ARTICULAR CARTILAGE INJURIES
Chondral Defects | Size-Based Treatment | Microfracture vs OATS vs ACI
ICRS CLASSIFICATION
Critical Must-Knows
- Articular cartilage has NO blood supply - cannot heal spontaneously
- Treatment algorithm based on DEFECT SIZE and patient factors
- Microfracture: less than 2cm² - creates fibrocartilage (Type I collagen)
- OATS: 2-4cm² - hyaline cartilage transfer, limited donor site
- ACI/MACI: greater than 4cm² - cultured chondrocytes, expensive, two-stage
Examiner's Pearls
- "Fibrocartilage from microfracture is mechanically inferior to hyaline
- "OATS donor site morbidity limits graft size
- "ACI requires intact cartilage rim (shoulders)
- "Osteochondral allograft for large defects with bone loss
Clinical Imaging
Imaging Gallery
Critical Exam Concepts
Size Determines Treatment
Size is the primary determinant. Less than 2cm² = microfracture. 2-4cm² = OATS. Greater than 4cm² = ACI/MACI. Know these thresholds cold.
Fibrocartilage vs Hyaline
Microfracture produces fibrocartilage (Type I collagen) which is mechanically inferior to native hyaline cartilage (Type II collagen). This is why larger defects need better options.
Patient Selection
Young, active, single contained defect = best candidates. Avoid in malalignment, instability, or meniscal deficiency - must address concomitant pathology.
Contraindications
Kissing lesions (bipolar defects), diffuse OA, inflammatory arthritis, BMI greater than 35, and uncorrected malalignment are relative contraindications. Address alignment first.
Quick Decision Guide
| Defect Size | Depth | First-Line Treatment | Key Consideration |
|---|---|---|---|
| Less than 1cm² | Any depth | Debridement +/- microfracture | May do well with debridement alone |
| 1-2cm² | Full thickness | Microfracture | Best for contained lesions |
| 2-4cm² | Full thickness | OATS (mosaicplasty) | Limited by donor availability |
| Greater than 4cm² | Full thickness | ACI/MACI | Two-stage, expensive |
| Large with bone loss | Into subchondral | Osteochondral allograft | Fresh allograft for viability |
DMOACCartilage Treatment Ladder
Memory Hook:Doctors Make Orthopaedic Algorithms Clear - size determines step!
PADSMicrofracture Technique
Memory Hook:PADS protect the healing fibrocartilage!
SLIMPrerequisites for Cartilage Surgery
Memory Hook:Keep your cartilage patient SLIM (address all these first)!
DONOROATS Considerations
Memory Hook:Get your DONOR site right for successful OATS!
Overview and Epidemiology
Why Cartilage Cannot Heal
Articular cartilage is avascular, aneural, and alymphatic. It relies on diffusion from synovial fluid for nutrition. Without blood supply, there is no inflammatory healing response. This is why we must create a vascular channel (microfracture) or transplant cells (ACI) to achieve repair.
Epidemiology
- 60% of knee arthroscopies show cartilage damage
- Peak incidence 10-50 years
- Sports injuries common cause
- Traumatic vs degenerative defects
- Males more commonly affected
Natural History
- Full thickness defects do NOT heal
- Partial thickness may not progress
- Size correlates with symptoms
- Surrounding cartilage at risk
- Untreated leads to OA
Pathophysiology and Mechanisms
Hyaline Cartilage Zones
Superficial (tangential) zone: Type II collagen parallel to surface. Resists shear.
Middle (transitional) zone: Oblique collagen. Resists compressive forces.
Deep (radial) zone: Perpendicular collagen. Anchors to tidemark.
Calcified cartilage: Above subchondral bone. Tidemark separates from deep zone.
Calcified Layer in Microfracture
The calcified cartilage layer MUST be removed during microfracture. Leaving it in place prevents integration of repair tissue with subchondral bone and leads to delamination. Curette to bleeding bone, but preserve subchondral plate.
Classification Systems
International Cartilage Repair Society
| Grade | Description | Depth | Treatment Implication |
|---|---|---|---|
| 0 | Normal | Intact | No treatment |
| 1 | Superficial | Softening/fibrillation | Conservative |
| 2 | Less than 50% | Abnormal | Debridement/microfracture |
| 3A | Greater than 50% | Not to calcified layer | Microfracture/OATS |
| 3B | Greater than 50% | To calcified layer | Microfracture/OATS |
| 3C | Greater than 50% | Through calcified layer | Restorative procedure |
| 3D | Greater than 50% | Blistering | Restorative procedure |
| 4 | Full thickness | To subchondral bone | Size-based algorithm |
Clinical Assessment
History
- Mechanism: Acute trauma vs insidious
- Pain: Activity-related, mechanical symptoms
- Locking/catching: Loose body?
- Swelling: Effusion pattern
- Previous surgery: Failed treatment?
