On the function of the pars flaccida: retraction of the pars flaccida and buffering of negative middle ear pressure.
Retraction of the elastic pars flaccida (PF) is the first reaction to a middle ear (ME) negative pressure in adults. Such retraction reduces middle ear volume, thereby elevating the ambiant pressure. PF retraction acts therefore as a buffering mechanism which counteracts ME negative pressure. Measurements of the volume of the retracted PF were performed by filling the retracted PF space with a liquid. Measurements of 15 PF retraction pockets stages I showed them to have a volume of 1-3 m3 (average 2 mm3) and 23 retraction pockets stage II presented a volume of 4-8 m3 (average 5-8 mm3). It was calculated that retraction of the PF stage I and II may counteract 1-80 mm H2O of negative ME pressure-depending on the degree of retraction and the extent of mastoid pneumatization. The range of PF retraction suggests that normal pressure in the ME fluctuates, up to 8 mm H2O in average pneumatic ears (10 cm2) extending eventually to 80 mm2 in ears with sclerotic mastoids. Other mechanisms which may also serve the ME as pressure buffers by reducing its volume-once a negative pressure sets in-are retraction of the pars tensa, swelling of the mucosa or flooding the ME cavity with an exudate.
Sadé
J
J
Ear Research Laboratory, Sackler School of Medicine, Tel-Aviv, Israel.
D000328:Adult; D004432:Ear, Middle / Q000502:physiology*; D006801:Humans; D011312:Pressure; D014432:Tympanic Membrane / Q000502:physiology*