Who is a suitable candidate for STACi

The most immediate question for most patients is straightforward: does my situation actually fit? STACi suits a fairly specific profile, and checking the key parameters early saves time before pursuing a specialist assessment.

The core eligibility criteria

• Age. Most candidates are between 16 and 55, though biological age and overall joint health matter as much as the number.
• Defect size. The cartilage lesion must be greater than 2 cm². Smaller defects are more commonly managed with procedures such as microfracture or mosaicplasty; STACi is designed for the focal but meaningful losses that those techniques are less well-suited to address.
• Lesion depth. The defect should be full-thickness — ICRS grade III or IV — meaning the cartilage damage reaches down to or through the underlying bone. Partial-thickness lesions do not fall within the typical STACi indication.
• Osteoarthritis status. This is the most important exclusion. Diffuse or advanced osteoarthritis — where cartilage loss is widespread across the joint rather than contained in one area — places the knee outside the cartilage-repair pathway entirely. STACi, like all cell-based repair, is intended for focal defects in an otherwise preserved joint.
• Prior conservative management. Physiotherapy, structured exercise, weight management, and analgesia must have been genuinely attempted and found insufficient before cell-based repair is considered, consistent with the NICE TA477 framework for this class of treatment.

Structural prerequisites

Two mechanical factors are assessed before any cartilage repair is planned. First, the ligaments must be stable: a knee that remains unstable under load will stress a repair construct before it can mature. Second, limb alignment must be adequate — if the leg's mechanical axis concentrates load through the repaired area, the graft is unlikely to survive long-term. Where either problem exists, it may need to be addressed surgically before or alongside the cartilage procedure, which is a real-world barrier worth understanding from the outset.

For patients who have already undergone a previous cartilage repair on the same knee, STACi has been noted as a possible revision option, though outcome data in that setting remain limited.

What happens during a STACi procedure

Inside the operating theatre, the entire procedure unfolds across four sequential steps — all within one anaesthetic, without any material leaving the room for external processing.

Step 1 — Cartilage harvest. A small sample of cartilage is taken from a non-weight-bearing area of the knee, leaving the load-bearing surfaces undisturbed.

Step 2 — Cell release. The sample is treated with a digestive enzyme in theatre. This breaks down the surrounding matrix and frees the patient's chondrocytes — the cells responsible for producing cartilage — without altering their biological character.

Step 3 — Bone-marrow concentration. At the same time, bone-marrow aspirate is taken from the operative field and concentrated to isolate mesenchymal stem cells (MSCs). These cells act as biological directors: they produce growth factors that signal and support the chondrocytes as they work to regenerate cartilage tissue.

Step 4 — Assembly and implantation. The chondrocytes and MSCs are combined and loaded onto a three-dimensional collagen scaffold. Unlike a flat sheet, the 3D structure supports cell growth both across the surface and into the depth of the defect — closer to the architecture of natural cartilage. The construct is then implanted into the prepared lesion site before the operation concludes.

The scaffold serves a dual purpose: it holds the cell mixture securely in position and provides a physical framework within which new cartilage tissue can mature over the months following surgery.

Defect size thresholds and lesion types

Numbers tell much of the story here. The SUMMIT trial — the landmark randomised study establishing MACI's superiority over microfracture — used ≥3 cm² as its threshold, and at both 2-year and 5-year follow-up, KOOS pain and function scores favoured MACI for lesions at or above that mark. That benchmark helps place cell-based repair on the treatment ladder: for defects under 2 cm², techniques such as AMIC or OATS are more commonly appropriate; once a focal lesion exceeds 2 cm², cell-based repair becomes the preferred pathway.

The 2 cm² lower boundary applies to STACi as it does to MACI and conventional ACI — this consistency reflects the underlying biology rather than any procedure-specific quirk.

Where STACi diverges is at the upper end. MACI's flat collagen membrane has practical limits when a defect becomes large or geometrically irregular; STACi was partly developed to extend cell-based repair beyond those boundaries. No documented upper size ceiling exists. A 4 cm² lateral condyle defect has been managed successfully in a single operative sitting — a scale that exceeds the territory where MACI has typically been applied, and one where the 3D scaffold's architecture becomes progressively more relevant to matching the lesion's geometry.

