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🦴 MRI Knee Anatomy

A Student Teaching Guide — Structures, Sequences & Pathology

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1. Introduction to Knee MRI

Why MRI for the Knee?

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2. The Three Compartments of the Knee

┌─────────────────────────────────┐
│  1. Medial compartment          │
│  2. Lateral compartment         │
│  3. Patellofemoral compartment  │
└─────────────────────────────────┘

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3. MRI Planes — What Each Shows

Standard MRI Sequences

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4. Key Anatomical Structures on MRI

4A. Cruciate Ligaments

ACL arises from posterior lateral femoral condyle → anterior tibial plateau (oblique course) PCL is shorter, stronger; arises from posterior tibial plateau → medial femoral condyle

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4B. Collateral Ligaments

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4C. Menisci

1. Circumferential (bucket-handle) — most common; causes locking
2. Radial — extends inward from free edge
3. Horizontal — splits meniscus into upper/lower leaves
4. Flap — unstable fragment
5. Degenerative — irregular, diffuse; seen in older patients

MRI tip: Abnormal signal reaching the articular surface = tear. Signal confined to meniscal substance = intrasubstance degeneration (Grade 1–2, not a true tear).

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4D. Articular Cartilage

• Best sequences: PD fat-sat, 3D GRE, dGEMRIC
• Normal cartilage: smooth, uniform intermediate signal
• Femoral condyles, tibial plateaus, patella undersurface, trochlea all covered

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4E. Extensor Mechanism

Quadriceps muscle
      ↓
Quadriceps tendon
      ↓
    Patella
      ↓
Patellar tendon
      ↓
 Tibial tuberosity

• Best visualized on sagittal images
• T2/PD: normal tendons are low signal (dark); tears show high signal disruption

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4F. Bursae & Other Soft Tissues

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5. Axial MRI Series — Slice-by-Slice Guide

• Distal femoral condyles (top slices)
• Patellofemoral trochlear groove (mid slices)
• Proximal tibia and joint space (lower slices)

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6. Quick Reference: "Where is it on MRI?"

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7. Common Pathologies at a Glance

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8. Teaching Pearls for Students

💡 ACL rule of 45° — On a true sagittal MRI, the ACL should be parallel to the roof of the intercondylar notch (Blumensaat's line). If it is not, suspect a tear.

💡 "Dark on all sequences" — Normal ligaments and menisci are dark (low signal) on T1, T2, and PD. Any bright signal = pathology until proven otherwise.

💡 Meniscal "bow-tie" sign — On consecutive sagittal 5mm slices, you should see the meniscus as a bow-tie on ≥2 slices. Only 1 = bucket-handle tear.

💡 PCL integrity — The PCL is a reliable landmark; it is the last structure to tear. If the PCL is intact, the knee is likely not dislocated.

💡 Patellofemoral assessment — Always check patellar tilt and trochlear dysplasia on axial sequences.

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References

1. Harrison's Principles of Internal Medicine, 21st Edition — p. 10396
2. Bailey and Love's Short Practice of Surgery, 28th Edition — pp. 511, 585

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