would there not be permanent hand damage?

Oh! The same thing happened to me as a kid, I stepped on a floorboard with nails. Also, it seems they are careful when nailing the hands to the wall, Esther tells Brian that he was perfect, "both hands clean between the third and fourth metacarpals" (that's between the middle and ring fingers) avoiding touching bones, veins and tendons. Sarah made a bit of tendon damage when she pulled the hands out but Esther (which is a "big-time doctor") said she would be fine.