The one-surgery versus two-surgery question

For most patients weighing up these two procedures, the defining question is straightforward: one anaesthetic or two?

Conventional autologous chondrocyte implantation — ACI, and its matrix-applied variant MACI — is built around two separate operations. The first harvests a small cartilage sample from a non-weight-bearing part of the knee; the cells are then sent to an external laboratory where they are cultured and expanded over several weeks. A second, larger procedure brings the patient back to theatre to implant the lab-grown cells into the damaged area. STACi (Single Treatment Autologous Chondrocyte Implantation) removes that gap entirely: harvest, cell preparation, and implantation happen in one uninterrupted session, with a dedicated laboratory team working in the operating theatre alongside the surgeon.

The biological target is identical for both — a focal, well-defined cartilage defect in a knee that is otherwise structurally sound. Neither approach is suited to diffuse, whole-joint arthritis.

Why does any repair procedure matter? Articular cartilage has very limited capacity to heal itself. Focal defects that go untreated tend to progress, and over time that progression can tip toward osteoarthritis and the prospect of joint replacement. The procedural structure — one stage or two — is what separates these two pathways; the underlying biological ambition is the same.

How two-stage ACI and MACI work

Stage 1 begins with a short arthroscopic procedure under general anaesthetic. The surgeon takes a small cartilage biopsy — typically a few hundred milligrams — from a non-load-bearing zone of the knee, usually the periphery of the femoral condyle. That sample is then couriered to an external laboratory, where technicians isolate the chondrocytes and culture them over several weeks until the cell population is large enough for implantation. Because the cells are removed from the body, expanded outside it, and chemically processed before being returned, this step constitutes ex-vivo manipulation — meaning the cells are substantially altered from their natural state. That classification matters legally: in the UK, any product derived from such manipulation meets the definition of an Advanced Therapy Medicinal Product (ATMP) under medicines regulation, requiring a product licence and specific manufacturing standards.

Stage 2 — typically four to twelve weeks later — brings the patient back to theatre for a second general anaesthetic. In the modern MACI variant, the cultured chondrocytes are pre-seeded onto a Type I/III collagen membrane, which is then trimmed to fit the prepared defect and secured in place, either through open surgery or, increasingly, arthroscopically. The collagen membrane acts as both a cell carrier and a structural scaffold, giving the implanted cells a three-dimensional environment in which to produce cartilage matrix.

In England, NICE technology appraisal TA895 recommends MACI for symptomatic articular cartilage defects of the knee, establishing a formal NHS funding pathway for eligible patients. Published five-year clinical and MRI data show good defect fill and sustained symptom relief; outcome studies of earlier ACI generations extend beyond ten years and support a meaningful reduction in the likelihood of progressing to knee replacement.

The practical implication of the two-stage structure is two separate anaesthetics, two recovery periods, and an interim wait that can span several months in total.

How STACi works in a single session

The moment the surgeon completes the STACi biopsy, a dedicated laboratory team — physically present in the operating theatre — takes over at the bench alongside the surgical field.

Cartilage chips harvested from a non-weight-bearing zone of the knee are cut into small fragments and enzymatically digested to release the chondrocytes. Crucially, the cells are not cultured, not expanded, and not chemically altered outside the body: they are simply freed from the tissue matrix and kept in solution. Because no substantial ex-vivo manipulation takes place, STACi falls outside the Advanced Therapy Medicinal Product (ATMP) regulatory classification that applies to conventional two-stage ACI — a practical distinction that has implications for how the procedure is regulated and commissioned in the UK.

At the same time, bone marrow is aspirated from bone within the operative field. The laboratory team concentrate the aspirate chairside, isolating and counting the mesenchymal stem cells (MSCs) it contains. These are not chondrocytes and do not appear to convert directly into cartilage cells; their value is paracrine — they release growth factors that stimulate and direct the co-implanted chondrocytes toward matrix production and repair. Think of them as a biological support crew rather than additional building-block cells.

The chondrocyte–MSC suspension is then loaded onto a collagen and hyaluronan scaffold, which provides a three-dimensional, supportive microenvironment — broadly analogous to the collagen membrane used in MACI, but populated with freshly harvested rather than laboratory-expanded cells. The construct is implanted into the prepared defect and secured before the patient leaves theatre.

Platelet-rich plasma (PRP) is sometimes incorporated as an additional growth-factor source during STACi, though its role at this stage is an emerging adjunct rather than an established standard component of the procedure.

Which procedure suits which patient

Several practical questions tend to dominate the consultation when a patient is weighing up STACi against two-stage MACI: age, how many damaged areas there are, and whether the leg's mechanical axis is straight.

The shared prerequisite — a focal defect in a sound knee

Neither procedure is appropriate when arthritis is widespread across the joint. Both target focal, well-defined chondral lesions in a knee whose surrounding cartilage, ligaments, and alignment are otherwise viable. Patients with diffuse or whole-compartment osteoarthritis are generally not candidates for either technique; joint preservation surgery or arthroplasty is a more realistic pathway at that stage.

'Am I too old?'

