High Yield Overview
MRI Cartilage Assessment Techniques
—Cartilage Thickness (Knee)
2-4mm
—MRI Sensitivity
70-95%for defects
—Best Sequence
3PD fat-sat or D GRE
—Resolution Required
0Less than .5mm for grading
ICRS Cartilage Grading
Grade 0: Normal
Grade 1: Superficial lesion, softening
Grade 2: Partial thickness (less than 50%)
Grade 3: Deep defect (greater than 50%, not full thickness)
Grade 4: Full thickness to subchondral bone
Key: Grade 3-4 lesions may benefit from cartilage repair procedures
Critical Must-Knows
- Normal cartilage: intermediate signal on all sequences
- PD fat-sat optimal for surface assessment
- 3D GRE (DESS, MEDIC) for morphologic detail
- Modified Outerbridge/ICRS classification for grading
- T2 mapping detects early biochemical changes
Examiner's Pearls
- "Cartilage defects: focal high T2 signal, surface irregularity
- "Full-thickness defect: subchondral bone exposed
- "OCD: unstable if fluid signal beneath fragment
- "Articular surface best seen on fluid-sensitive sequences
- "Bone marrow oedema often accompanies cartilage injury
Clinical Imaging
Imaging Gallery
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Exam Warning
Cartilage assessment requires understanding of optimal sequences and grading systems. Know the ICRS classification and how to identify full-thickness defects. For OCD lesions, determine stability based on fluid signal at the fragment-bed interface.
Cartilage MRI Sequences
Sequences for Cartilage Assessment
| Sequence | Strengths | Limitations |
|---|---|---|
| PD fat-sat | Good contrast, widely available, reliable | Resolution may limit small defects |
| 3D GRE (DESS, MEDIC) | High resolution, isotropic, reformats | Longer scan time |
| T2 mapping | Biochemical assessment, detects early changes | Requires special software, time-consuming |
| T1rho mapping | Sensitive to proteoglycan loss | Research application, limited availability |
| dGEMRIC | Proteoglycan content assessment | Requires contrast, lengthy protocol |
Standard Clinical Protocol
PD fat-sat in multiple planes is the workhorse sequence for cartilage assessment in clinical practice. 3D gradient echo sequences (DESS, FLASH, MEDIC) provide higher resolution and multiplanar reformats. T2 mapping and other compositional techniques are research/specialised tools.
Normal Cartilage Appearance
Normal Cartilage MRI Characteristics
| Feature | Description |
|---|---|
| Signal (T1) | Intermediate, homogeneous |
| Signal (T2/PD) | Intermediate, slight laminar appearance |
| Signal (fat-sat) | Intermediate, contrasts with dark bone and bright fluid |
| Thickness | 2-4mm in weight-bearing areas of knee |
| Surface | Smooth, sharp interface with joint fluid |
| Deep margin | Well-defined interface with subchondral plate |
Cartilage Layers
Normal hyaline cartilage shows subtle zonal stratification: superficial zone (tangential collagen), middle zone (random), and deep zone (radial collagen). On high-resolution MRI, this may appear as subtle signal variation. The tidemark (calcified cartilage interface) appears as a thin dark line.
Cartilage Grading Systems
ICRS Cartilage Lesion Grading
| Grade | Description | MRI Appearance |
|---|---|---|
| Grade 0 | Normal | Intact surface, normal signal |
| Grade 1 | Nearly normal | Superficial softening, surface intact |
| Grade 2 | Abnormal, less than 50% depth | Partial thickness defect, less than half thickness |
| Grade 3A | Severely abnormal, greater than 50% depth | Deep defect, not full thickness |
| Grade 3B | Severely abnormal, to calcified layer | Defect reaches calcified zone |
| Grade 3C | Severely abnormal, to subchondral plate | Full depth to bone, plate intact |
| Grade 3D | Severely abnormal, with blistering | Delamination, fluid-filled blister |
| Grade 4 | Severely abnormal, full thickness | Defect through subchondral plate |
Cartilage Defect Types
Types of Cartilage Lesions
| Lesion Type | Cause | MRI Features |
|---|---|---|
| Focal chondral defect | Trauma, degeneration | Well-defined cartilage loss, normal adjacent cartilage |
| Diffuse thinning | Osteoarthritis | Generalised thickness loss, bone changes |
| Osteochondral lesion | Trauma, OCD | Cartilage + subchondral bone involvement |
| Delamination | Shear injury | Fluid undermining cartilage from bone |
| Cartilage flap | Trauma | Displaced cartilage fragment |
Chondral Defect MRI Features
Focal area of cartilage loss or thinning. High T2 signal from joint fluid filling defect. Sharp or fibrillated margins. Underlying bone marrow oedema common. Size and location important for treatment planning.
