What the 'one-stage vs two-stage' label really means here

If you have been told you need ACI or MACI, it is worth knowing straight away that both procedures involve two separate surgical visits — not one. The label 'one-stage versus two-stage' that often appears in clinical conversations actually covers two distinct debates, and conflating them causes unnecessary confusion.

The first debate is between two-stage cell-based therapies (ACI and MACI) and genuinely single-stage options such as microfracture, AMIC (matrix-augmented microfracture), and OATS (osteochondral autograft transfer). These single-stage procedures are completed in one operation. ACI and MACI are not: both require an initial arthroscopic biopsy to harvest a small cartilage sample, a laboratory phase lasting several weeks during which your chondrocytes are cultured and expanded, and then a second procedure to implant the cells.

The second debate sits within two-stage cell therapy itself. MACI is third-generation ACI — it seeds the expanded chondrocytes onto a collagen I/III scaffold membrane rather than fixing them under a periosteal patch harvested from elsewhere in the knee. This reduces technical complexity and the risk of graft overgrowth, but the two-stage structure remains identical.

An emerging true one-stage ACI concept does exist, in which chondrocytes are enzymatically liberated intraoperatively and reimplanted in the same session, but this approach currently lacks the high-level trial evidence that supports standard MACI.

Choosing between ACI and MACI is therefore a within-class refinement. The more consequential upstream question — whether two-stage cell therapy is the right pathway at all — hinges largely on lesion size.

Defect size: the most important number in your decision

Lesion size, measured in square centimetres on MRI, is the single number that most reliably determines whether the two-stage commitment of ACI or MACI is warranted — or whether a simpler, single-stage alternative will suffice.

For defects roughly below 2–4 cm², procedures such as AMIC (matrix-augmented microfracture) or OATS (osteochondral autograft transfer) remain clinically appropriate. Microfracture — historically the most common first-line option for small defects — is now used more cautiously: it produces fibrocartilage rather than hyaline-like tissue, and that repair material tends to break down over two to three years, which can compromise the subchondral bone and make future repair more difficult.

Once a defect reaches approximately 3 cm² or larger, the evidence tips decisively in favour of cell-based therapy. The SUMMIT phase 3 RCT (144 patients; mean lesion size 4.8 cm²) found MACI statistically and clinically superior to microfracture on KOOS pain (P=0.001), function, activities of daily living (P<0.001), quality of life (P=0.029), and symptoms (P<0.001) at two years — and that advantage held at five-year follow-up. ACI and MACI are generally used across the 2–10 cm² range; for very large or post-traumatic lesions, osteochondral allograft (OCA) is typically the next consideration.

Size alone does not make the decision. A specialist will weigh lesion location, depth, the state of the surrounding cartilage, and your activity demands alongside the measurement. But if your MRI shows a defect of 3 cm² or more, that number is usually what moves the conversation from single-stage options towards two-stage cell therapy.

ACI or MACI: what changes between the two generations

Once cell therapy has been identified as the right pathway, the choice narrows to which generation of the technique is most appropriate.

First-generation ACI covers the implanted chondrocytes with a periosteal patch — a thin sliver of tissue taken from the tibia during the same operation. That patch acts as a biological lid to keep the cells in place, but harvesting it requires an open arthrotomy (a larger incision than keyhole surgery), and the tissue can thicken over time, causing graft hypertrophy that occasionally needs further treatment.

MACI (third-generation ACI) replaces the periosteal patch with a collagen I/III scaffold membrane onto which the expanded chondrocytes are seeded before implantation. The membrane distributes cells more uniformly across the defect, eliminates the need for a separate patch harvest, substantially reduces the risk of graft overgrowth, and makes arthroscopically-assisted delivery feasible in many cases.

One trade-off exists: standard MACI seeds cells at a lower density than first-generation ACI, which can result in softer repair tissue. High-density ACI (HD-ACI), developed to address this by implanting 5 million chondrocytes per cm², produces hyaline-like rather than fibrocartilage in animal models — a promising refinement, though it has not yet demonstrated superiority over standard MACI in randomised controlled trials.

Direct head-to-head RCTs comparing first-generation ACI against MACI remain sparse; most of the available comparisons come from registries and observational studies. Both approaches carry substantial long-term track records, so this evidence gap should be seen as a reflection of how clinical practice evolved rather than as uncertainty about whether cell therapy works.

