The genetronic replicator was a revolutionary medical device developed by Dr. Toby Russell. The genetronic replicator could scan the genetic information in a damaged organ, and replicate an entirely new, healthy organ for replacement.
It was in the prototype stages in 2368, when Dr. Russell brought it to the USS Enterprise-D in hopes of using it to treat a paralyzed Lieutenant Worf. At that point, it had very low success rates during holographic simulations. Starfleet had turned down Russell's request to test it on a humanoid patient, and Dr. Crusher agreed that there was still too little data to risk its use.
She later permitted the surgery after Worf made it clear he would rather die than live paralyzed. The replicator created a new spinal column for Worf, which Crusher and Russell successfully implanted. However, there were complications, and Worf died on the operating table. Fortunately, the redundancies of Klingon anatomy allowed Worf to be revived. (TNG: "Ethics")