What makes a kneecap cartilage defect suitable for ACI

Kneecap cartilage damage ranges from a small, sharply defined crater to widespread thinning across the entire joint surface — ACI addresses only one of those presentations.

The patellofemoral joint pairs the underside of the kneecap (patella) with the trochlear groove at the front of the femur. Cartilage defects here behave differently from injuries on the femoral condyle further inside the knee: contact pressures are high, the geometry complex, and any repair is subject to loading forces that shift with every degree of knee flexion.

Cell-based repair — ACI and its scaffold-based successors MACI and STACi — is suited to focal, full-thickness cartilage damage. In clinical grading terms, that means an ICRS grade III or IV lesion: a crater-like defect that reaches all the way through the cartilage layer. What qualifies the injury is not just its depth, but its boundaries. The surrounding cartilage must be healthy and intact, and the subchondral bone plate beneath the defect should be sound. When damage has penetrated into bone itself — intralesional osteophytes or subchondral cysts — osteochondral procedures such as OAT or OCA are generally preferred over cell-based repair.

Diffuse cartilage loss spread across the whole patellofemoral surface sits outside the scope of cell-based restoration. Candidacy assessment specifically separates focal from widespread damage before any surgical planning begins; patients with advanced, degenerative joint changes are typically directed towards a different care pathway.

Younger patients who sustain an acute patellar dislocation frequently develop the kind of focal osteochondral injury — a discrete fragment separating from the joint surface — that fits this profile most closely, making dislocation-related damage one of the more common presentations arriving at a cartilage specialist clinic.

How defect size drives the treatment decision

Defect area — measured in square centimetres on imaging and confirmed at arthroscopy — is the first branching point in any patellofemoral cartilage repair plan.

For smaller lesions, roughly under 2–4 cm², marrow-stimulation (microfracture) or osteochondral autograft transfer (OATS/mosaicplasty, typically suited to 1–2 cm² defects, with a mosaic configuration extending to around 4 cm²) remain recognised options. OATS involves a meaningful donor-site trade-off: healthy cartilage must be harvested from a less-loaded area of the same knee, which carries its own recovery burden. Microfracture's limitation is biological rather than technical — it stimulates fibrocartilage rather than the hyaline-like tissue of native cartilage. That fibrocartilage tends to soften and break down within two to three years, and the drilling process can disrupt the subchondral bone plate in a way that makes subsequent, more durable repair procedures technically harder to perform.

Once a defect reaches 3 cm² or larger, the evidence shifts clearly towards cell-based repair. The SUMMIT randomised controlled trial — the most directly applicable dataset for this size range — found MACI superior to microfracture on KOOS pain and function scores at both the two-year and five-year reviews.

At the upper boundary of size and complexity, where extensive bone loss accompanies the cartilage damage, fresh osteochondral allograft (OCA) is the appropriate reconstruction: it addresses both the cartilage and bone layers simultaneously, where cell-based repair alone cannot.

Why malalignment must be addressed at the patellofemoral joint

Correcting biomechanical alignment at the patellofemoral joint is not a preparatory nicety — it is a prerequisite that directly determines whether a cartilage repair succeeds or fails.

The kneecap tracks through the trochlear groove across a wide arc of knee flexion, generating substantial compressive and shear forces as it does. Any graft placed into a patellofemoral defect must withstand those forces every time the patient climbs stairs, rises from a chair, or squats. When the kneecap is not tracking centrally — when it sits too high (a condition called patella alta) or when the tendon anchoring it pulls too far outward relative to the groove — load concentrates on the repair site rather than distributing across the joint surface. That maldistribution of stress is a recognised cause of graft failure, not an uncommon bad luck event.

The outward pull of the patellar tendon is measured as the tibial tuberosity–trochlear groove distance, or TT-TG distance. When this distance is elevated, shear forces on any cartilage repair increase significantly. Both patella alta and elevated TT-TG distance must therefore be identified before surgery.

Cartilage surgery literature is explicit: where patellar malalignment is present, it should be corrected simultaneously with, or prior to, ACI. The standard corrective procedure is a tibial tuberosity transfer — sometimes called a Fulkerson-type anteromedialization — which repositions the tendon attachment to restore normal load distribution across the joint.

Biomechanical assessment is consequently a mandatory part of the candidacy workup; reviewing the defect on MRI alone is insufficient. Objective kinematic analysis — including AI-powered markerless motion capture using MAI Motion® — can quantify patellar tracking and load patterns to complement imaging findings and inform the full surgical plan.

