Articular cartilage — the smooth, white tissue that covers the ends of bones in the knee — does not heal on its own. Once it is damaged beyond a critical threshold, the body replaces it with inferior fibrocartilage rather than true hyaline cartilage. Without intervention, a significant cartilage defect progresses: the exposed bone erodes, inflammation increases, and the condition advances toward osteoarthritis.

Autologous chondrocyte implantation (ACI) was developed specifically to address this problem. Instead of accepting fibrocartilage substitutes, ACI aims to regenerate hyaline-like cartilage — biologically closer to what was lost. It is the treatment of choice for large, symptomatic cartilage defects in active patients who want to preserve their joint and return to an active lifestyle.

What Is ACI and How Has It Evolved?

ACI has undergone three generations of development since its introduction in the 1990s:

Three generations of ACI

  • First generation: Chondrocytes injected under a periosteal flap. Effective, but complicated by periosteal hypertrophy requiring reoperation.
  • Second generation: Periosteal flap replaced by a collagen membrane. Reduced complications, simplified technique.
  • Third generation (MACI / matrix-assisted ACI): Cells seeded onto a biodegradable scaffold before implantation. Uniform cell distribution, simplified surgical workflow, superior handling.

We use a third-generation technique with an atelocollagen-based scaffold — a highly purified collagen material that is biocompatible, biodegradable, and supports chondrocyte adhesion and function. In Japan, this approach has been standardized and is the basis of the JACK procedure (Japan Autologous Chondrocyte implantation using the Kenkyujo-scaffold), which has made ACI a routinely reproducible clinical procedure in specialized centers.

The Two-Stage Procedure

ACI requires two separate surgeries separated by approximately 6–8 weeks of cell culture:

1

Stage 1: Arthroscopic biopsy (30 minutes)

Small fragments of healthy cartilage (200–300mg) are harvested arthroscopically from a non-weight-bearing area of the knee — typically the intercondylar notch or the periphery of the femoral condyle. The biopsy site heals without consequence. The cartilage fragments are sent to a certified laboratory.

2

Laboratory phase (6–8 weeks)

In the laboratory, chondrocytes (cartilage cells) are isolated from the biopsy and expanded in culture to approximately 10–50 million cells. They are then seeded onto the atelocollagen scaffold, where they begin to attach and produce extracellular matrix. The cell-seeded scaffold is quality-tested before release.

3

Stage 2: Implantation (1–2 hours)

Through an open or mini-open approach, the cartilage defect is prepared — the unstable edges are trimmed to create a stable containment rim and the base is debrided to bleeding bone. The scaffold, now populated with your own chondrocytes, is cut to fit the defect precisely and secured with fibrin glue and, where needed, absorbable sutures. The wound is closed in layers.

Who Is ACI For?

Ideal ACI candidate

  • Symptomatic focal cartilage defect, typically 2–10 cm² in area
  • Full-thickness (ICRS Grade III–IV) defect on the femoral condyle, trochlea, or patella
  • Age generally under 55 — though evidence supports benefit in older patients too
  • Failed or inappropriate for single-stage options (microfracture, OATS) due to defect size
  • Good overall joint environment: well-aligned limb, stable ligaments, intact menisci
  • High functional demands — wants to return to sport or physically demanding activity

ACI is not appropriate for generalised osteoarthritis. It treats focal defects — areas where cartilage is missing but the surrounding joint is reasonably preserved. Addressing concomitant problems (malalignment, ligament instability, meniscal deficiency) at the time of ACI or in a staged fashion is critical for achieving the best outcome.

The whole-joint principle: ACI works best when the joint environment is optimized. A realignment procedure (HTO) for varus malalignment, or ligament reconstruction for instability, should be performed at the same time or before ACI — not left uncorrected.

What the Evidence Shows

Based on articles retrieved from PubMed, a 2026 review by Nakata et al. (Hiroshima University) in Modern Rheumatology comprehensively summarized the clinical evidence on ACI. Key findings: while younger patients tend to achieve higher absolute outcome scores, middle-aged and older patients frequently achieve clinically meaningful improvements in pain and daily function. Age alone is not a contraindication to ACI. The review specifically highlights that atelocollagen-based scaffolds developed in Japan have standardized ACI procedures and made reliable clinical application possible. DOI: 10.1093/mr/roag048

A systematic review by Kunze et al. (2025, Hospital for Special Surgery) analyzing 52 studies and 2,387 athletes found that matrix-assisted ACI (MACI) achieved the highest rate of return to the same or greater level of sport compared to microfracture or OATS, with an odds ratio of 2.15 in favor of same-level return. DOI: 10.1177/03635465241280975

Recovery Timeline

After ACI implantation

  • Weeks 1–6: Non- or partial weight-bearing, continuous passive motion (CPM), range of motion exercises
  • Weeks 6–12: Progressive weight-bearing, stationary cycling, swimming
  • Month 3–6: Strengthening, proprioception, light jogging
  • Month 6–9: Sport-specific training
  • Month 9–18: Gradual return to contact or high-impact sport

ACI requires patience. The newly formed cartilage matures over 12–18 months, and the rehabilitation protocol is designed to protect it during that period. Outcomes continue to improve for 2–3 years after implantation. Patients who commit fully to the rehabilitation process — and resist the temptation to return to sport prematurely — achieve the best results.

Frequently Asked Questions

Why not just do microfracture instead?

Microfracture produces fibrocartilage — not true hyaline cartilage. For small defects (under 2 cm²), this may be adequate. For larger defects or in active patients who want to return to sport at a high level, ACI produces significantly better outcomes and more durable repair tissue. Prior microfracture can compromise the subchondral bone and may reduce ACI success if done first — this is an important reason not to delay seeking specialist advice.

Am I too old for ACI?

Age is not a hard cut-off. The 2026 Hiroshima review found that patients over 40 and 50 can achieve clinically meaningful improvements. What matters more is the biological environment: is there enough surrounding cartilage to contain the defect, is the joint reasonably well preserved, and are concomitant problems addressed?

Is ACI available under Japanese national health insurance?

Yes. ACI using the approved atelocollagen scaffold (JACC) has been covered under Japan's NHI since 2013. Foreign residents enrolled in the Japanese health insurance system pay the standard 30% co-pay.

Can I travel to Japan specifically for ACI?

Yes — and this is something I can coordinate for international patients. Stage 1 surgery and the first consultation can sometimes be scheduled together. The 6–8 week laboratory phase provides time to return home before coming back for Stage 2. Postoperative follow-up is manageable with a combination of in-person and remote communication.

Diagnosed with a cartilage defect?

First consultation is free — 30 minutes, in person or by video. I will review your MRI and explain whether ACI is the right option for your situation.

Book a Free Consultation

References

Nakata K, et al. Autologous Chondrocyte Implantation (ACI) for the Knee: Historical Development, Age Related Outcomes, and Future Directions. Mod Rheumatol. 2026. DOI: 10.1093/mr/roag048

Kunze KN, et al. High Rate of Return to Sport for Athletes Undergoing Articular Cartilage Restoration Procedures for the Knee: A Systematic Review. Am J Sports Med. 2025;53(10):2471–2482. DOI: 10.1177/03635465241280975