Hi DB Miller!

D8090 - Comprehensive orthodontic treatment of the adult dentition

There is not a direct crosscode we are aware of, so you can either bill the "D" code on the medical claim (many insurers these days will process "D" codes when they are medically necessary services), or you can try the CPT code below and include a narrative report describing the procedure:
41899 - Unlisted procedure, dentoalveolar structures

Of course, if you did office visits/exams, radiographs, etc, those may be separately billable under their own CPT codes.

Hope this helps!