High Yield Overview

AUTOLOGOUS CHONDROCYTE IMPLANTATION (ACI)

Two-Stage Cell Therapy | Hyaline-Like Cartilage | Large Lesions

Clinical Imaging

Imaging Gallery

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Critical ACI Exam Points

Two-Stage Procedure

ACI requires two surgeries: Stage 1 harvests cartilage biopsy (200-300mg). Cells cultured for 4-6 weeks to expand. Stage 2 implants expanded cells. This is a key disadvantage vs single-stage OATS.

MACI Advantage

Third-generation MACI uses a collagen scaffold seeded with cells. This eliminates periosteal harvest and reduces hypertrophy complications. MACI is the current standard of care.

Hyaline-Like Repair

ACI produces hyaline-like cartilage with predominantly Type II collagen. Biopsy studies show 80%+ hyaline tissue at 2 years. This is superior to microfracture fibrocartilage.

Long-Term Durability

Unlike microfracture which degrades at 2-5 years, ACI results are durable to 10-20 years. Peterson series showed 84% good/excellent at 20 years follow-up.

ACI vs Other Cartilage Procedures

Mnemonic

ACIACI - Key Concepts

Memory Hook:ACI = your own cells, cultured, then implanted - Autologous, Cultured, Implanted!

Mnemonic

MACIMACI - The Modern Standard

Memory Hook:MACI adds Matrix to ACI - cells on a collagen scaffold!

Mnemonic

1-2-3Generations of ACI

Memory Hook:ACI evolved 1-2-3: Periosteum → Membrane → Matrix scaffold (MACI)!

Overview and Epidemiology

Why This Topic Matters

ACI/MACI is the gold standard for large cartilage lesions (over 2cm²) where microfracture outcomes are poor and OATS donor capacity is limited. Understanding the generations of ACI, the two-stage nature, and long-term outcomes is essential for examination success.

Indications

Large lesions over 2cm²
Failed microfracture or OATS
Young active patients (under 45)
Single or multiple focal defects
Full-thickness cartilage damage (ICRS 3-4)

Contraindications

Diffuse OA
Inflammatory arthropathy
Uncorrected malalignment
Meniscal deficiency (unless treated)
Kissing lesions (relative)
Age over 55 (relative)

Pathophysiology and Mechanisms

Hyaline-Like vs Fibrocartilage

ACI produces hyaline-like cartilage with predominantly Type II collagen and organized matrix structure. Biopsy studies show 80%+ hyaline tissue at 2 years. This is biomechanically superior to the fibrocartilage (Type I collagen) produced by microfracture, explaining the superior long-term durability.

Histological Comparison

Cell Culture Process

Harvest: 200-300mg cartilage biopsy
Enzymatic digestion: Release chondrocytes
Expansion: 4-6 weeks in culture
Result: 12-48 million cells
Matrix: MACI = cells seeded on collagen

Maturation Timeline

3 months: Soft cartilage repair tissue
6 months: Increasing firmness
12 months: Near-normal stiffness
18-24 months: Full maturation
Rehabilitation reflects this timeline

Classification Systems

Evolution of ACI Techniques

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MACI is Current Standard

MACI (third-generation) is now the standard of care. The collagen scaffold is seeded with cells in the lab, delivered as a ready-to-implant product. This eliminates periosteal harvest, reduces hypertrophy, and allows minimally invasive implantation with fibrin glue.

Clinical Assessment

History

Prior cartilage surgery (failed microfracture/OATS)
Symptom duration and progression
Mechanical symptoms (catching, locking)
Activity level and sport demands
Willingness for two surgeries and extended rehab

Examination

Effusion - common with cartilage damage
Tenderness - localized to compartment
Crepitus - may indicate damage
Alignment - assess for malalignment
Ligament/meniscal integrity

Patient Selection

ACI requires significant patient commitment: two surgeries, 4-6 week wait for cell culture, and 12-18 month rehabilitation. Ensure patient understands and is prepared for this timeline. Non-compliant patients are poor candidates.

