Why kneecap cartilage defects are different to treat

Pain on stairs, when getting up from a chair, or after sitting through a long meeting — these are the signals that the kneecap joint is under pressure. The patellofemoral joint, where the kneecap (patella) glides in the groove at the front of the thigh bone, transmits forces several times body weight during loaded everyday movements. When the cartilage lining that surface wears through completely, the result is a full-thickness defect — ICRS or Outerbridge Grade 3 to 4 — that cannot heal on its own.

Repairing these defects is technically more demanding than addressing damage elsewhere in the knee. The curved patellar surface, the high compressive loads it carries, and the way the kneecap tracks through its groove all influence whether a repair can take hold and remain durable. A graft placed on a malaligned or overloaded kneecap faces mechanical stress from the outset — a factor that shapes both patient selection and surgical planning.

MACI — matrix-induced autologous chondrocyte implantation — is a cartilage restoration procedure, not a pain-relief injection or arthroscopic debridement. It is typically considered for focal, full-thickness lesions larger than 2 cm² in patients who have not improved with conservative management or injection-based pathways.

Who is a suitable candidate for patellar MACI

Candidacy for patellar MACI centres on a combination of lesion characteristics, patient profile, and joint mechanics — all assessed together rather than in isolation.

The strongest candidates are typically under 50, physically active, and living with a symptomatic, full-thickness (Grade 3 or 4) chondral defect larger than 2 cm² that has not responded to conservative management such as physiotherapy, activity modification, or injection-based support. Lesion size matters beyond that threshold: the SUMMIT Phase 3 RCT found MACI significantly superior to microfracture (p=0.001) for KOOS pain and function, with that advantage most pronounced for defects measuring 3 cm² or more. For smaller lesions, single-stage procedures such as OATS or AMIC may offer equally valid outcomes, and a consultant assessment will weigh those options directly.

Alignment is an essential part of the candidacy evaluation and is covered in detail later in this guide. In brief, any evidence of patellar malalignment must be identified and factored into planning before a recommendation is made, as unaddressed mechanical problems significantly increase graft failure risk.

Two factors shift the preferred approach away from cell-based repair altogether. Where the defect extends into subchondral bone — with cysts or intralesional osteophytes beyond simple marrow oedema — osteochondral reconstruction such as OAT or OCA is typically preferred over MACI. Bipolar lesions, meaning cartilage damage across both the patellar and trochlear surfaces, remain treatable but carry a higher failure risk and usually require combined-procedure planning from the outset.

What the two-stage MACI procedure involves

MACI involves two separate operations, and understanding why both are necessary makes the overall timeline easier to plan around.

The first stage is arthroscopic — a keyhole procedure, usually performed as a day case under general or regional anaesthetic. The surgeon takes a small biopsy of healthy cartilage from a low-load region of the knee, typically a non-weight-bearing border. This takes around 30 to 45 minutes. The biopsy is then sent to a specialist laboratory, where the chondrocytes — the cells that produce cartilage matrix — are isolated and expanded over approximately four to six weeks. Once multiplied to a sufficient number, those cells are seeded onto a Type I/III collagen membrane. The resulting construct is the MACI graft.

The second stage is an open or mini-open procedure. The surgeon exposes the defect site, trims the graft to match its contour precisely, and secures it with fibrin glue. Because the cells are already carried on the membrane, no periosteal patch needs to be harvested — a technical step required in first-generation ACI that MACI eliminates.

From biopsy to implantation, the total process typically spans six to eight weeks. MACI is therefore a planned, resource-intensive commitment rather than a one-visit repair — something worth factoring into a patient's work and activity diary before agreeing to proceed. The phase that begins after Stage 2, covering protected weight-bearing and rehabilitation, is described in the following section.

What the outcomes evidence shows at 10 years

Long-term follow-up data for MACI now extends to a decade, and the overall picture is broadly reassuring for patients who achieve a stable graft.

A 2024 prospective series tracking 204 MACI patients found 92% satisfied with pain relief and 76% satisfied with sport participation at 10-year review. Patient-reported outcome measures improved significantly within the first two years, then held steady through to decade-long follow-up — suggesting early gains are durable rather than transient. On MRI, around 1 in 10 grafts (9.3%) showed signs of failure through delamination; MOCART scores measuring graft morphology were stable from 2 to 10 years. Patients who reach the 2-year mark with a well-integrated graft appear likely to maintain it.

A 2024 study isolating 82 patellofemoral MACI patients (41 patellar, 41 trochlear) at ≥10 years confirmed equivalent functional gains at both sites. One finding distinguished the patellar group: these patients recovered significantly greater peak isokinetic knee extensor torque than the trochlear group — a detail with direct relevance to rehabilitation planning and return-to-activity decisions.

A 2025 series on third-generation MACI (porcine collagen membrane) in 50 patellofemoral patients — mean age 32 — provides earlier-stage data specific to the PFJ. IKDC scores improved from 43.4 to 68.6 and Kujala scores from 58.9 to 77.1 (both p<0.001), representing significant gains in pain and kneecap-specific function; 77.8% reached the minimally important clinical difference for IKDC. Graft survival was 98% at 1 year, 96% at 2 years, and 85.7% at 4 years — a later decline worth acknowledging honestly, and one whose predictors in isolated patellar lesions are still being characterised. Bipolar defects carried a higher failure risk in this cohort.

