How To Read A Knee X-Ray

The 2 Main Bones

knee x-ray

The large thigh bone is known as the femur, which lies between two shin bones.  The larger shin bone is called the tibia and the smaller shin bone is called the fibula, which does not connect to the knee joint.  The inside of the femur and tibia form the medial joint and the outside of the femur and tibia form the lateral compartment. X-rays show bones but cartilage, ligaments, and muscles can only be seen on an MRI.

The 3 Compartments

1. Medial                                   

2. Lateral                               

 3. Patellar

Physicians are able to measure the space between the femur, both medially and laterally. and should make a 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, that means there is good joint space or a 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.


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 the 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 kneecap shows irregularities with chronic wear.  It is usually involved when the medial and lateral joint is narrowed as well.

Knee Arthritis

Abnormal Knee – “Bone On Bone”

Knee arthritis dr. Lox

“Normal” Knee

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 on 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 by prior lateral meniscus surgery which then accelerates the development of knee arthritis.

knee x-ray

A 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 every person 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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