Does defect size make STACi the stronger option?

For patients with a larger cartilage lesion — say, 4 cm² or more — the instinct is often to assume that greater damage means a more drawn-out treatment pathway. In practice, defect size is not a contraindication to STACi. A documented case involving a 4 cm² lateral condyle defect was managed successfully in a single operative sitting, illustrating that scale alone does not rule the approach out.

STACi was developed in part to extend cell-based cartilage repair to patients previously assessed as unsuitable for conventional ACi or MACi because their defect was too large or geometrically irregular — a gap in provision that the single-stage approach directly addresses. Where two-stage procedures apply a flat periosteal patch (classic ACi) or a flat collagen sheet (MACi) across the repair site, larger and deeper lesions can expose the limitations of those constructs. The three-dimensional scaffold used in STACi supports cell ingrowth in both surface spread and depth, a structural characteristic that grows more clinically relevant as lesion size increases.

What happens in a single-stage STACi procedure

Inside the operating theatre, STACi unfolds as a coordinated sequence rather than a simplified version of what came before. A small quantity of cartilage is taken from a non-weight-bearing area of the joint — a site chosen to avoid any functional compromise — and a laboratory team working alongside the surgical team cuts the harvested fragment into fine pieces, then uses a digestive enzyme to free the chondrocytes without altering their biological character. Simultaneously, bone marrow is aspirated from within the same operative field; the marrow-derived mesenchymal stem cells (MSCs) are isolated, concentrated, washed, and counted under a microscope before being combined with the freshly freed chondrocytes.

The blended cell population is then loaded onto a three-dimensional collagen scaffold and implanted into the prepared defect site — all before the patient leaves the theatre. Taylor and Lee (2019), who described STACi as the next generation of autologous chondrocyte implantation, identified this unbroken chain of harvest, processing, and implantation under a single anaesthetic as the defining departure from conventional two-stage protocols, which require a separate biopsy appointment, weeks of external cell culture, and a second open procedure.

A further practical advantage emerges when more than one defect requires attention. Multiple sites — for instance, a medial condyle lesion alongside a patellofemoral defect — can be treated within the same session. Two-stage protocols would require serial operations for each area, extending the total treatment timeline considerably.

Why a 3D scaffold changes the outcome in larger lesions

The structural case for a three-dimensional repair in larger lesions comes down to geometry. A periosteal patch or flat collagen sheet functions more like a lid than a filler: it spans the surface of a defect but cannot conform to the walls and floor of a deep or irregular void. As lesion depth increases, so does the risk of incomplete fill at the margins — a limitation that grows more pronounced, not less, when the defect is also wide.

By contrast, a sponge-like three-dimensional scaffold allows cells to migrate inward from multiple planes, more closely replicating the layered zonal organisation of native hyaline cartilage than a surface-only construct can. This matters most in defects that are not only large in area but deep or geometrically irregular — characteristics that occur disproportionately in the bigger lesions previously excluded from conventional ACi or MACi.

Adjacent evidence from scaffold-based repair helps contextualise what these structural properties mean clinically. AMIC — a single-stage technique combining microfracture with a collagen scaffold, without ex vivo cell culture — demonstrates durable outcomes in defects larger than 2 cm² at seven years. One study did identify elevated failure rates in large defects beyond five years with a collagen type I scaffold specifically, separating scaffold material from scaffold geometry as an independent risk variable. The practical implication is that material selection carries long-term weight alongside structural design — a distinction relevant to any scaffold-based strategy, STACi included.

Cell-based therapy's case against marrow stimulation at this scale is anchored by the SUMMIT trial, which showed superior KOOS pain and function scores with MACI versus microfracture in defects of 3 cm² or greater at both two and five years — the outcome floor any single-stage cell-based approach must reach.

