Normally the definite article (le/la/les) is used with body parts (j'ai mal à la tête)
However, in the following special cases:
- when special emphasis on the body part is desired, or
- when another person is involved
the possessive adjective (sa,son,ta,ton etc.) is used instead of the definite article (le/la/les) to reduce ambiguity.
Also, when the body part is the subject of the verb (rather than the object), possessive adjectives are used: