Why defect size determines which cartilage repair fits

Large focal cartilage defects do change your options — and understanding why requires only two pieces of information: how deep the damage goes and how wide an area it covers.

Depth is described using the ICRS grading scale. Grades I and II affect the surface layers; grades III and IV extend more than halfway through the cartilage or reach subchondral bone. MACI is most relevant at grades III–IV, where the full cartilage thickness is compromised and the underlying bone is at risk.

Area — measured in square centimetres — is what determines which repair technique makes biological sense. Articular cartilage carries no blood supply and almost no capacity to heal itself; left untreated, a full-thickness focal defect progressively fails under load, accelerating the path towards joint destruction and, eventually, replacement surgery.

For defects below roughly 2–4 cm², marrow-stimulation techniques such as microfracture and smaller osteochondral autograft transfers are established options. Their limitation is the repair tissue they produce: fibrocartilage, a scar-like material inferior to native hyaline cartilage that tends to break down within two to three years under sustained load. That degradation profile makes microfracture an increasingly poor fit as defect size grows.

Above approximately 4 cm², the biological case shifts firmly in favour of cell-based repair. The 4 cm² figure is a clinical consensus threshold rather than a hard regulatory boundary — the FDA indication for MACI does not set a rigid upper size cut-off — but it marks the point at which marrow stimulation is consistently outperformed in published clinical series.

Who is a suitable MACI candidate

Several factors combine to determine whether MACI is the right procedure — defect characteristics, the broader health of the joint, mechanical loading, and the patient's own capacity for the recovery ahead.

Defect grade and size. MACI targets ICRS grade III–IV lesions — full-thickness damage reaching or exposing subchondral bone — in a focal rather than diffuse pattern. The practical lower threshold sits at around 3–4 cm², where marrow-stimulation techniques consistently underperform; the SUMMIT trial enrolled patients from 3 cm² upwards and demonstrated MACI's superiority across exactly this size range. A practical upper boundary of approximately 4 cm² applies to the standard flat collagen membrane scaffold, beyond which newer three-dimensional scaffold approaches may be more appropriate. One genuine advantage of MACI is its ability to address multiple separate lesions in the same knee during a single implantation — a meaningful consideration when two or three focal areas are present.

Surrounding joint health. Candidacy depends on the cartilage outside the target defect being broadly intact. Diffuse arthritic change across multiple compartments shifts the conversation away from focal repair and towards alignment surgery or, ultimately, joint replacement.

Limb alignment. Varus or valgus malalignment concentrates load directly onto the repaired area and can undermine the graft before it matures. A surgical assessment must evaluate alignment; where correction is needed, a high tibial or distal femoral osteotomy may be planned alongside — or in advance of — the cartilage procedure.

Age and rehabilitation commitment. Adults across a wide age range may qualify; age alone is not a bar. Younger, active patients with post-traumatic or focal degenerative lesions generally derive the clearest documented benefit, but the more important eligibility question is whether the patient can commit to a prolonged structured rehabilitation programme — full return to demanding activity after MACI typically takes well over a year, and outcomes in patients who cannot sustain that commitment are likely to fall short of published series.

What the evidence shows for defects at and above 4 cm²

The strongest direct evidence for MACI in larger knee cartilage defects comes from the SUMMIT randomised controlled trial. Patients with injuries of 3 cm² or greater showed statistically significant improvements in KOOS pain and function scores at both two and five years when treated with MACI rather than microfracture — and the advantage was most pronounced in the larger-defect group. A subsequent meta-analysis confirmed those findings, reinforcing that cell-based repair outperforms marrow stimulation as defect area increases.

One caveat is worth naming plainly: SUMMIT enrolled patients from 3 cm² upwards, not from 4 cm². Evidence for defects specifically above 4 cm² is therefore extrapolated from the broader ≥3 cm² result rather than drawn from a dedicated randomised cohort. That extrapolation is biologically reasonable — the repair mechanism does not suddenly fail at 4 cm² — but it does mean the direct data becomes thinner rather than absent.

Longer follow-up studies strengthen the overall picture. Five-year MRI-based analyses demonstrate measurable cartilage fill within implanted defects; minimum ten-year outcome data from cell-based repair series show clinical gains that hold well past the initial healing window, with meaningful proportions of patients remaining free of joint replacement at last review. As a body of evidence, these studies support the view that once the biological repair matures, it is durable.

