This is the 5516th most frequent Russian word.
позвоночника
"Позвоночника" means "spine" or "vertebral column" in English.
This sentence uses 'позвоночника' in the possessive case, referring to the spine to denote ownership of its parts.
Мой друг сломал один из позвонков позвоночника.
My friend broke one of the vertebrae of the spine.
Here, 'позвоночника' is used in the genitive case to indicate the object of care, showing attention towards health.
Во время тренировки нужно заботиться о здоровье позвоночника.
During exercise, one should take care of the health of the spine.
In this sentence, 'позвоночника' is used to specify the location of pain, describing relation in genitive case.