Investigations

Investigation Protocol

First LineX-rays

Weight-bearing AP, lateral, Rosenberg, skyline. Assess alignment, joint space, OA changes. Rule out diffuse disease.

EssentialMRI

Cartilage sequences for lesion mapping. Size, location, depth. Subchondral bone status. Associated meniscal/ligament pathology.

If NeededAlignment Films

Full-length standing films if malalignment suspected. Plan osteotomy if significant deviation.

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MRI for ACI Planning

MRI helps identify all cartilage lesions for treatment planning. Unlike OATS (limited by donor), ACI can address multiple lesions simultaneously. Identify any lesion that needs treatment and plan to address at implantation.

Management Algorithm

When to Choose ACI/MACI

Ideal candidates:

Lesion over 2cm² (especially over 4cm²)
Failed prior microfracture or OATS
Multiple lesions
Good subchondral bone (no significant loss)
Motivated, compliant patient
Age under 45-50

Alternative treatments:

Under 2cm²: Consider microfracture first
1-4cm²: OATS reasonable single-stage option
Bone loss present: OCA may be better
OCA unavailable: ACI good for large lesions

This framework guides appropriate treatment selection based on lesion and patient characteristics.

Pre-operative Planning

Pre-operative Steps

InitialStage 1 Planning

Confirm indication for ACI. Plan biopsy location (non-weight-bearing area). Coordinate with cell laboratory. Patient counseling about timeline.

4-6 weeksCell Culture Period

Cells expanded in specialized laboratory. Patient rehab from biopsy (usually minimal). Plan stage 2 surgery date.

Pre-implantStage 2 Planning

Confirm cell culture successful. Plan implantation approach. Prepare MACI scaffold or membrane. Fibrin glue available.

Biopsy Site Selection

The biopsy for cell harvest should be taken from a non-weight-bearing area with healthy cartilage. Common sites include the superior margin of the intercondylar notch, the superomedial trochlear margin, or the peripheral edge of the lesion itself.

Surgical Technique

Stage 1: Cartilage Harvest

Biopsy Procedure

Step 1Approach

Arthroscopic or mini-open approach. Locate biopsy site (non-weight-bearing, healthy cartilage).

Step 2Harvest

Use curette or gouge to harvest 200-300mg of full-thickness cartilage. Typical size: 2-3 pieces, 4-5mm each.

Step 3Storage

Place in specialized transport medium. Send to cell culture laboratory. Cells must remain viable during transport.

Step 4Assessment

Assess the cartilage lesion(s). Measure size for implantation planning. Identify any additional pathology.

Stage 1 is a relatively minor procedure. Patients typically recover quickly and await cell culture completion.

Complications

Graft Hypertrophy

First-generation ACI with periosteal flap had 20-30% hypertrophy rates requiring secondary debridement. MACI has significantly reduced this complication. If hypertrophy occurs, arthroscopic debridement is usually effective.

Postoperative Care and Rehabilitation

Weight-Bearing Protocol

Progression

Touch weight-bearingWeeks 0-6

Toe-touch only with crutches. Brace for protection. Allows early cartilage maturation without load.

Partial weight-bearingWeeks 6-12

Progressive weight-bearing 25% to 75%. Wean brace. Crutches until comfortable.

Full weight-bearingWeeks 12-16

Full weight-bearing without aids. No impact activities. Low-load exercises.

Cartilage Maturation

ACI cartilage takes 12-18 months to fully mature. Early in healing, the tissue is soft and vulnerable. Weight-bearing and activity progression reflects this maturation timeline - slower than OATS or microfracture.

Outcomes and Prognosis

Long-term Outcomes

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Superior to Microfracture Long-term

The STAR trial (RCT) showed ACI superior to microfracture at 5 years. While microfracture results deteriorate at 2-5 years, ACI maintains improvement. For larger lesions, this difference is even more pronounced.

