I find the negative numbers for the underside not matching to be confusing. Personally I would rather say that (for example) 19 is just above the radiocarpal joint in the middle outside of the forearm, and that -19 is the spot directly opposite that.
Yes this means that this will have gaps in the numbering where you can’t (shouldn’t) put an implant but if someone says L5 or -L5 you know exactly where they are talking about.
I would also like to see a more generic indicator… So “I am thinking of putting it in P5” would mean any of L5, L-5, R5 or R-5.
Otherwise I think this can work.