This is a post on the EAC Site.

For some engineers, specifying a screw thread is no more than determining diameter and length and then searching through a parts catalog. For others, a custom thread is the difference between a successful surgical outcome with enhanced quality of life or a disappointing result that may require a revision surgery. A bone screw, whether used in a reconstructive, spinal, or other application, can be such an example.

A bone screw typically begins with a conical tip. The major diameter of the screw quickly increases to provide purchase in order to draw the screw into the bone. However, the minor diameter increases more gradually to reduce insertion force until the final portion is engaged into the bone. At this point the increasingly large tapered minor diameter provides a press fit into the outer (cortical) layer of the bone. This thread geometry is illustrated in Figure 1. The overall thread geometry is critical to providing stability to the affected area to allow bone growth and ultimately healing.

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