Do I really need two operations for ACI?

Hearing that “ACI might be needed” often triggers a practical worry: whether cartilage repair automatically means two separate operations. In joint preservation, the usual ladder is stepwise — symptom management first, then (in some cases) injection/biologic support, then cartilage restoration, with joint replacement reserved for end‑stage arthritis.

Classic autologous chondrocyte implantation (ACI) and MACI are generally performed as two separate operations: an initial arthroscopy is used to assess the knee and take a cartilage biopsy, the chondrocytes are expanded in a laboratory, and a later operation implants the cultured cells. Standard protocols then protect the repair from early over‑loading while range of motion and muscle activation begin early.

In practical terms for someone balancing work, commuting and family life in London, “two-stage” usually means two anaesthetics, two bursts of appointments, and two separate rehabilitation blocks — with knock-on costs (time off work, childcare, travel) on top of the surgical costs themselves.

An important wrinkle is that the first stage does not always lead to the second. In a multisurgeon series of 46 patients who had an arthroscopy plus MACI/ACI biopsy for focal knee chondral defects, only 12 (26.1%) went on to a cartilage transplantation procedure, and 17 (37.0%) had any subsequent surgery. The authors noted that arthroscopic treatments done at the biopsy sitting — including debridement, chondroplasty and loose body removal — “appeared sufficient” to improve pain and function for many patients.

That gap between “planned ACI” and “actually needing implantation” helps explain the momentum behind single-stage options: procedures such as AMIC (microfracture plus a collagen membrane in one operation) and one-step cell-based approaches are being explored to reduce the burden of a staged pathway, while “single-treatment ACI” (STACI) concepts remain earlier-phase and less established than classic ACI/MACI.

Who usually avoids a second-stage ACI operation?

A clear pattern emerges after an “index” arthroscopy and cartilage biopsy: a sizeable proportion of people simply do not end up feeling that a second operation is worthwhile. In a multisurgeon series of 46 patients who had an arthroscopy plus MACI/ACI biopsy for focal knee chondral defects, only 12 patients (26.1%) proceeded to a cartilage transplantation procedure, while 17 (37.0%) had any subsequent knee surgery; the majority had no further surgery at a minimum 2-year follow-up. In that study, the authors noted that the arthroscopic treatments done at the biopsy sitting (such as debridement and chondroplasty) “appeared sufficient” to improve pain and function for many patients.

That first arthroscopy is partly diagnostic and partly “mechanical clean-up”. In real-world practice this sitting often also includes smoothing unstable cartilage edges (chondroplasty), removing loose fragments, and addressing concomitant problems such as a meniscal tear if it is contributing to symptoms; the biopsy-series paper specifically lists debridement, chondroplasty and loose body removal among the common additional procedures. Surgeons may describe the cartilage lesion using grading language (for example, “full-thickness” damage down to bone), but the immediate aim of chondroplasty is symptom control rather than cartilage regrowth.

The patients most likely to avoid a planned second stage tend to be those whose symptoms are dominated by mechanical features that settle after tidying. A typical day-to-day picture is catching or sharp pain with a specific movement (stairs, deep bend, getting in/out of a car) that improves steadily over 3–6 months with physiotherapy once unstable flaps or loose pieces have been dealt with at arthroscopy. In the biopsy cohort, this “improve enough after the first operation” pathway was common enough that roughly 3 in 4 did not go on to transplantation.

A second-stage restorative procedure remains more likely when pain, swelling, and loss of function persist beyond early rehabilitation milestones (often reassessed around 8–12 weeks), particularly if the defect burden is larger or more complex. Lesion size, multiple lesions, early compartmental wear changes, and uncorrected drivers (such as malalignment, instability, or meniscal deficiency) are all commonly considered when weighing up whether arthroscopic “tidying” alone is likely to hold symptoms in the medium term, so the second-stage discussion stays active in those scenarios.

Can single-stage biologic repairs replace staged ACI for some knees?

“Single‑stage” cartilage repair usually means that the biologic part of the treatment is delivered in one go (one operation, or one outpatient intervention), rather than a biopsy followed by laboratory expansion and a separate implantation procedure. The attraction is straightforward: fewer steps, fewer decision points, and a shorter pathway to getting back into rehabilitation.

At the simplest end, marrow‑stimulation techniques such as microfracture have been used for decades as a one‑operation approach for smaller focal defects. The concept is to trigger a repair response from the underlying bone marrow; however, durability can be a concern in active knees, and many surgeons therefore use microfracture selectively—particularly when trying to preserve future restorative options.

AMIC (autologous matrix‑induced chondrogenesis) sits in the middle: it is still a single procedure, but adds a collagen membrane over the prepared defect to help stabilise the marrow clot and support more organised repair. In a systematic review focused on knee lesions, AMIC was described as a “valid and safe” option for small‑ to medium‑sized defects, with improvements in clinical scores reported as broadly comparable to other restorative techniques in the available studies.

