US Patent 8709048 Rod system for gradual dynamic spinal fixation

A rod system includes two rods each with a head and a shaft, two elastic members each fitted around one shaft against one head, and a case defining a first inner portion, a second inner portion, and a third inner portion in communication with the first and the second inner portions. A first rod with a first elastic member is seated in the first inner portion so the first elastic member abuts the bottom of first inner portion and the first head protrudes into the third inner portion. A second rod and a second elastic member is seated in the second portion so the second elastic member abuts the bottom of the second inner portion and the second head protrudes into the third inner portion and abuts the first head.