When MACI applies at the kneecap and trochlea

Damage to the cartilage on the kneecap (patella) or the groove it runs in (the trochlea) does not automatically make someone a candidate for MACI — but for a specific subset of patients it is precisely the right fit.

MACI is designed for focal, full-thickness cartilage loss: damage that has eaten through the entire cartilage layer down to the underlying bone, classified as ICRS grade III or IV. Widespread surface wear across multiple areas of the joint falls outside this remit. The procedure targets a defined lesion with healthy cartilage at its edges, not generalised deterioration.

Defect size acts as a practical filter. Lesions in roughly the 2–10 cm² range sit within MACI's working window. Smaller defects — particularly under 2 cm² — may be managed with single-stage options such as osteochondral autograft. Very large or complex lesions can exceed what the flat collagen membrane used in MACI can address, in which case osteochondral allograft reconstruction becomes the more appropriate route.

The single most important structural criterion is the condition of the subchondral bone — the bone plate immediately beneath the cartilage. Where MRI shows significant cysts or bony involvement, MACI is not suitable; the decision shifts instead to osteochondral procedures that replace both cartilage and bone together. Patellar dislocation is a well-recognised cause of focal patellofemoral cartilage injury in younger patients, and where the subchondral bone underneath remains largely intact, MACI can be a realistic option.

Beyond the structural picture, age, body weight, activity goals, and the history of any prior cartilage treatments all contribute to the final candidacy assessment, which requires consultant review and detailed MRI evaluation.

Why the patellofemoral joint presents extra challenges

The kneecap does not sit still. With every bend and straighten, it glides up and down the trochlear groove, and the forces pressing down on its cartilage shift direction and magnitude constantly — rising sharply during stair-climbing, squatting, or landing from a jump. This is quite different from the femoral condyle, where loading patterns are relatively consistent during walking. For a MACI repair, those variable compressive and shear forces mean the implant faces a more demanding mechanical environment from the moment rehabilitation begins.

The geometry compounds the difficulty. The patella and trochlea are curved surfaces, not flat ones, and conforming a collagen scaffold precisely to that curvature — and keeping it fixed while the cells integrate — requires a higher degree of surgical precision than condyle repair demands.

Malalignment sits at the centre of patellofemoral MACI risk. If the kneecap tracks too far to one side or tilts abnormally, concentrated stress falls unevenly across the repair site. Studies in the cartilage literature are consistent on this point: unaddressed tracking problems are a principal cause of implant failure, which is why alignment correction is a prerequisite, not an afterthought.

The evidence base for patellofemoral MACI, while supportive, is narrower than for condyle repair. Most large outcome datasets — including the SUMMIT trial data — are condyle-dominant, and patellofemoral subgroup analyses tend to be smaller and less statistically powered. Patellar and trochlear outcomes are rarely separated in published studies, which limits how precisely site-specific expectations can be set at the time of consultation.

Alignment and tracking: what must be sorted first

Restoring cartilage on a kneecap that is not tracking correctly is much like redecorating a wall without first fixing the damp — the repair will not hold because the underlying problem remains. For many patients with patellofemoral cartilage damage, correcting that mechanical environment is not an optional extra; it is a prerequisite for MACI to have a reasonable chance of lasting.

Malalignment occurs when the kneecap tracks too far to the outer side, tilts abnormally, or instability from a prior dislocation allows it to shift under load. Each scenario places concentrated, asymmetric stress on exactly the area that MACI is trying to rebuild. Left uncorrected, the scaffold and the developing cartilage within it are exposed to the same aberrant forces that caused the original damage — a principal reason for implant failure in the patellofemoral compartment.

Two procedures are most commonly used to restore correct mechanics. Anteromedialization (AMZ) osteotomy repositions the bony attachment of the patellar tendon on the tibia, shifting the line of pull inward and upward to offload the damaged area and reduce lateral tracking pressure. Where patellar dislocation is the primary aetiology, reconstruction of the medial patellofemoral ligament (MPFL) re-establishes the soft-tissue restraint that prevents the kneecap from sliding outward.

In practice, patellofemoral MACI is frequently performed as a combined surgical episode alongside one or both of these procedures. When instability is the dominant concern and overall tracking is broadly normal, MPFL reconstruction alone may suffice; more pronounced lateral maltracking typically calls for AMZ osteotomy, sometimes with MPFL work in addition. Staging — correcting alignment first and returning for MACI later — is reserved for cases where the degree of malalignment or other patient factors make a single combined episode inadvisable.

