There are two reasons. First most traditional hip replacements are inherently unstable. Permanent hip position precautions are required to avoid dislocations. Secondly , bearing surfaces can wear out or fracture. Studies in young active patients indicate a 20-30% failure rate by 8 years with standard polyethylene bearings. Crosslinked polyethylene and ceramic on ceramic bearings are more durable to wear. However , they are still prone to crack with severe repetitive impact activities. Only large metal-metal bearings are stable and durable enough to allow full activities without restrictions.