What real human teeth look like is a fact, and not subject to copyright. But some particular designer's or artist's image of human teeth would be protected by copyright, unless it is too old -- (prior to 1923 under US law), or unless it lost copyright in some other way. To create a derivative work based on such a copyrighted work would require permission from the copyright holder. Otherwise it is copyright infringement, and the holder could sue.
As to the car models, the original designs would be protected by copyright, and a maker of a model should have permission in some form, or again the copyright holder could sue.
Of course, a copyright holder is not required to sue, and if the value of the work is too small a suit might not pay. But that is not a matter of legal right, but of the holder's choice and business decision. A copyright holder could always change such a decision at a later time, when perhaps the value had increased.
The exact laws on how to file copyright suits, and exceptions that might apply, vary by country. The question does not indicate what county it applies to.) But the basic principals are the same in all countries that adhere to the Berne Copyright Convention, which is all but a very few countries in the world. See this Wikipedia article for more details.