Evidence Base and Key Trials

Peterson 20-Year Follow-up

Peterson L, Vasiliadis HS, Brittberg M, et al • AJSM (2010)

Clinical Implication: ACI provides durable long-term results, supporting its use in young patients who need lasting cartilage repair.

Limitation: First-generation technique, single center.

STAR Trial - ACI vs Microfracture

Saris DB, Vanlauwe J, Victor J, et al • AJSM (2014)

Clinical Implication: RCT evidence supports ACI over microfracture for larger lesions, especially long-term.

Limitation: High dropout rate at 5 years, heterogeneous lesion sizes.

MACI vs ACI

Basad E, Ishaque B, Bachmann G, et al • KSSTA (2010)

Clinical Implication: MACI preferred over first-gen ACI due to reduced complications with equivalent efficacy.

Limitation: Relatively short follow-up for long-term durability comparison.

ACI for Failed Prior Surgery

Minas T, Gomoll AH, et al • CORR (2009)

Clinical Implication: ACI remains viable after failed microfracture, though primary treatment may be preferable if ACI is ultimately planned.

Limitation: Retrospective comparison.

Return to Sport After ACI

Mithoefer K, Della Villa S, et al • AJSM (2009)

Clinical Implication: Counsel patients that return to high-level sport is not guaranteed but majority return to some activity.

Limitation: Heterogeneous patient populations and sport levels.

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Failed Microfracture (~2-3 min)

EXAMINER

"A 32-year-old female athlete has persistent symptoms 2 years after microfracture for a 3cm² medial femoral condyle lesion. MRI shows incomplete fill with subchondral changes. What are your options?"

EXCEPTIONAL ANSWER

This is a failed microfracture with a lesion size that was probably too large for microfracture to begin with (3cm² exceeds optimal size of under 2cm²). For a 3cm² lesion after failed microfracture, my options are: **First choice: ACI/MACI** - This size is ideal for ACI (over 2cm²) - Two-stage procedure but produces hyaline-like cartilage - Prior microfracture does not preclude ACI - May need to address subchondral changes **Alternative: OATS** - Would require 3-4 plugs for 3cm² lesion - At upper limit of donor capacity - Single-stage advantage **Alternative: OCA** (if available) - Single-stage, no donor morbidity - Good for failed prior surgery - Limited availability may be issue Given her age (32), athlete status, and lesion size, I would recommend MACI: - Stage 1: Arthroscopic biopsy from non-weight-bearing area - 4-6 weeks cell culture - Stage 2: MACI implantation with scaffold I would also confirm alignment is normal and address any malalignment with osteotomy if needed.

VIVA SCENARIOChallenging

Scenario 2: ACI Technique (~3-4 min)

EXAMINER

"Describe the MACI implantation technique for a 4cm² trochlear lesion."

EXCEPTIONAL ANSWER

MACI for a trochlear lesion requires careful attention to technique: **Pre-operative:** - Confirm cell culture successful (12-48M cells on collagen scaffold) - MACI scaffold arrives ready to implant - Plan approach - trochlear lesions often need mini-arthrotomy **Surgical Steps:** **Exposure:** Mini-arthrotomy through medial or lateral parapatellar approach, depending on lesion location. Evert patella or sublux to expose trochlea. May need to flex knee for access. **Debridement:** - Debride unstable cartilage to stable vertical shoulders - Create well-contained defect - Curette to remove calcified cartilage layer - Preserve subchondral plate - do not violate excessively - Dry the defect base **Template and Sizing:** - Create template using sterile foil - Trace exact defect shape - Cut MACI scaffold to match template precisely **Implantation:** - Apply fibrin glue to defect base - Orient scaffold correctly - cell-seeded side DOWN (toward bone) - Place scaffold into defect - Apply additional fibrin glue around edges - Ensure secure attachment **Confirmation:** - Cycle knee through ROM multiple times - Confirm scaffold remains stable and does not delaminate - Ensure edges are well-sealed For trochlear lesions specifically, the contouring is challenging due to the sulcus geometry. May need multiple pieces of scaffold for complex shapes.

