What ACI and MACI Actually Involve

Both ACI and MACI are two-stage knee procedures — and that point matters, because MACI is sometimes mistaken for a simpler or shorter treatment than its predecessor. In practice, both require two separate operations with a laboratory phase in between.

Stage 1 is identical for both. A surgeon takes a small arthroscopic biopsy of healthy cartilage — typically from the intercondylar notch or the periphery of a femoral condyle, areas that bear little load. That sample contains chondrocytes, the only cell type found in healthy articular cartilage, and the sole biological agent doing the repair work in both therapies. The cells are then sent to a specialist laboratory, where they are expanded in culture for at least four weeks before they are ready for re-implantation.

The waiting period between stages is a practical reality patients should factor into planning: there is no accelerated route through the laboratory phase.

Stage 2 — re-implantation — is where the two procedures diverge, and where the collagen scaffold or membrane becomes relevant. That carrier is not itself biologically active; it is a vehicle for the cells. Crucially, neither procedure is suited to diffuse osteoarthritis affecting the whole knee surface. Both are designed for focal articular cartilage defects — discrete areas of damage in an otherwise functional joint — and patient expectations should be framed accordingly from the outset.

Which Patients Are Suitable

Not every focal cartilage defect warrants a two-stage cell-based approach, and understanding the broad eligibility criteria can help patients gauge where they might sit before a formal consultant assessment.

Defect size is the starting point. Both ACI and MACI are generally considered for lesions above approximately 2 cm² — below that threshold, single-stage options such as OATS or microfracture are typically addressed first, since they can adequately fill a smaller defect without the added burden of a laboratory phase. At the upper end, lesions larger than 4.5 cm² are associated with measurably higher failure and reoperation rates; very large defects may prompt discussion of alternative or combined approaches.

The bone beneath the cartilage matters. MACI in particular is suited to defects where the underlying subchondral bone is largely intact. Where significant bone loss is present, osteochondral procedures — such as OATS or osteochondral allograft — are generally more appropriate.

Age is an independent risk factor. Older patients face higher failure rates with both cell-based techniques; this does not make either procedure categorically unsuitable, but it is a meaningful factor in the consent and shared decision-making conversation.

Previous marrow-stimulation surgery should be disclosed. A prior microfracture procedure is associated with higher ACI failure rates and will influence the surgical and referral planning — it is not a contraindication, but it changes the risk picture.

How the Two Procedures Differ at Stage 2

The divergence at Stage 2 comes down to one practical difference: what the surgeon does with the expanded cells once they reach the operating theatre.

In traditional ACI, chondrocytes are delivered as a liquid suspension that is injected beneath a membrane — periosteal tissue taken from the tibia, or a collagen patch — which must then be sutured watertight around the defect perimeter. The technique is technically demanding: any gap in the seal risks cell leakage, and the quality of the outcome is sensitive to surgical precision throughout. Where a periosteal membrane is used, there is also a meaningful risk that the tissue overgrows into the joint — a complication known as graft hypertrophy that can require further surgery.

MACI removes that suturing step entirely. The cells are pre-seeded onto a Type I/III collagen scaffold in the laboratory, so by the time the surgeon encounters the defect, the graft is a single handled construct that can be press-fit or secured with fibrin glue. There is no injected suspension to contain and no periosteal harvest from a separate site.

The clinical consequences of this difference are documented in a registry study of 662 knees comparing collagen-membrane MACI against periosteum-covered techniques in defects of 4 cm² or larger. Graft hypertrophy occurred in 10.7% of periosteum-covered cases versus 1.0% with collagen-membrane implantation; treatment failure fell from 9% to 3%. Functional improvement at follow-up was broadly similar between groups — a point worth noting, because it suggests the periosteal approach, when it works, is not inherently inferior in outcomes. The difference lies in the complication profile and operative burden, not in the biological ceiling either technique can reach.

For centres with lower cartilage-surgery volumes, the reduced technical complexity of the scaffold approach also matters: the narrower margin for error with a sutured liquid-suspension technique favours experienced specialist teams.

What the Long-Term Evidence Shows

ACI carries the longer evidence trail of the two. A systematic review spanning nine studies and 771 patients — followed to a mean of 11.4 years — placed the overall success rate at roughly 82%, with a failure rate of 18% and a reoperation rate of 37%. Those figures sit in a useful context: the patient population included older individuals and larger lesions, both of which independently push outcomes downward, so the headline success rate reflects a broad real-world range rather than a carefully selected cohort.

MACI's long-term data are more recently assembled but point in a similar direction. In a cohort of 168 patients followed for between 10 and 17 years — mean age 37 at surgery — patient-reported outcomes remained significantly improved from baseline, with an all-cause reoperation rate of 9.0% and progression to total knee arthroplasty in 7.4% of cases. Those figures are broadly comparable to the established ACI benchmarks, though the ACI evidence base is older and larger.

The SUMMIT trial provided the clearest comparative test of cell-based therapy, though it measured MACI against microfracture rather than against first-generation ACI. For defects of 3 cm² or more, MACI produced significantly better KOOS pain and function scores at both two and five years — the evidence underpinning cell-based approaches when defect size rules out simpler options.

