Fillings and crowns are no obstacle for major ablution. These pieces replace your teeth. Washing them is like washing your own teeth. They do not possess and obstacle for major ablution.
One should wash every accessible part of the body during major ablution. The concerning verse goes like this:
“…Allah does not desire to put you on any difficulty, rather He wishes to purify you and complete His favors upon you so that you may be grateful.” (Al-Ma’ida/ The Feast 5:6)
It was also said in the 78th verse of chapter Pilgrimage: “He has not laid upon you any hardship in this religion.” (Al-Hajj/ The Pilgrimage 22:78)
One of the main principles in Islamic Law is “to ease the hardships”. Fillings and crowns are necessary things health-wise. It is impossible to get them off and wash beneath them during ablution. So it is sufficient to wash them like washing your own teeth. It is not compulsory to wash inaccessible parts of the body.
 See. Ibn Abidin, Radd al-Mukhtar, 1307, Istanbul, vol.I, p.141