Here are some intraoperative photos from a recent left knee arthroscopy.  Below is the joint surface of the patella (knee cap).  You can see the fraying of the cartilage which represents early arthritis.  This degree of arthritis may not show up on an x-ray or MRI but, if present, may still cause pain for a patient.

Here is a meniscus tear in the medial meniscus.  You can see that the torn flap of cartilage that is getting pinched between the joint surface.
Another view of the torn meniscus.

Same knee.  Another torn flap of meniscus being trimmed with the metal device seen in the left side of the photo.

This torn flap of meniscus cartilage is getting trapped between the surfaces of the joint.  Just like a stone that got into you shoe, it can cause significant pain when you walk on it.

Here you can see that the torn flap of cartilage has bee trimmed and can no longer cause pain in the knee. 

On the other side of the joint, you can see that the lateral meniscus is normal, with a smooth edge.  No arthritis seen here.

Normal lateral meniscus.

Thanks,

JTM, MD