Rod system for gradual dynamic spinal fixation

A rod system, comprising:

• a first rod having a first head and a first shaft;
• a first elastic member fitted around the first shaft and against the first head;
• a second rod having a second head and a second shaft;
• a second elastic member fitted around the second shaft and against the second head;
• a case defining:
• a first inner portion with a first outer opening;
• a second inner portion with a second outer opening;
• and
• a third inner portion in communication with the first and the second inner portions;
• a first retainer ring fixed to the first inner portion, the first retainer ring abuts the first elastic member so the first elastic member is sandwiched between the first retainer ring and the bottom of the first inner portion;
• a second retainer ring fixed to the first inner portion, the second retainer ring having an inner radius larger than a radius of the shaft;
• and
• a first stopper ring between the first and the second retainer rings, the first stopper ring preventing movement to the first rod, the first stopper ring comprising an absorbable material that can be decomposed by bodily fluid;
• wherein:
• the first rod with the first elastic member is seated in the first inner portion so the first elastic member abuts the bottom of the first inner portion and the first head protrudes into the third inner portion;
• and
• the second rod with the 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.