All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Using a toothbrush takes the place of Siwaak; by doing so, one achieves the Sunnah.
Shaykh Ibn ‘Uthaymeen, was asked in Fataawa Noorun ‘ala Ad-Darb:
"Is using toothpaste sufficient instead of Siwaak, and will the person who uses it with the intention of purifying his mouth be rewarded? Meaning, will it be equal in reward to the use of Siwaak which the Prophet, sallallaahu ‘alayhi wa sallam, encouraged us to use when cleaning our teeth?"
He answered saying:
"Yes, the use of a toothbrush and toothpaste suffices instead of Siwaak; rather, it cleans and purifies more. Therefore, if a person does so, he will have acted in accord with the Sunnah, because what is important is not the tool but the act and the result. Using a toothbrush and toothpaste gives a better result than using the Siwaak alone." [End of quote]
For more benefit, please refer to Fatwa 281623.
Allah knows best.