As you've discovered, there were some concerns about heat build-up between the bone and the metal. If the temperature of this interface gets too high, bone necrosis (death) can occur. But surgeons have overcome this problem with modified techniques to dissipate the heat.

There were also some questions about maintaining an adequate blood supply from the shaft of the femur (thigh bone) up into the femoral head. But studies using nuclear imaging show that an even mount of blood flow is preserved.

Metal-on-metal hip resurfacing (MOMHR) has not been approved for use in woman of childbearing age. There is concern that debris from the metal can cross the placenta and affect the growing fetus. Cobalt and chromium ions have been found in umbilical cord blood to prove this can happen.

So far, there's been no negative effect seen in children who have been exposed to ion particles. But we don't know if long-term studies would show the same benign effect. More study is needed before MOMHR will be approved for this group of women.