What prevents kidney stones from damaging the urethra?

Water: The best way to prevent damage from kidney stones is preventign them therefore drinking atleast 2 to 3 liters of water will help. If you already have stones, the majority of them are small and will pass through the bladder and urethra without causing any damage. However if you have large stones that are painful, it will require the help of a urologist.

Usually do not: Kidney stones small enough to pass down the ureter & into bladder are much smaller than caliber of urethra & thus do not cause damage to the urethra. Kidney stones too large to pass down the ureter are treated with laser, shockwave lithotripsy or percutaneous nephro-lithotomy and fragments passed in urine are also too small to damage urethra.