How to Read an MRI Knee: A Surgeon's Systematic Approach

Visual Element: An interactive slider comparing a T1-weighted coronal slice (anatomy) with a T2-weighted coronal slice (pathology) of the same knee, highlighting the signal differences.

As an orthopaedic surgeon, the ability to independently and accurately interpret Magnetic Resonance Imaging (MRI) is not just a "nice-to-have" skill—it is a fundamental requirement. Relying solely on the radiologist's report is a dangerous habit that can lead to missed diagnoses, inappropriate surgical planning, and ultimately, poor patient outcomes.

Radiology reports, while invaluable, can be hedged, may miss critical clinical context (e.g., specific mechanism of injury), or can simply be incorrect. You, the surgeon, have the advantage of having examined the knee. You know where it hurts. You know if there is a mechanical block to extension. You must be the master of your own imaging.

This comprehensive guide outlines a systematic, "surgeon-focused" approach to reading a knee MRI, ensuring you never miss the primary pathology—or the subtle secondary injuries that change the management plan.

Part 1: The Basics of MRI Physics (For Surgeons)

Before diving into anatomy, you must understand what you are looking at. You don't need a PhD in physics, but you need to know your sequences.

T1 Weighted Images

• Physics: Short TE (Time to Echo) and TR (Repetition Time).
• Appearance: Fluid is DARK. Fat is BRIGHT.
• Utility: This is your "Anatomy" sequence. It provides the best definition of bony architecture and is excellent for evaluating the marrow signal (e.g., osteonecrosis, stress fractures, infiltrative processes).
• Mnemonics: "T1 is for Anato-1-my".

T2 Weighted Images (Fat Suppressed / PD)

• Physics: Long TE and TR.
• Appearance: Fluid is BRIGHT. Fat is usually DARK (if fat-suppressed).
• Utility: This is your "Pathology" sequence. Most pathology in the knee (tears, edema, cysts, bruising) has high water content and will light up like a Christmas tree.
• Variations:
  • Proton Density (PD): Often the workhorse for meniscal pathology. It offers a balance between anatomical detail and fluid signal.
  • STIR (Short Tau Inversion Recovery): The ultimate fat suppression. If there is edema anywhere, STIR will find it. Great for bone bruising.

The Planes

• Sagittal: The most useful plane. Best for ACL, PCL, Menisci, and Extensor mechanism.
• Coronal: Best for Collateral ligaments (MCL/LCL), Meniscal roots, and Osteochondral defects.
• Axial: Best for Patellofemoral joint (tracking, dysplasia) and Patellar tendon.

Part 2: The "Inside-Out" Systematic Checklist

Visual Element: A schematic diagram illustrating the "Inside-Out" review path: Bone -> Central Pivot -> Menisci -> Collaterals -> Extensor Mechanism -> Soft Tissues.

To avoid "satisfaction of search" (finding one thing and stopping), you must adopt a rigid, unshakeable routine. The "Inside-Out" method is a robust framework.

1. Bone (All 3 Planes)

Start here. Bone bruising patterns tell you the story of the injury mechanism even before you look at the ligaments.

• Bone Bruising Patterns:
  • Pivot Shift Injury: A "kissing contusion" on the lateral femoral condyle (middle third) and posterior lateral tibial plateau. This is pathognomonic for an ACL rupture.
  • Dashboard Injury: Contusion on the anterior proximal tibia. Suspect PCL.
  • Patellar Dislocation: Contusion on the medial patella and lateral femoral condyle.
• Fractures: Look for occult tibial plateau fractures (Segond fracture, reverse Segond) or tibial spine avulsions.
• Osteochondral Defects (OCD): Inspect the articular surface carefully. Is there fluid undercutting the fragment? Is the fragment displaced?
• Infiltrative Processes: Check the marrow signal on T1. Is it replaced by dark signal (tumor, infection)?

2. The Central Pivot (Sagittal & Coronal)

Anterior Cruciate Ligament (ACL)

• Sagittal View: This is your money view. The ACL should look like a taut, dark band running from the posterior femur to the anterior tibia.
  • Direct Signs: Discontinuity, "horizontalization" of the distal fibers (Blumensaat's line mismatch), or a "cloud-like" amorphous mass.
  • Indirect Signs: Anterior tibial translation (>7mm), PCL buckling.
• Coronal View: Look for the "Empty Notch Sign" where fluid fills the space usually occupied by the ACL on the lateral wall.