The essential requirement throughout is that the damage remain focal. Defect size, in isolation, is not a contraindication to STACi.

How STACi differs from MACI across four areas

The four dimensions below mark where the two procedures genuinely diverge — useful reference points when weighing which pathway fits a given situation.

Staging

MACI requires two separate operations. The first is an arthroscopic biopsy to harvest chondrocytes; those cells are then sent to an external laboratory, cultured on a porcine collagen membrane over several weeks, and returned for the second operation — the implantation itself. STACi compresses the entire sequence into one anaesthetic: harvest, processing, and implantation happen in the same theatre session.

Cell biology

MACI delivers chondrocytes alone, seeded onto the collagen membrane. STACi combines those same autologous chondrocytes with bone-marrow-derived MSCs isolated intraoperatively. The MSCs contribute a biological signalling layer — producing growth factors that direct and support the chondrocytes as they work to regenerate cartilage. MACI does not include this MSC component.

Scaffold geometry

MACI's flat membrane is well suited to surface-spread repair across a lesion. STACi's three-dimensional collagen scaffold supports cell ingrowth in depth as well as across the surface, more closely replicating the architecture of natural cartilage — an advantage that grows progressively relevant as defect depth increases. This structural difference also makes the periosteal flap required in first-generation ACI unnecessary; the 3D scaffold takes on that containing role without a secondary harvest site.

Access and funding

MACI is approved under NICE technology appraisal TA477 and is NHS-commissioned at designated tertiary centres, meaning eligible patients can access it without direct cost. STACi is not NHS-commissioned; it is available in the UK on a private basis only, with a guide price of approximately £28,000. That is a practical rather than a clinical distinction — suitability for one procedure over the other turns on the lesion and patient profile, not on the funding route.

Evidence, limitations, and realistic expectations

The 85–90% patient satisfaction figure commonly cited for cartilage repair procedures derives from ACI and MACI trial data — including multi-year follow-up studies and the SUMMIT randomised trial. That figure is applied to STACi by clinical analogy: the biological rationale is sound and the procedure has documented case support, but no randomised trial or registry study has yet compared STACi outcomes directly against MACI at meaningful follow-up durations.

That gap is worth naming plainly. How durably the MSC-chondrocyte combination performs on a 3D collagen scaffold at five or ten years is still being established through registry follow-up; robust long-term data are not yet available. Revision candidacy — using STACi after a prior cartilage repair has failed — is mentioned as a potential indication, but outcome data for that specific scenario have not been published.

This is not unusual for a next-generation technique. Standard ACI has decades of evidence behind it; STACi has years. The biological case for combining chondrocytes with MSCs on a three-dimensional scaffold is well-supported in principle and in early clinical experience, and the recovery pathway mirrors that of established cell-based repair — but the full evidence arc is still building.

For anyone at decision stage, the most useful question to put to the operating surgeon is what their own case series shows at five-year follow-up. Registry data are still maturing, so individual surgical experience currently forms the most concrete basis for realistic outcome expectation.

Finding a STACi specialist in the UK

Finding the right surgeon matters at least as much as identifying the right procedure — STACi sits at a specialised intersection of cell biology and knee reconstruction, and not every orthopaedic surgeon offering cartilage repair will have trained in the single-stage technique specifically.

The relevant expertise filter is experience with cell-based cartilage repair, not general orthopaedic surgery. A surgeon familiar with both MACI and single-stage cell-based implantation is the profile to seek — someone who can genuinely weigh the staging and scaffold differences for a given lesion rather than defaulting to whichever pathway they perform most often.

Search MSK is a UK specialist directory listing surgeons who offer advanced cartilage repair, including single-stage cell-based techniques; filtering by region and specialty narrows the results to clinicians positioned to assess STACi as a practical option.

Given the evidence gap relative to MACI and the private-only access, the most productive first appointment will focus specifically on whether the lesion's depth, geometry, and surrounding joint health suit what the 3D scaffold and MSC combination are designed to address — the central question this article has built towards.