Why the single-stage vs two-stage label matters

Search for 'MACI single-stage' and you will find contradictory answers — some sources describe it as a one-operation solution, others list two surgical visits. The confusion is understandable, but getting it wrong can mean an unexpected second operation and an additional anaesthetic that the patient never anticipated.

The facts are straightforward. Both first-generation ACI and third-generation MACI follow the same two-visit model: an initial arthroscopic biopsy harvests 200–300 mg of healthy cartilage from a non-weight-bearing area of the knee; those cells are then laboratory-expanded over three to six weeks before a second operation implants them into the defect. What MACI changes is the delivery format — chondrocytes are pre-seeded onto a collagen type I/III scaffold (Chondro-Gide) rather than injected beneath a membrane patch — not the number of surgical episodes. Two anaesthetics, two recoveries, one technique; that is true of both procedures.

A genuinely single-stage cell-based pathway does exist. In this approach, cartilage is harvested and enzymatically digested intraoperatively to free the chondrocytes without altering their character, while bone-marrow–derived mesenchymal stem cells are concentrated from the same surgical field and combined with the chondrocyte fraction before being seeded onto a scaffold — all within one theatre visit. This 'next-generation ACI' concept (sometimes labelled STACI) eliminates the culture delay and the second anaesthetic entirely, but it sits on a less mature evidence base than two-stage MACI and is not yet in routine commercial use across most markets.

Establishing this distinction at the outset shapes every subsequent decision: stage count affects patient logistics, cost, and recovery planning just as much as the biological rationale for choosing a cell-based repair in the first place.

ACI vs MACI: what the third generation actually changed

Moving from first-generation ACI to MACI resolved several practical surgical problems without abandoning the biological rationale that makes cell-based repair work.

First-generation ACI

In first-generation ACI, the expanded chondrocytes are introduced as a cell suspension beneath a periosteal patch — harvested from the tibia — or, in the second-generation variant, beneath a collagen membrane cover. Both require the patch to be sutured in place via an open arthrotomy, a technically demanding step. Periosteal hypertrophy (overgrowth of the cover tissue) was a recognised complication that sometimes necessitated a further arthroscopic procedure.

What MACI changed

MACI pre-solves the delivery problem by seeding the expanded chondrocytes directly onto a Type I/III collagen membrane — Chondro-Gide — before the patient reaches theatre. A sterile aluminium template is placed in the prepared defect to obtain an exact impression; this transfers to the membrane, which is trimmed to shape and fixed 2 mm below the surrounding articular surface with fibrin glue. No separate periosteal harvest is required, removing one source of donor-site morbidity and making the intraoperative steps more reproducible. Delivery can be via a standard open arthrotomy, a smaller mini-arthrotomy, or — in suitable anatomical locations — arthroscopically, an option reviewed by Edwards and colleagues in 2014.

What neither generation changes is the tissue quality produced: both ACI and MACI aim to regenerate hyaline-like cartilage rather than the inferior fibrocartilage typical of marrow-stimulation techniques.

Evidence grounding both approaches

Ten-year follow-up data reported by Minas and colleagues (Clin Orthop Relat Res, 2014 — the John Insall Award paper) confirmed that ACI provides durable, sustained clinical benefit across a minimum decade of follow-up, underpinning confidence in the cell-based family as a whole. For MACI specifically, the SUMMIT randomised controlled trial showed that patients with lesions of 3 cm² or larger achieved significantly improved KOOS pain and function scores compared with microfracture at both two and five years — a result that has become the benchmark for scaffold-based cell therapy in the knee. The scaffold format simplified the operation; the biology of repair belongs to the same proven family.

How defect size shapes the decision

Defect area is the single most practical variable when comparing repair options, because the tissue quality each technique produces — and its durability over time — depends heavily on how much surface needs to be restored.

Three approximate size bands

Below roughly 2 cm², marrow-stimulation techniques such as microfracture and single-stage osteochondral autograft transfer (OATS or mosaicplasty) remain clinically acceptable. OATS suits lesions of around 1–2 cm², with mosaic configurations extending that ceiling to approximately 4 cm², though donor-site morbidity from the graft harvest is a meaningful trade-off that should factor into any decision for a younger, active patient.

Between 2 and 4 cm², the evidence begins to favour cell-based repair, and the conversation turns firmly toward two-stage options such as MACI or first-generation ACI.

At 3 cm² and above, the SUMMIT randomised controlled trial provides the clearest guidance available: for lesions of this size, MACI produced significantly superior KOOS pain and function scores compared with microfracture at both two and five years. That finding has become the reference point for scaffold-based cell therapy in the knee for larger symptomatic focal defects.

Why microfracture carries extra risk for larger defects in younger patients

Microfracture stimulates marrow bleeding to lay down fibrocartilage rather than hyaline-like tissue. For defects above roughly 2–4 cm², that fibrocartilage repair typically degrades within two to three years, and — critically — the procedure breaches the subchondral bone plate. That breach can compromise the bone bed that more advanced cell-based procedures depend on, narrowing future surgical options if microfracture fails.