Associated Findings
Bone marrow oedema (reactive). Subchondral cysts. Subchondral sclerosis. Osteophytes. Loose bodies (displaced fragments). These features suggest mechanical damage and degeneration.
Osteochondritis Dissecans (OCD)
OCD Stability Assessment on MRI
| Finding | Stable | Unstable |
|---|---|---|
| Fluid signal at interface | Absent | High T2 signal surrounding fragment |
| Fragment displacement | In situ | Displaced or hinged |
| Cystic change at bed | Minimal/absent | Prominent cysts |
| Articular surface | Intact or minimal breach | Disrupted, defect present |
| Fragment size | Often smaller | Large fragments more concerning |
Mnemonic
FLUID = UnstableOCD Stability Signs
F
F = Fluid signal around fragment
L
L = Loose body in joint
U
U = Underlying cysts at bed
I
I = Interface disruption
D
D = Displaced fragment
Memory Hook:High T2 signal completely surrounding the OCD fragment indicates fluid undermining and an unstable lesion requiring surgery
OCD Location
Knee: Most common on lateral aspect of medial femoral condyle (classic location). Ankle: Medial or lateral talar dome. Elbow: Capitellum. Hip: Femoral head (rare). Location affects treatment approach and prognosis.
Compositional Imaging
Quantitative Cartilage MRI Techniques
| Technique | What It Measures | Clinical Application |
|---|---|---|
| T2 mapping | Collagen integrity, water content | Detects early degeneration before morphologic change |
| T1rho mapping | Proteoglycan content | Research, early OA detection |
| dGEMRIC | Glycosaminoglycan content | Post-repair assessment, requires Gd |
| Sodium MRI | Proteoglycan (fixed charge density) | Research, requires special coil |
| gagCEST | Glycosaminoglycan | Emerging technique |
T2 Mapping in Practice
Elevated T2 values indicate collagen disorganisation and increased water content - early cartilage degeneration. Normal articular cartilage T2: 25-50ms. Elevated T2 precedes morphologic changes visible on standard sequences. Used for early OA detection and monitoring repair tissue.
Post-Repair Assessment
Cartilage Repair MRI Assessment (MOCART)
| Parameter | Assessment |
|---|---|
| Defect fill | Complete, hypertrophy, incomplete (less than 50%, less than 100%) |
| Integration | Complete, demarcating border, split-like (delamination) |
| Surface | Intact, damaged but greater than 50%, damaged less than 50% |
| Structure | Homogeneous, inhomogeneous, cleft formation |
| Signal intensity | Normal (isointense), abnormal |
| Subchondral lamina | Intact, not intact |
| Subchondral bone | Intact, oedema, cysts, sclerosis |
| Adhesions | Present, absent |
| Effusion | Present, absent |
Repair Tissue vs Native Cartilage
Ideal repair: fills defect, integrates at borders, smooth surface, isointense to native cartilage. Fibrocartilage repair (microfracture): often lower signal than hyaline, may show poor integration. MACI/ACI: aim for hyaline-like tissue. T2 mapping helps assess repair tissue quality.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A 16-year-old presents with knee pain after sports. MRI shows a well-defined osteochondral lesion on the lateral aspect of the medial femoral condyle with high T2 signal surrounding the fragment."