How the MACI procedure works at the patellofemoral joint

The procedure unfolds in two distinct stages, and understanding that timeline is as important as understanding the surgery itself.

Stage one: the biopsy

A short arthroscopic procedure — carried out under anaesthetic — retrieves a small piece of healthy cartilage, typically from a low-load area of the knee where removal causes minimal functional disruption. The tissue is sent to a specialist laboratory, where the cartilage cells (chondrocytes) are isolated, then expanded over several weeks until there are enough to populate a repair site. This wait — commonly four to six weeks — is built into the process rather than being a delay; it is the time the cells need to multiply to clinically useful numbers.

Stage two: implantation

Once the cells are ready, they are seeded onto a bilaminar porcine collagen membrane — a scaffold that holds them in place while new cartilage forms and integrates with the surrounding tissue. At the patella or trochlea, the damaged cartilage surface is carefully prepared, the scaffold is cut to match the exact shape of the defect, and it is secured using fibrin glue, fine sutures, or a combination of both.

The flat scaffold design works well for surface-level patellofemoral defects, but it does not restore depth lost through significant subchondral bone damage — which is precisely why intact bone is a prerequisite for MACI candidacy, as discussed earlier.

A note on next-generation approaches

Emerging single-stage variants — sometimes called STACi — aim to compress the two stages into one by implanting a three-dimensional scaffold that carries cells at the time of initial surgery. Evidence for this approach in the patellofemoral joint specifically is still building, and it is best described as an evolving option rather than an established alternative for this compartment.

What the evidence shows about MACI outcomes here

Across the available evidence, the clearest single signal comes from the SUMMIT randomised controlled trial, which found that MACI produced significantly better KOOS pain and function scores than microfracture at both 2 and 5 years in patients with defects of 3 cm² or larger. For a technique sometimes perceived as complex and resource-intensive, that superiority — sustained across five years of follow-up — is a meaningful clinical endorsement for larger lesions.

Long-term data reinforce the picture. Studies with minimum 10-year follow-up, including Minas et al. (Clin Orthop Relat Res, 2014), show durable clinical improvement in well-selected patients, and Ebert et al.'s 5-year study added MRI confirmation of sustained cartilage fill — not just symptom scores. Behrens et al. reported comparable durability at 5 years in a MACI/MACT cohort. This consistency across methodologies and follow-up lengths is one of the more reassuring features of the evidence base.

Microfracture, by contrast, produces fibrocartilage rather than hyaline-like repair tissue, and that substitute tends to break down within two to three years — often while damaging the subchondral bone plate that any future repair would depend on. The clinical trajectory diverges over time: MACI outcomes hold or improve in appropriately selected patients, while microfracture outcomes typically decline, which explains its falling use as a first-line choice for larger or symptomatic defects.

Patellofemoral-specific subgroup analyses are, as touched on earlier, thinner than condyle data — the direction of effect is consistent, but site-specific figures should be treated as indicative rather than definitive. Across all sites and datasets, Basad et al. identified patient selection quality as the factor that most consistently separates good outcomes from poor ones: correct alignment, appropriate defect size, and intact subchondral bone are not supplementary criteria — they are the foundation the repair is built on.

Starting the assessment at MSK Doctors

Three clinical questions sit at the centre of any patellofemoral MACI assessment: how deep is the damage, is the subchondral bone intact, and is the joint tracking correctly? Each needs a considered answer before candidacy can be confirmed.

A detailed MRI review is the starting point — characterising defect dimensions, cartilage quality, and subchondral bone integrity. Where alignment and patellar tracking are in question, objective biomechanical data can sharpen the picture; the MSK Doctors team uses MAI Motion® markerless motion capture at this stage to add functional context to the imaging findings.

That workup leads into a consultant discussion covering the full range of options — MACI, alternative restoration techniques, or alignment correction alone where that is more appropriate — rather than a predetermined recommendation.

For patients outside London, the primary assessment centres are Sleaford, Lincolnshire (which houses an Open MRI scanner) and Grantham. London-based patients can access equivalent specialist assessment through the London Cartilage Clinic. In both settings, patients can book directly without a GP referral and without NHS-style waiting times.

The right outcome depends entirely on the right diagnosis first — a consultation at mskdoctors.com is the practical next step.

1. [1] Autologous Chondrocyte Implantation. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150
2. [2] Knee Cartilage Replacement Therapy. https://en.wikipedia.org/?curid=4984243 https://en.wikipedia.org/?curid=4984243