When a defect is too large for simpler options

Surgeons typically raise OCA and MACI together once imaging and examination point to a focal cartilage defect larger than roughly 4 cm² — the threshold at which the evidence base for simpler techniques runs thin. Microfracture produces fibrocartilage that breaks down within two to three years and damages the subchondral bone plate beneath, making it poorly suited to defects of this scale. OATS, which transfers small cylindrical plugs of healthy bone and cartilage from a donor site on the same knee, is generally limited to lesions up to around 2 cm² (or 4 cm² in a multi-plug mosaic arrangement) before donor-site morbidity becomes a significant concern.

The patients arriving at this decision point are commonly younger adults with high-grade (ICRS III–IV) full-thickness lesions — often the result of trauma, osteochondritis dissecans, or avascular necrosis — for whom total knee replacement is not yet appropriate. Both OCA and MACI are designed specifically for this population and this defect scale.

The two procedures differ fundamentally in structure. OCA — osteochondral allograft transplantation — is a single operation in which a size-matched plug of donor bone and cartilage is implanted to replace the defect. MACI takes a two-stage route: the patient's own cartilage cells are harvested, cultured in a laboratory over one to two months, and then implanted on a collagen scaffold in a second procedure.

Which of the two is more appropriate turns primarily on one question: whether the damage extends into the subchondral bone beneath the cartilage surface.

The deciding factor: is the bone underneath affected?

Beneath the articular cartilage lies the subchondral bone — a dense layer that acts as both structural foundation and vascular supply for the cartilage above it. In a straightforward focal chondral defect, this bone remains intact; only the cartilage surface is lost. In more complex lesions — particularly those arising from osteochondritis dissecans, avascular necrosis, or significant trauma — the bone itself becomes cystic, partially resorbed, or necrotic.

This distinction drives the clearest decision rule in published cartilage-repair algorithms. Hinckel et al's 2021 treatment algorithm, one of the most widely cited frameworks in the field, states that OCA is the preferred choice over MACI whenever subchondral bone loss, cysts, or necrosis are present. The reason is structural: a fresh osteochondral allograft is a composite graft — it carries both a cartilage surface and a bone base, replacing both layers simultaneously in a single procedure. Where the bone foundation is deficient, the allograft rebuilds it.

MACI, by contrast, addresses only the cartilage layer. The procedure implants cultured cells onto a collagen scaffold that integrates with existing bone — but it cannot generate or restore bone stock that is already missing. Significant subchondral bone loss is therefore a contraindication to MACI, not simply a relative caution.

For large lesions where the subchondral bone remains healthy and intact, both procedures are genuinely viable. In those cases, the decision moves on to other factors: the location of the defect within the knee, the patient's age, and the practicalities of a one-stage versus two-stage surgical timeline.

One operation or two: what each procedure involves

From a practical standpoint, the two procedures involve very different treatment episodes — a distinction that shapes a patient's planning as much as the clinical decision itself.

OCA is a single-stage operation. The surgeon receives a size-matched plug of fresh donor osteochondral tissue and implants it directly into the prepared defect. Most patients then follow a protected weight-bearing protocol — typically six to eight weeks on crutches — while the donor bone integrates with the surrounding host bone. Range-of-motion exercises generally begin during this protected phase rather than after it.

MACI requires two separate procedures with a waiting period in between. At Stage 1, a small cartilage biopsy is taken from a low-load area of the same knee and sent to a specialist laboratory, where chondrocytes are cultured on a Type I/III collagen membrane over roughly one to two months. At Stage 2, the cell-seeded scaffold is fixed into the defect site. Return to sport is typically nine to twelve months from the first biopsy appointment — making the full treatment episode substantially longer than with OCA.

Alignment correction is commonly added to both procedures, but more routinely alongside MACI. In one retrospective cohort of patellofemoral defects, a tibial tubercle osteotomy was performed at the time of approximately 98% of MACI cases, compared with around 66% of OCA cases. The difference has a mechanical explanation: cell-seeded scaffolds depend more heavily on load redistribution to survive and integrate, particularly at the patella where contact forces are high. For patients considering MACI in that location, an osteotomy is often an expected component of the overall surgical plan rather than an optional addition — something to explore with a specialist during pre-operative planning.

OCA introduces considerations around donor tissue matching and availability, since the allograft must be sourced and processed before implantation. MACI removes the allograft element but adds the cell-culture interval, a second anaesthetic, and a longer recovery commitment overall.

