Dr. Ross Shelden will be a star witness in the Mel Gibson investigation, TMZ has learned ... and we're told he has powerful evidence that Mel Gibson hurt Oksana Grigorieva.
Sources familiar with the treatment tell TMZ Dr. Shelden, who practices in the San Fernando Valley, took photos of Oksana's mouth the day after the January 6 blowout, in which Mel allegedly knocked out Oksana's teeth.
We're told Dr. Shelden took photos of Oksana's mouth and turned them over to her lawyers. We're also told Dr. Shelden has made it clear ... it was not a case where Oksana lost her veneers, which Gibson's people have claimed. We're told Dr. Shelden's analysis is that Oksana lost a tooth and another was chipped.
TMZ broke the story ... Oksana told her dentist -- whom we now know is Dr. Shelden -- that Mel knocked her teeth out, yet he did not report the incident to authorities which he is required to do by law. We now know why Dr. Shelden didn't report the incident -- before telling him about Mel, Oksana insisted on confidentiality and Dr. Shelden felt he was duty-bound to honor it.
We've also learned Dr. Shelden has not been interviewed by L.A. County Sheriff's investigators yet but he will indeed be interviewed as part of the investigation. We've also learned Oksana's lawyers have given Dr. Shelden written authorization to share all of his medical records related to Oksana with the Sheriff's Department.