I recently had arthroscopic surgery because my knee kept locking. A loose cartilage fragment was found and removed. Why wasn't the fragment seen on an X-ray or MRI?

Locking can occur with meniscus cartilage tears, ligament tears or floating loose bodies. Because your particular problem was cartilage and not bone, it would not be seen on a plain X-ray. Some loose bodies are made of a combination of bone and cartilage. The bone content of these can be partially seen to identify the problem. Your history of locking would lead me to suspect a loose body present if no ligament instability or meniscus cartilage tear was seen during clinical examination of your knee. An MRI study has the capability of spotting a loose body. The images that are developed by MRI are created by the comparative difference of water content of each tissue. Cartilage has different amounts of water content than bone. These two tissues, side by side, would be clearly distinct. If the cartilage loose body had the same water content as the tissue it was lying on and had an unusually flat shape, it might not be seen on the MRI because of the lack of contrast.