In Niven's universe, the technology to indefinitely sustain any human organ outside of the body was developed in the early 21st century, greatly simplifying organ transplants.
An example in the Known Space universe was that anyone who wore eyeglasses was considered a reasonable candidate for an eye transplant (one or both); whereas in the real world, today's nearsighted population can solve the problem (temporarily) by wearing corrective lenses or (more permanently) by undergoing laser surgery.