Wearing Tefillin the Wrong Way

Question

If:
A) You don't have your Tefillin and the only pair available for you to borrow is for the opposite hand you generally use (ie, your dominant hand), or
B) You have a cast or splint on your arm or hand that normally wears your Tefillin (ie, your non-dominant hand),

Do you wear the Tefillin incorrectly (in the first case, whether on your dominant hand, or on your non-dominant hand by flipping it around, or on your non-dominant has by wrapping it backwards; in the second case by wearing it on your dominant hand or on top of the cast/splint)?

Do you just not wear the Shel Yad?

Do you not wear Tefillin at all (ie, if the Shel Yad is not Halachically wearable, does it negate the value - is it Me'Akev - of wearing the Shel Rosh)?

Answer 1

The Mishna in Menachot 4:1 states:

תפלה של יד אינה מעכבת של ראש ושל ראש אינה מעכבת של יד
The hand Tefillah does not prevent the head Tefillah, nor does the head Tefillah prevent the hand Tefillah.

So you definitely still wear the head Tefillah no matter what happens to your arms.

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The Mishna Berura writes (OC 27 sk 1) that if one put tefillin on the wrong arm he does not fulfill his obligation. I am not aware of anyone that argues.

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The Aruch HaShulchan (OC 27:17) writes that if someone must borrow tefillin of the other hand, he should wear it on his own weak arm but turn it upside down such that the knot remains on side towards the body, even though the ma'avarta (the slot wherby the strap goes through the box) will end up on the bottom of the box instead of the top.

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Shulchan Aruch HaRav (OC 27:8) rules that if someone has a wound over the entire area of the arm where one can place the hand tefillin, then he is fundamentally exempt from wearing the hand tefillin, but if he wants to he can place the tefillin over a bandage without saying a bracha (and making sure to cover it to avoid marat ayin).