ACI Knee Surgery: Procedure, Recovery, and Outcomes for Cartilage Repair

What is ACI Knee Surgery?

Autologous Chondrocyte Implantation (ACI) is a highly specialized, two-stage surgical procedure designed to repair damaged articular cartilage in the knee, utilizing a patient's own cartilage cells to regenerate new, healthy tissue.

Understanding Articular Cartilage and Its Importance

The knee joint, like many other joints in the body, is lined with a smooth, slippery tissue known as articular cartilage. This specialized connective tissue, primarily composed of chondrocytes embedded in an extracellular matrix, covers the ends of bones (femur, tibia, patella) where they meet, allowing for frictionless movement and acting as a shock absorber.

Unlike many other tissues in the body, articular cartilage has a very limited capacity for self-repair due to its avascular nature (lacking blood supply). Injuries to this cartilage, whether from acute trauma (e.g., direct impact, sports injury) or chronic degeneration, can lead to pain, swelling, stiffness, and mechanical symptoms like catching or locking. Untreated, these defects can progress to osteoarthritis, a debilitating condition characterized by widespread cartilage loss and joint degeneration.

What is ACI (Autologous Chondrocyte Implantation)?

ACI is an advanced biological technique aimed at repairing isolated, full-thickness cartilage defects in the knee. The term "autologous" signifies that the cells used for the repair come from the patient's own body, minimizing the risk of immune rejection. "Chondrocyte implantation" refers to the process of growing these cartilage-producing cells in a laboratory and then implanting them into the damaged area.

The primary goal of ACI is to restore the integrity of the articular surface, reduce pain, improve joint function, and potentially delay or prevent the onset of osteoarthritis by regenerating a durable, hyaline-like cartilage repair.

Who is a Candidate for ACI?

ACI is not suitable for all cartilage defects. Ideal candidates typically meet specific criteria:

• Isolated, Full-Thickness Cartilage Defects: The procedure is most effective for single, well-defined defects rather than widespread cartilage loss (e.g., advanced osteoarthritis).
• Defect Size: Typically used for defects larger than 2 cm², which are too large for simpler repair techniques like microfracture.
• Age: Generally performed in younger, active individuals (typically under 55-60 years old) who have good bone quality and joint alignment.
• Healthy Surrounding Tissue: The surrounding cartilage and subchondral bone should be healthy.
• Absence of Other Major Joint Pathologies: Significant ligament instability, meniscal tears, or malalignment issues must be addressed either prior to or concurrently with the ACI procedure to ensure success.
• Failed Conservative Treatment: Patients have usually attempted and failed non-surgical treatments such as physical therapy, activity modification, and injections.

The ACI Surgical Procedure: A Two-Stage Approach

ACI is uniquely performed in two distinct surgical stages, separated by several weeks.

Stage 1: Chondrocyte Harvesting (Arthroscopic Procedure)

• Initial Biopsy: This stage begins with a minimally invasive arthroscopic procedure. Small, healthy cartilage biopsies (typically 200-300 mg) are harvested from a non-weight-bearing area of the patient's knee, such as the intercondylar notch or the superior aspect of the femoral condyle.
• Cell Cultivation: These cartilage samples are then sent to a specialized laboratory. In the lab, the chondrocytes are isolated from the tissue matrix and cultured in a nutrient-rich environment. Over a period of approximately 4-6 weeks, these cells multiply significantly, expanding into a population of millions of new chondrocytes.

Stage 2: Implantation (Open Arthrotomy or Advanced Arthroscopic Techniques)

• Defect Preparation: After the chondrocytes have been cultivated, the patient undergoes a second, usually open, surgical procedure (arthrotomy). The damaged cartilage in the knee is meticulously debrided (cleaned out) down to the underlying subchondral bone, creating a stable bed for the new cells.
• Cell Delivery: The cultured chondrocytes are then delivered to the defect site using one of two primary methods:
  • First-Generation ACI (Periosteal Patch): A small piece of periosteum (the membrane covering the bone, harvested from the patient's tibia) is sutured over the cartilage defect, creating a "pouch." The liquid suspension of cultured chondrocytes is then injected underneath this patch.
  • Second/Third-Generation ACI (Collagen Membrane/Scaffold): More commonly used today, the cultured chondrocytes are either seeded onto a biocompatible membrane (e.g., collagen or synthetic polymer) in the lab (MACI - Matrix-Applied Chondrocyte Implantation) or injected as a suspension beneath a pre-fabricated membrane that is precisely cut and secured to the defect. This scaffold provides a more stable environment for cell growth and integration.
• Sealing: The membrane/patch is sealed in place with fibrin glue to prevent cell leakage. The newly implanted chondrocytes then begin to produce new cartilage matrix within the defect over time.

