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Hello, I'm Dr. Mark. I am board certified (USA), and have twenty years experience. Please allow me to review your question.
Do you have pain when moving your shoulder around, such as mimicking a throwing motion?
Well, the issue here doesn't sound like a MRI of the rotator cuff will be helpful.
It appears that if your deltoid muscle isn't working at all -- the axillary nerve, or the C5 nerve in the neck could be the culprit.
The EMG and nerve conduction study can be helpful to diagnose this.
While rotator cuff tears can cause shoulder pain and immobility, it should not cause the deltoid muscle to not work at all.
They can check for tears as well, but that doesn't seem to be the urgent issue here.
Hopefully this is just neuropraxia, a temporary nerve issue, that can improve over a few weeks to a few months.
Yes, you're at higher risk if the muscles around the shoulder are weaker than normal.
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