The problem with two operations — and what STACi removes

Two surgeries, two anaesthetics, and weeks of waiting in between — for many patients, that has been the practical reality of autologous chondrocyte implantation (ACI). The first operation is an arthroscopic biopsy: a small sample of healthy cartilage is harvested and dispatched to an off-site laboratory. There, over several weeks, the chondrocytes — the specialised cells responsible for producing and maintaining the cartilage matrix — are expanded in culture until sufficient numbers are available. Only once that process is complete does the second procedure take place: reimplantation.

The delay was never arbitrary. Growing enough cells to fill a defect takes time outside the body. But the culture period carries a biological cost: chondrocytes expanded over weeks in an external laboratory can undergo phenotypic drift, gradually losing the properties that make them effective cartilage producers — a process known as dedifferentiation. The longer the culture, the greater the risk that the cells implanted in the second operation are not quite the same cells that were harvested in the first.

STACi (Single-Treatment Autologous Chondrocyte Implantation) was developed specifically to close that gap. Rather than sending harvested tissue to an external laboratory, the laboratory comes to the patient: a specialist team works within the operating theatre itself, processing cells and preparing the implant during the same anaesthetic episode. Prof Paul Lee, whose clinical work at MSK Doctors underpins this approach, describes STACi not as a departure from ACI but as its next logical evolution — the same biological rationale, with the procedural interruption removed.

What happens inside the operating theatre during a STACi procedure

Once the patient is under anaesthetic, the procedure begins with an arthroscopic survey of the knee. The surgeon maps the damage precisely — in a documented case, a focal defect of approximately 4 cm² on the lateral femoral condyle — and harvests a small fragment of healthy cartilage from a non-weight-bearing area of the joint.

At this point, the intraoperative laboratory team takes over. Working in the operating theatre alongside the surgical team, they cut the cartilage fragment into small pieces and apply a digestive enzyme that breaks the individual chondrocytes free from the surrounding tissue matrix — a process sometimes described as 'jail-breaking' the cells. The aim is liberation, not manipulation: the chondrocytes are released intact, without alteration to their cellular character.

Simultaneously, the surgeon aspirates bone marrow from within the surgical field. The lab team processes this separately to isolate mesenchymal stem cells (MSCs) — progenitor cells capable of contributing to tissue regeneration. The MSCs are concentrated, washed to remove unwanted material, and counted under the microscope to confirm the yield before the two preparations are combined.

With both cell populations ready, the team seeds them together onto a collagen scaffold — a structured support matrix that gives the cells a framework to inhabit. The scaffold is cut to a custom template of the measured defect, ensuring a close anatomical fit at the repair site.

The completed construct is then implanted, all within the same anaesthetic episode. Nothing leaves the theatre for external processing; nothing waits for a later date. The single sitting encompasses what, in conventional ACI, would have been two separate operations weeks apart.

Patients best suited to STACi

Candidacy for STACi turns on two variables: age and the nature of the cartilage damage.

On age, the general guideline is under 40, with clinical flexibility extending to approximately 45 where biological age supports it. Chronological age sets the frame; biological age informs the final decision. Younger, biologically active patients tend to have the cellular environment that favours genuine regeneration — the repair process relies on the body's own biology to consolidate the implanted construct.

On defect type, STACi is designed for focal cartilage lesions — discrete areas of damage within an otherwise functional joint. The procedure has been performed for moderate-sized defects, with a documented example at approximately 4 cm², placing it broadly within the range associated with cell-based repair techniques. Patients with damage confined to one or two identifiable sites are the target group.

A useful capability to note is that STACi can address more than one focal lesion in a single sitting. Combined repair — for example, simultaneous treatment of a medial femoral condyle defect and patellofemoral damage — has been performed without a second operative episode. For patients with two discrete problem areas in the same knee, this may meaningfully reduce the overall treatment burden.

Who does not fit is equally clear. Where arthritis is diffuse or affects the whole joint, STACi is not appropriate. The procedure restores focal defects; regenerating an extensively damaged joint in one sitting is not feasible, and patients in that situation are better assessed for a different pathway.

How STACi differs from standard ACI and MACI

The distinction that matters most for patients who have already encountered MACI is where the manufacturing step takes place. In MACI, harvested chondrocytes are sent to an accredited external laboratory, where they are expanded on a Type I/III collagen membrane over several weeks before being returned for surgical implantation. The scaffold arrives in theatre ready-made; the cellular work is done offsite. In STACi, that manufacturing step moves into the operating room itself — the scaffold is assembled intraoperatively using cells isolated from the patient's own tissue within the same anaesthetic episode, as outlined in the preceding section.

The biological argument for this sequence turns on what happens to chondrocytes during prolonged ex-vivo culture. Extended time outside the body is associated with dedifferentiation — a process in which chondrocytes progressively lose the specialised characteristics that make them effective cartilage producers, reverting towards a less differentiated cell type. By using the cells immediately after harvest, STACi aims to sidestep this shift entirely. STACi is therefore best understood as a next-generation variant with a sound biological rationale — not a wholesale replacement for every ACI or MACI indication, and one whose longer-term comparative evidence base is still developing.

One avenue under active investigation as a potential complement is platelet-rich plasma (PRP). Because PRP delivers a concentrated payload of growth factors that may support the regenerative environment around the implanted construct, it is being studied as a means to further enhance repair quality. It is not yet part of the standard STACi protocol, and its role remains at the research stage.

What the evidence currently shows — and where gaps remain

ACI has one of the longer follow-up records in cartilage surgery: landmark series by Peterson and Minas documented durable functional improvement over nine years or more, establishing the cellular approach as something considerably more than a short-term palliative. STACi is the direct descendant of that technique — same autologous cell source, same regenerative logic — and it inherits the credibility that lineage carries.

What STACi does not yet have is a published randomised controlled trial directly comparing its outcomes with standard two-stage ACI. The available evidence is expert clinical description, grounded in procedural literature and in the biological reasoning covered in the preceding section. That reasoning is scientifically coherent and consistent with established cell biology; whether it translates into measurably superior long-term cartilage quality is a question comparative trial data will need to address.

One limitation runs across cartilage repair as a whole, not STACi in particular: no current technique fully reconstitutes native hyaline cartilage. The realistic and clinically validated goal — across ACI, MACI, AMIC, and STACi alike — is durable repair tissue that restores function, reduces pain, and delays joint deterioration. That is a meaningful outcome, even where tissue-level perfection is beyond reach.

Getting an assessment for STACi

Determining whether STACi is the right option requires a detailed clinical assessment — defect size, precise location within the joint, lesion grade, biological age, and the history of any prior procedures all bear on the decision. It is not a pathway that can be self-selected from a checklist.

Prof Paul Y. F. Lee, who developed the STACi approach, carries out these assessments directly. For patients in London, that assessment is available through the London Cartilage Clinic on Harley Street, the London access point for advanced cartilage and joint-preservation evaluation within the MSK Doctors group.

Anyone considering cartilage repair is welcome to book an initial consultation at londoncartilage.com.

1. [1] Autologous chondrocyte implantation — Wikipedia. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150
2. [2] Chondrocyte — Wikipedia. https://en.wikipedia.org/?curid=1309267 https://en.wikipedia.org/?curid=1309267
3. [3] Articular cartilage repair — Wikipedia. https://en.wikipedia.org/?curid=19042351 https://en.wikipedia.org/?curid=19042351