Why grade 4 cartilage damage does not heal on its own

A grade 4 finding on an MRI report or arthroscopy note is not something that responds to rest, physiotherapy, or injections alone — and the reason is structural rather than a matter of severity grading.

Articular cartilage has no blood supply. Because it is avascular, it cannot mount the cellular repair response that bone or soft tissue can after injury. When a cartilage lesion is superficial, the tissue around it may stabilise; when it penetrates all the way through the cartilage layer into the subchondral bone beneath — the definition of grade 4 under both the ICRS and Outerbridge classifications — the body has no reliable mechanism to fill the gap with healthy, load-bearing tissue.

Grade 3 lesions involve deep fissuring or softening that does not yet expose bone; grade 4 crosses that threshold. The distinction matters clinically because it is the point at which the joint surface loses its structural integrity under load.

It is also worth being clear about what grade 4 damage is not. A focal, contained defect on an otherwise preserved femoral condyle or trochlea is a restoration target — different in kind from the diffuse, multi-compartment loss seen in advanced osteoarthritis. The patients this article is written for are those with a discrete lesion on a joint that is otherwise worth preserving.

Left untreated, symptomatic full-thickness defects carry a meaningful risk of progressive joint degeneration, particularly in patients under 50. That biological reality — not surgical preference — is what makes active cartilage restoration the appropriate next step.

Who qualifies for MACI

The typical MACI candidate has a painful, focal, full-thickness cartilage defect on an otherwise well-preserved knee — not widespread joint degeneration — and has either failed conservative management or undergone previous cartilage repair without adequate relief.

Surgeons and the evidence base assess candidacy across several criteria:

• Defect size. MACI is most clearly supported for lesions measuring 3 cm² or larger. The Phase 3 SUMMIT trial enrolled patients with a mean defect size of 4.8 cm², and its findings — together with consensus guidance — indicate that smaller defects below roughly 2–3 cm² are better addressed initially with simpler procedures such as microfracture or osteochondral autograft transfer.
• Defect grade. The lesion should be ICRS or Outerbridge grade 3–4 (deep or full-thickness); grade 4, where damage extends through to the subchondral bone, is the primary indication.
• Symptomatic burden. An incidental MRI finding is not sufficient. The SUMMIT trial required a baseline KOOS pain score of 55 or below, confirming that meaningful functional limitation must be present before proceeding.
• Age. Trial populations have skewed young — mean ages of 33–42 years across published studies. Patients broadly in the 18–50 age range sit within the core evidence base; older patients with large grade 4 defects can be assessed individually but are outside the population where MACI data is strongest.
• Aetiology. Trauma and osteochondritis dissecans (OCD) are the predominant causes treated with MACI. Diffuse degenerative osteoarthritis is an exclusion — the procedure targets contained defects, not global joint disease.
• Joint environment. Containment, ligament integrity, meniscal health, and limb alignment are all evaluated before a final decision. Instability or significant malalignment should generally be addressed at the same time or beforehand; the evidence does not define hard cut-offs for BMI or alignment degrees, and surgeons exercise individual judgement here.

Where a prior marrow-stimulation or cartilage repair procedure has already failed, MACI is also used as a salvage option — a role supported by its eligibility criteria in published trials.

How the MACI procedure works

MACI unfolds in two separate procedures with a laboratory interval between them — understanding that timeline from the outset helps patients plan realistically.

Stage 1 — cartilage cell harvest

The first procedure is a day-case arthroscopy, typically lasting under an hour. A small sample of healthy cartilage — roughly the size of a grain of rice — is taken from a low-load area of the knee joint where the tissue is intact. The cells (chondrocytes) are not genetically altered; they are simply collected and prepared for transport to a specialist laboratory.

Laboratory phase

Over the following three to six weeks, laboratory scientists culture and multiply the harvested chondrocytes, then seed them evenly onto a type I/III collagen membrane. This is the 'matrix' that gives MACI its name. The membrane acts as a scaffold — holding the cells in place and providing a structure into which they can integrate once implanted.

Stage 2 — implantation

The second procedure involves a small open incision (mini-arthrotomy) rather than a fully arthroscopic approach. The prepared defect site is cleaned and shaped, and the cell-seeded membrane is fixed into place, typically under general or spinal anaesthesia.

Current MACI uses this collagen membrane in place of the periosteal patch — a strip of tissue taken from the shinbone — used in first-generation autologous chondrocyte implantation. Registry data from 662 knees confirms the switch matters: graft hypertrophy fell from 10.7% with the periosteal technique to 1.0% with the collagen membrane, and delamination rates dropped from 14.1% to 5.9%. That is a meaningful safety improvement, not a minor refinement.

Because the two stages are separated by weeks of laboratory work, patients need to plan for two anaesthetics and the recovery period following each.

Outcomes: what the evidence actually shows

The strongest clinical signal comes from a single landmark trial: the Phase 3 SUMMIT study showed that patients treated with MACI did meaningfully better than those treated with microfracture — and stayed better. In 144 patients with a mean defect of 4.8 cm², MACI produced statistically and clinically superior KOOS pain and function scores at two years (p=0.001), with advantages extending to activities of daily living, quality of life, and symptoms. Five-year follow-up confirmed the benefit held, not a short-term artefact of early post-operative recovery.

