What patients are actually choosing between

Most patients referred for cell-based cartilage repair in the UK will be offered MACI. That is the practical starting point. What they are less likely to hear is that MACI did not arrive independently — it is the third-generation refinement of autologous chondrocyte implantation (ACI), a technique developed in the 1980s and 1990s that first established the principle of growing a patient's own cartilage cells in the laboratory before returning them to the damaged joint.

Both procedures target the same problem: a focal articular cartilage defect — typically between 2 cm² and 10 cm² — in a patient too young or too active to accept the trade-offs of joint replacement. Neither is appropriate for diffuse, end-stage osteoarthritis. They sit firmly in the restoration tier of the treatment pathway, not at its end.

The shared biology is identical: harvest a small cartilage sample, expand the chondrocytes over several weeks, then implant them into the prepared defect. Where the two techniques diverge is in how those cells are delivered — and whether that difference translates into meaningfully different outcomes is what the evidence addresses.

How each procedure works — and how they differ

The timeline both procedures impose is identical. Stage one is an arthroscopic biopsy: a small cartilage sample — typically 200–300 mg — is taken from a low-load area of the knee. The harvested chondrocytes are then sent to a specialist laboratory and expanded over three to five weeks before stage two, the implantation itself. Patients commit to this two-stage structure regardless of which technique is used.

The implantation stage is where the two diverge. In first-generation ACI, the expanded cells are injected as a liquid suspension beneath a periosteal flap — a thin strip of tissue taken from the upper tibia — which is stitched over the defect to contain them. This is technically demanding; the suturing must be watertight, and the approach is open or mini-open rather than fully arthroscopic. Graft hypertrophy — the flap thickening excessively as it heals — was reported in up to 36% of periosteal-cover cases, often requiring a further procedure to trim it back. Switching to a collagen membrane in later ACI variants brought that rate down meaningfully, though not to zero.

MACI removes the suturing step entirely. The cultured cells are pre-seeded onto a Type I/III collagen scaffold in the laboratory, trimmed to match the defect shape, and fixed in place with fibrin glue. The cells are distributed evenly across the scaffold from the outset. The result is a less invasive implantation, one that is technically more reproducible and supports an earlier, more structured rehabilitation programme than earlier ACI iterations — a refinement in delivery rather than a departure in biology.

The head-to-head evidence at one year

Only one prospective RCT has placed ACI and MACI in direct comparison, and its findings remain the most controlled data point available. Published in the Bone & Joint Journal in 2005, the trial enrolled 91 patients — 44 receiving collagen-cover ACI (ACI-C) and 47 receiving MACI — and assessed outcomes at one year.

The headline result is equivalence. The modified Cincinnati knee score rose by 17.6 points in the ACI-C group and 19.6 points in the MACI group; the difference was not statistically significant (p=0.32). Reoperation rates were identical at 9% in each group at twelve months.

Tissue-quality measures told a marginally different story. Arthroscopic ICRS good-to-excellent scores favoured ACI-C (79.2% versus 66.6%), and hyaline-like cartilage on biopsy was found somewhat more often in the ACI-C group (43.9% versus 36.4%). Neither difference reached statistical significance. The more plausible explanation is technique maturity: MACI was at an earlier stage of clinical adoption in 2005, and implantation has been refined considerably since. Interpreting these figures as evidence of an inherent biological advantage for ACI-C would be a step the data do not support.

This single trial reports one-year outcomes in a relatively small cohort. What happens at five, ten, or fifteen years requires looking at independent long-term cohorts for each procedure — and that evidence, assembled separately for ACI and MACI, is where the more durable picture emerges.

ACI beyond a decade — what the long-term data shows

Eight in ten patients who underwent ACI show successful outcomes at an average of more than eleven years — the headline finding from a 2016 systematic review drawing on nine studies and 771 patients, with a mean follow-up of 11.4 years. Lysholm scores improved by an average of 24.9 points and IKDC scores by 16.5 points from baseline, improvements sustained across a follow-up period well beyond what most surgical trials report.

