Complex sentence: By the time the device was perfected, Theo had healed to the point at which it was not necessary for him; however, several thousand other children around the world benefited from the WDT valve before medicine technology progressed beyond it.
Split into simpler sentences: By the time the device was perfected, Theo had healed to the point at which it was not necessary for him. However, several thousand other children around the world benefited from the WDT valve before medicine technology progressed beyond it.