What Missi has is called luxating patellas, where the knee cap slides in and out of place. They are very common in Yorkies and be genetic or from stress from jumping on and off furniture.
Unfortunately, dogs with luxating patellas are prone to ACL tears. If the ACL is completely torn, Missi will need surgery. Her patellas will be fixed, too, which will prevent crippling arthritis later on.
Here are a couple of good links for you:
Ruptured Anterior Cruciate Ligament (ACL) in Dogs Luxating Patella: A Knee Problem in Dogs