The medial meniscus is displaced how can that happen? what are the symptoms of having a meniscus displacement?

Knee locking: There is most likely pain also. This is a surface that allows the knee to move smoothly. It is usually associated with a tear from a trauma.

Usually means it is: Healthy meniscii generally do not displace; for this to happen you need to either tear the meniscus from its attachments or have what is known as a bucket handle tear, a discoid meniscus can also be symptomatic.The commonest symptom is locking of the knee.