One operation instead of two: the STACI concept

For most patients referred for autologous chondrocyte implantation, the first question is blunt: does this really require two separate operations? Under conventional ACI and MACI, the answer has always been yes — two general anaesthetics, two hospital admissions, and several weeks between them while a specialist laboratory cultures the harvested cells. That waiting period is not merely inconvenient. An untreated full-thickness cartilage defect can enlarge during the inter-stage window, and the extended culture process carries its own biological cost: chondrocytes expanded outside the body for weeks can undergo phenotypic drift, gradually losing the cell identity that makes them effective cartilage producers — a process known as dedifferentiation.

STACi (Single-Treatment Autologous Chondrocyte Implantation) was developed to answer that question differently. Described by Taylor and Lee as the next generation of ACI, it collapses the two-stage pathway into a single uninterrupted surgical episode. Harvest, cell preparation, and implantation all occur within one anaesthetic sitting: a dedicated laboratory team works in the operating theatre itself, enzymatically liberating chondrocytes from a small biopsy taken from a non-weight-bearing area and combining them with concentrated bone marrow-derived mesenchymal stem cells, before the preparation is seeded onto a collagen scaffold and fixed into the defect — all before the patient leaves the operating room.

The principle underlying the approach is the same one ACI established: the patient's own cartilage cells, returned to the joint to regenerate damaged tissue. What changes is the architecture of care around them.

How STACI works: harvest, prepare, implant in one sitting

The procedure moves through four overlapping stages without interruption. It begins with a targeted biopsy from a non-weight-bearing area of the patient's own knee — a zone not called upon for load-bearing — yielding a fragment of healthy cartilage that becomes the seed material for the repair.

That fragment passes directly to the theatre laboratory team, who free the chondrocytes from the surrounding matrix using an enzymatic digest: the binding protein between cell and scaffold is dissolved, the cells are released, and they are not altered by culture or expansion. Simultaneously, bone marrow is drawn from within the surgical field and concentrated to isolate mesenchymal stem cells (MSCs). The rationale for pairing these two cell populations has published scientific precedent: Bekkers and colleagues demonstrated in 2013 that combining freshly isolated chondrons with MSCs in a single-stage model could produce cartilage repair — the co-implantation biology that STACi applies.

The combined preparation is loaded onto a collagen scaffold. Unlike the periosteal flap of first-generation ACI — a flat sheet sewn over the defect — or the pre-seeded membrane used in MACI, the scaffold has depth, supporting cell growth throughout the full thickness of the cartilage layer rather than merely across its surface. Think of it less as a patch and more as structural scaffolding: the architecture holds the cells in the right geometry while integration proceeds.

Before the scaffold is fixed into position, the RegenRepair™ framework's 'Structure first' principle takes effect: coexisting mechanical problems — malalignment, ligament instability, or meniscal damage — are corrected in the same operative session. Placing a biological repair into a mechanically unsound environment reduces its prospects; addressing both in one sitting also extends eligibility to patients who would not have qualified for standard single-lesion ACI.

This theatre-based surgical pathway is distinct from NanoACi — a needle-delivered clinic variant that applies similar biology without keyhole surgery or general anaesthetic, suited to patients whose joint structure requires no operative preparation. The biological rationale is shared between the two; what separates them is the scale of mechanical groundwork the joint needs before the repair can be placed.

Who is a suitable candidate

The eligibility picture can be sketched in practical terms before a formal assessment fills in the detail.

The lesion itself. STACi is suited to focal, full-thickness cartilage loss rated ICRS grade 3 or 4 — a discrete, well-defined defect, not degeneration spread across the whole joint surface. A single defect of roughly 4 cm² on the lateral femoral condyle, for instance, is within scope; generalised arthritis involving multiple compartments is not. Multiple focal lesions in the same knee can, in some cases, be addressed concurrently — medial condyle and patellofemoral damage treated in one sitting, for example.

Age and biological health. The typical candidate is under 40. The ceiling sits at around 45, but chronological age is a guide rather than a hard rule: what matters is the biological condition of the joint — its remaining cartilage quality, healing capacity, and the absence of widespread degenerative change.

Joint mechanics. The joint must be structurally sound, or have problems — malalignment, ligament instability, meniscal damage — that can be corrected in the same operative session. A mechanically compromised environment that cannot be addressed simultaneously would undermine the repair.

Where NHS access falls short. NICE technology appraisal TA477 sets four eligibility criteria for NHS ACI; patients who cannot meet all four — for example, someone who has undergone a prior marrow-stimulation procedure such as microfracture, or who presents with two focal lesions rather than one — are often declined on the NHS despite having a joint that is biologically appropriate for single-stage repair. A patient in that position, aged 38 with a 3 cm² defect and a previous microfracture, sits in exactly the borderline zone where a specialist assessment determines whether STACi is viable privately. The assessment visit exists to clarify those cases — not as a formality, but because the decision involves weighing lesion geometry, prior procedures, and joint-wide health together.

