Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- The use of braces due to need, such as setting right a defect or straightening crooked and unaligned teeth, is permitted.
2- However, using braces purely for beautification purposes is to be avoided.
In his answer to your question, Mufti Muhammad Ibn Adam Al-Kawthari, Director and researcher at the Institute of Islamic Jurisprudence (Darul Iftaa, www.daruliftaa.com), Member of the Al-Qalam Shari`ah Scholars Panel, and advisor on Islamic Banking, states:
Moreover, one should keep in mind that if an area of the body required to be washed in either a ritual bath (ghusl) or ritual ablution (wudu) is covered, and as a result, water fails to reach it, then one’s ghusl or wudu will be valid if it has been covered due to “reasonable need”. However, if water is prevented from reaching the area without a need, then one’s ghusl or wudu will not be valid. (Radd al-Muhtar, 1:104)
As such, even though water may not reach the covered portion of the teeth, one’s ghusl is valid with braces when using them out of need, and as mentioned, wanting to straighten crooked teeth is considered reasonable need.
Almighty Allah knows best.