My initial thought was the mortality rate was d/t the force required and location. So, I did a bit of research and came up with this:
"First rib fractures were once thought to be a harbinger of severe trauma, since the first rib is very well protected by the shoulder, lower neck musculature, and clavicle and was thought to require a much higher impact force to fracture than other ribs. These data are now in question, but until further studies are done, fractures of the first rib should raise suspicion of significant chest trauma. Mortality rates as high as 36% have been previously reported with first rib fractures, which are associated with injury to the lung, ascending aorta, subclavian artery, and brachial plexus."
The complete article is at
http://www.emedicine.com/emerg/byname/fractures-rib.htm