Are there ways to just have your lower leg numbed during knee arthroscopy?

Yes: Some types of knee arthroscopy can be performed under local anesthesia with adequate sedation. But a discussion with the surgeon and anesthesiologist is required and at times during the surgery it must be converted to a general anesthic if the procedure is not being tolerated or the relaxation of the knee is not enough.

Anesthesia: I believe what you are referring to is a spinal anesthetic which is an excellent choice for temporary "lower leg numbness" that allows the surgeon to perform knee arthroscopy.