While maybe not so relevant to how static magnetic fields can assist pain relief or injury recovery, this research review article by Q Magnets nonetheless, does give an important insight into biological effects.
When tooth decay advances to the inner pulp, it can require extraction or root canal work to preserve the tooth.
Conservative alternatives include direct pulp capping or partial pulpotomy.
Either way, increasing the formation of dentine within the tooth is important, a process called dentinogenesis.
Dentinogenesis can be facilitated through stem cell proliferation, migration and differentiation, which previous studies have shown can be improved with an external static magnetic field (SMF) (Chiu 2007 & Kim 2015).