Examination
- Effusion: Common with acute injury
- Tenderness: Focal joint line
- ROM: Usually preserved unless OB/locking
- Alignment: Varus/valgus assessment
- Stability: ACL, meniscal tests
Address Concomitant Pathology
Cartilage procedures in isolation will fail if you do not address ACL insufficiency, meniscal deficiency, or malalignment. A comprehensive assessment is mandatory. Combined procedures (e.g., ACL + microfracture, HTO + cartilage procedure) may be needed.
Key Clinical Distinction
Traumatic defects: Single, contained, healthy surrounding cartilage. Good candidates for restorative procedures.
Degenerative defects: Multiple lesions, poor surrounding cartilage, kissing lesions common. Poor candidates for isolated cartilage surgery - consider arthroplasty or osteotomy.
Investigations
MRI Assessment
Gold standard for cartilage evaluation.
Sequences: Proton density, T2 mapping, dGEMRIC.
Assessment: Location, size, depth, bone edema.
Kissing lesions: Bipolar damage - worse prognosis.
Surrounding cartilage: Quality affects surgical planning.
MOCART Score
Magnetic Resonance Observation of Cartilage Repair Tissue (MOCART) evaluates repair tissue quality after cartilage surgery. Assesses: fill, integration, surface, structure, signal intensity, subchondral bone, effusion.
Management Algorithm
Treatment Selection by Defect Size
Decision Pathway
First-line for small lesions. Creates access to marrow elements. Forms fibrocartilage. Best for contained defects on femoral condyle.
Hyaline cartilage transfer. Autologous osteochondral plugs from non-weight-bearing area. Limited by donor availability and morbidity.
Cultured chondrocytes. Two-stage procedure. Expensive. Requires intact cartilage rim. Best long-term hyaline restoration.
Fresh allograft transfer. For lesions with subchondral bone involvement. Cell viability requires fresh tissue.
Surgical Technique
Microfracture Technique (Steadman)
Surgical Steps
Debride unstable cartilage to stable vertical walls. Curette the calcified cartilage layer. Preserve subchondral plate.
Use curved arthroscopic awl. Create holes 3-4mm deep, 3-4mm apart. Start peripherally, work centrally. 3-4 holes per cm².
Reduce arthroscopic pressure. Observe fat droplets (marrow element) from holes. This is the "super clot" precursor.
CPM 6-8 hours/day for 6 weeks. NWB or TTWB 6-8 weeks. Avoid impact for 4-6 months.
Fibrocartilage vs Hyaline
Microfracture produces TYPE I collagen (fibrocartilage) which is mechanically inferior to native TYPE II collagen (hyaline cartilage). Fibrocartilage has only 25% of the stiffness and durability of hyaline. This is why larger defects need better solutions.
Complications
| Procedure | Complication | Incidence | Management |
|---|---|---|---|
| Microfracture | Intralesional osteophyte | Common | Revision restorative procedure |
| Microfracture | Fibrocartilage deterioration | 5+ years | Consider second-line procedures |
| OATS | Donor site morbidity | 5-10% | Limit harvest size and number |
| OATS | Plug subsidence | 5% | Proper depth preparation |
| ACI | Hypertrophy | 15-30% | Secondary debridement |
| ACI | Delamination | 10% | Proper rim preparation |
| ACI | Arthrofibrosis | 5% | Appropriate CPM and therapy |
| All | Failure | Variable | Address malalignment, revise to higher tier |
Why ACI Fails
Graft delamination and hypertrophy are the main ACI-specific complications. Delamination occurs when the calcified layer is not removed or when the surrounding cartilage rim is inadequate. Hypertrophy (overgrowth of repair tissue) may require debridement but is usually a sign of successful healing.
Postoperative Care
Rehabilitation Protocol
CPM 6-8 hours/day. NWB or TTWB with crutches. Avoid shear forces. Focus on ROM and quadriceps activation.
Gradual weight-bearing progression. PWB to FWB by 12 weeks. Pool exercises, stationary bike.
Full weight-bearing. Progressive strengthening. Low-impact activities (swimming, cycling). No impact sports.
Impact activities gradually introduced. Sport-specific training. Full return 9-12 months for high-level athletes.
Early Loading is Catastrophic
Premature weight-bearing damages the immature repair tissue and leads to failure. Strict adherence to NWB/TTWB protocol is essential. CPM promotes nutrition diffusion and prevents adhesions.
Outcomes and Prognosis
Procedure-Specific Outcomes
Microfracture: 70-80% good/excellent at 5 years. Durability limited - best as bridge procedure. Fibrocartilage degrades over time.
OATS: 80-90% good outcomes. Limited by plug number. Donor site morbidity in 5-10%.
ACI/MACI: 80-90% satisfaction at 10+ years. Best long-term hyaline restoration. Expensive, two-stage.
Osteochondral Allograft: 75-85% survival at 10 years. Best for large defects with bone loss.
Prognostic Factors
Good prognosis: Young age, single defect, femoral condyle location, normal alignment, intact menisci, first-time procedure.
