Why do I have, a DEEP GROOVE - IN MY TEETH parallel to my gum-line.

[Ilikeboats]

I am 68 years old - I have had 8 cavities throughout my lifetime.
These “grooves” were not noticeable, to me, 3-4 years ago.
These grooves are NOT on my inner & outer front teeth - both top & bottom.
The grooves are only on the "sides" of my teeth.

a. Hygienist said, on multiple occasions, that “My teeth were really hard”.
         1. Do “Harder” teeth require “Special Tips” & not just “Universal Tips?
         2. Some tips are used where all surfaces of the tip are active (Ultrasonic) and some where only the
                    sides of the tip are active (Piezoelectric). Which type should be used?
b. I have noticed there is little water flushing while cleaning… Does this cause "Recessions"??
c. Is the “natural frequency, of the teeth” considered when selecting tips and type of cleaning, Ultrasonic or
          Piezoelectric?
d. Hygienist says I should have my teeth cleaned three times per year.