The four NHS criteria that open the door to MACI

Four specific tests — not a general impression of suitability — determine whether MACI is available to a patient on the NHS. NICE Technology Appraisal TA477, published in 2017, sets out these criteria, and all four must be met together:

• No previous surgery to repair articular cartilage defects in the affected knee
• Minimal osteoarthritic damage, confirmed using a validated measure
• A cartilage defect measuring more than 2 cm²
• Treatment carried out at a tertiary referral centre

These are not advisory thresholds that a clinician weighs against other factors. NICE's cost-effectiveness modelling places the cost per QALY for ACI/MACI below £20,000 — within the NHS threshold — specifically in patients who satisfy all four conditions. Outside that population, the economics shift unfavourably, which is why the criteria are structural rather than indicative.

Before any surgical option enters the conversation, patients must have already worked through best supportive care without sufficient relief. That pathway typically includes physiotherapy, weight management, intra-articular injections, and analgesia. MACI is a restorative procedure for people those measures have not adequately helped — not an early-stage intervention.

The sections that follow examine each criterion in more detail: what the defect-size threshold means in clinical terms, why prior cartilage surgery changes the calculation so significantly, and what "minimal osteoarthritic damage" looks like on assessment. Together they build the fuller picture of who a specialist is likely to consider suitable.

Defect size and grade: the clinical thresholds that matter

Not all cartilage defects qualify for MACI — grade and size together determine which surgical option belongs at which stage of the decision tree.

The target population is full-thickness cartilage loss: grade III or IV on the Modified Outerbridge Scale, where the cartilage is either severely thinned down to the subchondral bone or absent altogether in a localised zone. Shallower lesions — grades I and II — do not reach the threshold for cell-based repair.

Size thresholds follow a clear sequence. Two square centimetres is roughly the area of a thumbnail; below that boundary, or within the 2–4 cm² range, microfracture or mosaicplasty are the recognised first surgical steps. Both remain in use, though microfracture is now in declining favour: it produces fibrocartilage rather than true hyaline cartilage, which tends to break down within two to three years, and the procedure risks damaging the subchondral bone plate in ways that can complicate any subsequent repair. It is not a contemporary first-line choice.

Once a defect reaches or exceeds 2 cm² and microfracture has either failed or is unlikely to succeed, MACI becomes the surgery of choice. A published UK consensus of 104 specialist surgeons concluded that microfracture is less effective above this threshold. Evidence from the SUMMIT trial strengthens that position specifically for defects ≥3 cm², where MACI produced significantly better KOOS pain and function scores at two and five years. MACI's licensed indication covered the 3–20 cm² range (grades III–IV); traditional ACI, supplied under MHRA hospital exemption by the OsCell laboratory, extends coverage to patients with single or multiple full-thickness defects where MACI is not directly available.

Pre-operative MRI provides the planning estimate, but it consistently underestimates the true defect area once debrided to clean margins at Stage 1 arthroscopy — so the confirmed size in theatre is likely to be larger than any scan suggests. That intraoperative measurement also feeds directly into location-based risk assessment: patellofemoral lesions carry a meaningfully higher risk of poorer outcomes than condyle defects of equivalent size, which is one reason the clinical team weighs location alongside area rather than treating the scan figure as a standalone qualifier.

The two-stage pathway: what the procedure actually involves

MACI commits a patient to two separate procedures, separated by weeks of laboratory work — and understanding that structure upfront shapes how realistically people can plan around treatment.

Stage 1 is an arthroscopic procedure in which a small sample of healthy cartilage cells is harvested from a non-load-bearing zone of the knee, typically within the medial intercondylar notch. The tissue is collected through a minimally invasive approach and sent to a specialist laboratory. In the UK, traditional ACI is prepared by the OsCell John Charnley Laboratory under an MHRA hospital exemption. There, the harvested chondrocytes are cultured and expanded over approximately two to six weeks, then seeded onto a Type I/III collagen membrane — the scaffold that will carry them back into the joint. That interval between stages is driven by the cell preparation biology, not administrative scheduling.

Stage 2 is the implantation. The cell-seeded membrane is fixed into the prepared defect site, with the surgical approach — open arthrotomy or arthroscopic — determined by defect location and geometry. The defect bed is first debrided to stable, healthy margins before the graft is secured in place.

One point worth clarifying for patients who have come across 'single-stage' options: no single-stage, cell-based procedure is recommended under NICE TA477. AMIC — autologous matrix-induced chondrogenesis — is single-stage but works by augmenting microfracture with a scaffold rather than using cultured cells, placing it on an entirely separate clinical pathway. The outcomes evidence for the MACI approach, including the SUMMIT trial results set out in the previous section, relates specifically to this two-stage cell-based process.

Patient factors: age, BMI, multiple defects, and mechanical problems

Meeting the four NICE criteria confirms eligibility in principle; what follows is the nuance layer — the patient-level variables a specialist weighs to judge likely success and surgical risk.

Age rarely acts as a hard cut-off. The typical patient treated with MACI in published series is around 37 years old, with a recorded range of 15 to 63. Age informs prognosis and urgency: in younger patients, early repair matters because the alternative — progressive deterioration toward joint replacement — carries a particularly long cost over a lifetime ahead. Older patients are not automatically excluded, but age enters the clinical conversation alongside other factors.

