Can a TPA process a dental benefit claim using their own internal policies on limitations and exclusions even if these are not spelled out in the employee's summary plan description?

Are you asking about a dental plan or a dental expense in a FSA under section 125. A dental plan should be operated in accordance to the plan specifications determined by the employer. The TPA acts as an agent of the plan sponsor, the employer. A TPA operates at its own peril if it begins to lose sight of whose dental plan it is administering.In a FSA account the IRS regulations are used as a guide as to what is an eligible expense. If the expense is questionable to the TPA, a written instruction by the plan sponsor, the employer,is sufficient to settle the issue. An employer should hire a TPA for their expertise. We have never been overridden by a client.