Are the rod's they use now for scoliosis surgery more flexible then the harrington rod?

Modern hardware is as inflexible as Harrington rods were. The big difference is how they connect to the vertebrae. Harrington rods had hooks which looped over or under the last vertebrae to be fused, both bottom and top. It was possible for the hooks to dislodge from the vertebrae which could cause pain and perhaps another surgery. Harrington rod patients also tend to develop "flatback syndrome." Neither of these happen with modern hardware.