Good question...and one that is highly debated in the literature. After decades of using the metal-backed implants surgeons are trying the new all-poly (molded plastic) implants.
They say the metal backed implants can get worn unevenly causing the bone to deteriorate. The implant can loosen, too. On the other hand there's concern that the polypropylene type won't hold up under daily use by active adults.
Researchers at the Lenox Hill Hospital in New York City report results of the all-poly implant for a group of active, younger (less than 60 years old) adults. A majority of the patients said that walking distance was unlimited. A smaller number reported walking limited to 10 blocks or less.
Many of these active adults were also involved in swimming, tennis, and golf.