VIVA SCENARIOCritical

Scenario 3: Patient Selection (~2-3 min)

EXAMINER

"A 52-year-old man with a 2cm² medial femoral condyle lesion asks about ACI. His MRI also shows Grade 2 changes in the patellofemoral joint and mild medial joint space narrowing. Is he a candidate?"

EXCEPTIONAL ANSWER

This case requires careful consideration of several factors that may affect ACI candidacy. **Concerning features:** 1. Age 52 - at upper limit for ACI (typically under 45-50) 2. PF changes (Grade 2) - suggests early generalized OA 3. Medial joint space narrowing - may indicate bipolar disease 4. 2cm² lesion - at lower threshold where simpler options exist **My assessment:** This patient may not be ideal for ACI for several reasons: 1. **Age**: At 52, healing potential is reduced, and longevity of repair is a concern. ACI is best for patients who need 10-20+ years of benefit. 2. **Generalized changes**: The PF changes and medial narrowing suggest this may be early OA rather than isolated focal cartilage damage. ACI works best for focal defects in otherwise healthy joints. 3. **Lesion size**: At 2cm², simpler options like microfracture or even debridement may be reasonable first-line treatments. **My recommendation:** I would counsel this patient that ACI may not be the best option. I would consider: 1. **Conservative management** optimization 2. **Debridement +/- microfracture** as first-line 3. If he fails these, consider **unloader brace** or **HTO** if varus 4. Eventually may need **UKA or TKA** ACI should be reserved for younger patients with truly focal defects in otherwise healthy joints where long-term biological repair is the goal.

MCQ Practice Points

Stage Question

Q: How many surgical stages are required for ACI? A: Two stages - Stage 1 is cartilage biopsy for cell harvest. Cells are cultured for 4-6 weeks. Stage 2 is implantation of expanded cells. This is a key disadvantage compared to single-stage OATS.

Cell Expansion Question

Q: How many cells are implanted during ACI? A: 12-48 million cells - The initial biopsy (200-300mg) contains approximately 200,000-300,000 chondrocytes. Cell culture expands this to 12-48 million cells for implantation.

MACI Definition Question

Q: What does MACI stand for? A: Matrix-induced Autologous Chondrocyte Implantation - MACI is third-generation ACI where cells are seeded on a collagen scaffold, eliminating the need for periosteal harvest or watertight membrane suturing.

Hypertrophy Question

Q: Which generation of ACI had the highest rate of graft hypertrophy? A: First-generation (periosteal ACI) - Hypertrophy rates of 20-30% were seen with periosteal flaps. This was significantly reduced with collagen membranes (second-gen) and MACI (third-gen).

Long-term Results Question

Q: What is the longest reported follow-up for ACI outcomes? A: 20 years (Peterson series) - The original Brittberg/Peterson series reported 84% good/excellent results at 20 years, demonstrating exceptional durability of the technique.

Lesion Size Question

Q: What lesion size is typically considered ideal for ACI? A: Over 2cm² - ACI is typically reserved for lesions over 2cm² where microfracture outcomes are suboptimal and OATS donor capacity may be limiting. It is especially valuable for lesions over 4cm².

Australian Context and Medicolegal Considerations

Australian Practice

MACI available through specialized centers
Requires TGA-approved cell culture facilities
PBS does not cover cell therapy costs
Significant out-of-pocket expense for patients
Some private insurers may provide partial coverage

Documentation Standards

Document two-stage consent thoroughly
Record cell culture laboratory and tracking
Document scaffold orientation at implantation
Record all associated procedures (osteotomy)
Consent must cover extended rehabilitation timeline

Medicolegal Considerations

Key documentation requirements:

Two-stage consent with clear explanation of process
Document cell culture tracking and viability confirmation
Record surgical technique details (scaffold orientation, fixation)
Consent must include: extended rehabilitation (12-18 months), two surgeries, cost implications, realistic sport expectations
If failure occurs, documentation of proper indication and technique is protective