One evidence gap bears naming: no large randomised trial has focused specifically on isolated patellar MACI, so these prospective series represent the best available guide to what patients at this site might expect.

Recovery timeline after patellar MACI

Patellar MACI demands a longer recovery than most patients initially expect — and being clear about that from the outset helps people plan around it more realistically.

In the weeks immediately following implantation, the graft requires protection while it begins to integrate with the surrounding bone and cartilage. This means crutches and restricted weight-bearing for roughly six to eight weeks, alongside early passive range-of-motion exercises designed to stimulate graft maturation without overloading it. The balance between motion and protection in this phase is managed by the surgical and physiotherapy team together.

Quadriceps strength — specifically the ability to generate extensor force at the knee — becomes the central benchmark as rehabilitation progresses. For patellar MACI in particular, isokinetic strength testing offers an objective measure of readiness to advance between activity phases, rather than relying on time alone. Research tracking strength trajectories after MACI confirms that recovery follows a gradual arc; those indices guide when patients can safely move from walking to low-impact activity such as cycling or swimming, and eventually to more demanding loading.

As established by the long-term data reviewed above, most functional gains accumulate within the first 12 to 24 months — which places realistic return-to-full-activity expectations at that horizon, not six months.

Combined-procedure cases — where MACI is performed alongside an alignment correction such as tibial tubercle osteotomy — tend to involve longer protected phases and more complex rehabilitation sequencing.

Because granular, patellar-specific rehabilitation data remain limited, an individualised plan developed with the treating consultant and physiotherapist is essential rather than optional.

When MACI needs to be combined with alignment correction

For many patients with patellar chondral damage, MACI alone is not the complete answer. The graft sits in a joint where mechanical forces are governed by alignment — and if those forces are abnormal, even a well-integrated graft faces a higher risk of failure over time.

The key structural measure is the tibial tubercle–trochlear groove (TT-TG) distance, which describes how far laterally the patellar tendon pulls on the kneecap relative to the groove it should track through. When this is elevated, a tibial tubercle osteotomy (TTO) is frequently performed alongside MACI implantation: the procedure repositions the tendon attachment point to reduce the compressive and shear forces acting across the graft during everyday loading.

Where patellar instability — recurrent dislocation or subluxation — has contributed to the cartilage injury, medial patellofemoral ligament reconstruction (MPFLR) is the indicated adjunct. Combined MACI and MPFLR procedures are well-described in the published literature and are indicated when instability forms part of the diagnosis, not treated as an optional extra.

Bipolar defects — simultaneous lesions on both the patella and the trochlea — may require a combined cartilage strategy, such as patellar MACI alongside trochlear osteochondral allograft (OCA). These cases are recognised and documented, but carry less extensive population-level outcome data than isolated MACI, and the 2025 patellofemoral series noted bipolar lesions as a specific higher-risk category. The approach is necessarily individual.

Whether alignment correction is performed in the same operative session as Stage 2 implantation or staged separately depends on defect complexity and overall joint mechanics. For a patient facing these decisions, the practical implication is that the MACI graft's durability depends on the whole mechanical environment being addressed — not the cartilage defect in isolation — and that is precisely why the pre-operative assessment covers alignment, stability, and defect characteristics together.

1. [1] SUMMIT Trial: MACI versus Microfracture by Lesion Characteristics. (2013). https://doi.org/10.1177/2325967113S00029 https://doi.org/10.1177/2325967113S00029
2. [2] Surgical Approach to Full Thickness Cartilage Defect With Patellar Instability: Combined MACI and MPFL Reconstruction. (2023). https://doi.org/10.1177/26350254231161371 https://doi.org/10.1177/26350254231161371
3. [3] Poster 227: Prospective Clinical and Radiological Follow-Up Beyond 10 Years in Patients Undergoing MACI in the Patellofemoral Joint. (2024). https://doi.org/10.1177/2325967124s00196 https://doi.org/10.1177/2325967124s00196
4. [4] 10-Year Prospective Clinical and Radiological Evaluation After Matrix-Induced Autologous Chondrocyte Implantation and Comparison of Tibiofemoral and Patellofemoral Graft Outcomes. (2024). https://doi.org/10.1177/03635465241227969 https://doi.org/10.1177/03635465241227969
5. [5] Comprehensive Patellofemoral Cartilage Restoration: Trochlear OCA, Patellar MACI, and TTO. (2026). https://doi.org/10.1177/26350254261452268 https://doi.org/10.1177/26350254261452268
6. [6] Excellent outcomes after open flake refixation and ACI following acute patella dislocation. (2022). https://doi.org/10.1007/s00167-022-06899-3 https://doi.org/10.1007/s00167-022-06899-3
7. [7] Patellofemoral Joint Chondral Defects Treated With Third-Generation MACI on Porcine Collagen Membrane: Minimum 2-Year Follow-up. (2025). https://doi.org/10.1177/23259671251341474 https://doi.org/10.1177/23259671251341474
8. [8] The progression of isokinetic knee strength after MACI: implications for rehabilitation and return to activity. (2014). https://doi.org/10.1123/JSR.2014-0159 https://doi.org/10.1123/JSR.2014-0159