What does slight superior displacement of the mensical root mean?

Meniscus injury: The anterior and posterior meniscal roots are the sites where the knee meniscus attaches to the central tibial plateau. Your symptoms of locking and buckling sound like the symptoms of a torn meniscus. The meniscus, a cartilage in your knee that acts as a "shock absorber" between the two bones can be torn in various places. Sounds like you had a tear at the root.