That makes sense. Can the upper and lower jaws continue to downswing throughout life? Is that what happened to FDR, for example? How does this guy turn into what we now think of him as looking like?

I am in particular concerned about this because I was recently treated for TMD with a splint (to decompress the joints) and unfortunately had already planned a long trip abroad before I could properly finish, so the doctor has me atypically wearing my bulky splint for four months to prevent regression until I get another piece that will train me to keep my bite as it is. I plan on mewing after this, of course, after I can get a needed frenectomy (unless I can't!), but I swear: my face seems like it's aged how you described over the last two months or so I've had this thing in my mouth and my antegonial notch is growing. Before, I couldn't mew at all, but now it's totally impossible. I usually hang my mouth open all day because it's too much of a strain on my lower lip and chin to close.

Can bad oral posture cause regression? That fast? (I am pretty sure I have the connective tissue disorder Marfan syndrome.) I am early 20s and consistently mistaken for a few years older than I am.

@silver the splint caused your sudden change. Changes in bite height/position, even artificial, will cause the mandibular condyles to remodel in an unfavourable way. @sugr1 coincidentally wrote about this issue just yesterday:  /community/community/main-forum/i-just-made-it-to-39mm-inter-molar-width-and-its-awsome/#post-8561

If you read up about Mandibular Advacement Splints eg Somnomed for the treatment of sleep apnoea, you will see very good research by dentists, sleep physicians etc that show ALWAYS with long term use the bite of the patient changes and they become more class 3. Why? Because every night they are promoting Mandibular condyle changes which become permanent making the lower jaw forward posture a permanent change.

@Progress My doctor told me that condyle remodelling is on purpose! This splint really did help me though. I had crazy myofascial pain, chiefly, and other awful typical TMD symptoms, and it all went way down. My doctor showed me on a CT scan that the ball of the TMJ on one side was literally ground down flat from grating against an incorrect position. Apparently the idea behind their therapy was to relax the jaw muscles to allow the joint to decompress, which it seemed to.

Can I mew my way out of the problems it's caused? Is the splint therapy going to do any irreversible damage?

EDIT: It's not a mandibular advancement splint! It's just a little soft-ish plastic thing that fits over my bottom teeth and prevents the teeth from closing all the way. There's a slight bump at the first molars, we're talking 2mm at most, that slopes down toward the back of my mouth.

EDIT 2: I should add I am sleeping with an NTI splint and the posterior part of the contact point is sloped up about 15° towards the back of my mouth.