MACI always means two operations

Yes — MACI always requires two separate operations. There is no single-stage MACI pathway, and no version of the procedure that compresses biopsy and implantation into one anaesthetic.

The sequence is fixed by biology. At Stage 1, a surgeon takes a small arthroscopic biopsy of healthy cartilage — typically from a low-load area of the knee. That sample is sent to a specialist laboratory, where the chondrocytes are expanded over four to eight weeks until sufficient cell numbers are available. Only then can Stage 2 take place: the cultured cells, now seeded onto a porcine Type I/III collagen membrane, are implanted into the prepared defect and secured with fibrin sealant.

A common misconception is that MACI represents a simpler alternative to first-generation ACI. In one narrow sense it is — the collagen membrane is technically easier to handle at Stage 2 than injecting loose cells under a periosteal flap. But the staging burden is identical. Both procedures require two anaesthetics, two recovery and rehabilitation blocks, and an intervening wait during which the patient lives with their untreated defect. The label 'simpler' applies to surgical technique at the second operation, not to the number of operations.

What happens between the two operations

Stage 1 takes around 30 minutes. Under arthroscopic guidance, a surgeon harvests a small sample of healthy cartilage — typically 200–300 mg — from a low-load region of the knee, commonly the intercondylar notch or the medial femoral condyle margin. The sample is then sent under controlled conditions to a specialist laboratory.

The laboratory phase spans four to eight weeks. Technicians isolate the chondrocytes and culture them until cell numbers are sufficient to populate the collagen scaffold used at Stage 2. During this time the patient's primary cartilage defect remains untreated — a fact that carries genuine clinical weight.

That interval is not risk-free. A 2023 retrospective review found that longer delays between biopsy and implantation may allow chondral lesions to expand in both number and size, with measurable downstream effects on KOOS subscores and return-to-sport outcomes. Scheduling backlogs and patient hesitation can both extend the gap, and the evidence suggests that allowing it to drift is not clinically neutral.

Stage 2 is the larger operation. The defect is debrided and sized, the cell-seeded membrane trimmed to fit, and the graft secured with fibrin sealant — either arthroscopically or through a small open incision depending on defect location. Recovery from Stage 2 is substantially longer than from the biopsy.

Two separate rehabilitation courses follow the two procedures. Consensus guidance from a US Delphi panel of orthopaedic surgeons places full weight-bearing at seven to nine weeks for tibiofemoral cases, with 90-degree range of motion targeted by week four. Return to impact sport typically takes twelve months or more from Stage 2 as the graft matures — meaning the full timeline from biopsy to returning to play commonly runs fourteen to eighteen months.

Most Stage 1 patients never reach Stage 2

Roughly two-thirds to three-quarters of patients who undergo a MACI biopsy never reach Stage 2. That figure is not a failure of planning — it is one of the more practically significant findings in the cartilage repair literature.

In a multisurgeon series of 46 patients who had an arthroscopic biopsy for focal knee chondral defects, only 12 (26.1%) went on to Stage 2 implantation. The arthroscopic treatments performed at the biopsy sitting — debridement, chondroplasty, loose body removal — appeared sufficient to resolve symptoms in the majority. A 2024 cohort study (n=71) found a somewhat higher but still minority rate: 35.2% required implantation. Across both series, the consistent message is that Stage 1 alone is a meaningful therapeutic event, not simply a cell collection.

Who is most likely to need Stage 2? The 2024 data identified two independent predictors. Larger defect size carried an odds ratio of 1.43 per cm² (p=0.031), and being aged 26 or over at the time of biopsy carried an odds ratio of 3.55 (p=0.042). Defect size also separated the groups in absolute terms: patients who went on to implantation had lesions averaging 5.2 cm², compared with 3.3 cm² in those who did not progress.

For patients with smaller defects or younger age at presentation, Stage 1 is not a staging exercise that has been abandoned — it may simply be the appropriate endpoint. This gap between planned MACI and executed MACI is, however, one clinical reason why single-stage alternatives attract growing interest: avoiding a second procedure that evidence suggests most Stage 1 patients will not need.

How well does MACI actually work over time?

Mid-term, the evidence for MACI is genuinely strong. The SUMMIT randomised controlled trial — the most cited benchmark in the field — showed that MACI produced superior KOOS pain and function scores compared with microfracture for focal defects of 3 cm² or greater, a finding that held at both two and five years. Across published series, 75–93% of patients report meaningful improvement in pain and function at this timeframe. Mechanistically, the advantage over marrow-stimulation techniques relates to tissue quality: MACI targets hyaline-like cartilage repair, which is structurally closer to native articular cartilage and more durable under load than the fibrocartilage generated by microfracture.

The longer-term picture is more sobering, and the data deserve to be stated plainly. A prospective New Zealand series followed 15 patients to ten years and found MRI graft fill falling from 90% at two years to 49% at ten years — meaning that, on average, roughly half of the repaired area remained covered a decade after implantation. Every patient in the cohort showed bone oedema and bone cyst formation by ten years. Biopsy samples told a similar story: 73% showed fibrocartilage rather than hyaline tissue, the opposite of what MACI is designed to produce. Outcome scores appeared to plateau at around five years and then decline.

