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
Clinical 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
| D | Debridement First step, remove loose fragments |
| M | Microfracture Less than 2cm², creates fibrocartilage |
| O | OATS 2-4cm², transfers hyaline plugs |
| A | ACI/MACI Greater than 4cm², cultured chondrocytes |
| C | Chondral allograft Large defects with bone loss |
| D | Debridement First step, remove loose fragments | A | ACI/MACI Greater than 4cm², cultured chondrocytes |
| M | Microfracture Less than 2cm², creates fibrocartilage | C | Chondral allograft Large defects with bone loss |
| O | OATS 2-4cm², transfers hyaline plugs |
Hook:Doctors Make Orthopaedic Algorithms Clear - size determines step!
PADSMicrofracture Technique
| P | Prepare the defect Remove calcified layer, stable shoulders |
| A | Awl the base 3-4mm depth, 3-4mm apart |
| D | Drainage of blood Fat droplets (marrow access) is key |
| S | Slow rehab NWB/TTWB 6-8 weeks, CPM |
| P | Prepare the defect Remove calcified layer, stable shoulders | D | Drainage of blood Fat droplets (marrow access) is key |
| A | Awl the base 3-4mm depth, 3-4mm apart | S | Slow rehab NWB/TTWB 6-8 weeks, CPM |
Hook:PADS protect the healing fibrocartilage!
SLIMPrerequisites for Cartilage Surgery
| S | Stability No ligament laxity (ACL intact) |
| L | Limb alignment Correct varus/valgus first |
| I | Intact meniscus Or address meniscal deficiency |
| M | Motivation Compliant patient for rehab |
| S | Stability No ligament laxity (ACL intact) | I | Intact meniscus Or address meniscal deficiency |
| L | Limb alignment Correct varus/valgus first | M | Motivation Compliant patient for rehab |
Hook:Keep your cartilage patient SLIM (address all these first)!
DONOROATS Considerations
| D | Defect contained Needs stable surrounding cartilage |
| O | One or few plugs Limited donor availability |
| N | Non-weight bearing zone Harvest from periphery |
| O | Orthotopic placement Curvature match important |
| R | Resurfacing complete Flush with articular surface |
| D | Defect contained Needs stable surrounding cartilage | O | Orthotopic placement Curvature match important |
| O | One or few plugs Limited donor availability | R | Resurfacing complete Flush with articular surface |
| N | Non-weight bearing zone Harvest from periphery |
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
Brittberg - First-in-Human Autologous Chondrocyte Implantation
- Landmark first clinical series of ACI in 23 patients (defects 1.6 to 6.5 cm²)
- Cultured autologous chondrocytes injected under a sutured periosteal flap
- 14 of 16 femoral condylar transplants good to excellent at 2 years
- Patellar lesions did markedly worse than femoral condylar lesions
- Biopsy showed hyaline-like cartilage in 11 of 15 femoral grafts
Steadman - Microfracture for Traumatic Chondral Defects (11-Year)
- 72 patients (75 knees), age 45 years and younger, isolated traumatic full-thickness defects
- Average 11.3-year follow-up (range 7 to 17 years), 95% retention
- Lysholm improved from 59 to 89, Tegner from 3 to 6
- 80% of patients rated themselves improved at 7 years
- Younger age predicted greater functional improvement
Knutsen - ACI vs Microfracture RCT (5-Year)
- 80 patients randomized to first-generation ACI (n=40) vs microfracture (n=40)
- Both groups improved significantly at 2 and 5 years
- No significant clinical or radiographic difference between groups at 5 years
- 9 failures (23%) in EACH group by 5 years
- No correlation between histological repair quality and clinical outcome
Knutsen - ACI vs Microfracture, 14 to 15-Year Follow-up
- Long-term follow-up of the original 80-patient RCT
- No significant difference between ACI and microfracture at 14 to 15 years
- 17 failures (ACI) vs 13 (microfracture); more TKAs in ACI group (6 vs 3)
- Around half of all patients had radiographic OA (Kellgren-Lawrence 2 or higher)
- Authors raise concern that neither procedure reliably prevents OA
Saris - Characterized Chondrocyte Implantation vs Microfracture (36-Month Clinical)
- 118 patients randomized to characterized chondrocyte implantation (n=57) vs microfracture (n=61)
- Significantly greater overall KOOS improvement with CCI at 36 months (P=0.048)
- More treatment responders with CCI (83% vs 62%)
- Benefit greatest when symptom onset was under 3 years
- Companion 2008 paper (PMID 18202295) showed superior structural/histological repair at 1 year
SUMMIT Trial - MACI vs Microfracture (2-Year RCT)
- 144 patients with symptomatic defects 3 cm² or larger (mean lesion 4.8 cm²)
- MACI significantly better than microfracture for co-primary KOOS pain and function at 2 years (P=0.001)
- Fewer treatment failures with MACI (12.5% vs 31.9%)
- Histology and MRI repair-tissue quality similar between groups
- No unexpected safety signals
Hangody - Autologous Osteochondral Mosaicplasty (10-Year Experience)
- 831 patients undergoing mosaicplasty over 10 years
- Good to excellent results in 92% of femoral condylar implantations
- Lower success at the patella/trochlea (79%) than femoral condyle
- Long-term donor-site morbidity only 3% (Bandi score)
- Arthroscopic biopsies confirmed survival of transplanted hyaline cartilage
Levy/Bugbee - Fresh Osteochondral Allograft of the Femoral Condyle
- 122 patients (129 knees), median 13.5-year follow-up
- Graft survivorship 82% at 10 years, 74% at 15 years, 66% at 20 years
- Durable improvement in pain and function scores
- Age over 30 years and 2 or more prior knee surgeries predicted failure
- 47% underwent a reoperation but most retained the graft
Mithoefer - Microfracture Evidence-Based Systematic Review
- Systematic review of 28 studies, 3122 patients
- Consistent functional improvement in the first 24 months
- Conflicting evidence on durability beyond 2 years
- Variable defect fill on MRI correlated with functional outcome
- Limited hyaline repair tissue and possible functional deterioration over time
ICRS Cartilage Injury Classification (Reference Standard)
- Standardized arthroscopic grading (0 to 4) of chondral lesions
- Grade-3 subdivided (3A to 3D) by depth into the calcified layer
- Includes ICRS Cartilage Repair Assessment (CRA) for repair tissue
- Globally adopted reporting standard for cartilage studies
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
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.