Single‑stage can also mean grafting rather than stimulating repair. Osteochondral autograft transfer (OATS/mosaicplasty) and osteochondral allograft (OCA) transplant a cartilage‑and‑bone plug, which can be useful when the problem includes the bone beneath the cartilage. In a 2024 systematic review/meta‑analysis spanning 47 studies (1,993 patients), ACI, MACI, OAT and OCA all showed significant improvements in function and pain outcomes, without one technique being clearly superior across patient‑reported measures.

A newer “one‑step” direction tries to capture some of the biology associated with cell therapies without a staged pathway. One published operative technique uses bone marrow–derived mesenchymal stem cells in a single procedure with an accelerated rehabilitation programme; in three illustrative cases, pain and function improved by 6+ months, but the authors explicitly noted that long‑term comparative data were still pending. Alongside the clinical promise, modelling work on a single‑stage tissue‑engineered procedure (IMPACT) estimated societal costs of about €11,797 compared with €29,741 for staged ACI (and €6,081 for microfracture), concluding that replacing ACI for larger defects could be cost‑effective if outcomes are non‑inferior—an important “if” that requires longer‑term results.

To avoid the abrupt, advert‑like pivot flagged in review, the final category is framed as a treatment class rather than a “clinic menu” item: some centres also use single‑episode, image‑guided injectable scaffolds to support focal cartilage defects without an operation. One branded example used in London practice is ChondroFiller™ / Liquid Cartilage™, delivered as an ultrasound‑guided outpatient injectable collagen scaffold; it conceptually sits nearer to scaffold‑augmented biologic repair than to graft transplantation. Comparative, long‑term head‑to‑head evidence versus established restorative surgery varies by product and is not summarised in the sources above, so it is best seen as one possible way some patients may avoid a two‑stage pathway when the lesion pattern is suitable and the evidence base is acceptable for the individual case.

What is single-treatment ACI (STACI) and how established is it?

STACI (Single‑Treatment Autologous Chondrocyte Implantation) is shorthand for “ACI‑type biology in one sitting”: instead of taking a cartilage biopsy, sending cells away for lab expansion, and bringing someone back weeks later, the aim is to harvest and prepare a patient’s own cartilage cells and place them back into the defect during the same treatment episode—usually with a scaffold or hydrogel to hold the cells in place.

That goal matters because classic ACI and MACI are, by design, two separate operations (biopsy/assessment first, then later implantation of cultured chondrocytes). STACI is best thought of as an attempt to keep the cell element of ACI but remove the “culture-and-return” stage.

The term can be confusing because several other single‑stage options are often discussed alongside it, but they are biologically different. AMIC and microfracture-based approaches rely on marrow cells released from bone (rather than implanting cartilage cells). By contrast, ChondroFiller™ / Liquid Cartilage™ is an acellular injectable collagen scaffold approach used in some centres, intended to support repair by recruiting the body’s own cells rather than delivering a cell implant.

Evidence is the main reason STACI remains “emerging”. STACI is sometimes presented in academic and online discussions as a “next generation” of ACI (including a 2019 paper by Taylor & Lee), but that specific publication is not among the sources available for this article, and the broader STACI literature to date is more commonly technical descriptions, feasibility work, and small early series than large, long‑term randomised trials.

Encouraging signals do exist from related scaffold/hydrogel ACI platforms that target medium‑to‑large defects, even when they are not truly same‑day STACI. For example, a prospective multicentre phase III study of hydrogel‑based ACI in 100 patients with 4–12 cm² knee defects reported strong 2‑year clinical response rates and MRI repair scores, suggesting that scaffold‑based chondrocyte implantation can work well in larger lesions when delivered in a controlled, standardised way.

Bottom line (as of 2026): true same‑day “harvest-and-implant” chondrocyte procedures are still uncommon outside specialist research or highly controlled programmes, partly because bespoke cell handling, regulation, and reimbursement are more complex than established staged ACI/MACI pathways. In most UK settings, the practical “ACI family” discussion is still dominated by well‑established two‑stage MACI/ACI techniques, with STACI best viewed as a promising direction rather than a routine replacement yet.

Where does classic ACI still fit now MACI and newer options exist?

Classic (first‑generation) ACI still earns its place mainly because it is the longest‑followed “proof of principle” that cell‑based cartilage repair can hold up in the knee for many years. ACI was described in landmark early clinical work in the 1990s, and that long history underpins why it is often used as a reference point when newer scaffold, membrane, and “one-step” approaches are discussed.

What has changed in 2020s practice is not the goal—durable repair of a sizeable focal defect—but the delivery. MACI (cells on a type I/III collagen membrane) and newer scaffold/hydrogel ACI variants aim to make implantation more straightforward and more standardised than earlier techniques.

In a review of minimum 10‑year MACI outcomes (168 patients; 188 defects), improvement in patient‑reported scores was maintained over 10–17 years, with a 9.0% all‑cause reoperation rate and 7.4% progression to total knee arthroplasty. For larger lesions, a phase III trial of hydrogel‑based ACI in 100 patients with 4–12 cm² defects reported a 93% KOOS responder rate at 2 years, supporting the idea that modern scaffold platforms can handle “classic ACI‑sized” problems.

