Why MACI applies to the patellofemoral joint

Anterior knee pain — the ache behind the kneecap when climbing stairs, the grinding or catching sensation during sport — often points to damage in the patellofemoral joint: the articulation between the patella (kneecap) and the trochlear groove of the femur. Patients researching cartilage repair for this area quickly encounter a distinction that matters: not every technique is approved for this compartment.

MACI (matrix-induced autologous chondrocyte implantation) is one of the few cell-based treatments with both FDA approval and NICE endorsement explicitly covering the patella and trochlea, typically for defects in the 2–10 cm² range. That regulatory standing reflects accumulated evidence, including long-term follow-up data published as recently as 2024 (Ebert et al., PubMed 39101611) showing durable outcomes at ten years or more.

The technique's applicability here is not accidental. Earlier generations of autologous chondrocyte implantation relied on a periosteal patch for fixation — a method poorly suited to the curved, high-load patellofemoral surfaces. MACI uses a Type I/III collagen membrane onto which the patient's own chondrocytes are seeded, a design that makes implantation at these anatomically complex sites more practical.

The process unfolds in two stages: an initial arthroscopic biopsy to harvest cartilage cells, followed by four to six weeks of laboratory culture, then a second procedure to implant the cell-loaded membrane directly into the defect. Understanding this framework helps set realistic expectations before exploring who is a suitable candidate.

Who is a suitable candidate

Your consultant will consider several factors before recommending MACI — and understanding them in advance helps you arrive at that conversation with realistic expectations.

Age and BMI

The FDA-approved window is 18 to 55 years. Safety and effectiveness have not been established outside this range, so patients above 55 are generally guided towards alternative approaches. Body weight matters too: a BMI below 35 is required, because higher body mass places greater mechanical load through the patellofemoral joint, including on a developing graft.

Defect characteristics

MACI targets full-thickness cartilage loss — ICRS grade III or IV — where damage extends down to the subchondral bone surface. The defect typically needs to be larger than 2 cm²; smaller focal lesions may be more appropriately managed with single-stage techniques such as OATS or AMIC. Cavitary bone loss up to approximately 8 mm depth is acceptable. Where there is significant subchondral involvement — intralesional osteophytes or substantial cysts — the preferred option shifts to osteochondral reconstruction (OAT or OCA), which addresses the bone and cartilage layers together.

Contraindications

Inflammatory arthritis, multi-compartment osteoarthritis, meniscal insufficiency, and unresolved ligamentous instability all rule MACI out. Uncorrected patellar malalignment is a particularly important consideration in patellofemoral presentations: the abnormal load it creates would stress the graft before it can integrate.

Correctable malalignment, however, does not exclude MACI — it means that realignment surgery needs to be planned alongside the implantation. The concomitant procedures routinely used to protect the graft in exactly these situations are covered in the next section.

Failing any single criterion does not close off all restorative options; several alternative pathways exist, and a consultant assessment is the appropriate place to identify which fits.

The added complexity of patellofemoral mechanics

The patellofemoral joint operates under substantial mechanical demand. Stair climbing, squatting, and rising from a chair can generate compressive forces across the kneecap several times body weight — a loading profile considerably higher than the tibiofemoral compartment experiences, and one that makes the local mechanical environment a critical factor in graft survival.

Patellar tracking sits at the heart of this. The kneecap acts as a pulley, gliding centrally through the trochlear groove as the knee bends and straightens under load. Maltracking — where the patella rides laterally rather than centrally — is common in precisely the patients who develop patellofemoral cartilage damage, and when it goes uncorrected, the new graft absorbs disproportionate stress along its edge before it can fully integrate.

Cartilage implantation alone cannot resolve this mechanical mismatch. Where imaging confirms correctable malalignment, surgeons typically plan concurrent procedures alongside MACI — tibial tubercle osteotomy (TTO) to reposition the bony attachment of the patellar tendon, or MPFL reconstruction to restore medial soft-tissue restraint. The intent is to create a loading environment in which the repair tissue can mature; the additional surgical steps represent thorough planning rather than added risk.

Pre-operative assessment is correspondingly detailed: patellar alignment on plain X-ray, MRI, and where indicated CT, supplemented by evaluation of soft-tissue restraints. Static imaging cannot fully characterise how the patella actually tracks under load, which is why dynamic biomechanical assessment — of the kind MSK Doctors consultants obtain through the UKCA-registered MAI Motion® markerless motion capture system — can add clinically meaningful data when planning the surgical approach.

One nuance from the long-term patellofemoral evidence (Ebert et al., 2024) is worth noting: patella-specific repairs may yield somewhat better quadriceps and hamstring strength recovery than trochlear-site repairs, though overall clinical outcome scores between the two sub-sites remain broadly equivalent at ten or more years of follow-up.

What the 10-year outcome data shows

Ten or more years of follow-up data now exist specifically for MACI in the patellofemoral joint, and the headline figure is striking. Ebert et al. (2024, PubMed 39101611) found that 90.2% of patients reported satisfaction with pain relief at the ten-year mark, and 85.4% were satisfied with their ability to participate in sport — not merely an improvement on baseline, but high-bar satisfaction scores that go beyond clinical scales.

