the dental usage is one of the latest transferrals. the astrological is the earliest use.

I'll just throw out some citations that may serve to illustrate the... er... point.

Astrol The Cusp or very entrance of any house, or first beginning. (1647)

In this figure Capricorn is upon the cuspe of the ascendent. (1651)

general And mid the loftiest [mountains] we could well discern One that was shining in a cusp of snow. (1847)

Astron About the middle of the eclipse, the air was very clear, and the cusps well defined. (1764)

Arch In all the concave bends of the stone-work, a small pointed ornament occurs, which is very common in Gothic windows+I have ventured to apply to it [the name] of cusp, by which mathematicians denote a figure of this sort. (1813)