What causes a knee joint to lock up? Is a displacement ot the mensicus root considered a tear? What is medium size joint effusion mean?

A meniscus tear. The most common cause of a locked knee is a large meniscus tear. When this happens, a portion of the meniscus gets stuck in a bad spot. This doesn't allow the knee to move freely, causing it to lock. Displacement of the meniscus root is a certain kind of tear. Sometimes, it can be repaired which can be hard. See an experienced surgeon. An effusion is fluid in the knee, likely from the tear.
Meniscus Tear Often. A significant meniscus tear can cause the knee to "lock up" which means that there is less than full extension or full flexion of the knee, not necessarily that the knee can't move at all. A bucket handle meniscus tear is the most common cause of the problem. A joint effusion means that there is increased fluid (synovial, blood, pus, etc.) in the knee and they are often grade small, med., large.
Meniscus tear. A meniscus tear can cause a knee to lock up. A root tear is a tear near the insertion of the meniscus. A medium effusion means there is fluid inside the knee usually caused by irritation from the tear. Arthroscopic surgery is indicated to remove or repair the tear.