The one-operation vs two-operation difference

If both procedures repair the same thing — a focal articular cartilage defect, meaning a localised area of damaged joint surface rather than widespread wear — why would a patient undergo two operations when one might suffice? That question sits at the heart of the difference between MACI and STACi.

With MACI (Matrix-Induced Autologous Chondrocyte Implantation), the answer lies in where the cells are prepared. Stage one is a biopsy: a small cartilage sample is taken arthroscopically and sent to an external laboratory, where the harvested cells are cultured and expanded over several weeks. Only once that process is complete can stage two take place — a separate open procedure to implant the expanded cells on a collagen scaffold. Two operations, two anaesthetics, and a waiting period between them are built into the design.

STACi (Single Treatment Autologous Chondrocyte Implantation) resolves this by relocating cell processing from the external laboratory into the operating theatre itself. Harvesting, cell preparation, and implantation all occur within the same anaesthetic episode. For patients, the practical difference is direct: one surgery rather than two, no inter-stage waiting period, and a single episode of anaesthetic exposure.

What MACI involves

Stage one begins in the arthroscopy room. Under keyhole access, the surgeon takes a small cartilage biopsy — typically 200–300 mg — from the superomedial margin of the femoral trochlea, a low-load-bearing area chosen to minimise any donor-site consequence. The sample is preserved at 4°C and dispatched to an accredited external laboratory.

At the laboratory, technicians isolate the chondrocytes from the surrounding matrix, then culture and expand them over several weeks. Expanding the cell population to sufficient numbers for a meaningful repair is the core reason two operations are required: the process simply cannot be compressed into a single theatre session. Once enough cells have been produced, they are seeded onto a Type I/III collagen scaffold — a thin, flexible membrane that provides the cells with a structured environment and acts as the delivery vehicle during implantation.

Stage two is a separate open procedure, scheduled once laboratory turnaround is complete. The scaffold, now carrying the patient's own expanded chondrocytes, is trimmed to fit the defect and fixed in place. Because the cells have been cultured and multiplied outside the body, they meet the regulatory definition of a 'manipulated' advanced therapy medicinal product — a classification that carries its own manufacturing, quality, and traceability requirements.

MACI is generally considered for focal defects of roughly 2–10 cm² in active patients where smaller-scale interventions are unlikely to give durable results. The evidence base is well established: published series include five-year RCT and MRI outcome data, and Minas et al. (2014, Clinical Orthopaedics and Related Research) reported minimum ten-year follow-up demonstrating durable clinical benefit.

What STACI involves

The entire STACi procedure takes place within a single anaesthetic episode — what its proponents describe as bringing the 'lab in' rather than sending cells out. A dedicated intraoperative laboratory team works alongside the surgeon, processing tissue as it is harvested, so that no waiting period is built into the pathway.

The surgeon harvests cartilage from a non-weight-bearing area of the joint. The intraoperative team cuts this tissue into small pieces and treats it with a digestive enzyme — a step sometimes described as 'jail-breaking' the chondrocytes from the surrounding extracellular matrix. Critically, the cells are freed but not cultured or expanded; they retain their native character and remain unmodified beyond physical separation from the matrix. It is the absence of this expansion step that makes a single visit possible.

Simultaneously, bone marrow is aspirated from within the surgical field. The intraoperative team concentrates, isolates, and counts the mesenchymal stem cells (MSCs) this yields. These MSCs are characterised in the clinical literature as paracrine directors — producing growth factors and guiding chondrocyte differentiation rather than forming repair tissue directly. The combined chondrocyte-plus-MSC population is then loaded onto a collagen scaffold and implanted into the prepared defect before the patient leaves theatre.

Because the chondrocytes are enzymatically isolated but not substantially altered, STACi may fall outside the stricter 'manipulated cell' classification applied to MACI under advanced therapy medicinal product (ATMP) regulations — a distinction with potential implications for how the procedure is licensed and delivered. Taylor and Lee (2019) describe STACi in this context as the next generation of autologous chondrocyte implantation. It remains an emerging technique, and long-term comparative outcome data against MACI are not yet established.

Why STACI adds MSCs and what that means for repair

Standard MACI delivers only the expanded chondrocytes — no stem cells — seeded onto the collagen scaffold. Repair depends on those cultured cells alone integrating into the defect and producing new cartilage matrix.

STACi's addition of MSCs changes that biological picture. Mesenchymal stem cells do not act as structural building blocks in this context; they work through paracrine signalling — secreting growth factors and molecular instructions that influence how the chondrocytes around them behave. The chondrocytes remain the primary repair cells, but they are working in a richer, more directive environment than a chondrocyte-only scaffold can provide.

The clinical rationale is that this combined cell population more closely recreates the conditions under which durable hyaline-like cartilage forms. Natural cartilage repair does not happen in a growth-factor vacuum: chondrocytes require active differentiation signals to integrate and form organised tissue rather than simply filling a void. The paracrine contribution of MSCs is thought to narrow the gap between implanted material and the native tissue response that effective repair demands.

Whether this biological advantage translates into superior long-term outcomes compared with MACI has not been established in comparative studies — the MSC addition remains a well-reasoned rationale pending that evidence.

Regulatory classification and what it means for patients

The regulatory distinction noted in the previous section has concrete consequences for how MACI is manufactured and delivered. Because MACI cells are cultured and expanded in an external laboratory, they fall under the Advanced Therapy Medicinal Product framework — a tier of medical regulation that requires accredited manufacturing facilities, batch-level quality testing, and strict chain-of-custody controls throughout cell processing. Meeting those requirements is resource-intensive: it contributes directly to MACI's cost and limits its availability to centres with the licensed infrastructure to handle cell products at that standard.

STACi's intraoperative processing — isolating cells without expanding them — may sidestep that tier of regulation, which could simplify licensing and reduce some of the logistical overhead that makes MACI a two-stage commitment. That said, a lighter regulatory footprint is not the same as a clinical advantage. Comparative long-term outcome data between the two approaches are not yet established, and the appropriate procedure for any patient depends on lesion size, general fitness for surgery, and specialist assessment rather than on which pathway involves fewer regulatory steps. Where MACI's evidence base and manufacturing infrastructure are already in place, the two-stage model remains a well-supported option for suitable candidates.

Evidence base, current limitations, and finding a specialist

The gap in the evidence runs in one direction. MACI's clinical record is well-established — the published follow-up data referenced earlier in this piece represent years of peer-reviewed outcome tracking — while STACi's evidence base is still forming. The key published reference for the single-treatment approach is Taylor & Lee (2019), which set out the procedural rationale and early findings; no head-to-head comparison between STACi and MACI has been published, and long-term outcomes for STACi have yet to be established in the same way.

That asymmetry does not settle the question in MACI's favour by default. A technique with a coherent biological rationale, a single anaesthetic episode, and no inter-stage waiting period can still be the right choice for the right patient. What the evidence gap does mean is that specialist assessment carries real weight. Both procedures are performed at centres with specific cell-therapy expertise rather than general orthopaedic units, and finding a surgeon experienced in cell-based cartilage repair is the practical first step. Search MSK lists specialists across the UK who offer these procedures — filtering by region and specialty can help identify a consultant suited to your situation.

The appropriate choice depends on individual factors: defect size and location, prior surgical history, and what the treating centre is equipped to offer. Those questions require clinical assessment in person. The evidence base sets the context; it does not make the decision.