What the evidence shows — and what is still emerging

At ten-to-seventeen years of follow-up, 168 patients who had received matrix-induced autologous chondrocyte implantation showed sustained improvements in patient-reported outcomes, an all-cause reoperation rate of 9%, and a 7.4% rate of progression to total knee replacement. That cohort — mean age 37 at surgery — provides the most directly applicable long-term benchmark available, because STACi as a named protocol has not yet been evaluated in a large-scale randomised controlled trial.

That absence does not place STACi outside evidence-based practice; it situates it alongside many procedural innovations that are supported by their component biology, validated by adjacent long-term studies, and accumulating their own clinical data in parallel. Across the ACI, MACI, osteochondral autograft transfer, and fresh osteochondral allograft literature, a 2024 meta-analysis found significant improvements in both function and pain at a mean follow-up of 57 months — a consistent signal across the scaffold-based and cell-based approaches whose biology STACi shares. The 2013 Bekkers study, which demonstrated that freshly isolated chondrons combined with MSCs in a single-stage model could produce cartilage repair, provides additional scientific grounding for the specific co-implantation approach.

Closer to the single-stage concept itself, a 2020 series reported reduced pain and improved function at six months in patients who underwent single-stage bone marrow MSC transplantation, alongside lower costs compared with two-stage ACI. Three patients were included; this is proof-of-concept, not a controlled trial, and long-term follow-up data are pending.

What recovery looks like in practice. Range of motion recovers well following ACI-type procedures. Quadriceps strength is a different matter: biomechanical review of ACI outcomes indicates that strength remains below that of healthy controls beyond two years post-operatively. Full functional recovery — particularly return to demanding sport — takes considerably longer than pain relief alone would imply, and realistic timelines for both should form part of any pre-operative discussion.

Cost, NHS access, and why patients choose STACI privately

The financial comparison is more straightforward than many patients expect. NHS two-stage ACI, where NICE TA477 criteria are met, carries an estimated total pathway cost of approximately £16,000. STACi is offered privately from £12,500 all-inclusive — a single invoice covering assessment, the operative session, and immediate post-operative care. Early health-economic modelling, published in 2017, identified single-stage cartilage repair as favourable compared with two-stage approaches on cost-efficiency grounds, a finding consistent with the structural logic of collapsing two hospital admissions into one. A small 2020 series reporting on single-stage bone marrow MSC transplantation similarly noted reduced financial costs alongside early clinical improvement.

For many patients who arrive at a private consultation, the NHS question has already been settled. As the eligibility discussion above outlines, prior marrow-stimulation procedures or defect patterns outside the four-criterion NICE framework can disqualify individuals whose joints remain biologically appropriate for repair. Waiting times compound this: even patients who do qualify may face substantial delays, during which a cartilage defect can enlarge and the biological window for effective repair narrows.

The all-inclusive pricing carries a practical dimension beyond the headline figure. Rather than navigating separate referral chains for imaging, assessment, and the operative episode itself, the private pathway consolidates those steps — removing the administrative fragmentation that can characterise complex multi-stage NHS cases. For a patient declined NHS ACI at 38 or 39, facing a defect that is not yet irreparable, the private route functions less as a preference than as a considered clinical alternative.

Getting assessed and what to expect next

The patient who benefits most from a STACi assessment is often one the NHS pathway has already turned away: mid-to-late thirties, a substantial focal defect, biologically capable of repair but outside the four strict NICE criteria or facing a wait long enough to let the lesion enlarge. For that person, the question a consultation answers is not simply 'is STACi possible?' but whether the joint — reviewed against MRI, assessed for mechanical alignment, and weighed against personal history and realistic expectations — is still at a point where regenerative repair is worthwhile. Sometimes the answer is STACi; sometimes it is a different technique entirely; occasionally it is that the biological window has already narrowed too far.

At the London Cartilage Clinic on Harley Street, that assessment is led by Prof Paul Y. F. Lee, who developed the STACi RegenRepair™ framework and performs the procedure in London. Patients based outside London can be assessed through the MSK Doctors group's Lincolnshire and Grantham sites under the same clinical approach. To book a cartilage consultation, visit londoncartilage.com.

1. [1] Single-Stage Cell-Based Cartilage Regeneration Using a Combination of Chondrons and Mesenchymal Stromal Cells. (2013). https://doi.org/10.1177/0363546513494181 https://doi.org/10.1177/0363546513494181
2. [2] Minimum 10-Year Outcomes of Matrix-Induced Autologous Chondrocyte Implantation in the Knee. (2024). https://doi.org/10.1177/03635465231205309 https://doi.org/10.1177/03635465231205309
3. [3] Early health economic modelling of single-stage cartilage repair. Guiding implementation of technologies in regenerative medicine. (2017). https://doi.org/10.1002/term.2197 https://doi.org/10.1002/term.2197
4. [4] 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