Real-world volume reinforces this. A 2026 registry study of 662 knees with Outerbridge grade III–IV defects measuring ≥4 cm² found that approximately three-quarters of patients treated with third-generation collagen-membrane MACI achieved clinically meaningful improvement at 24 months.

On the question of what happens over years rather than months, an MR arthrography cohort study — 26 patients followed 8–16 years post-operatively — produces a finding worth stating plainly: changes visible on the scan do not mean the knee is getting worse. Radiological graft scores fell significantly over time (mMOCART from 59.2 to 38.6; p=0.00003), yet functional scores on the Cincinnati Knee Rating System barely shifted — from 8.9 to 8.3, a change that did not reach statistical significance (p=0.06). Imaging and clinical experience diverge, and the clinical picture is the one that matters most to patients.

Longer-term durability is supported by the John Insall Award-winning minimum 10-year ACI outcome study (Minas et al.) and several independent 5-year MACI series, collectively suggesting the cell-based approach sustains benefit beyond what shorter trials can demonstrate.

At short follow-up, the evidence does not yet clearly distinguish MACI from newer single-stage techniques such as AMIC or minced cartilage implantation — a matched cohort study found statistically equivalent two-year improvements in VAS pain and KOOS outcomes across all three methods. Long-term revision rates stratified by lesion grade also remain incompletely characterised. Both are areas where comparison data needs to mature, not reflections of the procedure's established track record for large grade 4 defects.

Recovery timeline after MACI

Recovery from MACI is measured in months, not weeks — patients should enter the process expecting a staged rehabilitation pathway of roughly 12 to 18 months before the graft reaches full biological maturity.

The early phase depends on where in the knee the graft was placed. For tibiofemoral MACI — procedures on the femoral condyle or trochlea — the joint is protected from full loading in the first weeks post-operatively. A US expert Delphi consensus, developed among orthopaedic surgeons with direct MACI experience, sets full weight-bearing for these patients at seven to nine weeks. Range of motion progresses in parallel: 90° of knee flexion is the target by week four, with full range expected by weeks seven to nine.

Patellofemoral MACI — procedures at the patella or trochlea — follows a different early loading logic. Immediate weight-bearing is typically permitted, though bracing is used and the loading parameters differ from tibiofemoral implants. The distinction matters practically: applying the wrong protocol risks either unnecessarily prolonging immobility or overloading a healing graft.

Return to sport has no fixed date in the consensus protocol. Readiness is determined by biological progress, functional testing, and physiotherapy assessment — not by a point on the calendar.

Physiotherapy is not optional during this period. Supervised rehabilitation shapes range of motion, muscle strength, and joint stability from the immediate post-operative weeks through to return to activity, and the quality of that input influences graft integration as much as the surgical technique itself.

How MACI fits within the wider cartilage repair pathway

Deciding which cartilage repair technique is most appropriate depends primarily on how large the defect is, whether a two-stage procedure is acceptable, and what — if anything — has been tried before.

Smaller defects: single-stage surgical and non-surgical options

For focal lesions at the smaller end of the spectrum — broadly up to 2 cm² — osteochondral autograft transfer (OATS, or mosaicplasty when multiple plugs are placed) transplants intact hyaline cartilage and underlying bone from a donor site in a single operative session.

Microfracture occupied this space for many years, but its limitations are now well characterised. The fibrocartilage it produces is mechanically inferior to hyaline cartilage; deterioration typically becomes apparent within two to three years, and drilling through the subchondral bone plate can compromise later repair options. Current practice increasingly restricts its use rather than offering it as a routine starting point.

For patients with a smaller focal defect who prefer a non-surgical route, a ChondroFiller injection — delivered under ultrasound guidance as an outpatient procedure — places an acellular collagen scaffold directly into the lesion, recruiting the patient's own progenitor cells. It requires no biopsy, no theatre admission, and no culture interval.

Bridging the gap: AMIC

Autologous matrix-induced chondrogenesis (AMIC) combines microfracture with an overlying collagen scaffold in a single operation — a practical middle ground for patients who want to avoid the two-stage process but have a defect too extensive for microfracture alone. Two-year outcome data suggests results comparable to MACI at that timepoint, though longer-term follow-up data has not yet matured.

Very large or post-traumatic defects

Where defects are exceptionally large or involve bone loss alongside cartilage damage, fresh osteochondral allograft (OCA) replaces both cartilage and subchondral bone in one procedure, without the donor-site constraints of autograft techniques.

Emerging single-stage cell-based variants

Single-stage ACI approaches — sometimes referred to as STACI or next-generation ACI — aim to remove the biopsy interval from the standard MACI pathway. Comparative data against established two-stage MACI currently remains retrospective and limited; this is a developing area rather than a validated alternative.

Putting it together

For the symptomatic, full-thickness grade 4 defect measuring roughly 3–10 cm² on a mechanically sound knee, MACI carries the most robust evidence of any currently available technique — a phase 3 randomised trial plus long-term follow-up — but it asks something of the patient in return: two procedures, a laboratory interval, and a rehabilitation commitment measured in months rather than weeks. Patients who understand and accept that structure, and whose anatomy and health profile make them suitable candidates, have a strong evidence base behind them.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral — book an assessment at lincolnshireknee.co.uk.

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