BMI is weighed similarly. The average in long-term MACI outcome data sits at approximately 26; higher BMI is a recognised decision factor that may affect both surgical access and the mechanical environment around the graft. Weight management forms part of the best supportive care pathway that already precedes any surgical consideration, so patients are often advised to address this before being listed.

Multiple defects are present in roughly 34% of ACI patients. Multifocality does not automatically disqualify a candidate, but it adds complexity to surgical planning and tempers the prognosis discussion.

Mechanical problems — malalignment, instability, or meniscal deficiency — are not automatic exclusions either. A biological repair will only hold if the mechanical environment supports it; where those problems can be corrected surgically as part of the same treatment plan, they do not close the door on MACI. The ORKA (Oswestry Risk of Knee Arthroplasty) score gives clinicians a structured way to stratify this risk: even patients scoring 3 or 4 on the scale may still be considered, provided the correctable concomitant pathologies are addressed. Patients who have been told they have 'other knee problems too' should not read that as an automatic disqualification — the relevant question is whether those problems are surgically manageable alongside the cartilage repair itself.

Firm rules out: when MACI is not appropriate

Some patients arrive at assessment already suspecting MACI may not be right for them. Understanding the firm exclusions — and why they exist — is more useful than a vague sense of uncertainty.

Diffuse or severe osteoarthritis is an absolute contraindication. The distinction matters mechanically: MACI depends on repairing a focal area of loss within a joint whose surrounding cartilage, bone, and biology remain fundamentally intact. Widespread OA changes the whole joint environment — altered load distribution, an inflammatory milieu, progressive bone remodelling — in ways that prevent a localised graft from succeeding. The surgery targets a circumscribed defect, not a globally degraded joint; where that degradation is diffuse, the biological conditions for a durable repair are simply absent.

Prior cartilage repair surgery significantly undermines candidacy. NICE TA477 makes this one of its four explicit criteria because the biological environment in a knee that has already undergone marrow stimulation or an earlier repair procedure is measurably different. Cell viability, residual tissue quality, and the structural integrity of the defect bed are all affected by prior intervention. This is not a bureaucratic rule — it reflects the evidence base in which ACI's favourable cost-effectiveness was established, a population that excludes those with previous repair surgery. Patients who have had microfracture or an earlier ACI should raise that history at the outset of any cartilage assessment.

Mechanical problems that cannot be surgically corrected close the door differently from those discussed in the preceding section. Correctable malalignment, meniscal deficiency, or instability can be addressed alongside MACI; the exclusion applies when those problems are not fixable. A graft placed into a mechanically hostile environment will not hold, regardless of how carefully the cartilage repair itself is performed.

Where MACI is not appropriate, the clinical conversation does not end there. For larger or post-traumatic defects, fresh osteochondral allograft (OCA) may offer a more suitable reconstruction. For patients with early diffuse joint changes, alignment correction through osteotomy can redistribute load and preserve the joint longer. For end-stage OA, an honest discussion about replacement is the right path. A specialist assessment exists precisely to map which of those options fits the individual joint — a ruling-out here is a redirection, not a dead end.

Getting a MACI candidacy assessment in London

Reaching this point in the decision tree typically begins with a GP or sports-medicine referral, or through direct self-referral to a specialist cartilage service. NICE TA477 requires treatment at a tertiary referral centre, so the assessing clinician will be working at that level of provision — in practice, a dedicated cartilage or joint-preservation unit rather than a general orthopaedic outpatient clinic.

The assessment covers considerably more than a binary eligibility check. A clinician will take a full clinical history, apply functional scoring, arrange weight-bearing X-rays, and review any existing MRI. That MRI provides a working estimate of defect size and is the key planning tool, but MRI-based sizing tends to underestimate the post-debridement area; the definitive measurement is confirmed intraoperatively at the biopsy stage. All four NICE criteria are weighed in conjunction with the patient-level factors discussed earlier — age, BMI, ORKA score, and the mechanical state of the joint. Expect the consultation to map the full treatment hierarchy for this particular knee, not MACI in isolation: where conservative options still apply, where alternative surgical approaches belong, and under what conditions MACI becomes the most appropriate step.

For patients seeking this level of specialist review in London, London Cartilage Clinic on Harley Street offers cartilage assessment and surgical planning led by Professor Paul Y. F. Lee; appointments can be booked at londoncartilage.com.

1. [1] Third-Generation ACI vs Microfracture for Focal Chondral Defects: Systematic Review of RCTs. (2022). https://doi.org/10.1016/j.arthro.2022.02.011 https://doi.org/10.1016/j.arthro.2022.02.011
2. [2] Comparison of Knee Articular Cartilage Defect Size Between Measurements on Preoperative MRI Versus During Arthrotomy. (2023). https://doi.org/10.1177/23259671231193380 https://doi.org/10.1177/23259671231193380
3. [3] Differences in Clinical and Functional Outcomes Between OCA and ACI for Focal Articular Cartilage Defects. (2022). https://doi.org/10.1177/23259671211058425 https://doi.org/10.1177/23259671211058425
4. [4] Minimum 10-Year Outcomes of Matrix-Induced Autologous Chondrocyte Implantation in the Knee. (2024). https://doi.org/10.1177/03635465231205309 https://doi.org/10.1177/03635465231205309