Hi there,
I have had a locked jaw for 5 years now with an opening of 10mm ( 5mm before a discectomy 10 months ago), and i do experience tenderness under my jaw. my Doc said that because my jaw is so immobilised and hasn't functioned/moved properly for all this time, that my salivary glands are not able to work properly as well which in turn, causes them to swell and become tender.
Hope this helps,
Take care,
Carey