Poor prognosis: Age greater than 40, bipolar lesions, patellofemoral location, malalignment, revision surgery.
Evidence Base and Key Studies
Steadman Microfracture 11-Year Follow-up
- 127 patients followed 11 years post-microfracture
- 75% good to excellent outcomes
- Durability concerns beyond 5 years
- Best results in young patients with acute defects
ACI vs Microfracture RCT (Brittberg)
- 118 patients randomized to ACI-C vs microfracture
- Better structural repair with ACI at 36 months
- Clinical outcomes similar at 36 months
- ACI showed superiority in lesions greater than 4cm²
MACI vs Microfracture (SUMMIT Trial)
- 144 patients randomized MACI vs microfracture
- MACI superior KOOS at 2 years
- MRI showed better defect fill with MACI
- More consistent outcomes with MACI
OATS Long-Term Outcomes
- 831 patients with mosaicplasty
- 92% good to excellent at 10 years
- Donor site morbidity 3%
- Best for 2-4cm² defects
OCA 25-Year Survivorship
- 122 knees with fresh OCA
- 78% survivorship at 15 years
- 59% survivorship at 25 years
- Best for large lesions with bone loss
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Young Athlete with Chondral Defect
"A 25-year-old soccer player presents with knee pain after a twisting injury. MRI shows a 1.5cm² ICRS Grade 4 chondral defect on the medial femoral condyle. The knee is otherwise normal. How would you manage this?"
Scenario 2: Large Defect with Malalignment
"A 32-year-old woman has persistent medial knee pain. MRI shows a 4.5cm² full-thickness defect on the medial femoral condyle. Long leg films show 5° of varus. She has failed 6 months of conservative treatment. How would you proceed?"
Scenario 3: Failed Microfracture
"A 28-year-old returns 3 years after microfracture for a 2cm² medial femoral condyle defect. He has recurrent symptoms. MRI shows poor fill and subchondral cysts. Alignment is normal. What are your options?"
MCQ Practice Points
Size Thresholds
Q: What is the upper size limit for microfracture? A: Less than 2cm². Beyond this, consider OATS (2-4cm²) or ACI/MACI (greater than 4cm²).
Cartilage Type
Q: What type of cartilage does microfracture produce? A: Fibrocartilage (Type I collagen). Native articular cartilage is hyaline (Type II collagen). Fibrocartilage is mechanically inferior.
ACI Requirement
Q: What is required for successful ACI? A: Intact surrounding cartilage rim (shoulders). This is needed to contain the implanted chondrocytes and prevent leakage.
OATS Limitation
Q: What limits the size of defects treatable with OATS? A: Donor site availability. Harvesting too many plugs causes donor site morbidity. Typically limited to 2-4cm² defects.
Calcified Layer
Q: Why must the calcified cartilage layer be removed in microfracture? A: Prevents integration of repair tissue with subchondral bone. Leaving it causes delamination of the fibrocartilage layer.
Allograft Timing
Q: Why must osteochondral allografts be fresh? A: Chondrocyte viability. Cells die with freezing - must implant within 28 days of harvest for living cartilage.
Australian Context
Clinical Practice
- Microfracture widely available at all centres
- OATS requires training and equipment
- MACI available at specialized centres
- OCA limited by tissue banking logistics
- Combined procedures (HTO + cartilage) common
Funding and Access
- Debridement and microfracture covered under public system
- MACI PBS listed for specific indications
- Private health insurance variability
- Long wait times in public system
- Patient contribution often required for biologics
Orthopaedic Exam Relevance
Cartilage surgery is a common viva topic. Know the size-based algorithm cold. Be prepared to discuss why you would choose one procedure over another, address concomitant pathology, and understand the biological differences between repair tissue types.
ARTICULAR CARTILAGE INJURIES
High-Yield Exam Summary
Size-Based Algorithm
- •Less than 2cm² = Microfracture
- •2-4cm² = OATS/Mosaicplasty
- •Greater than 4cm² = ACI/MACI
- •Large + bone loss = OCA
Tissue Produced
- •Microfracture = Fibrocartilage (Type I)
- •OATS = Hyaline (transplanted)
- •ACI = Hyaline-like (Type II)
- •OCA = Hyaline (transplanted)
Prerequisites (SLIM)
- •Stability (ligaments intact)
- •Limb alignment (correct malalignment)
- •Intact menisci
- •Motivation for rehab
Microfracture Technique
- •Debride to stable vertical walls
- •Remove calcified cartilage layer
- •Awl holes 3-4mm deep, 3-4mm apart
- •Confirm fat droplets (marrow access)
Postoperative Protocol
- •CPM 6-8 hours/day for 6 weeks
- •NWB or TTWB 6-8 weeks
- •No impact for 4-6 months
- •Full return 9-12 months
Key Studies
- •SUMMIT trial: MACI superior to microfracture
- •Steadman: Microfracture 75% good at 11 years
- •Hangody: OATS 92% good at 10 years
- •Size matters for treatment selection