Here is a link to an informative article by Dr. Su.

http://www.amjorthopedics.com/PDF/041100446.pdf

Nice article written in plain english so even thickos like me can get it.

Nice little article!

Very good article.  Here's a question, are revision increases in <50 mm femoral heads due to elevated metals from excessive wear (less surface area on head) or due to femoral neck fracture because there is a smaller neck from the get go? 

Su might actually address this in the article, but I couldn't quite tell. 

Thanks