MRI Cartilage Assessment
Imaging Articular Cartilage for Surgical Decision-Making
Modified Outerbridge MRI Grading
Grade 0: Normal cartilage — smooth surface, uniform thickness, normal signal
Grade 1: Focal signal abnormality with intact surface (softening/swelling)
Grade 2: Partial-thickness defect involving less than 50% of cartilage depth
Grade 3: Partial-thickness defect involving more than 50% depth, or full-thickness without bone exposure
Grade 4: Full-thickness cartilage loss with exposed subchondral bone
Key: Arthroscopy remains the gold standard — MRI provides a non-invasive estimate of cartilage integrity but tends to underestimate partial-thickness lesions
Critical Must-Knows
- Normal articular cartilage is 2-4mm thick and appears as a smooth, intermediate-signal layer on most sequences.
- PD fat-suppressed and T2 fat-suppressed are the standard sequences for morphological cartilage assessment.
- MRI is highly sensitive (70-95%) for full-thickness cartilage defects but less reliable (40-70%) for partial-thickness lesions.
- T2 mapping and dGEMRIC are quantitative techniques that detect biochemical cartilage changes BEFORE morphological damage is visible.
- The modified Outerbridge classification (Grades 0-4) is used to grade cartilage lesions on MRI.
Examiner's Pearls
- "3T MRI provides superior cartilage imaging compared to 1.5T due to improved signal-to-noise ratio and spatial resolution.
- "MR arthrography (direct, with intra-articular gadolinium) improves detection of partial-thickness cartilage lesions.
- "Normal cartilage shows a layered appearance on high-resolution imaging: the deep radial zone has lower signal than the superficial transitional zone.
- "Post-cartilage repair MRI (MOCART score) assesses: fill grade, integration, surface, signal, subchondral bone, effusion.
- "Cartilage cannot heal spontaneously — even small defects will progress without intervention, making early detection critical.
Exam Warning
MRI cartilage assessment is commonly examined in the context of knee and hip pathology. You must be able to: explain optimal sequences for cartilage imaging, grade cartilage lesions using the modified Outerbridge classification, discuss the role of quantitative techniques (T2 mapping, dGEMRIC), and explain how MRI findings guide surgical decision-making between microfracture, OATS, and ACI. A common exam trap is overestimating the sensitivity of MRI for partial-thickness lesions.
SOFBECartilage MRI Grading
Memory Hook:SOFBE: Signal, One-half, Full, Bone, Eburnation — the progression of cartilage destruction on MRI.
PIGOptimal Cartilage Sequences
Memory Hook:PIG sequences for cartilage: PD fat-suppressed is the workhorse, 3D GRE for quantitative mapping, and Gadolinium arthrography for tricky partial-thickness defects.
FISSBEMOCART Score Components
Memory Hook:FISSBE is the MOCART score framework for assessing cartilage repair on follow-up MRI.
Overview
Articular cartilage is a unique tissue that lacks intrinsic healing capacity due to its avascular nature and limited chondrocyte proliferative potential. Once damaged, cartilage defects tend to progress to larger lesions and eventually to osteoarthritis. MRI is the primary non-invasive modality for assessing articular cartilage, providing information about defect size, depth, location, and the condition of surrounding subchondral bone that guides surgical decision-making.
The challenge of cartilage MRI is that the tissue is thin (2-4mm), has subtle internal structural variation, and partial-thickness lesions can be difficult to distinguish from normal signal heterogeneity. This has driven the development of dedicated high-resolution cartilage sequences and quantitative imaging techniques that can detect biochemical changes in cartilage before macroscopic damage occurs.
Why Cartilage Assessment Matters
Cartilage defects of greater than 2cm² in the weight-bearing zone of the knee produce symptoms and risk progression to osteoarthritis. Treatment options depend on defect characteristics: microfracture for smaller defects (less than 2cm²), osteochondral autograft transfer (OATS) for 1-4cm² defects, and autologous chondrocyte implantation (ACI/MACI) for larger defects (greater than 2-4cm²). MRI provides the preoperative defect characterisation needed for surgical planning.
