What the evidence actually says

For most patients with a focal cartilage defect of 3 cm² or larger, the best available evidence favours MACI over microfracture — but the comparison is not one-size-fits-all, and defect size is the single most important factor shaping that conclusion.

The clearest evidence comes from the Phase 3 SUMMIT randomised controlled trial, which enrolled patients with knee cartilage defects ≥3 cm² across 14 sites. MACI produced statistically and clinically significantly greater improvements in KOOS pain and function scores at two years compared with microfracture. Critically, that was not a short-term effect: the SUMMIT Extension Study followed 128 of the original 144 randomised patients (89%) out to five years, and MACI's advantage was fully maintained at that point.

Defect size, however, is a genuine moderating variable rather than a detail. For lesions below approximately 2–3 cm², microfracture and mosaicplasty remain guideline-acceptable first-line options — MACI is not automatically indicated for every cartilage injury. A 2024 network meta-analysis by Muthu et al. reviewed randomised trial data across multiple cartilage repair techniques and found no single procedure that outperforms all others across every defect size and patient profile. The analysis does increasingly disfavour isolated microfracture for larger or mechanically demanding lesions, but reinforces the point that treatment decisions should be size-matched and individualised rather than made by blanket comparison.

Why microfracture loses ground over time

The fundamental problem with microfracture is biological, not technical. When the procedure works as intended, it stimulates marrow cells to fill the cartilage defect — but those cells form fibrocartilage, not the native hyaline cartilage that originally lined the joint. The distinction matters: think of fibrocartilage as closer to scar tissue than to original tissue. It lacks the organised collagen architecture that allows hyaline cartilage to absorb and redistribute load across a lifetime of use. Under the repetitive forces of daily activity and sport, fibrocartilage wears faster and offers less resilience.

That biological vulnerability plays out predictably in the long-term survival data. Gopinatth et al. (2024) tracked microfracture outcomes in medium-to-large chondral defects and found a survival rate of 88.8% at five years — acceptable on the surface — but by ten years that had fallen to 67.9%, and by twelve years to just 45.6%. This is not a stable plateau: it is a trajectory of progressive breakdown. The systematic review by Orth et al. (2020) reinforces the picture, placing microfracture failure rates at 11–27% within five years and 6–32% at ten years across studies, with repair tissue consistently characterised as fibrocartilaginous rather than hyaline.

Subchondral osseous overgrowth — where bone proliferates beneath the repair site — is implicated in 93% of microfracture failures. Damage to the subchondral bone plate during the procedure can also reduce the bone quality needed for any future cartilage repair, narrowing a patient's options downstream.

Microfracture is not without a legitimate historical role. For smaller, well-contained lesions in lower-demand patients, it has been an acceptable single-stage option and remains listed in guidelines accordingly. The evidence problem sharpens specifically with defect size and time: the larger and more mechanically loaded the lesion, the steeper the deterioration curve.

MACI's long-term track record

Beyond the five-year SUMMIT results, the question of how MACI performs over a decade or more is addressed by a 2024 systematic review by Wang et al. examining minimum 10-year follow-up data. The review confirmed sustained significant improvements in patient-reported outcomes, high patient satisfaction, and satisfactory cartilage defect fill on MRI — placing MACI firmly in the category of long-horizon treatments rather than procedures that merely defer a problem.

The patellofemoral compartment — the joint surface between the kneecap and the groove in the femur — merits a separate note. A 2024 study by Ebert et al. reported outcomes from patella and trochlea MACI implants at ≥10 years: approximately 92% patient satisfaction and a graft failure rate of 9–13%. The findings are broadly positive, though clinical scores in this group sit modestly lower than those seen after tibiofemoral MACI, despite MRI appearances being largely similar. Location, in other words, introduces a degree of variability even within the same technique.

