Displacement pattern of the normal pars flaccida in the gerbil

Anatomical boundary of the tympanic membrane pars flaccida of the Meriones unguiculatus (Mongolian gerbil)

The pars flaccida of the Meriones unguiculatus (Mongolian gerbil) was in previous studies shown to bulge almost spherically when pressurized, a behavior suggesting that it is suspended by a fixed circular boundary. The question arises whether this "functional" boundary is based on an underlying circular anatomical boundary, an important issue for modeling the middle-ear mechanics. In this article, the boundaries of the Mongolian gerbil pars flaccida were visualized in situ with otomicroscopy and in slides with light microscopy and by micro-CT radiology. For the major part of its circumference, the pars flaccida was found to be suspended by rigid bone, that is, the tympanic legs. The remaining boundary is made up of the terminal portion of the handle of the malleus and the soft tissue of the terminal arches. The attachment to these structures is simple and uncomplicated, and the geometry is regular and symmetric: deviating by only 3.5% from a perfect circular shape. The findings justify the use of a fixed circular boundary as a good approximation for the modeling of pars flaccida deformation in the Mongolian gerbil.

A nonlinear finite-element model of the newborn middle ear

A three-dimensional static nonlinear finite-element model of a 22-day-old newborn middle ear is presented. The model includes the tympanic membrane (TM), malleus, incus, and two ligaments. The effects of the middle-ear cavity are taken into account indirectly. The geometry is based on a computed-tomography scan and on the published literature, supplemented by histology. A nonlinear hyperelastic constitutive law is applied to model large deformations. The middle-ear cavity and the Young's modulus of the TM have significant effects on TM volume displacements. The TM volume displacement and its nonlinearity and asymmetry increase as the middle-ear cavity volume increases. The effects of the Young's moduli of the ligaments and ossicles are found to be small. The simulated TM volume changes do not reach a plateau when the pressure is varied to either -3 kPa or +3 kPa, which is consistent with the nonflat tails often found in tympanograms in newborns. The simulated TM volume displacements, by themselves and also together with previous ear-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.

Pars flaccida retraction and mastoid size: relationship in clinically normal specimens

BACKGROUND

The relationship of pars flaccida retraction with epitympanic aeration and mastoid size is ill-defined. Both pars flaccida retraction and small mastoid size are traits of nearly all clinical entities of otitis media.

OBJECTIVES

To determine, in clinically normal specimens, the relationship of pars flaccida retraction with mastoid pneumatisation and epitympanic aeration.

STUDY DESIGN

Post-mortem anatomical dissection of 41 bequeathed adult crania without clinical otitis.

METHODS

Pars flaccida retractions were viewed with an operating microscope and judged using Sade's classification. Mastoid sizes were determined radiographically (by plain Law lateral images). Ten crania, five with the largest mastoids and five with the smallest mastoids, were studied by computed tomography.

RESULTS

No specimen had a retraction worse than Sade grade two (i.e. retracted onto the neck of the malleus). Retractions were unrelated to the extent of mastoid pneumatisation. All epitympani were found to be normal on both anatomical dissection and computed tomography imaging.

CONCLUSION

Pars flaccida retraction is probably related to prior, presumably transient, non-aeration of Prussak's space.

Pars flaccida displacement pattern in otitis media with effusion in the gerbil

HYPOTHESIS

Our hypothesis is that otitis media with effusion causes stiffness loss in the pars flaccida of the tympanic membrane. This loss of stiffness may be persistent and may trigger the development of retraction pockets and cholesteatoma.

BACKGROUND

Otitis media with effusion is a very common disease in childhood. It can cause minor to moderate hearing loss and delayed speech development. Otitis media with effusion is a risk factor for retraction pocket formation.

METHODS

Otitis media with effusion was induced unilaterally in 15 gerbils by obstructing the eustachian tube with glue. The contralateral ears served as normal controls. As a measure of mechanical stiffness, pars flaccida peak displacement versus pressure was used. The displacement measurements were made with moire interferometry. This is a noncontacting optical technique with which the displacement of an object can be measured in real time.

RESULTS

The mean peak displacement of the pars flaccida in the group with otitis media with effusion was increased threefold as compared with normal controls. This difference was statistically significant.

CONCLUSION

There was a loss of mechanical stiffness in the pars flaccida caused by otitis media with effusion. This loss of stiffness may be persistent and may predispose for retraction pocket formation and cholesteatoma development.

Thickness of the gerbil tympanic membrane measured with confocal microscopy

Thickness data for the gerbil tympanic membrane, an extremely thin biological membrane, are presented. Thickness measurements were performed on fresh material using fluorescence images taken perpendicular through the membrane with a commercial confocal microscope. Thickness varies strongly across the membrane. Similar thickness distributions in all samples (pars tensa n = 11; pars flaccida n = 3) were observed. The pars tensa has a rather constant thickness of about 7 microm in the central region curving as a horse shoe upwards around the manubrium. In the most superior parts of the pars tensa thickness becomes gradually twice as large. Thickness increases also steeply from the central region towards the edges (about 35 microm near the annulus and 20 microm near the manubrium). A pronounced, local thickening of about 30 microm is present close to the edge and extends as a ring along the entire annular periphery of the pars tensa. Overall, the pars flaccida is thicker than the pars tensa and has a rugged surface. Its central region has a mean thickness of about 24 microm with a mean variation of about 4 microm. The average thickness in the inferior region is slightly larger than in the superior region. The pars flaccida thickens steeply, up to about 80 microm, near the edges.

Pars flaccida displacement pattern in purulent otitis media in the gerbil

HYPOTHESIS

Our hypothesis is that purulent otitis media and otitis media with effusion cause stiffness loss of the tympanic membrane. This loss of stiffness may be persistent and precede the development of retraction pockets and cholesteatoma.

BACKGROUND

Postinflammatory changes such as retraction pockets and cholesteatoma develop in the pars flaccida and in the pars tensa of the tympanic membrane. In our previous experimental studies, stiffness changes were shown to develop early in the pars tensa in response to purulent otitis media and otitis media with effusion. These changes are suggested to be precursors to a later development of retraction pockets and cholesteatoma. In the clinical situation, retraction pockets are often found in the pars flaccida only. The aim of the current study was thus to investigate whether stiffness changes appear also in the pars flaccida during purulent otitis media.

METHODS

Streptococcus pneumoniae type 3 was injected into the middle ear to induce purulent otitis media. As a measure of pars flaccida stiffness, peak displacement versus middle ear pressure was used. The peak displacement measurements were obtained from full-field moiré; interferometry, which is a noncontacting optical technique for deformation measurements.

RESULTS

Ears with purulent otitis media showed increased peak displacement of the pars flaccida at a middle ear pressure of 200 daPa, compared with normal controls.

CONCLUSION

There was a decreased mechanical stiffness of the pars flaccida in acute purulent otitis media. This decreased stiffness may predispose for future retraction pocket formation and cholesteatoma development.