I tore my ACL and I haven't done anything about it cause I'm in sports, but my knee is bony and protruding, can the musle shrink? I was skating in hockey, and I changed directions, quite quickly, my knee gave out and I fell to the ground hearing a popping
Likely Quad atrophy. After an acl tear or post- surgery it is common to have atrophy of the quadriceps muscle. Building that back up is prudent as that will help prevent other injuries and anterior knee pain. If you opt for acl surgery, a stronger quad will ease recovery and rehab too.
Commonly. Commonly after an injury your muscle will atrophy (or shrink) as a result of disuse or inactivity. This happens after surgery as well if the leg/knee is not immediately started in an exercise program. Even with aggressive exercise the leg may temporarily shrink, but you can build the muscle size and stength back up with time and dedication. In regards to your acl injury, just realize that continued sports participation may cause additional injury to either the meniscus or articular cartilage in your knee. It is best to have this reconstructed as soon as possible so that you may return to normal activities and sports with less risk of additional injury.