Some of the most effective LOR's are from people that got to know you themost. For example, a letter from a community college professor who got to knowyou very well during a two semesters small size class, a lab and office hourswill be more powerful than a university professor who only saw you a few timesin his lecture of 300 students.
Also, if you feel that the letter may bestrong, but it's a few years old, try contacting that person and ask them toprovide you with a more recent, maybe an updated letter. LOR's are consideredimportant by many dental schools during the admissions process. Try to shine inthis portion of the application, as well as others 🙂
Finally, I heard that ifLOR's are not opened, i.e. sealed in an envelope by the evaluator, they maypotentially carry move value, because they're written more freely without aconcern for being seen by the applicant.
If your school offers an option to put together a committee letter onyour behalf, this option is also viewed well by the admissions committee. Good luck.