How do I go about getting this treatment paid by them and not me paying for it?

I had a bowl of salad at restaurant and at one point I bite something hard. I spit it out to take a look at it and it happened to be broken piece of plate. The supervisor did the incident report. It happened almost at the end of March and the person who is handling my case on the company behalf saying that they are not going to pay for it since salad come from the vendor. After sending them an email four times, they now saying they will reevaluate my case again but have the letter signed saying I will released all the information that my dentist wrote. They will not pay for anything until my treatment is over and according to my dentist, root canal is not successful and I would need to get implant done.