Working out whether ACI, MACI or a single‑stage option fits you

A quick way to judge “fit” is to start with the shape of the problem and the shape of the solution: these options are mainly discussed when knee pain and swelling come from a localised “pothole” of missing cartilage (a focal, full‑thickness defect), rather than widespread wear‑and‑tear arthritis across the whole joint. The big practical divider is whether repair is done in two operations (ACI/MACI) or in one operation (single‑stage/“one‑step” techniques).

In the overall knee pathway, these procedures usually sit after symptom-management steps and, in certain cases, after simpler surgical options have been tried or judged unlikely to work well, but before joint replacement is considered. Selection also tends to assume the knee mechanics are addressable (for example, reasonably stable and aligned) and that there is not advanced osteoarthritis.

• ACI (autologous chondrocyte implantation): a two‑stage approach—cartilage cells are taken in a small biopsy, expanded in a lab, then implanted at a second operation under a cover.
• MACI (matrix‑induced ACI): also two‑stage, but the cultured cells are seeded onto a collagen membrane before implantation.
• Single‑stage cell-based options (often grouped as STACI/one‑step): cells or progenitor-rich material are harvested and placed in the defect in the same operation, typically using a scaffold/biologic carrier.

Size and pattern still matter, but exact thresholds vary between techniques and centres.

Evidence maturity differs as well: MACI has published follow‑up out to 10–17 years in systematic review data (2024), whereas many single‑stage approaches are still supported mainly by smaller, more variable clinical series—so comparisons can be uncertain.

Who ACI and MACI are usually offered to

Beyond the basic “focal full‑thickness defect” picture described earlier, ACI and MACI selection in practice tends to come down to three practical questions: is the problem local enough to repair, is the knee mechanically ready for a repair, and is the size in the range where cell‑based repair is usually considered. Reviews describe these procedures being used for symptomatic, localised defects—particularly in younger, active adults without advanced osteoarthritis and with a stable, reasonably well‑aligned knee.

How big is the defect (as a rule of thumb)?

Size is one of the main “sorting” factors, but it is not a hard cut‑off. Broadly, clinicians tend to separate smaller focal defects from larger focal defects when discussing whether an osteochondral grafting option or a cell‑based repair is more appropriate, alongside patient factors such as age, activity, and the exact defect characteristics.

Has anything been tried already?

ACI/MACI are frequently considered after a period of non‑operative care, or after a simpler surgical option has failed or looks unlikely to hold up for the defect size.

What makes surgeons cautious?

Common “pause points” include diffuse/advanced osteoarthritis, significant malalignment or instability that is not corrected, and practical factors such as smoking, high BMI, or inability to commit to rehabilitation.

Where does MACI’s label fit?

In the published surgical literature, MACI is described as a membrane‑based, two‑stage autologous chondrocyte technique, and it has also been reported as a way to treat multiple chondral lesions in selected knees—while surgeons still individualise decisions case by case.

A simple contrast: a 30‑year‑old footballer with a symptomatic focal condylar defect and an otherwise stable knee is closer to the “typical” ACI/MACI discussion than someone with long‑standing swelling, stiffness and multi‑area cartilage loss consistent with established osteoarthritis.

How ACI and MACI actually differ day to day

In practice, the choice between first‑generation ACI and MACI is often less about “which works” and more about what the two-stage journey feels like and how predictable the implantation step is for the surgeon.

With first‑generation ACI, the second operation is often performed via open arthrotomy, where a chondrocyte suspension is placed into the defect and sealed under a sutured periosteal or collagen flap. That “watertight patch” step is one reason classic ACI is described as technically demanding. MACI uses the same biopsy-and-culture concept, but the lab grows cells on a collagen membrane; at implantation, the surgeon trims a pre-seeded patch to shape and fixes it in place (often with fibrin glue, sometimes sutures), which can make handling simpler and may allow smaller incisions or arthroscopic techniques in some cases.

“Two-stage” means: an initial arthroscopy to confirm the defect and take a biopsy, then a waiting period for cell expansion, then a second operation for implantation—so there are typically two anaesthetics and two separate recovery periods to plan around.

ACI vs MACI at a glance

• Stage‑2 surgery: ACI injects/places cells under a sutured cover; MACI implants a pre-seeded collagen membrane cut to fit the defect.
• Practical knock-on: MACI is generally easier to reproduce in theatre because it avoids creating and sewing down a watertight flap.
• Complication profile: avoiding a periosteal flap is one reason MACI was developed, with the aim of reducing flap-related issues reported with earlier ACI techniques.

On outcomes, a 2024 systematic review/meta-analysis (47 studies, 1993 patients) reported that ACI and MACI (as well as osteochondral autograft and allograft procedures) were associated with substantial improvements in pain and function scores for tibiofemoral defects; overall, improvements were broadly similar in size between ACI and MACI rather than clearly favouring one. Separate 2024 long-term review data (mean age 37, 168 patients) found MACI benefits could persist out to 10–17 years in most series.

What single‑stage cell procedures (STACI) actually involve

Single‑stage cell procedures (often grouped as “STACI” or “one‑step” repairs) aim to do in one operation what ACI/MACI spreads across two: collect cells or cell‑rich material during the procedure and place it into the cartilage defect with a scaffold or biological “carrier”, rather than sending cells away for lab expansion and returning later for implantation. Conceptually, these are framed as a “next generation” of cell‑based cartilage repair intended to reduce the delay, logistics and cost linked to in‑vitro culture.

