Columns to new rows

I am new to PowerShell.

I have a file named c:\index.txt  The files contains 1 to many rows each with seven columns separated by commas.  The first and last column always have data.  Columns two through six can be blank. 

For each row in index.txt, I need to output column1-6,Column7 on a new line as long as column 1-6 has data.

index.txt original

ABC,DEF,123,456,1,2,foo1

A,D,1,,,,foo2

BC,F,13,,,,foo3

New desired file structure index.txt

ABC,foo1

DEF,foo1

123,foo1

456,foo1

1,foo1

2,foo1

A,foo2

D,foo2

1,foo2

BC,foo3

F,foo3

13,foo3