One nuance worth setting out plainly: MACI grafts can show declining MRI scores over time without a matching decline in how patients report feeling. In one long-term series of 26 patients assessed at 8 to 16 years, mean mMOCART scores fell significantly on imaging, yet clinical outcome scores did not fall to a statistically significant degree. That divergence matters in pre-surgical counselling — imaging follow-up after MACI may look more concerning than the patient's symptoms warrant.

The most cited head-to-head RCT comparing ACI directly against MACI dates to 2005, predating the collagen scaffold technologies that now define current MACI practice. No large, adequately powered contemporary trial has yet closed that comparative gap.

How Surgeons Decide Between ACI and MACI

Several factors feed into the ACI-versus-MACI consultation, and the most instructive are those not already covered by lesion-size thresholds and membrane safety — the two variables addressed in earlier sections of this article.

Defect location is one such factor. For lesions on the patella or trochlea, ACI and MACI produce significantly better pain scores than osteochondral allograft, even where overall patient satisfaction and return-to-sport rates are broadly equivalent between the approaches. A related concern often raised for patellofemoral defects — patellar dysplasia, graded by the Wiberg classification — has been specifically examined in patients undergoing cell-based repair. Wiberg type (A, B, or C) did not significantly affect patient-reported outcomes or graft survival across a 59-patient series; it should not be treated as a contraindication to either technique.

Cell density is an honestly open question. MACI's scaffold delivers chondrocytes at lower density than some ACI variants, and animal-model evidence suggests this can produce softer regenerated tissue. High-density ACI (HD-ACI), implanting 5 million chondrocytes per cm², was developed partly in response to that limitation. Whether the density advantage translates into meaningfully superior clinical outcomes in the knee over the long term remains under investigation rather than settled by current evidence.

Surgeon and centre volume rounds out the framework. MACI's single-construct delivery lowers the procedural margin for error, which matters in settings where two-stage cartilage surgery is performed at modest case numbers. Experienced high-volume ACI centres can achieve comparable results with refined periosteal technique; the practical advantage of MACI is most pronounced where sustaining the technical demands of a sutured liquid-suspension approach consistently is more difficult.

In practice, location, bone status, scaffold safety, and surgeon experience are weighed simultaneously in the consultation — no single variable overrides the others on its own.

Recovery, Rehabilitation and What to Expect

Recovery from either procedure is measured in months, not weeks. Both ACI and MACI require protected weight-bearing in the early post-operative period, followed by a structured physiotherapy programme that progressively reloads the joint as the graft matures. Precise milestones vary by defect size, location, and individual biology — surgeons typically set personalised timelines rather than applying a uniform protocol.

Return-to-sport timing is one of the least well-mapped areas in the evidence base. A 2025 systematic review of 85 studies found that only 15.3% reported return-to-sport as a postoperative percentage, making cross-study benchmarking unreliable. Patients planning around an athletic return are better served by seeking an individualised estimate from their treating team than by relying on published averages.

Sex-based differences in return-to-sport rates are documented. A 2025 meta-analysis of 22 studies (1,468 athletes) found males returning to sport at approximately 75% versus 56% for females after knee cartilage procedures including ACI and MACI. The evidence does not currently explain this disparity. It warrants discussion in pre-operative counselling — but should not be used to set lower expectations before surgery has taken place.

Post-operative imaging may not reflect how the knee actually feels. As the long-term imaging data covered earlier in this article show, MRI appearance and symptom experience can diverge over time after MACI. A scan that looks less than ideal does not necessarily signal repair failure, and patients are better served by tracking functional progress than by interpreting imaging findings in isolation.

Across longer follow-up, the published data support both procedures as durable joint-preservation strategies. In one cohort of 168 patients followed for 10 to 17 years, patient-reported outcomes remained significantly improved from baseline, with conversion to total knee arthroplasty in only 7.4% of cases. What the evidence does not yet settle — and what the choice between ACI and MACI ultimately turns on — is how cell density, scaffold design, and surgeon experience combine for a specific patient. That judgement requires a clinical assessment.

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2. [2] Collagen-membrane-covered matrix-associated chondrocyte implantation improves safety over periosteum-covered MACI for large knee cartilage defects. (2026). https://doi.org/10.1002/ksa.70352 https://doi.org/10.1002/ksa.70352
3. [3] Minimum 10-Year Outcomes of Matrix-Induced Autologous Chondrocyte Implantation in the Knee. (2024). https://doi.org/10.1177/03635465231205309 https://doi.org/10.1177/03635465231205309
4. [4] Comparison of Clinical Outcomes and Return to Sport Rates Between OCA and ACI/MACI for Articular Cartilage Lesions in the Patellofemoral Joint. (2024). https://doi.org/10.1177/2325967124s00050 https://doi.org/10.1177/2325967124s00050
5. [5] Patellar Dysplasia and Cell-Based Cartilage Repair Outcomes. (2024). https://doi.org/10.1177/2325967124s00463 https://doi.org/10.1177/2325967124s00463