How do teethbrake. Off?

Tooth decay. Teeth break-off (avulsion) when exposed to significant trauma, tooth decay or gingivitis. In trauma the tooth may be displaced or avulsed. In tooth decay, a bacterial infection of the enamel weakens the tooth and in severe cases may cause the tooth to die. In gingivitis, an infection of the gums and teeth weakens the attachment of the teeth to bone of the jaw via bone resorption.
Healthy ones, trauma. Healthy teeth can resist fairly significant stress, but a strong enough direct force can damage the tooth either partially or entirely. Poor dentition, gum disease, systemic disorders and some medications may cause teeth to be much more susceptible to decay and injury.