Guidelines, Registries & Global Practice
Global Epidemiology
- Focal chondral lesions seen in roughly 60% of knee arthroscopies
- Full-thickness (ICRS 3 to 4) defects in around 5 to 11% of arthroscopies
- Peak presentation 30 to 50 years; sports and trauma common in younger patients
- Medial femoral condyle and patella most frequent sites
- Untreated full-thickness defects are a recognized precursor to osteoarthritis
Registry & Approval Notes
- MACI (matrix-applied ACI) holds FDA approval (US, 2016) and EMA approval for symptomatic femoral condylar defects
- First-generation periosteal ACI largely abandoned for membrane/matrix techniques
- No single dominant cartilage-repair registry; data come mostly from RCTs and institutional series
- Fresh OCA depends on tissue-bank logistics and is concentrated in higher-resource centres
- Cell-therapy products vary widely in regulatory status by region
Society Guidance - Side by Side
| Body | Position on Microfracture | Position on ACI/MACI | Emphasis |
|---|---|---|---|
| NICE (UK) | Standard option for small defects | ACI recommended option for defects over 2cm² with no prior repair and intact rim | Defect size and treatment-naive status drive ACI eligibility |
| AAOS / ICRS (consensus) | First-line for small contained lesions | Preferred for larger lesions; correct alignment/stability first | Patient selection and concomitant pathology |
| ESSKA / European consensus | Acceptable for lesions under 2 to 2.5cm² | Favoured for larger lesions and active patients | Lesion size threshold and activity demand |
High vs Limited-Resource Practice
In high-resource settings the full ladder (microfracture, OATS, ACI/MACI, fresh OCA) is available, often combined with osteotomy or meniscal transplant. In limited-resource settings, debridement and microfracture predominate because they are single-stage, low-cost, and need no cell-culture facility or tissue bank. Cell-based therapy and fresh allograft are restricted by cost, cold-chain logistics, and regulatory approval rather than by clinical indication.
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. A strong candidate also acknowledges that long-term RCT data (Knutsen) show no clear winner and that no procedure reliably prevents osteoarthritis.
Differential Diagnosis
Distinguishing Causes of Focal Knee Pain and Effusion
| Condition | Key Distinguishing Feature | Best Investigation | Why It Matters |
|---|---|---|---|
| Focal chondral defect | Activity-related pain, mechanical symptoms, well-localized lesion | MRI (T2 map) / arthroscopy | Index diagnosis - size and depth drive treatment |
| Osteochondritis dissecans (OCD) | Subchondral bone fragment, younger patient, often stable lesion | MRI assesses fragment stability | May be amenable to fixation rather than resurfacing |
| Osteoarthritis | Joint space narrowing, osteophytes, bipolar/diffuse change | Weight-bearing radiographs | Contraindicates isolated cartilage repair |
| Meniscal tear | Joint-line tenderness, positive McMurray, no full-thickness cartilage loss | MRI | Often coexists - address concurrently |
| Subchondral insufficiency fracture / SPONK | Older patient, acute pain, marked bone marrow oedema | MRI | Resurfacing inappropriate; treat the bone |
| Patellofemoral pain / chondromalacia | Anterior pain, stairs/squatting, often no discrete defect | Clinical + skyline view | Address maltracking before any cartilage surgery |
Controversies and Areas of Uncertainty
Does microfracture cause harm?
Subchondral bone overgrowth (intralesional osteophyte) and subchondral cysts after microfracture may compromise later restorative surgery. Some surgeons now prefer marrow stimulation augmented with scaffolds (AMIC) or move straight to cell therapy in larger lesions.
Is ACI/MACI truly superior?
Industry RCTs (SUMMIT, Saris) favour cell therapy for larger lesions, but independent long-term RCTs (Knutsen 5 and 15 year) show no clear advantage over microfracture and no protection against OA. Cost-effectiveness remains debated.
Size thresholds are not absolute
The 2cm² and 4cm² cut-offs are pragmatic teaching figures. Real decisions weigh location, depth, bone involvement, patient age, activity, and prior surgery - not size alone.
Emerging biologics
Minced/particulated cartilage, scaffold-based one-step techniques, BMAC and PRP augmentation, and allogeneic cell products are under active study but lack long-term high-level evidence.
ARTICULAR CARTILAGE INJURIES
Clinical 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