Decoding the Image Part II: Understanding your knee imaging

Why the knee? Well, time and again, patients confide in me that they find themselves nodding along as their doctor points to MRI images, while secretly thinking, "What am I actually looking at here?" Sound familiar? We've all been there! That's exactly why we created this newsletter – to bridge the gap between medical imaging and understanding.

In this issue, we'll walk you through everything you need to know about knee X-Rays and MRIs in clear, simple terms. From ligaments to menisci, cartilage to bone, we'll help you understand what those mysterious gray and white images really mean for your health. Let's demystify your knee imaging together!

Understanding X-Rays: The Basics

X-rays show the bones of your knee joint. Think of them as shadows – dense structures like bones appear white, while soft tissues appear in shades of gray, and air appears black. They're particularly useful for seeing alignment, arthritis, and fractures.

Pictured left: A normal X-Ray of the knee

Key Landmarks on Knee X-Rays

1. The Three Main Bones

   • Femur (thighbone) – the large bone at the top

   • Tibia (shinbone) – the large bone at the bottom

   • Patella (kneecap) – the circular bone in front

2. Joint Spaces

   • Medial (inner) compartment

   • Lateral (outer) compartment

   • Patellofemoral (kneecap) compartment

3. Growth Plates (in children)

   • Appear as dark lines through the bones

   • Typically closes at the time of puberty

Different X-Ray Views

1. Front View (AP)

   • Shows overall alignment

   • Best for seeing joint spaces

   • Shows arthritis severity

2. Side View (Lateral)

   • Shows kneecap position

   • ACL injuries may show subtle signs

   • Shows fluid in joint

3. Sunrise/Merchant View

   • Shows kneecap tracking

   • Reveals patellofemoral arthritis

What we are looking for on a Knee X-Ray

• Narrowing of the joint space on one or both sides

• Irregular (or jagged) surfaces of the joint

• Obvious breaks or fractures

• Normal alignment of bones and positioning of the patella

Take a look at the X-Ray picture below and see if you can tell what is wrong. Hover over the picture with your cursor to reveal the answer.

The patella is dislocated laterally, it should be in the center of the knee

Look at the X-Ray below and take a guess on what you are looking at.

Notice the decrease in medial (inside) joint space in the picture on the left. This indicates an arthritic knee. The right is a normal X-Ray.

Understanding your MRI: A Detailed View

MRI’s are challenging to read, even for the most trained eyes. You really need to know your anatomy to decipher what you are looking at. There is a website that helps label everything as you scroll listed below, and I encourage everyone to take a look. Remember, MRIs shows soft tissues that X-rays can't capture. Each image is like a slice through your knee from different angles.

Here is a lateral view of the knee, meaning we are looking at the knee from the side with the slice from almost the exact middle of the knee.

Here is an axial view of the knee, meaning we are looking directly above your knee, right in the middle of the joint.

How to Read MRI Colors

There are 2 different types of MRI’s; T2 where the bone & fluid are light colored, and T1 where the bone and fluid is dark).

• For example: In a T2- weighted MRI:

  • Black = Air and dense bone

  • White = Fluid, swelling, and some tendons

  • Gray = Most muscles and normal tissues

  • Dark Gray = Tendons and ligaments

Key Structures to Identify on MRI

1. Menisci (In the joint space; absorbs impact and stabilizes the knee)

   • Appear as dark triangular structures

   • Located between femur and tibia

   • One on inner (medial) and outer (lateral) side

   • Should be uniformly dark without breaks

2. Ligaments

   • ACL (Anterior Cruciate Ligament)

     • Appears as a dark diagonal band in the center

     • Should be continuous without gaps

   • PCL (Posterior Cruciate Ligament)

     • Appears as a dark "C" shape behind the ACL

   • MCL (Medial Collateral Ligament)

     • Dark band on inner side of knee

   • LCL (Lateral Collateral Ligament)

     • Dark band on outer side of knee

3. Cartilage

   • Appears as a bright white layer covering bone ends

   • Should be smooth and even

   • Visible on both femur and tibia

4. Patella (Kneecap) Tracking

   • Position relative to groove in femur

   • Thickness and condition of cartilage underneath

Common Abnormal Findings

1. Meniscus Tears

   • White line going through normally dark meniscus

   • Irregular shape or fragment

   • May see "parrot beak" appearance

2. ACL Tears

   • Gap in the normal dark band

   • Wavy appearance

   • Fluid (white) where ligament should be

3. Arthritis

   • Narrowed joint spaces on X-ray

   • Bone spurs (osteophytes)

   • Irregular cartilage surface on MRI

4. Patellofemoral Problems

   • Abnormal tracking of kneecap

   • Cartilage wear under kneecap

   • Tilting of the patella

Tips for Viewing Your Images

1. Understanding Orientation

   • Remember: Left side of image is usually your right side

   • Top of image is usually the front of your knee

   • Dark joint spaces should be even

2. Questions to Ask

   • "What is this dark/white area?"

   • "How does this compare to normal?"

   • "Can you show me where the problem is?"

Again, I urge you to take a look at this awesome website if you need help navigating your knee MRI.

Red Flags to Look For

1. On X-rays:

   • Uneven joint spaces

   • Bone spurs or loose bodies

   • Abnormal alignment

   • Fracture lines

2. On MRI:

   • Bright white areas in normally dark structures

   • Breaks in continuous structures

   • Large amounts of fluid

   • Bone marrow changes (signal changes within the bone)

Special Considerations

1. For Athletes

   • Focus on meniscus and ligament integrity

   • Look for bone stress reactions

   • Cartilage damage patterns

2. For Older Adults

   • Arthritis progression

   • Bone quality

   • Alignment issues

Remember: While this guide helps you understand your knee imaging, always rely on your healthcare provider's interpretation for medical decisions. Different imaging machines and settings can produce slightly different appearances, so some variation is normal.

Examples of Abnormal Scans

Now, you play the role of the radiologist. After reading and learning everything above, give it your best shot! The answer will reveal when you hover over the image.

MRI of an ACL tear. Notice the ligament is not attached on the bottom bone (tibia)

MRI of a meniscal tear. Notice the irregularity in the lateral (outside) meniscus.

The change in color consistency within the femur indicates edema, or in this case a bone bruise.