Who the single-stage candidate looks like

Three practical filters determine which procedure fits: age, activity level, and the nature of the cartilage damage itself.

For most patients, the divide falls roughly at 40. STACI — the single-stage approach — is designed for younger, highly active patients whose biology is best placed to make use of freshly isolated cells implanted in a single operative session. Biological age carries more weight than the number on a birth certificate; patients up to around 45 may be considered case by case, but the principle holds: the younger and more biologically vigorous the joint environment, the stronger the case for a single-stage approach.

Activity level matters too. Patients who place high functional demands on their knee — those who play sport, do physical work, or simply want to return to an active life — tend to find the single-stage model well suited, because it compresses what would otherwise be two separate operations under general anaesthetic into one. Avoiding a second anaesthetic is not a minor convenience; for patients in full-time work or with caring responsibilities, removing a second surgical episode and a second recovery has genuine practical weight.

MACI remains the established cell-based option for patients up to approximately 55 who share the same underlying problem: a focal, full-thickness cartilage defect in the knee — a discrete hole or crater, typically between 2 and 10 cm², with healthy cartilage around it. Neither STACI nor MACI is appropriate where arthritis has spread across the joint; both are specifically designed for contained focal damage, not diffuse cartilage loss.

If you are under 40, active, and have a focal knee cartilage defect, the single-stage pathway is worth exploring. If you are in your forties or fifties with a similar defect pattern, MACI is likely the more established route — though the right answer always depends on a clinical assessment of the specific defect, its location, and your joint health overall.

What STACI does differently in theatre

During a STACI procedure, everything happens within a single operative session. While the surgeon prepares the damaged area of the knee, a laboratory team works simultaneously at a sterile bench in theatre. A small amount of healthy cartilage is harvested from a non-weight-bearing region of the joint, cut into fragments, and enzymatically digested to free the chondrocytes — a process that releases the cells without altering their character. At the same time, bone marrow is drawn from within the surgical field and concentrated to isolate mononuclear cells (MNCs), which include mesenchymal stem cells capable of supporting tissue regeneration. The two populations are then combined and seeded onto a three-dimensional scaffold before being placed into the defect.

The scaffold architecture is central to how the approach differs from MACI. MACI uses a flat Type I/III collagen sheet — effective, but essentially two-dimensional in how it supports cell distribution. The scaffold used in single-stage implantation has a sponge-like, three-dimensional structure, supporting cell growth in depth as well as across the surface. The biological rationale is that this geometry more closely replicates the layered organisation of native articular cartilage.

Prof Paul Lee, who developed this approach at London Cartilage Clinic, describes the process as bringing the laboratory into the operating theatre rather than sending tissue out to an external facility and waiting. The practical result is one general anaesthetic instead of two, and a treatment timeline shortened by the three-to-six weeks that external cell culture would otherwise require.

MACI and the ACI lineage it comes from

Cell-based cartilage repair did not arrive fully formed. First-generation ACI, developed in the early 1990s, delivered culture-expanded chondrocytes as a liquid suspension beneath a periosteal flap — a patch of tissue harvested from the proximal tibia and sutured over the defect in an open or mini-open procedure. The technique worked, but the periosteal flap introduced technical variability and a recognised risk of graft hypertrophy.

MACi resolved both problems. As the third-generation refinement, it pre-seeds the cultured chondrocytes directly onto a Type I/III porcine collagen membrane, which is shaped to the defect and secured with fibrin glue — no flap, no sutures. The result is a more consistent, less technically demanding implantation. MACI is NHS-funded for eligible patients in the UK and received FDA approval in the United States in 2016; its evidence base includes Phase 3 randomised trial data and minimum ten-year follow-up studies (Minas et al., 2014).

The procedure still requires two separate operative episodes: an arthroscopic biopsy, then three to six weeks of external laboratory culture before the implantation operation. That interval — unavoidable under the two-stage model — became the logical target for the next generation. STACI's defining move was to collapse the laboratory phase into theatre, not to abandon the cell-based biology that makes MACI effective.

Defect size, grade, and contraindications

Lesion size and grade shape candidacy as much as any other factor. Both MACI and STACI are designed for focal, full-thickness defects graded ICRS 3 or 4 — the range in which cartilage loss reaches subchondral bone while the surrounding joint remains largely intact, as the opening section established. The operative range is broadly 2–10 cm², though the strongest published evidence sits above the 3 cm² mark.

The SUMMIT Phase 3 randomised trial — the most rigorous head-to-head data available for MACI — showed a clear advantage over microfracture for defects of at least 3 cm², with superior KOOS pain and function scores at both two and five years. Real-world registry data reinforce this picture: a 2015 analysis of 2,690 MACI-treated patients (Foldager et al.) reported a mean defect size of 5.64 cm², with 70% of cases falling between 3 and 10 cm². The femoral condyle and patellofemoral surface are the primary treatment sites for both procedures.

Where STACI adds a practical advantage over the two-stage model is in patients with more than one focal defect. Because everything is managed within a single session, a surgeon can address medial femoral condyle damage and patellofemoral joint damage at the same sitting — something that would otherwise require staged operations under the MACI model. Where a malaligned knee is contributing to load on the defect, alignment correction via osteotomy may be considered alongside either procedure, but that remains a separate decision determined by individual assessment.

What the evidence shows — and where the gaps are

The clearest numbers available for single-stage implantation come from a prospective multicenter study of 40 patients treated with freshly isolated chondrocytes and bone marrow MNCs on a PolyActive scaffold. MRI confirmed successful lesion filling in all 40 patients at three months and in 32 of 32 at 24 months; biopsy in 31 of those patients found hyaline-like repair tissue in 22 — roughly 71%. KOOS and IKDC scores improved progressively, with pain gains apparent from three months and sport and recreation scores continuing to rise through 18 months. Two patients required implant removal following scaffold delamination.

Placing these figures against MACI's trial record requires honesty about the asymmetry. MACI's evidence base rests on Phase 3 randomised controlled trial data, large registry series, and minimum ten-year outcome studies — a depth the single-stage evidence has not yet matched.

No independent randomised controlled trial has directly compared STACi and MACI outcomes. The mechanistic case for single-stage implantation — the 3D scaffold architecture, MSC addition, and single-anaesthetic design — is well-reasoned, but it currently rests on biological plausibility and expert-practice experience rather than controlled comparative data. The published STACi evidence comes predominantly from the technique's developing centre and should be read as expert-practice evidence rather than as an independent systematic review.

Longer follow-up and independent trial data would substantially strengthen the single-stage case.

Getting the right assessment

Choosing between single-stage and two-stage repair is not a decision that imaging alone can settle. A scan confirms defect size, ICRS grade, bone involvement, and any alignment deviation — all necessary inputs — but those findings must be weighed alongside age, biological profile, activity demands, and prior treatment history, including any earlier marrow stimulation that may have altered the subchondral bone.

The practical decision framework is relatively clear: a patient under 40 with a contained focal defect, a preference for limiting operative episodes, and a favourable biological profile sits on the STACi side of the boundary; an older patient for whom MACI's longer evidence record carries more weight, or who does not meet the biological-age threshold, will typically follow the two-stage path. Neither choice is universally superior — that is the honest position the current evidence supports.

Lincolnshire Knee offers consultant-led cartilage assessments without a GP referral or long waiting times. Book an appointment at lincolnshireknee.co.uk.