EXCEPTIONAL ANSWER
This is an osteochondritis dissecans (OCD) lesion in the classic location (lateral aspect of medial femoral condyle). The high T2 signal surrounding the fragment indicates fluid undermining, which is a sign of instability. I would also look for: complete fluid rim, cysts at the bed, articular surface disruption, and any displaced fragments. An unstable OCD in a skeletally immature patient (open physes) may still heal with activity modification, but a large unstable lesion typically requires surgical intervention - options include drilling/microfracture if small, or fixation of the fragment if suitable.
KEY POINTS TO SCORE
Classic location: lateral aspect of medial femoral condyle
High T2 signal surrounding fragment = unstable
Also assess: cysts, surface integrity, displacement
Skeletal maturity affects prognosis
Unstable lesions usually require surgery
COMMON TRAPS
✗Missing the surrounding fluid signal
✗Not assessing all stability criteria
✗Not checking skeletal maturity
VIVA SCENARIOStandard
EXAMINER
"A 50-year-old presents for MRI knee before high tibial osteotomy planning. The surgeon asks you to grade the cartilage in all compartments."
EXCEPTIONAL ANSWER
I would systematically assess each compartment using PD fat-sat sequences: (1) Medial compartment - medial femoral condyle and medial tibial plateau, assess weight-bearing areas. (2) Lateral compartment - lateral femoral condyle and lateral tibial plateau. (3) Patellofemoral - patella facets and trochlea. For each surface, I would report: location of defects, depth using ICRS grading (0-4), size of defects (in mm or cm²), and presence of associated bone changes (oedema, cysts, osteophytes). For HTO planning, the lateral compartment cartilage is critical - surgery requires reasonable lateral cartilage to be successful.
KEY POINTS TO SCORE
Assess all three compartments systematically
Use ICRS or Outerbridge grading
Report location, size, and depth of defects
Assess associated bone changes
Lateral compartment status critical for HTO
COMMON TRAPS
✗Missing the patellofemoral compartment
✗Not using standardised grading
✗Not commenting on lateral compartment for HTO
VIVA SCENARIOStandard
EXAMINER
"A 30-year-old had microfracture for a chondral defect 18 months ago. Follow-up MRI is requested to assess repair tissue."
EXCEPTIONAL ANSWER
I would use the MOCART scoring system to assess: (1) Defect fill - ideally complete fill without hypertrophy. (2) Integration - borders should integrate with native cartilage without gaps. (3) Surface - smooth, congruent with adjacent cartilage. (4) Signal intensity - repair tissue should be isointense to native cartilage on PD sequences; lower signal suggests fibrocartilage. (5) Subchondral bone - assess for normalisation of oedema, resolution of cysts. (6) Structural homogeneity - homogeneous is ideal. A good outcome shows complete fill, good integration, smooth surface, and isointense signal. Microfracture typically produces fibrocartilage, so some signal difference from native hyaline cartilage is expected.
KEY POINTS TO SCORE
Use MOCART scoring system
Assess fill, integration, surface, signal
Microfracture = fibrocartilage (lower signal)
Compare signal to native cartilage
Subchondral bone normalisation important
COMMON TRAPS
✗Expecting hyaline-like signal after microfracture
✗Not using standardised scoring
✗Missing subchondral bone assessment
Cartilage MRI Quick Reference
High-Yield Exam Summary
Optimal Sequences
- •PD fat-sat: Clinical workhorse
- •3D GRE (DESS): High resolution
- •T2 mapping: Biochemical assessment
- •Normal cartilage: Intermediate signal
ICRS Grading
- •Grade 0: Normal
- •Grade 1: Softening (less than 50% depth)
- •Grade 2: Partial thickness (less than 50%)
- •Grade 3: Deep defect (greater than 50%)
- •Grade 4: Full thickness to bone
OCD Stability
- •Stable: No fluid signal around fragment
- •Unstable: Fluid surrounding fragment
- •Also assess: cysts, surface, displacement
- •Location: Lateral MFC classic
Post-Repair (MOCART)
- •Fill: Complete vs incomplete
- •Integration: Borders with native
- •Surface: Smooth vs irregular
- •Signal: Isointense = good