Very large or bony defects

Where cartilage loss exceeds approximately 10 cm², or where there is concurrent subchondral bone loss, fresh osteochondral allograft (OCA) transplantation may be more appropriate than ACI or MACI alone, since it replaces both the cartilage layer and the underlying bone in a single procedure.

Defect size, then, functions as a triage filter: it narrows the field before patient age, activity level, and tolerance for a two-stage pathway enter the calculation.

Patient factors: who is suited to two-stage cell therapy

Biology sets the floor — but whether two-stage cell therapy is genuinely appropriate depends as much on the person as on the size of their cartilage defect.

The patient profile that benefits most

Younger, active adults — broadly those under 50 — with a focal, isolated chondral defect and no significant osteoarthritis elsewhere in the knee tend to gain most from the durable, hyaline-like repair that ACI and MACI can produce. When the surrounding joint is heavily worn, restoring one localised area delivers diminishing returns.

Commitment to a staged pathway

Two-stage cell therapy means two operations, two anaesthetics, and a rehabilitation period that commonly runs to 12–18 months before return to demanding physical activity. Work commitments, family responsibilities, and practical funding considerations all legitimately shape whether that timeline is realistic for a given patient. For those who cannot commit to a second operation — for personal, practical, or logistical reasons — a single-stage approach deserves honest discussion as part of shared decision-making, weighing the stronger long-term evidence base for two-stage MACI against the reduced procedural burden of single-stage alternatives.

Prior marrow stimulation

A previous microfracture procedure raises the risk of poorer outcomes with ACI or MACI. The subchondral bone breach that microfracture creates can leave a scarred bed that complicates cell integration, making early-stage referral preferable where two-stage repair is being considered.

Alignment

Varus or valgus malalignment that concentrates load through the repaired compartment may undermine any cartilage restoration. An osteotomy — high tibial (HTO) or distal femoral (DFO) — may be considered alongside or in advance of the cartilage procedure to offload the area and protect the repair.

Single-stage cell-based repair: where STACI fits

Harvesting cartilage in theatre, enzymatically freeing chondrocytes from the matrix without altering their character, concentrating bone-marrow-derived mesenchymal stem cells (MSCs) from the same surgical field, then seeding everything onto a shaped collagen scaffold — all within a single anaesthetic episode. That is the single-stage, cell-based pathway described in the clinical literature under the label STACI (Single-Treatment Autologous Chondrocyte Implantation), and its central claim is the elimination of the 3–6 week laboratory culture phase and the second operative visit.

Taylor and Lee (2019) articulated this approach under the explicit heading 'the next generation of ACI', positioning it as a conceptual advance over conventional two-stage practice. Their paper represents an early series framing the technique rather than a large randomised trial, and no head-to-head RCT comparing STACI with two-stage MACI currently exists — a gap that should be stated plainly rather than papered over. STACI is therefore best understood as a promising next-generation concept with a less mature evidence base than the two-stage pathway supported by the SUMMIT trial.

How STACI differs from AMIC

Both STACI and AMIC (autologous matrix-induced chondrogenesis) complete repair in one operation, but they are biologically distinct. AMIC augments microfracture with a collagen membrane — it relies on marrow-stimulated progenitor cells migrating from subchondral bone, not on harvested chondrocytes. STACI, by contrast, uses the patient's own isolated chondrocytes and concentrated MSCs, preserving the cell-biology rationale of conventional ACI. For patients who are biologically suited to cell-based repair but face genuine barriers to a second operation, STACI currently represents the most chondrocyte-preserving single-stage option available.

Getting assessed at Lincolnshire Knee

An accurate picture of the lesion comes first. The practical variables — defect depth, subchondral bone integrity, joint alignment, and the condition of the surrounding cartilage — must be mapped objectively before any technique can be sensibly recommended, and symptoms alone are a poor guide to lesion severity or stage.

Cartilage MRI with T2 mapping gives measurable tissue characterisation that a standard scan may not capture. At Lincolnshire Knee, the onMRI™ platform applies AI-driven cartilage analysis to produce an objective lesion grade as part of the pre-treatment planning process — the kind of detail that distinguishes, for instance, a defect with intact subchondral bone from one where bone-loss is already complicating the repair picture. For patients where malalignment is part of the clinical story — particularly when an osteotomy adjunct is being considered alongside cartilage restoration — MAI Motion® gait and load assessment can identify compartment-loading patterns that would put a repair at risk.

Consultations are available at Sleaford NG34 and Grantham NG31 through the MSK Doctors group, with no GP or NHS referral required. To book an assessment, visit lincolnshireknee.co.uk.

1. [1] Knee cartilage replacement therapy. https://en.wikipedia.org/?curid=4984243 https://en.wikipedia.org/?curid=4984243
2. [2] Autologous chondrocyte implantation. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150