Measurement and modeling of boundary shape and surface deformation of the Mongolian gerbil pars flaccida

The shape of the pars flaccida (PF) boundary and its pressure induced deformation was measured in five Mongolian gerbil ears, using an opto-electronic moiré interferometer. To determine the PF boundary, membranes were deformed by middle ear (ME) pressures of -2 kPa and +2 kPa. The boundary of the PF was defined as the locus of points where the pressure induced deformation begins. To a very high precision, this boundary was found to be a circle with the same radius for both over- and underpressure deformations. Between animals the radius varied from 0.760 mm to 0.778 mm. We show that the shape of the PF, while being deformed by static pressure in the ME, can be modeled as part of a sphere. Volume displacement can then be calculated as the volume enclosed by the sphere cap, delimited by the circular PF boundary plane. Volume displacement was calculated for membranes deformed by 400 Pa ME overpressure and -400 Pa ME underpressure. The agreement of the experimental data with the sphere cap model is shown to be very good, and results in a volume displacement measuring accuracy which is better than 10%.