Phleboliths originate as thrombi in the pelvic veins and are the result of injury to the vein wall.
Although in children the pelvic veins and venous plexuses may be protected by valves, these have disappeared in adult life. In 20 dissections in adult cadavers remnants of osteal valves were sometimes seen, but a functioning bicuspid valve was noted only once.
These unprotected veins are subjected to large and abrupt rises in pressure during normal acts of coughing and straining. Because of this the veins are distended and damage to the intima with the formation of a protective thrombus can occur.
A locally high concentration of phosphatase in the perivesical tissues would explain why these thrombi become calcified in the pelvis and why phleboliths are so rarely found elsewhere.
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