Why would your boss need to prepare a defence ?
Carpal tunnel syndrome can happen to anybody for any reason, it happens to people who do nothing more strenuous than pick a pen up, or can happen to somebody who has spent a lifetime abusing their hands in heavy industry, or not happen.
I have had both hands done, was it because I have used my hands for pushing spanners and vice handles along with using a hammer all my life, or was it because I am just unlucky. Doesn't really matter does it, it happens (or not) so we have to deal with it, but to suggest it is somebodies fault and want to claim damages it just wrong.
Neil.