**9. Mechanical hypothesis**

Gauss first proposed the mechanical theory of fat embolization which requires that large fat cells in the bone marrow rupture into the venous circulation through torn venules at the fracture site in the setting of a favorable pressure gradient (increased intra-medullary pressure). These fat globules subsequently embolize to the lungs and obstruct the pulmonary capillaries (Gauss, 1924). Systemic embolization takes place via intra-cardiac shunts or PFO. Small fat droplets (7-10 micrometers) can pass through pulmonary capillaries (Parisi et al., 2002) causing systemic embolization in the absence of anatomic shunt. The clinical picture is dictated by the extent of the organ(s) involved. However, mechanical theory does not explain the following observations:


Therefore factors other than mechanical obstruction must be playing a role which led to the biochemical hypothesis.
