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The skull can be thought of simplistically as the connection of 5 convex  surfaces (front, top, back and two sides) into an oblong curved shape. (normocephaly) Each of the five surfaces has varying degrees of convexity but none of its surfaces are flat. The back of the head has a fair amount of convexity and it is the closest of all the surfaces to a hemisphere. (albeit not a strong one)

Because of its most convex shape and its location, the back of the head is most commonly affected by deformation creating various types of flattened contours. Plagiocephaly or flatness one side of the back of the head is the most recognized but an overall loss of convexity is probably just as if not more common. Such flatness is usually so evident that in the prone position a cup can rest comfortably on the back of the head.

This has led to the intraoperative cup test. One measure of the success of a custom skull implant for improvement in the shape of the flat back of the head is that it no longer has a flat surface on which the cup can rest. The cup will fall of the reshaped back of the head because its increased convexity is too narrow a surface for the base diameter of the cup to rest on in a stable fashion. (provided the cup has a broad enough base)

In the end the success of any aesthetic procedure is does it met the patient’s expectations or level of satisfaction. While there are always limits as to how much implant the scalp can stretch to accommodate it, if enough new back of the head projection or convexity is created the chances of patient satisfaction is high. While the cup test is a crude measure of the success of the procedure it does provide intraoperative evidence of a positive change in its shape. 

Dr. Barry Eppley

World-Renowned Plastic Surgeon

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