Hi Happyhopper, the words "implications" and "considerations" are keeping everyone silent. I think a resurfacing surgeon might answer those best.
But, my non-medical understanding is the head to neck ratio is a measurement that compares the size of the femoral head to the size of the femoral neck. In basic terms, the larger the diameter of the ball is compared to the diameter of the neck, the more range of motion can be achieved. In some people with FAI, the neck has grown "fatter" in certain areas and so the ratio is reduced. Simply put, the neck is too wide and it bangs into the edges of the socket. Other patients have developed over time a femoral head that has "slipped" away from being centered on the femoral neck. And so the neck will "bang" into the side much sooner on one side. In resurfacing, the new femoral cap is measured and placed to be right back in the center of the neck so it restores all the "clearances" for the neck. And sometimes non structural cysts are shaved off the neck too that were making the neck too wide.
But, there are so many other things involved in the head / neck ratio like the depth of the patients hip socket. I understand that women more often than men have hip dysplasia. A shallow hip socket will allow more range of motion theoretically but is less stable. And so that is another confusing issue with head neck ratio discussion. Can a shallow hip be reamed deeper to accommodate the new metal cup?
Sorry that is as far as I will try to go with a complex question.
Chuckm