Special cases when you use mon/ma/mes/etc with parts of the body (French Possessive Adjectives)

Normally the definite article (le/la/les) is used with body parts (j'ai mal à la tête) but there are exceptions.

See Using le, la, les with body parts and clothing (definite articles)

The possessive adjectives with body parts in French

 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:

BUT