Recovery and Rehabilitation After ACI

The recovery process following ACI is lengthy, demanding, and critical for the success of the procedure. It typically spans 6-12 months and is divided into several phases:

• Immediate Post-Operative Period (Weeks 0-6):
  • Non-Weight Bearing: Strict non-weight bearing on the operated leg is usually required to protect the delicate new tissue. Crutches are essential.
  • Continuous Passive Motion (CPM): A CPM machine is often used to gently move the knee joint, promoting nutrient flow to the healing cartilage and preventing stiffness.
  • Bracing: A knee brace is typically worn to limit range of motion and prevent unwanted stresses.
• Early Rehabilitation (Weeks 6-12):
  • Gradual Weight Bearing: Progressive weight bearing is introduced under the guidance of a physical therapist.
  • Range of Motion: Focus on restoring full, pain-free range of motion.
  • Gentle Strengthening: Isometric exercises for quadriceps and hamstrings begin.
• Intermediate Rehabilitation (Months 3-6):
  • Progressive Strengthening: More advanced strengthening exercises, including closed-chain activities, are introduced.
  • Proprioception and Balance: Exercises to improve balance and joint awareness.
  • Low-Impact Activities: Introduction of activities like cycling and swimming.
• Advanced Rehabilitation (Months 6-12+):
  • Functional Training: Sport-specific drills and plyometrics (if appropriate for the patient's goals and defect location) are gradually incorporated.
  • Return to Activity: A gradual, highly individualized return to high-impact activities or sports is permitted only after achieving specific strength, stability, and functional milestones, often taking 9-12 months or longer.

Expected Outcomes and Considerations

• Success Rates: ACI has demonstrated good long-term success rates, with studies showing 70-90% success in improving pain and function in appropriately selected patients over 5-10 years.
• New Cartilage Formation: The implanted chondrocytes produce a type of cartilage that is often described as "hyaline-like" or "repair cartilage." While not always identical to native hyaline cartilage, it is generally superior in durability and function compared to the fibrocartilage produced by simpler techniques like microfracture.
• Potential Complications: As with any surgery, risks include infection, bleeding, nerve damage, stiffness, and re-operation. Specific to ACI, potential complications include hypertrophy (overgrowth) of the periosteal patch (less common with newer membrane techniques), delamination of the repair tissue, or failure of the graft to integrate.
• Long-Term Outlook: While ACI can significantly improve symptoms and function, it does not guarantee a complete cure or prevent the eventual development of osteoarthritis in the long term, especially if underlying biomechanical issues are not fully corrected. Adherence to the rehabilitation protocol is paramount for optimal outcomes.

ACI vs. Other Cartilage Repair Techniques

ACI is one of several techniques for cartilage repair, each with its own indications:

• Microfracture: A simpler, single-stage procedure where small holes are drilled into the subchondral bone to stimulate bleeding and fibrocartilage formation. Best for smaller defects.
• Osteochondral Autograft Transplantation (OATS) / Mosaicplasty: Involves transferring plugs of healthy cartilage and bone from a non-weight-bearing area of the patient's knee to the defect. Best for small-to-medium defects.
• Osteochondral Allograft Transplantation: Involves transplanting a larger block of cartilage and bone from a cadaveric donor. Used for larger, more complex defects.

ACI is generally reserved for larger, isolated full-thickness defects where other methods are less likely to yield durable, hyaline-like repair tissue.

Conclusion

Autologous Chondrocyte Implantation (ACI) represents a sophisticated biological approach to knee cartilage repair. By harnessing the body's own cells, it offers a pathway to regenerate damaged articular surfaces, providing significant pain relief and functional improvement for carefully selected patients. Its two-stage nature and extensive rehabilitation period underscore the complexity and commitment required, but for the right candidate, ACI can be a highly effective treatment in preserving knee joint health and function.