If someone is missing the middle finger of their non-dominant hand, how would they wear tefillin? Just skip the 3 wraps around the middle finger? Wrap 3 times around another finger?
If one is missing his middle finger, he should wrap around the finger closest to the thumb, the index finger. That is the same finger a ring is placed on by marriage, and the wrappings represent our connection to Hashem [as we say "וארשתיך"].
(שו"ת בית ישראל מהאב"ד דעדעלין או"ח סי' ג)