The implication is that Mayfield performed a very skilled bit of sleight-of-hand, managing to palm and hide the "dissolving sutures" and later plant them on Columbo. Presumably, he also produced some shreds of "regular" suture and pretended to remove them from the patient.
Certainly this would be difficult, and his defense would have much fun at trial with the fact that the skilled onlookers failed to spot it -- but, surgeons are famed for their manual dexterity, and if anyone could do this, Mayfield could.
Ah, the great Santini. Which reminds me I can't wait until Season 5 comes out because "Now You See Him" is in there. I wonder when season 6 will be released, anyone know?