Does Food Stuck in Teeth Break the Fast?
Question

When a person wakes up in the morning and he is fasting, and there are bits of food left from his sahur in his mouth, what is the ruling if he swallows them?
Praise be to Allah.Eating invalidates the fast
“Undoubtedly eating is one of the things that invalidates the fast. Allah says (interpretation of the meaning): 
“and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night)” [al-Baqarah 2:187]
It is well known among the Muslims that fasting means refraining from eating, drinking and intercourse, and all other things that invalidate the fast .” (Majmu’ Fatawa Shaykh al-Islam, 25/219)
Eating means deliberately introducing any substance into the stomach via the mouth. (See: Hashiyat Ibn Qasim ‘Ala al-Rawd al-Murbi’, 3/389) 
It is not conditional that this eating be something beneficial or that the amount be large, rather if a person swallows something that does not benefit him (such as a pearl) or he swallows a small amount of something, then he has broken his fast. 
Does food stuck in teeth break the fast?
Swallowing bits of food that may be left between the teeth is regarded as eating, so it invalidates the fast . 
This applies if the fasting person swallows it by choice, and is able to expel it but he swallows it deliberately. But if it has already reached his throat and he swallows it and cannot expel it, then there is no sin on him and his fast remains valid, because in all cases where a thing invalidates the fast, that is conditional upon the fasting person doing it by choice. If he does it by force or involuntarily, then his fast is valid and there is no sin on him. 
Ibn Qudamah (may Allah have mercy on him) said in al-Mughni, 3/260: 
“If a person has food between his teeth, one of the following two scenarios must apply: 

It is a small amount that he cannot spit out, so he swallows it. This does not invalidate his fast, because it cannot be avoided. It is like saliva. Ibn al-Mundhir said: The scholars unanimously agreed on that. 
It is a large amount and he can spit it out. If he spits it out there is no sin on him, but if he swallows it deliberately, his fast is invalidated according to the majority of scholars, because he has swallowed food that he could have spat out willingly when he is mindful of his fast. So this breaks the fast just as if he deliberately started eating.” 

To sum up the answer, if he is able to expel the food stuck between the teeth but he does not do so and he swallows it, then he has invalidated his fast. If he swallows it involuntarily, then his fast is valid and there is no sin on him. 
And Allah knows best.