The 37% reoperation rate that sometimes appears in summaries of this review requires careful reading. A substantial proportion of those reoperations were planned second-look arthroscopies — staged assessments of graft integration rather than rescue procedures prompted by clinical failure. The more meaningful figure is the 18% failure rate: the proportion of patients in whom durable, functional cartilage restoration was not achieved. Conflating the two gives a misleading picture of how ACI performs over time.

As established by the generational differences in technique, graft hypertrophy is a complication concentrated in periosteal-cover ACI, with incidence reported at up to 36%; switching to collagen-membrane and matrix-based variants brings that rate down considerably. Failure rates across generations follow the same pattern — periosteal-cover ACI sits at the high end, while membrane-based approaches perform better — which is why pooled ACI statistics, spanning techniques that differ substantially, need to be interpreted with that heterogeneity in mind.

One further finding from the long-term data is relevant to patient selection: failure rates are higher in those who have previously undergone marrow-stimulation procedures such as microfracture. Prior microfracture can alter the subchondral bone environment in ways that compromise subsequent cell-based repair, making treatment history an important consideration during planning.

MACI beyond a decade — durability and the SUMMIT trial

Ten-year follow-up data for MACI, now available from two independent 2024 cohort studies, paints a reassuring long-term picture. Wang and colleagues, publishing in the American Journal of Sports Medicine, found that patients who had undergone MACI demonstrated significant and durable improvements in patient-reported outcomes at minimum 10-year follow-up — with an all-cause reoperation rate of approximately 9% and progression to total knee arthroplasty (TKA) in roughly 7.4% of cases. Those figures compare favourably with the historically reported ACI reoperation data, though the comparison must be made cautiously given the absence of a head-to-head long-term RCT.

A companion study by Weishorn and colleagues confirmed good long-term clinical outcomes and high overall patient satisfaction, whilst adding a practical caveat for selection: BMI emerged as a meaningful modifying variable, with heavier patients showing less favourable results. Together with older age and defect size exceeding 4.5 cm², body habitus forms part of the risk-stratification picture a consultant will weigh before recommending MACI.

For defects of 3 cm² or larger, MACI's position relative to marrow-stimulation techniques is settled by the SUMMIT trial (Brittberg 2018) — a 14-centre prospective RCT that found MACI statistically superior to microfracture on KOOS pain and function scores at 2 years, with that advantage sustained and significant at 5 years. This comparison defines MACI's role as the preferred restorative option over marrow-stimulation for larger focal lesions, not as a direct refutation of ACI, for which equivalent long-term head-to-head trial data does not exist.

One honest qualification applies across all this evidence: patients treated in routine clinical practice tend to carry significantly larger defects than trial populations, which may mean published success rates slightly overestimate outcomes in the broader clinical setting. Concomitant procedures — osteotomy for malalignment, ACL reconstruction — are often performed alongside MACI, but the criteria for optimal combination remain incompletely defined in the current evidence base.

Who is suitable and what the assessment involves

Practical selection for either procedure rests on a handful of well-evidenced criteria that, taken together, give most patients a clear picture of whether they are a strong candidate, a borderline one, or better suited to a different pathway.

The core indication is a symptomatic, focal cartilage defect — typically between 2 and 10 cm² — in a joint otherwise free of diffuse or end-stage osteoarthritis. Generalised OA lies outside the scope of cell-based repair; for those patients, joint-preservation or replacement conversations are more appropriate.

Candidate strength broadly increases when several factors align: the patient is an active adult, the defect is focal and contained, body weight is in a healthy range (as the 2024 Weishorn cohort data confirmed, higher BMI is associated with less favourable long-term results), and no prior marrow-stimulation procedure has been performed. Prior microfracture alters the subchondral bone environment in ways that compromise subsequent cell-based repair — a treatment history worth raising explicitly at assessment.

Malalignment is a linked but separate consideration. A limb that concentrates load on the repaired compartment places the graft under disproportionate stress; weight-bearing radiographs are standard, and where gait mechanics are in question, objective biomechanical assessment — including markerless motion capture — can help clarify whether an osteotomy is needed alongside cartilage repair rather than after it.

Patients who recognise themselves in this picture and want to understand their options fully can arrange a specialist assessment without a GP referral at mskdoctors.com.