The MRI picture is equally encouraging. Radiological assessment showed stable defect fill and good graft integrity from year two through to year ten, confirming that the benefit seen in early follow-up is not simply a pain-relief response but reflects durable tissue integration. Graft maturation scores remained stable across that eight-year window, which matters because it tells us the repair tissue is not silently deteriorating while patients feel well.

The numbers your consultant should share with you

Honest pre-operative counselling also means sharing the failure data. In the same long-term cohort, all-cause reoperation was approximately 9%, and conversion to total knee arthroplasty (TKA) occurred in 7.4–10.8% of cases over ten years. These are not alarm figures — the majority of patients retained a functioning graft — but they are the realistic background rates against which the decision to proceed should be made.

The arc of benefit appears consistent across timescales. A 2024 systematic review by Retzky et al. found clinically significant improvements in patient-reported outcome measures at short-term follow-up, establishing that gains accumulate early and are sustained through the long term rather than appearing only years later.

As noted when discussing candidacy, published data are confined to the 18–55 age group; outcomes outside that range remain unstudied. The evidence base within those parameters is now substantial enough to inform a well-grounded conversation about what MACI can and cannot reliably deliver for suitable patellofemoral candidates.

How MACI compares with other repair options for patellofemoral defects

Deciding which repair technique is right depends less on brand familiarity and more on two concrete variables: how large the defect is, and how deeply it has disrupted the subchondral bone beneath.

For defects smaller than roughly 2 cm² with intact subchondral bone, a ChondroFiller injection — an ultrasound-guided outpatient injectable collagen scaffold — offers a minimally invasive route that recruits the patient's own progenitor cells to support repair without the need for theatre or cell culture. This sits in a distinct pathway from MACI: appropriate for lower-grade or smaller focal lesions where a single outpatient appointment rather than a two-stage surgical programme is proportionate.

Once a defect exceeds 2 cm² and is full-thickness, the decision shifts. Microfracture — drilling small channels into the subchondral bone to stimulate marrow-derived repair — was historically the first-line response for cartilage damage of any size. Its limitation is now well-documented: the fibrocartilage it produces is mechanically inferior to hyaline cartilage and tends to break down within two to three years. It also damages the subchondral bone plate in a way that can compromise the results of any subsequent repair procedure. For larger patellofemoral defects, it is no longer considered a first-line modern choice. The SUMMIT trial confirmed this hierarchy directly, demonstrating that MACI produced superior pain and function scores over microfracture for lesions of 3 cm² or more at both two- and five-year follow-up.

OATS (osteochondral autograft transfer) addresses the 1–2 cm² range using a bone-and-cartilage plug harvested from a lower-load area of the patient's own knee. For smaller focal defects this is a proven single-stage option, but donor-site morbidity is a genuine limitation, and the technique does not scale practically to the larger patellofemoral lesions for which MACI is indicated.

Where subchondral bone involvement is substantial — intralesional osteophytes or cysts extending beyond 8 mm depth — cell-based therapies including MACI become less appropriate. These cases are typically redirected to osteochondral allograft (OCA), which replaces both the cartilage and the underlying bone using donor tissue and is better suited to defects where the bone architecture itself needs reconstruction.

The two-stage process and what recovery involves

Committing to MACI means accepting a structured programme that runs from biopsy to functional recovery over the better part of a year — and understanding that sequence in advance is part of preparing for it.

Stage 1 is a day-case arthroscopic biopsy: a small cartilage sample is harvested and sent to an FDA-licensed laboratory, where chondrocytes are expanded and seeded onto the collagen membrane over approximately four to six weeks. Stage 2 — open implantation of the cell-seeded construct — is the more substantial procedure and typically requires a short hospital admission rather than same-day discharge.

The rehabilitation programme

Recovery unfolds through a physiotherapy-guided loading programme lasting nine to twelve months. Early weight-bearing is restricted while the graft integrates, and progression is governed by graft response rather than any fixed calendar. The long-term MRI data confirm that graft maturation continues well beyond the initial healing phase; abbreviating or skipping rehabilitation stages risks compromising a repair before it has consolidated.

Return-to-sport is genuinely individualised and no universal consensus figure exists. A patient with an uncomplicated trochlear repair and no concomitant procedure might reasonably target low-impact activity somewhere in the six-to-nine-month window. Someone who has also undergone a tibial tubercle osteotomy to correct patellar maltracking will typically need additional protected loading time before returning to sport-level demand — the corrective bone work must heal in parallel with the cartilage repair. The questions worth raising at any orthopaedic consultation are: what milestones will guide progression, what sport-specific loading is the rehabilitation targeting, and how will graft maturation be confirmed on follow-up imaging.

Patients wishing to explore whether MACI is appropriate for their situation can see an MSK Doctors consultant at Sleaford or Grantham without a GP referral, through mskdoctors.com.