A custom mouthguard has a resilient U-shaped body with an anterior wall and a posterior wall. A post dam on the posterior wall forms a seal with palatal tissue to increase retention of the mouthguard in a wearer's mouth. The increased retention allows a wearer to speak and open mouth breath while wearing the mouthguarrd. The mouth guard also has an indexed region that serves to mutually stabilize maxillary teeth, mandibular teeth, mandible and TMJ components. Mouthguard methods and processes are also disclosed.