MRI vs Arthroscopy
Arthroscopy remains the gold standard for cartilage assessment — it allows direct visualisation, probing of the cartilage surface, and immediate treatment. However, MRI has the advantage of being non-invasive, showing the full extent of subchondral bone changes, revealing marrow oedema, and assessing areas inaccessible to the arthroscope. The correlation between MRI and arthroscopic findings is best for full-thickness defects (70-95% sensitivity) but less reliable for partial-thickness lesions (40-70% sensitivity).
Clinical Imaging
Imaging Gallery
Systematic Approach
Systematic Cartilage Assessment on MRI
Systematic Cartilage Assessment Framework
| Step | Assessment | Key Considerations |
|---|---|---|
| 1. Identify the sequence | PD-FS or T2-FS is most appropriate for cartilage morphology | Cartilage appears intermediate grey; fluid appears bright — this contrast is essential for detecting surface defects |
| 2. Assess cartilage thickness | Compare thickness across the articular surface; note focal thinning | Normal thickness varies by location: femoral condyle 2-3mm, patellar surface 3-5mm (thickest), tibial plateau 2-3mm |
| 3. Grade the defect | Apply modified Outerbridge classification (Grades 0-4) | Grade 1: signal change only. Grade 2: less than 50% depth. Grade 3: more than 50% depth. Grade 4: full-thickness with bone exposure |
| 4. Measure defect dimensions | Document the size (AP dimension, width, and depth in mm) | Size determines treatment: less than 2cm² = microfracture, 1-4cm² = OATS, more than 2-4cm² = ACI/MACI |
| 5. Assess subchondral bone | Look for oedema, cysts, sclerosis beneath the cartilage defect | Subchondral changes indicate chronicity and may affect surgical outcomes — large cysts may require bone grafting |
| 6. Check for associated pathology | Meniscal tears, ligament injuries, loose bodies, alignment | Cartilage defects rarely occur in isolation — associated pathology must be addressed simultaneously |
Cartilage Imaging Sequences
Standard Cartilage Imaging
PD fat-suppressed is the workhorse sequence for cartilage morphological assessment. It provides:
- High signal-to-noise ratio (long TR, short TE)
- Good cartilage-fluid contrast (cartilage is intermediate; fluid is bright)
- Sensitivity to surface defects and partial-thickness lesions
- Fat suppression eliminates competing signal from epiphyseal fat and marrow
3D Gradient Echo sequences (SPGR, FLASH, DESS, MERGE) provide isotropic thin-slice acquisitions (0.5-1mm) that enable multiplanar reformatting and quantitative thickness mapping. These are increasingly used for:
- Research studies requiring volumetric cartilage measurement
- Pre- and post-operative cartilage repair monitoring
- Cartilage thickness maps overlaid on 3D joint models
T2-weighted fat-suppressed can also be used for cartilage but has lower SNR than PD and may underestimate the extent of lesions that are better seen with PD contrast.
3T vs 1.5T: Higher field strength significantly improves cartilage imaging quality by increasing SNR and spatial resolution. 3T is preferred for dedicated cartilage assessment when available.
These remain the standard for clinical cartilage assessment.
Cartilage Repair Assessment
Monitoring cartilage repair on MRI is increasingly importantas the number of cartilage repair procedures grows. The MOCART (Magnetic Resonance Observation of Cartilage Repair Tissue) score is the standardised tool for post-operative cartilage repair assessment.
MOCART Score Assessment Criteria
| Variable | Best Outcome | Intermediate | Worst Outcome |
|---|---|---|---|
| Fill grade | Complete fill (100%) | Over-fill (hypertrophic) or slight under-fill (75-100%) | Significant under-fill (less than 50%) or complete graft failure |
| Integration | Complete integration — no visible cleft between repair and native cartilage | Partial integration — incomplete border with focal cleft | No integration — persistent cleft, delamination |
| Surface | Smooth surface, flush with native cartilage | Irregular surface, mild fibrillation | Fissured, severely irregular, or absent surface |
| Signal intensity | Isointense to native cartilage on all sequences | Mild hyperintensity (suggests more fibrous composition) | Markedly abnormal signal (suggests failure or fibrocartilage only) |
| Subchondral bone | Intact subchondral lamina with normal marrow | Mild irregularity or oedema | Subchondral overgrowth, large cysts, or persistent osteolysis |
Repair Tissue vs Hyaline Cartilage
On MRI, true hyaline cartilage repair tissue is isointense to surrounding native cartilage on all sequences. Fibrocartilage repair (as produced by microfracture) tends to have slightly different signal characteristics: often mildly hyperintense on PD/T2 with a less distinct layered appearance. This signal difference can help distinguish the type of repair tissue, which has prognostic implications — hyaline-like repair (from ACI/MACI) generally has better long-term durability than fibrocartilage.
