Bottom lip will not move down after submental lipo - what should I do? (Photos)

What you describe is a known complication from Chin or neck liposuction.
It almost certainly represents an injury to the buccal branch of the facial nerve.
This nerve crosses the mandible or the jaw bone and can get injured from various surgical procedures including liposuction in this area.
Nerves are quite strong and liposuction uses blunt instruments that are not sharp.
When nerves are injured from liposuction they are rarely cut.
This is a good thing because most of the time patient see a full recovery with enough time when nerves have not been severed.
The nerve itself may be intact but there's a fatty coating of nerves called the Myelin sheath that needs to be intact in order for the nerve to work.
. Nerves that are injured are referred to as having neurapraxia.
Neurapraxia is generally categorized as stage 1, 2 or 3.
Stage one neurapraxia is like when your legs are crossed and your foot goes numb for a while.
Stage III is when there's a real nerve damage and it takes months to recover.
Stage 2 is somewhere in between. This may be the case with you.
With stage three neuropraxis the nerve goes through something called nerve regeneration.
This is a slow process. The nerve can regenerate approximately 1 mm per day and sometimes less.
Sometimes facial nerve injuries can take as long as a year before recovery is complete.
I would anticipate you should regain movement faster than that but the final improvement may take several months.
There's nothing you can do to speed up the process.
The bad news is you got a facial nerve injury. The good news is function it will most likely return completely.
These injuries simply happen from time to time.
They are not particularly common.
Most likely your plastic surgeon didn't do anything wrong.
There are a few things surgeons can do to minimize this from happening.
One is to use very gentle types of liposuction equipment, using small cannula's and avoiding aggressive techniques.
Another way to minimize this from happening is to not push on the skin to force more fat to go into the cannula when working over the  area where this nerve is.
All plastic surgeons know facial anatomy well and know about the buccal branch of the facial nerve(or at least they should)
Best,
Mats Hagstrom M.D.