedures that damaged it, or by significant bone loss at depth — allograft is the only cartilage-and-bone repair option that avoids arthroplasty outright. For very large or multi-compartmental defects, the same logic applies: OCA may be the only realistic joint-preservation strategy available.

No randomised trial has directly compared OCA to MACI or ACI for intermediate-sized lesions, and that gap is unlikely to be closed soon given the heterogeneity of defect types in clinical practice. Defect size, depth of bone involvement, joint alignment, patient age, activity expectations, and prior surgical history all bear on the decision — and their interaction makes individual specialist assessment more useful than any generalised threshold.

That is the practical takeaway: when a lesion falls into uncertain territory, technique selection is a clinical judgement requiring direct examination, MRI characterisation, and sometimes cartilage-volume segmentation analysis. Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. Book an assessment at lincolnshireknee.co.uk.