Why do tympanostomy tubes block?

One hundred extruded Shah tympanostomy tubes were recovered from the ears of children following surgical treatment of otitis media with effusion. They were examined both macroscopically and microscopically for the presence of luminal blockage. The lumen was blocked by eosinophilic material with a polymorphonuclear leucocyte infiltrate in 56% of cases and the base of the extruded tube was covered by a cast of squames in 70% of cases. The occurrence of luminal blockage was associated with the presence of a thick middle ear effusion.