How ACI has evolved: from periosteal patch to scaffold

First-generation ACI, performed from the early 1990s, required a patch of periosteum — the tissue lining the shin bone — to be harvested and sutured over the implanted cells as a biological seal. The approach proved effective: long-term cohort data from Peterson and, more recently, Minas et al. (Clin Orthop Relat Res, 2014) confirm durable clinical benefit at nine years and beyond. Its consistent drawback was periosteal hypertrophy — the patch can overgrow after surgery and sometimes requires further arthroscopic trimming.

MACI (matrix-induced autologous chondrocyte implantation) addressed that limitation directly. The patient's cartilage cells are still harvested at a first arthroscopy, expanded in a laboratory, and reimplanted in a second procedure — but instead of being sealed under a periosteal flap, they are seeded onto a Type I/III collagen scaffold. That three-dimensional matrix supports cartilage growth in depth as well as across the surface, more closely replicating the layered architecture of native tissue. Five-year follow-up data from Behrens et al. (Knee, 2006) confirm maintained clinical benefit at the medium term, and the scaffold design has also widened candidacy to defects previously considered too large or complex for traditional ACI.

STACi, an emerging single-stage evolution developed at the London Cartilage Clinic, combines cell harvest and reimplantation into one operation, reducing the treatment burden considerably; the evidence base is still developing and should be discussed in full at consultation. Both MACI and STACi pathways are available through MSK Doctors.

Across all these scaffold variants the goal is consistent: durable, hyaline-like repair tissue, rather than the fibrocartilage that marrow stimulation produces and that tends to soften and break down within a few years.

What the long-term evidence shows for PF outcomes

Published follow-up data give a broadly reassuring picture of durability, though with important caveats about where that evidence was gathered.

The Minas et al. 10-year cohort (Clin Orthop Relat Res, 2014 — John Insall Award) and the Behrens five-year MACI series (Knee, 2006) confirm that clinical gains from chondrocyte implantation are not merely early post-operative phenomena: benefit is maintained across different ACI generations and at meaningful follow-up horizons. The SUMMIT randomised trial then adds the most directly applicable comparative data. At both two and five years, MACI produced significantly higher KOOS pain and function scores than microfracture in patients with defects of 3 cm² or greater. That result matters particularly because microfracture benefit is known to erode between two and three years as the fibrocartilage it produces softens — the SUMMIT five-year data show MACI maintaining ground that microfracture progressively loses.

One nuance from the German Cartilage Registry (Mehl et al., Knee Surgery, Sports Traumatology, Arthroscopy, 2019) is directly relevant to expectation-setting: degenerative patellofemoral defects carry a heavier symptom burden at baseline than traumatic ones. That finding does not exclude degenerative patients from cell-based repair, but it does mean pre-treatment discussions should be candid about what recovery looks like when cartilage loss has accumulated gradually rather than following a single acute event such as patellar dislocation.

The principal limitation is that most long-horizon RCT data come from femoral condyle populations rather than pure patellofemoral samples. The direction of benefit is consistent across studies, and these reports represent the best available evidence — but PF-specific trial data at ten years remain limited. A consultant will factor that gap openly into any personalised treatment plan rather than extrapolating without acknowledgement.

Getting assessed at MSK Doctors

A candidacy assessment for patellofemoral ACI at MSK Doctors brings together several clinical strands: a detailed history of how the problem developed, weight-bearing imaging to assess alignment, and MRI review — with consideration of advanced cartilage mapping sequences such as T2 mapping where clinically indicated. Biomechanical evaluation is part of that picture too; the group's UKCA-registered MAI Motion® markerless motion-capture system can quantify how load distributes across the joint during movement, helping the consultant determine whether malalignment correction is needed alongside any cartilage repair.

No GP referral is required, which matters for patients who have been waiting on NHS pathways, or who want a second opinion before committing to a treatment route. Consultations and diagnostics are available at the Sleaford, Lincolnshire hub — where the Regeneration Hub and an Open MRI scanner support the full range of regenerative-medicine pathways — and at the Grantham centre, home to the MFO Life Sciences Lab for research-linked care.

From that assessment, the consultant team can map out the full range of options relevant to each patient's defect size, biomechanics, and personal goals: scaffold-based repair (MACI or STACi), alignment surgery where needed, and any complementary biologic support.

To arrange an assessment without a referral, visit mskdoctors.com.

1. [1] Autologous chondrocyte implantation. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150