sentences:  Caution should be used for patients who have rheumatoid arthritis, are tall, thin, or small boned, those with osteonecrosis (poor blood supply) to the femoral head, or those with femoral head cysts > 1 cm on an x-ray taken before surgery.Metal-on-metal resurfacing systems are generally unsuitable for women of child-bearing age due to unknown effects of metal ion release on the fetus.Patients with any of these conditions may not be suitable candidates for hip resurfacing.There are hip resurfacing components that have a ceramic coating on metal femoral head component and cross linked polyethylene plastic as a liner for the socket or cup area making it not metal on metal.