There is no need to lie about a shoulder injury. Just state 'I'm physically unable to use the scanner'. Best of luck to you.

(I'm glad that so many are able to avoid the scanner by stating that they have an injury, but based on the fact that TSA reads here I am also worried that they will soon not permit that 'excuse' and state that we can still use the scanner, just as best as we can, thereby causing more issues for those of us who legitimately cannot assume the position)