Posterior Cruciate Ligament (PCL)

• Sagittal View: The "Black Banana." It should be thick, dark, and uniformly curved.
• Check: Trace it from the medial femoral condyle to the posterior tibial shelf. It is rarely torn in isolation without significant trauma.

3. The Menisci (Sagittal & Coronal)

The most commonly misdiagnosed structures. You must toggle between Sagittal and Coronal views constantly.

Medial Meniscus (MM)

• Sagittal: Look for the "Bow-tie" sign on the peripheral slices (should see 2 consecutive bow-ties). As you move centrally, the posterior horn is significantly larger than the anterior horn.
  • Trap: If the posterior horn is smaller or absent, suspect a displaced bucket handle tear.
• Coronal: Look for the triangular shape. Check the root attachment at the posterior tibial spine.
• Signs of Tears:
  • Linear signal intersecting the articular surface.
  • Double Delta Sign: Two triangles posteriorly (bucket handle fragment).
  • Ghost Sign: Absence of the meniscus in a slice where it should be.

Lateral Meniscus (LM)

• Sagittal: The anterior and posterior horns are roughly equal in height (bow-tie shape persists longer).
• Coronal: Check the popliteal hiatus. The popliteus tendon runs through the joint here—don't mistake it for a tear.
• Roots: The posterior root of the lateral meniscus is a common site for radial tears, especially in ACL injuries.

4. The Collateral Ligaments (Coronal)

Medial Collateral Ligament (MCL)

• Anatomy: It has superficial and deep layers.
• Review: Scan from the medial femoral epicondyle to the proximal tibia. Look for surrounding edema.
• Grading:
  • Grade 1: Periligamentous edema, intact fibers.
  • Grade 2: Partial disruption, some fiber continuity.
  • Grade 3: Complete disruption, often with retraction ("wavy" appearance).

Lateral Collateral Ligament (LCL) & Posterolateral Corner (PLC)

• Anatomy: The "Dark Side of the Knee." Includes LCL, Popliteus tendon, and Popliteofibular ligament.
• Review: Find the fibular head on the coronal view and trace the dark band of the LCL up to the femur.
• Pearl: Always compare symmetry with the contralateral side if available, or look for asymmetry in the soft tissue edema.

5. The Extensor Mechanism (Sagittal & Axial)

Patella & Trochlea

• Axial View: Assess patellar tilt and subluxation. Look for Trochlear Dysplasia (a flat or convex trochlear groove).
• Cartilage: Check the retro-patellar cartilage facets. This is the most common site for early OA in young patients.

Tendons

• Quadriceps Tendon: Check the multi-layered insertion.
• Patellar Tendon: Look for thickening or signal change at the inferior pole of the patella (Jumper's Knee).

6. The "Corners" and Soft Tissues

Finally, do a "perimeter check."

• Popliteal Fossa: Look for a Baker's Cyst. Trace its neck between the medial gastrocnemius and semimembranosus.
• Loose Bodies: Check the gutters and the posterior pouch.
• Incidentalomas: Check the field of view for distal femoral tumors or vascular malformations.

Part 3: Reporting and Synthesis

Once you have completed your checklist, synthesize the findings. Do not just list pathology; tell a story.

Bad Synthesis: "ACL tear. Bone bruise. MCL sprain."

Good Synthesis: "Acute complete mid-substance ACL rupture with characteristic pivot-shift bone bruising. Associated Grade 2 MCL injury and a vertical peripheral tear of the posterior horn of the lateral meniscus. The extensor mechanism is intact."

The "Second" Injury

Always ask yourself: If I fix the obvious thing, what will make my repair fail?

• Missed Meniscal Root tear? -> Accelerated OA.
• Missed PLC injury? -> ACL graft failure.
• Missed Ramp lesion? -> Instability.

Conclusion

Reading an MRI is a pattern-recognition skill that requires thousands of repetitions. Start today. Open the images before you read the report. Commit to a diagnosis, then check the report. If you disagree, go back and look again. Talk to your musculoskeletal radiologists—they are your best teachers.

By adopting this systematic "Inside-Out" approach, you move from being a passive consumer of information to an active diagnostic expert, ensuring the best possible care for your patients.