That fibrocartilage finding at biopsy does not mean the procedure had failed symptomatically for those patients — many continued to report functional benefit. What it does temper is the claim that MACI delivers durable biological restoration of native hyaline cartilage in the long run. The honest read is that MACI offers well-evidenced mid-term relief for appropriate defect sizes, with a more uncertain trajectory beyond five years.

Single-stage alternatives and the trade-off with evidence

The appeal of avoiding a second operation is obvious, and several techniques make it possible — though the case for them rests on different foundations.

AMIC (autologous matrix-induced chondrogenesis) adds a collagen scaffold over standard microfracture in a single sitting, offering a biological step up without requiring a return to theatre. It is a practical bridge for defects that have outgrown simple marrow stimulation but where full cell-based repair may be disproportionate. OATS (osteochondral autograft transfer) transplants a plug of bone and cartilage from a low-load donor area in the same operation; it suits smaller focal defects of roughly 1–2 cm², or up to around 4 cm² in a mosaic configuration, though donor-site morbidity warrants frank discussion before proceeding.

Minced cartilage techniques — including AutoCart and AMC — have attracted striking research momentum. Half of all publications in this area appeared between 2021 and 2024, and a 2026 bibliometric analysis found that 78.6% of studies published in 2025 involved autologous minced cartilage. The clinical rationale is straightforward: harvest and implantation happen in one sitting, the biology is autologous, and the technique is relatively accessible. The tissue produced tends towards fibrocartilage — the same limitation s4 noted for microfracture — and what the long-term data will eventually show is still accumulating. Popularity is not the same as validated durability.

STACi, offered at the London Cartilage Clinic as a UK-exclusive procedure, takes a structurally different approach: it retains MACI's cell-based biology but consolidates harvest and implantation into a single anaesthetic, using a 3D scaffold rather than a flat collagen membrane. Taylor and Lee's 2019 paper described this class as 'next-generation ACI'. Published evidence for STACi specifically remains limited by its novelty, and longer follow-up will be needed before it can sit alongside decade-level data.

The honest framing is not that single-stage approaches are inferior, but that evidence maturity differs substantially. MACI has systematic review data extending to 10–17 years; its single-stage competitors are, for the most part, still building theirs.

Which approach suits which patient — and next steps

Defect size and age are the two most reliable guides here. For focal lesions of 3 cm² or greater — where the SUMMIT trial data apply most directly — MACI remains the best-evidenced option when the patient can manage both stages and the subchondral bone plate is intact. For smaller lesions, or where a second anaesthetic represents a genuine practical barrier, alternatives are worth exploring: OATS for contained defects around 1–2 cm², AMIC as a single-stage scaffold step-up from plain microfracture, or — for patients earlier in the care pathway — a ChondroFiller injection, an ultrasound-guided outpatient treatment that places an injectable collagen scaffold without any theatre episode.

Age adds a specific variable. The 2024 cohort data identified being 26 or older as an independent predictor of requiring second-stage implantation (OR 3.55) — early discussion of staging burden is therefore particularly relevant for younger patients who may carry a repaired joint across several decades.

MACI is NICE-approved for NHS use but largely unavailable privately. Patients seeking a private cell-based route without a two-stage commitment may consider STACi, available at the London Cartilage Clinic at an all-inclusive price of £28,000.

The decision should be driven by defect biology, not by staging preference alone. Professor Paul Y. F. Lee sees patients for that full-range assessment at the London Cartilage Clinic — appointments can be arranged at londoncartilage.com — and whatever the eventual pathway, the right starting question is not how many operations but what the joint actually requires.

1. [1] Rates and predictors of reimplantation of MACI following first stage cartilage harvest: A cohort study (2024). (2024). https://doi.org/10.1016/j.knee.2024.04.006 https://doi.org/10.1016/j.knee.2024.04.006
2. [2] Consensus on Rehabilitation Guidelines among US Orthopedic Surgeons following MACI (2020). (2020). https://doi.org/10.1177/1947603520968876 https://doi.org/10.1177/1947603520968876
3. [3] Autologous Minced Cartilage on the Rise: A Bibliometric Mapping (2026). (2026). https://doi.org/10.14744/start.2026.11290 https://doi.org/10.14744/start.2026.11290
4. [4] Effect of Time Delay From Biopsy to Second-Stage Implantation on Outcomes following ACI/MACI (2023). (2023). https://doi.org/10.1177/2325967123S00264 https://doi.org/10.1177/2325967123S00264
5. [5] Costal Chondrocyte–Derived Pellet-Type ACI vs Microfracture: 5-Year Follow-up RCT (2024). (2024). https://doi.org/10.1177/03635465231222797 https://doi.org/10.1177/03635465231222797
6. [6] A Prospective Outcome, MRI and Biopsy Study of MACI Cartilage Transplantation (2017). (2017). https://doi.org/10.1177/2325967117S00186 https://doi.org/10.1177/2325967117S00186