Question: Do dental fillings prevent the performance of ghusl or not? Could you explain with proof texts and documents?
ANSWER
Statements in fiqh books are as follows:

If food remains are left between the teeth and one cannot wash under them, a ghusl will be acceptable. For water is fluid and can infiltrate under the remains. But if the remains have become solid, a ghusl will not be acceptable. For water cannot infiltrate under them. There is no darûrat or haraj in this. (Halabî-i kabîr)

[darûrat: a samâvî (involuntary) reason that forces one to do something, that is, a situation which arises beyond one’s will, is termed a darûrat; haraj: when it is difficult to prevent something from hindering the doing of a fard or from causing a harâm to be committed, the case is termed haraj.]

If there are solid food remains between the teeth, a ghusl will not be valid. (Qâdîhân)

If the food remains between the teeth become solid and thereby prevent water from filtering through, a ghusl will be unacceptable. (Al-Majmû’at-uz-zuhdiyya)

It is written in Durr-ul-mukhtâr: “There are those (scholars) who have given the fatwâ that anything between the teeth or in any tooth cavity would not harm a ghusl ablution, but if the stuff is solid and does not let water through, a ghusl will not be acceptable.”

In explaining this Ibni ’Âbidîn (rahmatullahi ’alaih) wrote:
“The reason why the fatwâ was given that it would do no harm was because water would infiltrate under it and wet the part beneath it.” The book Khulâsat-ul-fatâwâ says the same. As is understood from the fatwâ, if water does not go under it, a ghusl will not be accepted. The same is written in the book Hilya and in the annotation to Munyat-ul-musallî. (Radd-ul-mukhtâr)

Tahtâwî, explaining Marâqil-falâh, wrote: “If water goes under the food remains between the teeth and in the tooth cavities, a ghusl will be accepted. If they are too solid to let water through, a ghusl will not be accepted. The same is written in Fath-ul-qadîr.”

Again Tahtâwî wrote in his explanation of Durr-ul-mukhtâr:
“Because water will infiltrate under the food remains between the teeth and in tooth cavities, they do not prevent the performance of a ghusl. If you doubt whether water infiltrates under them, you must take them out, wash in between the teeth and the cavities.”

Though it is written in Majmû’a-i Jadîda’s supplemented second edition in 1329 A.H. that dental fillings do not prevent the performance of a ghusl, the statement in question is not included in its first edition in 1299 A.H. It was sneakily added in its second edition (1329 A.H.) by freemason Mûsâ Kâzim, who was the shaikh-ul-Islam appointed by the Party of Union and Progress (Ittihâd wa Taraqqî). You should not give credence to the men of Union and Progress.

It is wrong to assert: ‘‘If it is permissible to have a gold, silver, or plastic tooth mounted, a ghusl, in turn, is acceptable.” Hadrat Imâm-i Rabbânî “quddisa sirruh” declares, “In usûl-i fiqh of the Hanafî Madhhab, if a piece of information is communicated without being subject to any condition, it is understood to be subject to conditions.” In fiqh books, it is declared that it is permissible to eat the meat of a deer. This rule is dictated without a condition. Thinking that it is permissible to eat the meat of a deer, you cannot catch a deer, chop off its leg, and eat it. Moreover, if it is butchered by a disbeliever other than the ones with a heavenly book or if it dies of natural causes, it is not eaten, for it has become a carcass. If it is slaughtered without uttering the Basmala, it cannot be eaten, either. As is seen, there are several conditions to fulfill in order to eat the meat of a deer.

[usûl-i fiqh: the knowledge of usûl-i fiqh explains how learnings of fiqh are derived from âyats and hadîths.]

Since the hadîth-i sharîf “He who dies in a war becomes a martyr” is reported without a condition, it must be understood that it involves some conditions. For example, if a person who is not a Mu’min dies in a war, such a person does not become a martyr. Likewise, the rule “It is permissible for men to wear a silver ring” has been reported without a condition. But its weight must not be more than 4.8 g. Furthermore, an ablution or a ghusl will not be valid if the skin under a tight ring is not moistened. It is necessary to shift or take it off and thereby make water reach the skin under it. It would be wrong to draw the conclusion from the permissibility of wearing a silver ring that it is unnecessary to wash the skin under it. In the same way, when it is said, “It is permissible to tie a loose tooth with a gold wire,” we should think that it also has some conditions. It must be assessed in terms of whether water penetrates under it or not. Just as an ablution or a ghusl is not valid if water does not moisten the skin under a ring, a ghusl, either, is not valid if any dry place remains in the mouth. For this reason, a person with a filled tooth must imitate a madhhab, e.g., the Mâlikî Madhhab, which states that it is not fard to wash inside the mouth in ghusl.