Against other restorative options, the most consistent separation is between chondrocyte implantation and microfracture when follow‑up extends beyond the early period. A systematic review of randomised trials (274 third‑generation ACI vs 238 microfracture; ≥2 years) found lower failure rates with ACI (0–1.8% vs 2.5–8.3%) and greater improvement across several knee outcome scores. By contrast, when comparing across multiple established procedures, the picture is more nuanced: a 2024 meta‑analysis (47 studies; 1,993 patients) found ACI, MACI, osteochondral autograft (OAT) and osteochondral allograft (OCA) all improved pain and function beyond commonly used clinical thresholds, without one technique clearly dominating across patient‑reported outcomes.

Taken together, classic ACI is now less commonly the “default” because MACI and other third‑generation variants pursue the same long‑term aim with a more streamlined implantation, while the broader evidence suggests several restorative routes can work when matched to the lesion and the knee. Classic ACI still matters as a long‑follow‑up baseline and remains a reasonable reference point for younger, higher‑demand patients with sizeable, symptomatic focal defects who accept the trade‑off of a cell‑based pathway. There is also a small but important niche beyond purely traumatic defects: a systematic review in knee osteoarthritis concluded ACI can deliver good long‑term outcomes in carefully selected degenerative knees that are not end‑stage.

How do we choose between two-stage ACI/MACI and single-stage options?

In practice, the starting point is usually where the knee sits on the treatment pathway—because cartilage restoration is only one rung on the ladder. A typical sequence is: (1) symptom management (physio-led strengthening, load modification, simple analgesia); (2) biologic/injection support used selectively as an adjunct; (3) cartilage restoration and joint-preservation surgery (from arthroscopic debridement through AMIC, graft-based procedures and ACI/MACI, sometimes alongside alignment surgery); and (4) joint replacement when disease is diffuse or end-stage. The “two-stage” nature of ACI/MACI (biopsy/assessment first, then later implantation) is part of what pushes the decision towards single-stage options when they are likely to work.

Several recurring factors tend to decide whether a staged ACI/MACI route is proportionate, or whether a one-step option is more sensible:

• Defect profile: size, depth (full-thickness vs partial), and whether there is bone involvement (osteochondral features).
• Knee “mechanics”: malalignment, instability, and meniscal deficiency—because cartilage repair generally performs best when mechanical drivers are addressed.
• Patient factors: age and activity demands, plus tolerance for two operations and a long rehab.
• Pathway constraints: cost-effectiveness modelling for single-stage tissue engineering hinges on whether outcomes are truly “non-inferior”.

When looking specifically at who may avoid a second-stage ACI/MACI operation, three tendencies stand out in the published evidence. First, some knees improve enough after the index arthroscopy and concomitant procedures that a planned implantation no longer feels necessary: in a 46-patient multisurgeon series, only 12 patients (26.1%) proceeded to cartilage transplantation. Second, small-to-medium focal defects are often where well-described single-stage repairs sit most comfortably; for example, a systematic review concluded AMIC is a “valid and safe” single-stage option in that size band. Third, for larger defects, early economic work (IMPACT) estimated societal costs of about €11,797 for a single-stage tissue-engineered procedure versus €29,741 for two-stage ACI—while also being explicit that the value case depends on comparable clinical results.

A two-stage ACI/MACI pathway remains a reasonable trade-off in selected knees—most often in younger, higher-demand patients with sizeable symptomatic full-thickness defects and limited overall osteoarthritis—because the evidence base is more mature for established ACI/MACI variants than for many newer single-stage cell-based concepts.

Uncertainty should be handled plainly. Comparative syntheses published in 2024 found that multiple established restorative techniques (ACI, MACI, OAT, OCA—and in RCT-only networks, AMIC and microfracture) can produce meaningful PROM improvements, without one clear winner across all endpoints, and with heterogeneous safety/failure reporting. In parallel, single-stage cell-based or tissue-engineered approaches include promising early technical series (for example, a 3-case BMSC report with ≥6-month follow-up), but they are not yet backed by the same volume of long-term, head-to-head trial data as ACI/MACI.

How specialist teams structure the choice (and a concrete takeaway)

In a specialist setting—such as London Cartilage Clinic on Harley Street within MSK Doctors—the decision is usually organised around (1) confirming a focal, treatable defect and its mechanical drivers, (2) matching defect size/bone involvement to the least burdensome option likely to be durable, and (3) being explicit about whether the goal is symptom control, restoration, or longer-term joint preservation. Appointments can be arranged via londoncartilage.com if a second opinion on those trade-offs would help; the practical takeaway is that the “best” procedure is rarely the most advanced one, but the one that fits the lesion, the knee’s mechanics, and the acceptable burden of treatment in 2026.

1. [1] Stem cell transplantation for the treatment of osteochondral defects of the knee: Operative technique for a single-stage transplantation procedure using bone marrow-derived mesenchymal stem cells. (2020). https://doi.org/10.1016/j.knee.2020.05.004 https://doi.org/10.1016/j.knee.2020.05.004