If a person broke his arm, Heaven forbid, and the arm – which is the arm on which he normally wears Tefillin – is now covered by a cast from the upper arm until his fingers, should he wear the Tefillin Shel Yad over the cast?
Normally, of course, the Tefillin Shel Yad is worn directly on the skin, and not on one’s shirt. There are different views, however, as to the reason for this requirement. One possibility is that a "Hasisa" ("disruption") disqualifies the Misva; meaning, the obligation is fulfilled by wearing the Tefillin directly on one’s body, and if there is something in between the Tefillin and the skin, one is not considered to have worn Tefillin. This is the view of the Rosh (Rabbenu Asher Ben Yehiel, Germany-Spain, 1250-1327), who held that one must wear both the Tefillin Shel Rosh and Tefillin Shel Yad directly on his body. However, some Halachic authorities inferred a different view from the comments of the Rashba (Rabbi Shelomo Ben Aderet of Barcelona, Spain, 1235-1310). According to their reading of the Rashba’s comments, a "Hasisa" in between the Tefillin and one’s skin does not invalidate the Misva. One may not wear the Tefillin on his shirt only because the Tefillin Shel Yad should be covered, as indicated by the verse which states, "Ve’haya Lecha Le’ot Al Yadecha" ("It shall be FOR YOU a sign upon your arm" – Shemot 13:9). Since one must cover the Tefillin Shel Yad with his shirt sleeve, he must wear the Tefillin underneath his shirt, directly on the arm. However, in a case of one wearing a cast, this view would allow wearing Tefillin on the cast and then covering it with the shirtsleeve. Since there is no disqualification of "Hasisa" according to this view, one can fulfill the Misva by wearing the Tefillin over the cast and then covering it with his sleeve.
Given the difference of opinion in this regard, the Halachic authorities advise a person with a cast on his arm to wear the Tefillin Shel Yad over the cast, but without reciting a Beracha. Instead of reciting the Beracha of "Le’hani’ah Tefillin" over the Tefillin Shel Yad, he should instead recite the Beracha of "Al Misvat Tefillin" over the Tefillin Shel Rosh, having in mind for the Beracha to apply also to the Tefillin Shel Yad.
It should be noted that this entire discussion applies only in a case where the cast cannot be removed and it covers the area where the Tefillin box is worn. Ideally, if possible, one should ask the doctor to apply the cast in a way which allows him to at least wear the Tefillin box on the muscle of the upper arm. If this is not possible, then the Tefillin should be worn over the cast, as discussed.
Summary: If a person has a cast covering the entire arm on which he normally wears Tefillin, and the cast cannot be removed, then he should wear the Tefillin over the cast. In such a case, he recites the Beracha of "Al Misvat Tefillin" over the Tefillin Shel Rosh, and does not recite a Beracha over the Tefillin Shel Yad.