How MACI compares with single-stage alternatives

The two-stage commitment of MACI raises a practical question: does the extra investment — a second operation, weeks of waiting, and greater logistical demand — actually produce better results than a simpler single-stage approach?

At two years, the evidence suggests not necessarily. A matched-pair study comparing MACI, AMIC, and arthroscopic minced cartilage (16 patients per group, 48 total) found no statistically significant differences in VAS pain, KOOS-Pain, or KOOS-Symptoms across the three techniques. All three produced meaningful clinical improvement, and all three were considered equally recommendable at this time point.

The picture shifts over a longer horizon and with larger lesions. MACI's advantage over single-stage alternatives is most clearly established for defects of 3 cm² or greater; in smaller defects, AMIC or OATS may deliver comparable near-term outcomes without requiring a second procedure. For patients for whom a return to theatre is a significant practical or personal barrier, a single-stage option is a reasonable conversation to have with a specialist — particularly where the lesion falls below the size threshold where two-stage cell therapy has demonstrated clear superiority.

One further point is worth noting for active or working patients: gait analysis following successful MACI shows residual biomechanical differences compared with healthy controls — including reduced walking speed, a smaller peak knee flexion angle, and a reduced knee extension moment — even where clinical satisfaction is high. Patient-reported outcomes are a meaningful measure of recovery, but they do not always capture the full picture of how the knee functions under load.

Long-term durability: what 10-year data shows

Ten-year follow-up data from two independent studies provides reassuring evidence of sustained benefit after MACI. A systematic review of 168 patients (mean age 37 years) and a separate prospective cohort of 204 patients both confirm durable patient-reported outcome improvements at the decade mark, with 92% of patients satisfied with their pain relief.

The headline figures also contextualise what the long-term minority pathway looks like: an all-cause reoperation rate of 9% and a 7.4% rate of progression to total knee arthroplasty at 10–17 years. MRI-confirmed graft delamination — structural failure of the implanted tissue — occurred in approximately 9.3% of grafts at ten years. These figures are worth raising openly at the counselling stage; they represent a meaningful minority rather than rare events, and patients should understand that graft failure on imaging does not always produce immediate clinical symptoms, making follow-up imaging an important part of the long-term plan.

One particularly practical counselling point concerns the timing of recovery. In the published cohorts, outcomes appear to plateau at around two years and remain stable thereafter. For patients who are doing well at that mark, the evidence suggests that trajectory is likely to be sustained — a useful anchor for setting realistic expectations over the years ahead.

Prior marrow-stimulation procedures are a further relevant history point. Clinical evidence indicates that ACI and MACI failure rates are higher in patients who have previously undergone microfracture. Anyone with that history should make a point of disclosing it during specialist assessment, as it may influence both the technique recommended and the expected outcome.

Making the decision: questions to raise with a knee specialist

Before any consultation about ACI or MACI, it helps to arrive with a short list of the variables that will drive the discussion. Four questions cover the essentials.

How large is my defect on MRI, in square centimetres? This is the single number that most reliably anchors the pathway choice. Below roughly 2–3 cm², it is worth asking about single-stage alternatives — AMIC or OATS — before committing to two procedures. At or above 3 cm², MACI has the strongest published evidence base.

Have I previously had microfracture or any marrow-stimulation procedure? Published evidence indicates that failure rates after ACI and MACI are higher when prior marrow stimulation has been performed. This history should be disclosed from the outset, as it influences both the technique recommended and expected outcome.

Am I a candidate for a single-stage alternative? At two years, MACI, AMIC, and minced cartilage produce comparable results; the two-stage investment appears to differentiate itself over a longer horizon and in larger lesions.

Is next-generation single-stage ACI (sometimes called STACI) available? This emerging approach eliminates the laboratory waiting period by processing chondrocytes intraoperatively — but it currently lacks high-level trial evidence and should be raised as a future possibility rather than expected as a standard choice.

The practical synthesis the article has built points to two variables that most reliably guide this decision: defect size on MRI and microfracture history. Those two facts, brought clearly to a first consultation with a cartilage specialist, will move the conversation quickly to the options with the most relevant evidence for your situation.