How defect location changes the recommendation

Defect location within the knee — specifically whether the damage sits on the femoral condyle (the rounded end of the thigh bone) or on the patellofemoral joint (between the kneecap and the groove it tracks in) — acts as a second major stratifier, independent of lesion size.

For large femoral condyle defects where the subchondral bone remains intact, MACI has a well-established evidence base. Published series report that only 7.4% of patients progress to total knee replacement at 10 to 17 years post-surgery, and roughly 87.5% report meaningful symptom improvement — figures that reflect the relatively forgiving biomechanics of the condyle surface.

Patellofemoral defects, and in particular bipolar lesions — where the cartilage damage affects both opposing joint surfaces simultaneously — present a more demanding environment. In those settings, cell-based approaches such as MACI and ACI are generally preferred over OCA. The survivorship data for bipolar OCA illustrates why: unipolar grafts (one surface only) achieve approximately 82% survival at 10 years, whereas bipolar transplants — grafting both sides of a joint — drop to around 39% at the same timepoint. The contact stresses acting on two mismatched graft surfaces appear to accelerate failure in ways that do not affect single-surface repairs.

The patellofemoral joint also places greater mechanical demands on any repair. This is part of the reason that alignment correction is so commonly performed alongside cell-based procedures at that location — the repair needs load redistribution to survive, not just biological integration.

What long-term outcomes data actually shows

The most direct head-to-head evidence currently available comes from a 2025 retrospective cohort (Triana et al, n=95) that followed patients with patellofemoral cartilage defects for a mean of 63.8 months. Functional outcomes, return-to-sport rates, and patient satisfaction were broadly comparable between OCA and ACI/MACI groups — but one measure diverged: KOOS pain scores averaged 74.7 in the OCA group versus 83.6 in the ACI/MACI group, a statistically significant difference. On the KOOS scale, lower scores indicate worse pain, meaning OCA patients were reporting meaningfully more discomfort despite similar results on other functional measures. That tension is worth exploring with a specialist rather than being used as a reason to dismiss either option outright.

Within the same cohort, two outcome predictors emerged. Older patient age was associated with lower satisfaction after OCA; larger lesion size, conversely, was associated with worse outcomes after ACI/MACI. These interactions suggest that even within the large-defect category, the two procedures suit somewhat different patient profiles.

On longer-term durability, OCA survivorship is solid in the medium term — approximately 86.7% at five years and around 82% at ten years in published series including Gracitelli et al (2015, n=164) — but declines to roughly 67.5% at 20 years. Failure mechanisms include host bone non-union, chondrocyte death, structural fracture through the graft, and mechanical overload from uncorrected malalignment. For MACI, an approximately 9% reoperation rate in longer follow-up series reflects the procedure's durability in appropriate candidates, complementing the survivorship picture already discussed in the context of femoral condyle outcomes.

No large randomised controlled trial has directly compared OCA and MACI for large femoral condyle defects; available evidence is predominantly retrospective, and how well either procedure performs for defects exceeding 6 cm² remains genuinely uncertain — a limitation that is worth naming plainly when weighing the published figures above.

Questions to bring to your surgeon — and finding the right specialist

Choosing between these procedures hinges on details that only a specialist assessment can confirm — MRI findings alone rarely settle the question without a skilled review of subchondral bone depth, lesion geometry, and mechanical alignment. The earlier sections map those factors; the role of the consultation is to apply them to the individual case.

One point worth raising explicitly with any surgeon is whether STACi — an emerging three-dimensional scaffold evolution of MACI — might be relevant. STACi is beginning to appear at specialist centres for larger or structurally complex defects that a flat collagen membrane cannot fully accommodate, and its design in principle broadens the range of cases suitable for a cell-based approach. Long-term comparative data against OCA does not yet exist, however; it remains an evolving option rather than an established alternative, and any centre offering it should be able to explain where the evidence base currently stands.

Both OCA and MACI sit well outside routine general orthopaedic practice. Surgical volume matters in cartilage restoration: familiarity with graft sizing, fixation technique, and the judgement calls specific to complex cases develops through repeated exposure. Seeking a surgeon for whom cartilage restoration is a dedicated subspecialty — not an occasional procedure — is a reasonable criterion when shortlisting.

Finding a specialist

Search MSK lists knee cartilage specialists across the UK; filtering by region and specialty identifies surgeons whose practice includes OCA and MACI.