One long-horizon caveat deserves honest acknowledgement. Randomised trial data comparing first-generation ACI to microfracture at 14–15 years found no statistically significant difference in clinical scores between the two groups, and approximately 57% of patients in both arms had developed early radiographic osteoarthritis by that point. First-generation ACI used a periosteal patch rather than MACI's seeded Type I/III collagen membrane — the biological mechanisms are meaningfully different — but very long-horizon data specific to MACI are still maturing. The 15-year ACI findings are a reasonable reminder that cartilage repair does not guarantee immunity from osteoarthritis over decades, and expectations at the very longest time horizons should remain realistic.

Defect size and the decision framework

Below approximately 2 cm², microfracture and mosaicplasty (osteochondral autograft transfer, or OAT) remain guideline-acceptable first-line options for focal femoral condyle lesions — particularly in lower-demand patients where the biological limitations of fibrocartilage are less likely to become clinically decisive within a realistic time horizon.

In the 1–4 cm² range, OAT merits serious consideration as an alternative to cell-based repair. The 2024 network meta-analysis by Muthu et al. found that OAT produced significantly better ten-year functional outcomes than microfracture — a relevant data point for patients who prefer a single-stage autograft approach and whose lesion size falls within that window, accepting that donor-site morbidity at the harvest site is a meaningful trade-off to discuss with a surgeon.

At approximately 3 cm² and above, MACI is the evidence-supported choice for active patients — supported by the SUMMIT trial and confirmed at five years in the SUMMIT Extension Study, as outlined in earlier sections.

Size, though, is not the whole picture. Location matters: patellofemoral lesions (patella and trochlea) carry modestly different clinical expectations than tibiofemoral implants, as the ten-year patellofemoral data suggest. Patient age, activity demand, and history of prior marrow-stimulation procedures all influence which technique a specialist is likely to recommend.

Mechanical environment is arguably the most frequently underestimated factor. A well-chosen cartilage repair implanted into a malaligned or ligament-unstable joint faces abnormal loading from the outset, which compromises results regardless of technique. Where significant varus or valgus malalignment is present, corrective osteotomy — to redistribute load away from the damaged compartment — is typically considered a prerequisite rather than an optional adjunct. A thorough assessment of alignment and ligament stability therefore forms part of any pre-operative planning conversation.

Practical factors in the MACI pathway

Choosing MACI means committing to two separate procedures. The first is a biopsy — a small arthroscopic step to harvest cartilage cells from a low-load area of the knee. Those cells are then cultured in a laboratory and seeded onto a collagen membrane before the second, implantation procedure takes place.

The interval between the two stages carries a practical implication worth understanding: cartilage defects continue to expand during that window at a mean rate of approximately 0.11 cm² per month, and around 16.2% of patients develop at least one new high-grade cartilage lesion before implantation occurs. This is not a reason for alarm, but it does support moving through the pathway without unnecessary delay once a biopsy has been performed.

For some patients aged 18–55 with suitable anatomy, the implantation stage may be offered arthroscopically rather than via open surgery. This arthroscopic approach — known as MACI Arthro — reduces surgical morbidity compared with open implantation. It is not universally available or appropriate for every case, so whether it is an option depends on individual anatomy and the specialist's assessment.

Cost is a legitimate factor to raise with a clinical team. MACI is more resource-intensive than microfracture — involving laboratory cell culture and two surgical episodes — and this is relevant both for NHS commissioning discussions and for patients exploring private funding routes.

Finally, as touched on in the previous section, the mechanical environment of the joint must be optimised before or alongside implantation. Meniscal integrity, as well as alignment and ligament stability, forms part of the pre-operative checklist — deficiencies in any of these areas are addressed as part of surgical planning rather than assumed to resolve on their own.

Finding a specialist who can advise on your options

The variables that determine whether MACI, OAT, AMIC, or another approach is appropriate — defect size, location, mechanical alignment, and prior surgical history — can only be properly weighed by a specialist who has examined the knee and reviewed the imaging. A practical starting point for that consultation is asking whether the lesion sits in the 2–3 cm² zone where the evidence for any single technique remains genuinely contested, and whether alignment or ligamentous factors need addressing alongside the cartilage repair itself. Knee cartilage specialists across the UK — including those offering MACI and the full range of restorative options — are searchable through the Search MSK directory by region and specialty, making it straightforward to identify a clinician suited to your specific situation.