STACi is principally offered to biologically younger patients — broadly under 40, or up to roughly 45 in selected cases. The operative word is biological age: what matters clinically is the quality of the remaining cartilage and the joint's capacity to support regeneration, not the number on a birth certificate. Two-stage MACI does not carry the same strict age ceiling in the published evidence, which may make it the more appropriate choice for older patients with focal defects who still meet the structural selection criteria.

'What if I have damage in two places?'

For patients with discrete lesions at more than one site — for instance, concurrent medial condyle and patellofemoral damage — STACi can address both areas in the same operative session. That is a meaningful practical advantage: a two-stage approach in the same scenario would require either a more complex second operation or separate staged procedures. Published case material documents STACi managing a 4 cm² lateral condyle defect within a single sitting; two-stage MACI carries an upper indication range extending to approximately 10 cm², which may suit larger single lesions where the biology of a single-stage approach has not yet been established.

'Does my bowed leg matter?'

Malalignment concentrates load on the damaged compartment and can undermine any cartilage repair if left uncorrected. When a corrective osteotomy (such as a high tibial or distal femoral osteotomy) is planned alongside cartilage repair, the staging question becomes important. Because STACi does not involve ex-vivo cell culture, it can in principle be combined with an osteotomy in a single operation. Two-stage ACI or MACI, by contrast, typically requires the cartilage and alignment procedures to be carefully sequenced — often separated — adding further complexity to the overall pathway.

Assessment and imaging

Deciding between the two pathways requires detailed MRI evaluation to characterise defect depth, size, location, and the condition of the surrounding cartilage and bone. A consultant-led assessment interpreting cartilage-specific MRI sequences — including T2 mapping where available — informs this decision rather than imaging alone.

Evidence, NHS access, and private availability in the UK

Two-stage MACI carries the stronger published evidence record. NICE technology appraisal TA895 formally recommends MACI for symptomatic articular cartilage defects of the knee, establishing an NHS commissioning pathway for the two-stage, cell-based approach. Underpinning that recommendation is a substantive body of trial data: MACI has five-year randomised controlled trial results and MRI cartilage-fill evidence, and the earlier generations of two-stage ACI — on which MACI is built — have been followed in published series for nine to ten years, consistently supporting durable pain relief and delayed or avoided joint replacement.

STACi's published evidence sits at an earlier point on that development curve. The scientific case currently rests on case series and mechanistic rationale — including Taylor and Lee's 2019 description of the single-treatment technique — rather than on head-to-head randomised data directly comparing STACi with MACI. The biological reasoning is well-grounded, and case material documents successful outcomes in large focal defects, but the kind of long-term controlled trial evidence that underpins the MACI commissioning decision does not yet exist for STACi in the open literature. That is the honest position, and it does not negate the biological rationale; it means the evidence is at a different stage of maturity.

NHS access and private availability

For two-stage MACI, the NHS funding route is established through TA895. For STACi, no published commissioning guidance specifically addressing it — as distinct from traditional ACI procedures covered under NICE's existing framework — is currently in the public record. Its regulatory classification as a non-ATMP single-stage procedure may affect how commissioners treat it. STACi is available at specialist UK private centres; patients wishing to explore whether NHS access applies to their case should raise the question directly with their treating team.

Published literature does not currently report direct cost comparisons between the single-stage and two-stage pathways. Any cost discussion — including the potential single-anaesthetic advantage of STACi against the two-operation pathway of MACI — is best explored at consultation alongside clinical suitability.

Recovery, rehabilitation, and practical next steps

Recovery from knee cartilage repair is measured in months under both pathways — the procedures differ substantially in structure, but neither offers a rapid return to full activity.

One rehabilitation versus two

The clearest practical difference is the cumulative burden of staged surgery. Two-stage MACI involves two separate anaesthetic events and two distinct recovery periods, typically separated by six to twelve weeks of cell culture and preparation. Patients are effectively mobilising from the first procedure before the second has even been scheduled.

STACi's single-session structure collapses this into one continuous rehabilitation pathway — one anaesthetic event, one period of protected weight-bearing, one uninterrupted course of physiotherapy. For patients with work or family commitments, that continuity represents a genuine practical advantage, as well as the psychological benefit of not facing a second operation.

What 'demanding' actually means

Regardless of technique, the early weeks after cartilage implantation require protected weight-bearing — often with crutches for six to twelve weeks, depending on defect size, location, and the surgeon's protocol. Return to sedentary desk work typically falls within that same window; physically demanding occupations or return to sport may take nine to eighteen months. These are approximate clinical ranges that vary considerably by individual biology, defect characteristics, and how the repair matures.

Where MACI is delivered arthroscopically rather than via open surgery, initial mobilisation may be somewhat faster, though the overall rehabilitation arc remains similar. Biomechanical rehabilitation — quadriceps strengthening, gait re-education, and progressive loading — is essential throughout both pathways to protect the repair site as it consolidates.

Questions worth asking at your assessment

• Is my defect size and location better suited to a single-stage or two-stage approach?
• Would arthroscopic or open delivery be used, and what does that mean for my early recovery?
• Can any malalignment be corrected at the same time, and how would that affect rehabilitation?
• What cartilage MRI assessment is available to plan and monitor the repair?

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. Book an assessment at lincolnshireknee.co.uk.