How STACi and two-stage ACI/MACI differ in clinical practice

Choosing between STACi and conventional two-stage ACI or MACI is, in practical terms, a question of how many times a patient undergoes surgery. The two-stage pathway begins with a biopsy — typically 200–300 mg of cartilage harvested from the superomedial femoral trochlea — followed by weeks of external laboratory culture before a second open procedure under a separate anaesthetic. Each admission carries its own pre-operative preparation, recovery episode, and cumulative risk from anaesthesia. STACi removes that inter-stage interval entirely: there is one admission, one anaesthetic, and one recovery arc.

For eligibility, the distinction is equally clear. Conventional ACi and MACI were developed around defects that a flat periosteal patch or collagen sheet could span. Patients whose lesions were judged too large or too complex were routinely declined. STACi explicitly extends to that group — not as an exception but as part of its stated indication — making defect size less a barrier than a prompt to consider the single-stage route.

Direct head-to-head evidence comparing STACi against two-stage MACI in matched large-defect cohorts does not yet exist. That gap reflects how recently the technique has entered practice rather than a signal against its use; the procedural rationale and mechanistic advantages are clear, but long-term comparative outcome data will take time to accumulate. For now, the available clinical and structural evidence provides the basis for discussion between patient and surgeon — a conversation best had with access to specialist assessment.

Who is a suitable candidate — and who is not

Three practical questions help map whether STACi falls within range for a given patient.

Age. STACi is typically considered for patients under 40; individual assessment may extend suitability to around 45, depending on the overall condition of the joint. The upper boundary is not a fixed cut-off but a threshold beyond which age-related changes to surrounding cartilage and bone begin to affect the likelihood of durable repair — making the final determination one for specialist review rather than a number alone.

The nature of the damage. STACi is designed for focal cartilage lesions — discrete areas of damage within an otherwise functional joint. Widespread arthritis across the knee, where cartilage loss is extensive throughout multiple surfaces rather than concentrated in one site, falls outside scope. The relevant distinction is between a joint that retains enough healthy architecture to support focal repair and one where degeneration has become generalised.

Defect size and complexity. Both large and geometrically irregular lesions are within indication. Patients who were previously assessed as unsuitable for conventional ACi or MACI on grounds of defect size or complexity may find that prior verdict worth revisiting — STACi's expanded scope is a stated part of its design, not a borderline concession. Where damage involves more than one site, multiple areas can be addressed in the same operative session rather than through serial procedures.

The practical upshot: a confirmed focal lesion in a patient under 40 — or approaching that range — warrants a consultation, including those who have already been declined for two-stage techniques.

The evidence base and what remains unknown

The honest evidence position for STACi sits between established principle and maturing outcome record. Adjacent scaffold data — from AMIC studies following patients for up to seven years — shows that scaffold-based single-stage repair can sustain results in defects larger than 2 cm², though the same literature flags increased failure rates in large lesions beyond five years where collagen type I scaffolds were used specifically. That nuance is worth noting when scaffold selection is discussed, rather than as a general argument against single-stage repair.

What the current literature does not yet provide is a published outcome series extending beyond ten years for STACi itself. For a patient in their mid-thirties with a large focal lesion, this matters: durability through mid-life and beyond is the real clinical test, and that data will accumulate over the coming decade as more cases mature. Two-stage techniques were once in precisely the same position — their long-term series built gradually from early case experience rather than arriving fully formed.

For patients with a large focal defect, no realistic alternative of equivalent biological sophistication, and an age profile that rules out early joint replacement, the mechanistic and procedural rationale for STACi is generally considered clinically defensible in the absence of a full decade-long series. Whether that reasoning holds for a specific individual depends on the full picture of joint condition, prior procedures, and personal priorities — a judgement that requires specialist assessment rather than general guidance. Professor Paul Y. F. Lee at the London Cartilage Clinic works with patients at this decision point, including those who may have been declined for conventional cell-based repair before the single-stage option was available.

1. [1] Autologous chondrocyte implantation. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150