When the painkillers stop working, arthritic joint damage in the big toe can make even the weight of a bedsheet agonizing. Cartiva Inc’s implant – made of the same substance used in contact lenses, called hydrogel – was approved for big-toe surgeries by the U.S. Food and Drug Administration in July. It is designed to mimic natural cartilage and can achieve the same pain relief as surgery while retaining joint-mobility.