Hip and knee replacements, two of the fastest-growing U.S. medical procedures, are subject to huge - and apparently random - price variations within the same geographical areas, a new insurance industry study said on Wednesday. The study by Blue Cross Blue Shield health insurers adds to the evidence of massive disparities between what different hospitals and medical practices charge in the world's most expensive healthcare system. It examined claims in 64 healthcare markets over three years and found the biggest price swings for hip surgery in Massachusetts, where the same type of care varied by more than 313 percent, from a low of $17,910 to a high of $73,987.