This year one of the Molars cracked again and an abscess developed. I had the tooth extracted, the roots stayed in so I had them drilled out.

You didn't go to a holistic dentist did you?

If you had he would never have left the root and the 2 ligaments that hold the tooth in place. Root Canal teeth are dead and aren't serviced by blood, so your immune system couldn't handle any infection that might arise.

The 2 ligaments left were enough for an infection that can end up causing a cavitation, forget about the dead roots.