Evidence Base
Accuracy of MRI for Articular Cartilage Lesions in the Knee
- MRI sensitivity for full-thickness cartilage defects was 72-94% depending on the sequence and field strength.
- Sensitivity for partial-thickness (Grade 1-2) lesions was significantly lower at 40-68%.
- 3T MRI improved detection compared to 1.5T, particularly for partial-thickness lesions.
MR Arthrography for Cartilage Assessment
- Direct MR arthrography improved sensitivity for partial-thickness cartilage defects to 85-92% compared to 55-65% for non-arthrographic MRI.
- Intra-articular gadolinium provided a natural contrast agent that outlined subtle surface irregularities invisible on standard sequences.
- Correlation with arthroscopy was excellent for defect location and depth grading.
MRI is best for full-thickness defects; arthrography improves partial-thickness detection.
Australian Context
In Australia, MRI cartilage assessment is an integral part of the orthopaedic workup for osteochondral defects, particularly in the knee. Medicare-funded knee MRI is available for clinical indications including internal derangement and suspected cartilage damage, with 3T scanners increasingly available at major Australian imaging centres.
The Australian Orthopaedic Association has endorsed national guidelines for cartilage repair that recommend pre-operative MRI with dedicated cartilage sequences (PD-FS at minimum) to characterise defect size and subchondral bone quality before surgical planning. Quantitative cartilage imaging (T2 mapping, dGEMRIC) is available at Australian research institutions and selected tertiary imaging centres, though it is not yet part of routine clinical protocols.
The AOANJRR indirectly reflects cartilage health through its tracking of knee replacement rates — early detection and treatment of cartilage defects is a key strategy for delaying or preventing arthroplasty. Australian Medicare funds both microfracture and ACI/MACI procedures, with MACI (matrix-associated autologous chondrocyte implantation) being more widely available in Australia than in many other countries.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
"A 28-year-old footballer has an MRI showing a 3cm² full-thickness cartilage defect on the medial femoral condyle with underlying subchondral oedema."
"An examiner asks you about quantitative MRI techniques for assessing cartilage health beyond standard morphological imaging."
"You review a follow-up MRI 12 months after ACI/MACI of the medial femoral condyle. The repair tissue is hyperintense on PD-FS compared to the surrounding native cartilage."
MRI Cartilage Assessment — Exam Day Reference
High-Yield Exam Summary
Optimal Sequences
- •PD fat-suppressed: standard morphological cartilage sequence
- •3D GRE (SPGR/FLASH/DESS): thin isotropic slices for quantitative mapping
- •MR arthrography: improves partial-thickness defect detection (85-92% sensitivity)
- •3T preferred over 1.5T for cartilage assessment
Outerbridge MRI Grading
- •Grade 0: Normal cartilage
- •Grade 1: Signal change, intact surface (softening)
- •Grade 2: Partial-thickness less than 50% depth
- •Grade 3: Partial-thickness more than 50% depth or full-thickness without bone
- •Grade 4: Full-thickness with exposed subchondral bone
Treatment by Defect Size
- •Less than 2cm²: Microfracture (fibrocartilage result)
- •1-4cm²: OATS (true hyaline cartilage, donor site morbidity)
- •More than 2-4cm²: ACI/MACI (hyaline-like repair, two-stage)
- •Always address alignment (HTO), meniscal status, and ligament stability
Quantitative Techniques
- •T2 mapping: collagen integrity and water content (non-invasive, no contrast)
- •dGEMRIC: GAG content (requires IV gadolinium + 90-min delay)
- •T1rho: proteoglycan content (research, not widely available)
- •All detect biochemical changes BEFORE morphological damage
MOCART Score (Post-Repair)
- •Fill grade, Integration, Surface, Signal, Bone interface, Effusion
- •Isointense signal to native cartilage = best outcome
- •Hyperintense signal = more fibrocartilaginous or immature
- •Fill grade and integration are strongest outcome predictors