What is usually done for a acl tear? and does it heal one its own without surgery? can a piece of meniscus get stuck under the patella?

ACL reconstruction: Symptomatic instability resulting from a torn ACL is usually treated with ACL reconstruction. A piece of a torn meniscus can become stuck or lodged anywhere to include under the patella, especially if it becomes a free fragment or loose body (rare). More common in the area of the patellofemoral joint is synovium/scar tissue, or a cartilage flap from the patella or trochlea.

Usually surgery.: Surgery is usually recommended for an ACL tear. It generally does not heal on its own, and leaves the knee vulnerable to episodes of instability. Sometimes a stump of the old ACL can get stuck under the patella.