A biosynthetic limb was a prosthetic limb made for patients who had lost limbs through injury or disease. The limb could replace the functionality of the original one in terms of both sensation and movement, although it required both physical therapy and counseling for this to be achieved. There were some instances where biosynthetic limbs failed, not due to the limb itself but due to neural trauma in the wound of the damaged limb, which hindered the bio-electric signals required for the synthetic muscles to function as with a "real" limb.
Nog was given a biosynthetic leg in 2375 to replace the left leg that he lost during a critical mission on AR-558. (DS9: "The Siege of AR-558") He had difficulty with adjusting to his new leg, but it was found that the problem was neither his body nor the biosynthetic leg but rather a psychological problem, which was later resolved. (DS9: "It's Only a Paper Moon")