After reviewing a few North Carolina negligence and personal injury actions, I ran across several cases that mirror your situation, dmyokum. The one that is most applicable, perhaps, is Goodman v. Wenco Foods, Inc.
Goodman sued Wendy's, and the company that supplies Wendy's hamburger, for negligence and breach of implied warranty. Goodman bit down into a Wendy's hamburger and broke his tooth. He found a bone in the burger.
Goodman had a witness who was at the table with him when he broke his tooth on the bone in the burger, he showed the bone (and his tooth fragment) to the manager of Wendy's and he had three dentists examine his broken tooth.
For court he obtained Wendy's meat-grinding specifications, which limit the size of bone or cartilage allowable in their beef.
There were several cases cited during the course of the trial, as well as the Uniform Commercial Code N.C.G.S. section 25-2-314 (1986). Some of the cases cited were: Sharpe v. Worland, Coffer v. Standard Brands, Inc. and other North Carolina cases and Mix v. Ingersall Candy Co. (a California case), among others.
In one case cited, Adams v. Tea Co, 251 NC 565, 112 SE 2d 92 (1960), the court said that whenever a substance is natural to the food itself, there can be no liability because the consumer should anticipate and be on guard against the presence of such a substance (ie. fish and fish bones). In other words, it must be decided if a substance is natural or foreign to a food product and, if natural, can a consumer reasonably expect it in the food product.
Natural substances are not foreign substances and, therefore, they are not a defect in the food product and, therefore, the seller of the food product cannot be held liable - unless the natural substance found is large enough to be injurious to the health or there is a large quantity of such natural substances appearing that would make their presence unexpected by a consumer. Only when there is foreign matter present can some liability be assumed.
Although a restaurant has a duty of care to see that the food it serves does not injure the customers, consumers must have reasonable expectations and not expect any "perfect" product. Goodman lost his negligence and breach of warranty suit against Wendy's.
Without the substance that broke your tooth and, therefore, without evidence that it was a foreign substance that would be unexpected in your food instead of a natural substance that could reasonably be expected in your food, you may have a difficult time holding the restaurant liable for your broken tooth (although Sonic may pay just to avoid a potential lawsuit).