to have their teeth examined
to be examining their teeth
to be examined their teeth
to have examined their teeth
“ have + sth （宾语）+ 过去分词（宾语补足语）”意为“让 / 叫 / 使 / 请别人做某事”。宾语sth 后面用过去分词作宾语补足语，说明sth 与过去分词表示的动作之间是被动关系；their teeth和examine的关系是人们的牙齿被检查。
句意：人们需要定期去牙医那里检查牙齿。
