Non-linear finite-element modelling of newborn ear canal and middle ear

Early hearing screening and diagnosis in newborns are important in order to avoid problems with language acquisition and psychosocial development. Current newborn hearing screening tests cannot effectively distinguish conductive hearing loss from sensorineural hearing loss, which requires different medical approaches. Tympanometry is a fast and accurate hearing test routinely used for the examination of conductive hearing loss for older children and adults; however, the tympanograms are hard to interpret for newborns and infants younger than seven months old due to significant differences in the outer and middle ear. In this work, we used the finite-element method (FEM) to investigate the behaviour of the newborn canal wall and middle ear in response to high static pressures as used in tympanometry. The model results are compared with the analysis results of multifrequency tympanometry measured in healthy newborns and with available tympanometry measurements in newborns with presumed middle-ear effusion. Analysis results of multi-frequency tympanometry show that both susceptance and conductance increase with frequency. The equivalent volumes calculated from both tails of both the admittance and susceptance functions decreased as frequency increases. The volumes derived from susceptance decrease faster than do those derived from admittance. The 5th-to-95th percentile ranges of equivalent volume and energy reflectances are much lower than previous measurements in older children and adults. Non-linear finite-element models of the newborn ear canal and middle ear were developed. The ear-canal model indicates that the Young’s modulus of the canal wall has a significant effect on the ear-canal volume change, which ranges from approximately 27% to 75% over the static-pressure range of ±3 kPa. The middle-ear model indicates that the middle-ear cavity and the Young’s modulus of the tympanic membrane (TM) have significant effects on TM volume displacements. The TM volume displacement and its non-linearity and asymmetry increase as the middle-ear cavity volume increases. The simulated TM volume displacements, by themselves and also together with the canal model results, are compared with equivalent-volume differences derived from tympanometric measurements in newborns. The results suggest that the canal-wall volume displacement makes a major contribution to the total canal volume change, and may be larger than the TM volume displacement.