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.