How To Read A Knee X-Ray

Have you ever wondered what is really on your knee x-Ray when a doctor tells you it is “bone on bone”?

The 2 Main Bones

knee x-ray

The large thigh bone is known as the femur. Beneath it is the larger shin bone the tibia. The smaller shin bone the fibula does not connect to the knee joint.  The inside of the femur and tibia form what is known as the medial joint.

The outside of the femur and tibia form the lateral compartment.
What you see on x-rays are bones. The cartilage, ligaments and muscles do not show on x-rays, only on an MRI.

knee x-ray

Medial Knee Joint

knee x-ray

Lateral Knee Joint

The 3 Compartments

1. Medial                                   2. Lateral                                 3. Patellar

We can determine a lot about the distance or space between the femur, both medially and laterally. One should make a good habit of comparing both knees. Some doctors only look at the one you say is hurting you. This misses a good opportunity to compare to your supposedly “normal” knee, and often shows signs of change such as arthritis on the other side.

What Does “Bone on Bone” Mean?

The distance between the femur and the tibia is really the amount of space the cartilage should take up. For instance if there is a lot of room between the femur and tibia there is good joint space or normal amount of cartilage is inferred. If there is minimal space between the bones the joint is worn and the cartilage has thinned out. This is advanced arthritis of the knee. It may occur on the inside (medially), on the outside (laterally) or both compartments. Quite often patients are told they are “bone on bone” when in fact there is still good joint space.

Knee Arthritis

Reading a knee x-ray is mostly easy.  There are some things better to be left to a trained radiologist.  However, for the most part, it is not too hard.  Especially in telling if you have arthritis or not.  Look for distance between the femur and tibia on both the inside and outside.  Compare with your other knee.  Look for irregularities of the bone surface or bone spurs at the edge of the bone.  The knee cap shows irregularities with chronic wear.  It is usually involved when the medial and lateral joint is narrowed as well.

To sum it up, look for normal or narrowed medial and lateral joint space, irregular bone surfaces, bone spurs and patella (knee cap) irregularities.  A little practice makes a big difference.  Sometimes one side the medial is more affected than the lateral.  This can be from being overweight, prior trauma or knee surgery in that area.  The lateral is usually affected from prior lateral meniscus surgery, it accelerates the development of knee arthritis.

Knee Arthritis

Abnormal Knee – “Bone On Bone”


Knee arthritis dr. Lox

“Normal” Knee







knee x-ray

Patient was told his knees were “bone on bone”, but as you can see, there is still space between the joints.










So, as you can see, not everyone who is told that they are “bone on bone” is actually “bone on bone”.  Sometimes, the glass is half full, when others may view it as half empty.  We often see people who have plenty of joint space left, even though they are told they need a knee replacement.  Quite often, these patients are candidates for Regenerative Therapy.

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