Magnesium has several effects that improve dental health

Magnesium regulates the amount of calcium inside cells, which provides the hard mineral component for strong teeth.    

MAgnesium inhibits osteoclasts that would otherwise cause the progressive loss of dentine and cementum.   It does this by stimulating the production of CALCITONIN, the hormone responsible for inhibiting osteoclast.  Whereas this loss is a necesssary process, called tooth resorption, that occurs under pressure from permanent teeth upon "baby" teeth, it is, however, a pathological problem if it happens to permanent teeth. Stimulants of root resorption include pulp necrosis, trauma, periodontal treatment, orthodontics and teeth whitening, situations in which magnesium supplementation would be a benefit by inhibiting osteoclasts.

Decreases inflammation in periodontal disease, thus preventing periodontitis (destroys bone supporting your teeth).   MAgnesium is a component and activator of alkaline phosphatase (ALP), an enzyme, whose increased activity indicates the level of inflammation in periodontal disease. Increased inflammation in periodontal disease can change gingivitis to periodontitis when there is damage to bone supporting teeth