Where confidence does thin is above approximately 4 cm² — and the constraint there is technical rather than biological. The standard collagen membrane is a flat sheet; fitting it neatly into a large or irregularly shaped defect is much like trying to tile an uneven floor with a single rigid tile — workable for contained areas, increasingly impractical as the surface grows. This is a design limitation of the current scaffold format, not a failure of the cellular repair principle itself.

How MACI compares to other options for larger defects

Four realistic alternatives sit alongside MACI at the larger end of the defect spectrum, each with a distinct set of trade-offs.

Microfracture is effectively ruled out for defects above 4 cm². As covered earlier, it produces fibrocartilage rather than hyaline-like tissue; the additional concern at this scale is that the marrow-stimulation technique also damages the subchondral bone plate — a consequence that can narrow options for any future repair if primary treatment fails.

OATS / mosaicplasty can cover defects up to approximately 4 cm² in a mosaic configuration, but donor-site morbidity limits how much graft can safely be harvested, and fitting multiple plugs into a large or irregularly shaped lesion becomes technically impractical beyond that area.

Osteochondral allograft (OCA) is a legitimate single-stage option for very large or post-traumatic defects — particularly where subchondral bone is involved, which is the territory where the standard MACI membrane reaches its size limit. The trade-off is dependence on donor tissue availability and different biological considerations around graft incorporation.

AMIC — matrix-augmented microfracture — improves on standard marrow stimulation by adding a scaffold layer, making it a reasonable single-stage choice for moderate defects. For large lesions, however, its biological ceiling is lower than that of cell-based repair.

STACi (next-generation single-stage ACI) is the most direct emerging comparator: it applies the same cell-based principle as MACI but delivers chondrocytes within a three-dimensional scaffold better suited to large and geometrically complex defects. The evidence base is still developing, and STACi should be understood as a next-generation option rather than an established standard.

What the two-stage MACI procedure and recovery involve

MACI unfolds in three distinct phases spread across several months — understanding the sequence helps patients plan realistically from the outset.

Stage 1 — biopsy. A small arthroscopic procedure harvests a sliver of healthy cartilage from a non-weight-bearing area of the knee. The operation itself is brief, and most patients are well enough to go home the same day.

Laboratory phase. The harvested tissue travels to a specialist laboratory, where chondrocytes are isolated, expanded over several weeks, and seeded onto a porcine Type I/III collagen membrane — the Chondro-Gide scaffold. This waiting period is built into the biological process; it cannot be shortened.

Stage 2 — implantation. The cell-seeded scaffold is implanted during a second, more involved procedure, typically via an open or mini-open approach. The defect edges are trimmed back to stable cartilage, the membrane is cut to the exact defect shape, and fixed securely in place.

Recovery. Protected weight-bearing continues for approximately six to eight weeks after implantation to allow the scaffold to integrate without disruption. Return to normal daily activity typically occurs over six to twelve months. Full return to sport or high-demand physical activity is expected at around twelve to eighteen months — a timeline driven by how slowly articular cartilage matures, not by any particular fragility in the repair itself.

The two-stage structure and the length of rehabilitation are the most significant practical considerations. Whether a single-stage alternative might achieve comparable results with less procedural burden is a question worth putting directly to a consultant.

Getting assessed for large-defect cartilage repair in the UK

Patients researching cartilage repair in the UK will encounter 'MACI' frequently in the literature, but the branded Vericel product is not commercially available here. UK specialist centres offering equivalent matrix-assisted autologous chondrocyte procedures use ACI or platform technologies such as STACi — the biological principle is identical; only the specific scaffold product differs.

A thorough pre-operative assessment matters regardless of which technique is ultimately chosen. MRI with cartilage-specific sequencing characterises defect size, depth, and subchondral bone involvement — the factors that determine whether a defect sits within the MACI-equivalent candidacy range or beyond it. Where malalignment is suspected, weight-bearing limb alignment imaging adds essential mechanical context. Objective gait and biomechanical assessment can further clarify how load is distributed across the joint, which is relevant both to operative planning and to rehabilitation design.

For patients across Lincolnshire and the wider East Midlands, assessment at Lincolnshire Knee does not require a GP referral. Consultations take place at Sleaford NG34 and Grantham NG31, with AI-assisted MRI cartilage analysis (onMRI™, including T2 mapping) and objective biomechanical evaluation (MAI Motion®) available where clinically indicated.

Further information and self-referral are available at lincolnshireknee.co.uk.

1. [1] Knee cartilage replacement therapy. https://en.wikipedia.org/?curid=4984243 https://en.wikipedia.org/?curid=4984243
2. [2] Autologous chondrocyte implantation. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150