Rather than one standard operation, STACI is an umbrella for several technique families:

• HA‑based scaffold + BMAC (HA‑BMAC): bone marrow is aspirated and concentrated, then combined with a hyaluronic‑acid scaffold to fill the defect in a single sitting. In published long‑term series, this has been used in middle‑aged cohorts (mean ~48 years) with large lesions (median ~6.5–6.6 cm², reported up to 27 cm²) and sometimes more than one defect, with improvements in pain and function still present at around 8 years and 14 years. The main uncertainty is how well these results translate outside specialist centres and whether outcomes match ACI/MACI when patients and defects are closely comparable.
• Autologous minced cartilage + fibrin glue: small cartilage fragments (taken from the knee at the time of surgery) are minced and fixed into the defect with fibrin glue. Early clinical studies (for example, an 18‑patient prospective series with 12‑month MRI follow‑up, and a meta‑analysis totalling 52 patients/63 lesions at 12–24 months) report symptom and function improvements alongside generally good defect fill on imaging. The key uncertainty here is durability: most data are short‑term, and protocols vary between centres.
• MSC‑augmented scaffold approaches (for example LIPO‑AMIC): these combine marrow‑stimulation/matrix methods with an added cell source such as adipose‑derived MSCs. In one 5‑year series, IKDC and Lysholm scores improved substantially (IKDC roughly 36 to 87, Lysholm 44 to 94) with MRI showing complete defect fill in about 78%. The main uncertainty is again comparative evidence—there is limited head‑to‑head data against established cell‑based techniques.

Across STACI methods, the potential appeal is practical (a single hospital episode rather than two operations) and, in some reports, a wider “attempted” patient range (including larger or multiple lesions). However, most evidence still comes from small cohorts and heterogeneous methods, with long‑term follow‑up beyond 10 years currently concentrated in selected HA‑BMAC series rather than across all one‑step approaches.

How ACI, MACI and single‑stage options compare in practice

Three decision pivots usually separate these options in clinic: defect size, how “two‑stage” fits life plans, and how much long‑term evidence matters for that knee (for example, a 37‑year‑old with a single focal lesion vs a 48‑year‑old with multiple lesions). This section deliberately ends on those trade‑offs rather than a directory pointer.

Which option fits my age, defect size and activity goals?

In published literature, selection across cartilage restoration options (including ACI, MACI, osteochondral autograft transplantation (OAT) and osteochondral allograft (OCA)) is commonly discussed in relation to patient and defect characteristics such as lesion size, location, and activity demands. In long‑term single‑stage series using HA‑BMAC, lesions have been reported as relatively large (median ~6.5–6.6 cm², up to 27 cm²) and cohorts have been middle‑aged (mean ~48 years), suggesting there can be overlap with—sometimes an extension beyond—traditional cell‑based indications.

How strong is the evidence on outcomes and durability?

For MACI, a 2024 review (168 patients) reported durable improvements with follow-up out to 10–17 years, alongside about 9% reoperation and 7.4% conversion to total knee arthroplasty. For single‑stage approaches, the most mature long‑term follow‑up is in selected HA‑BMAC cohorts at around 8 years and 14 years, but head‑to‑head comparisons with ACI/MACI remain limited.

One operation or two—and what tends to drive that choice?

ACI/MACI usually means two anaesthetics (biopsy then implantation), whereas STACI approaches are designed around a single operation. Earlier periosteal/flap‑style ACI techniques were developed further partly to reduce flap‑handling issues; MACI’s membrane approach aims to simplify implantation.

Risks, rehab, and “the rest of the knee”

Across all techniques, complications such as stiffness, infection, thrombosis and graft failure are recognised risks of knee surgery. Rehabilitation commonly involves protected weight‑bearing for weeks and a return to higher‑impact sport over months, with programme quality often as important as the implant choice. Where the knee is malaligned or unstable, cartilage repair is sometimes combined with procedures that correct mechanics (for example, osteotomy or ligament surgery), while diffuse advanced osteoarthritis generally shifts the discussion towards partial or total knee replacement rather than focal repair.

Questions to ask your surgeon and how to find the right specialist

A productive first appointment often comes down to pinning down the diagnosis (MRI plus exam findings), clarifying the goal (pain control, sport, delaying replacement), and checking whether the knee’s “mechanics” are suitable for a repair (alignment, stability, meniscus). Keeping the discussion anchored to the scan report and a specific defect description (site, size in cm², and ICRS/Outerbridge grade) helps avoid vague “cartilage wear” labels.

Useful questions to take into a consultation include:

• “What is the exact defect location (e.g., femoral condyle/trochlea/patella), size (cm²) and grade?”
• “Based on my MRI, age and activity goals, does ACI, MACI, a single‑stage option, AMIC, OATS/OCA—or non-surgical care—fit best, if any?”
• “What are the realistic trade-offs in my case: durability, number of operations, and the consequences of delaying?”
• “If the knee is malaligned or unstable, would an osteotomy or ligament procedure be needed alongside cartilage repair?”
• “How many of these procedures do you perform in a typical year, and how do you track outcomes and revisions?”
• “What does rehabilitation involve (weight-bearing limits, physio milestones), and what might restrict work or sport?”

Care is often shared: an orthopaedic knee surgeon typically leads on procedure choice, while a physiotherapist guides the month-by-month rehab plan; in some pathways a sports-medicine doctor contributes to diagnosis and non-operative options.

Directory tools such as Search MSK are best treated as an optional step—use them to find UK clinicians by region and by treatments offered (for example ACI/MACI, single‑stage procedures, osteotomy), then compare like-for-like approaches in consultation. Not every hospital offers every technique; being declined for one option can still leave a clear pathway for symptom control, joint preservation, or (where appropriate) knee replacement.