Will physical therapy to treat tmj actually work when there is already bone necrosis?

Depends: What is the goal of the physical therapy. Is it to replace a displaced cartladge? Is it to restore motion in the jaw (opening wider)? Is it to for pain relief? Other issues? If there is necrosis, there are major issues in the joint. You should see a reputable ent/oral surgeon/dentist/orthodontist who specializes in this area and if need be get several opinions. This is difficult!

TMJ necrosis: It is not uncommon to see boney changes in the tmjs, even in people with no pain or dysfunction. Im not clear on whay is meant by necrosis, but if the boney changes are what is commonly normal adaptive wear, then physical therapy modalities can be usefull for decreasing pain and increasing functional movements.

Helps With Pain: It is very important keep in mind that TMJ physical therapy was designed to reduce symptoms of tmj, physical therapy by itself cannot cure the TMJ problem. However, reversing necrosis is not likely. Some include cervical mobilization exercises, TMJ mobility exercises, TMJ stability exercises, etc. On the whole, these exercises form a thorough program for people experiencing TMJ disorder pain.

Seperate but related: A joint that has bone necrosis is in late stage joint derangement. When the joint is dysfunction the muscles that move the joint are compromised. The physical therapy would address the muscle problem, but it will provide limited results if you do not address/correct the internal joint injury/damage.

Not likley necrosis: Bone necrosis means dead bone. Don't get dead bone in routine TMD, you get various stages of degenerative bone disease. Physical therapy often helpful, reversible treatment. Harmless. Most important see TMJ expert. Any dentist can be TMJ expert with proper training/experience. Most commonly, oral surgeons, prosthodontists, orofacial pain specialists. Ask your MD, your dentist and dental society.