Учебники / Middle Ear Mechanics in Research and Otology Huber 2006

2.3 Morphologic processing

After the interferometry measurements the temporal bones were fixated and decalcified with EDTA according to standard procedures and were studied in light and electron microscope.

3. Results

Allperforationswereclosed,asjudgedbyotomicroscopy,between9and14 days post myringotomy.

3.1 3-D shape

Tenratandtenmouseearswererecorded.TheTMismountedinthemeasuring set-up so that the baseline coincides approximately with the plane of the annulus. Thereby the moiré fringes outline the height of the various portionsoftheTMinrelationtotheannulus.Thepeakislocatedaroundor at the umbo. The mean peak height for the group was 6.8 × 10–4 m (range, 6–8).

Ten TMs with closed perforations were recorded. There were no significant di erences in the patterns of the studied TMs in any of the two groups, as compared with the normal controls. The mean peak height for thegroupwas7.3×10–4 m(range,7–8).Therewasnostatisticaldi erence as compared with the normal controls.

3.2 Displacement

Seven control TMs and eight TMs with closed perforation were successfully recorded in a pressure sequence going up to +350 daPa and down to –350 daPa. As the pressure in the external ear canal is increased a first moiré fringe becomes visible in the center of the posterior portion of the pars tensa followed by a first fringe in the anterior portion. With increasing

148pressure the fringe expands and its contour moves in centrifugal direction sothatitgetsclosertotheannulusandtothehandleofmalleus.Meanwhile a second fringe may appear in the same location as the first one did, and so on. The contours of the moiré fringes were compared in the two groups without any notable discrepancy between the two groups.

Fig. 1 Displacement interferogram at an ear canal pressure load of 350 daPa of a) normal left TM and b) myringotomized right TM recorded at two weeks after the intervention. Ad a) number two fringe is present in anterior and posterior portion of TM. Ad b) number two fringe is present in posterior portion of TM and hinted in anterior portion.

Fig.2Mean peak displacement versus pressure in the normal control group and in the myringotomized group. Results obtained during increasing pressurization from zero to 350 daPa and from zero to –350 daPa. Standard deviation indicated with error bars for a few pressure levels. Almost coinciding curve plots.

3.3 Morphology

In light microscopy the pars tensa of the rat TM appears homogenous over its entire surface whereas the site of closed myringotomy appears thicker. The thickness is almost constant in its various portions. In transmission electron microscopy the three layers are readily identified: the dominating lamina propria with its densely packed fibrous bundles and on both sides consisting of thin (outer and inner) epithelial layers (fig. 3). The overall thickness of the pars tensa is approximately 5 m. In the region of a closed perforation a five-fold thickening is present. The increased thickness consists of invading fibroblasts and large amounts of extra cellular substance. This substance of scar tissue contains fiber-like structures that do not appear densely packed as those in the lamina propria. This tissue does not only cover the site of the myringotomy, but is also spread out around this

150site and is here located on the medial side of the remaining lamina propria. The epithelial layers appear normal.

Fig. 3 Transmission electron microscopy, 2.500x, of a) a normal pars tensa, and b) a TM with closed perforation. Note the border of the lamina propria at the perforation site (arrow). Arrowhead in fig. a indicates the outer epithelial layer. The thickness is fivefoldincreasedatthemyringotomysiteduetoinvadedfibroblastsandextracellular matrix (asterisk). lp = lamina propria.

4. Discussion

Apurposeofthisstudywastoevaluatethedegreeofthemorphologicaland functional competence that was restituted in the post-traumatic healing process. As of today chronic TM perforations are most commonly treated with surgical transplants of temporalis fascia, perichondrium or cartilage. Several other substances have been tried out, such as dermal allograft (8), various types of growth factors (9), human type IV collagen, live yeast cell derivatives etc. So far no real alternative to conventional surgical procedures have been presented.

Our aim for a series of investigations is to try out possible remedies for chronic TM perforations as an alternative to open surgery. In order to do so some basic data on the healing of perforations are needed. In the present report we evaluated a set-up for measurements of the strength of the rat TM. The rat was chosen since, among other reasons, there is an

established chronic perforation model in rat (10). A moiré interferometry 151 set up was modified in order to perform such measurements on the rat

TM.

Our second aim was to establish reference knowledge on the functional outcome of closure and healing of fresh perforations where no infection is at hand and when healing conditions are unimpaired. By using a laser beam for myringotomy the size and location of the perforation thus created was accurately done in a reproducible way. It is the first time data are presented on the strength of the recently closed perforation the TM, with the exception of our previous pilot study on gerbil ears treated with stemcells(11).Themostexcitingfindinginthepresentreportisthealmost completely restored static mechanical function, i.e. pressure resistance in

the TM already after two weeks. The histology sections reveal a fivefold increase of thickness due to scar tissue formation without an obvious system of orientation. It appears that the lack of fiber orientation is compensated by the large amount of fibers that are produced at the myringotomy sites. The result as tested with moiré interferometry indicates that this action of nature is appropriate in the sense that the strength or resistance to pressures is restored. This healing process, with an “overproduction” of fiberlike structures that do not exhibit an obvious system of orientation, is of utmost clinical importance. Even this recently closed perforated TM has to withstand the challenges of pressure gradients that occur in every day life (sneezing, coughing etc). It may be that over a longer time period the fiberswillbecomereorganizedinamoree cientwaysimilartothatofthe laminapropria,andtherebythethicknessoftheTMmayberestored.Such an e ect may be of importance for hearing. It has been found empirically that “underlay” myringoplasty, where the transplanted tissue is placed medial to the remnant of the TM, is preferable to overlay. The reason has never been proven. This procedure results in a structural situation similar to the repair process as found in the present study.

5. Conclusions

The static sti ness properties of the rat TM can be measured with moiré interferometry. The functional properties as regards pressure resistance are mainlyrestoredalreadyattwoweeksafterlasermyringotomyinthemodel. The lack of systematic fiber orientation in the closed perforation appears to be compensated by increased amount of tissue.

152References

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669–683.

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153

BOOST OF TRANSMISSION AT THE PEDICLE OF THE INCUS

IN THE CHINCHILLA MIDDLE EAR

Mario A. Ruggero, Andrei N. Temchin, Yun-Hui Fan, Hongxue Cai

Institute for Neuroscience and Hugh Knowles Center (Dept. of Communication Sciences and Disorders), Northwestern University, Evanston, IL, USA

Email: mruggero@northwestern.edu, a-temchin@northwestern.edu, yhfan@northwestern.edo, h-cai@northwestern.edu

Luis Robles, Dept. of Physiology and Biophysics, Faculty of Medicine, Universidad de Chile, Santiago, Chile, Email: lrobles@med.uchile.cl

We re-examined ossicular vibrations in the widely opened middle ear of the chinchilla, a species with a relatively narrow hearing bandwidth, similar to that of humans. The magnitude of vibration velocity at the head of the stapes is relatively constant (–0.3 mm/s/Pa) up to 24 kHz, thus exceeding the bandwidth of hearing in chinchilla (<20 kHz). Phase lag relative to pressure in the external ear canal increases approximately linearly between 0 and 20 kHz, with a slope equivalent to a pure delay of 76 μs. The present findings support the contention that, in general, the middle ears of amniotic vertebrates do not limit the bandwidth of hearing. The magnitude of vibration of the incus lenticular plate is similar to that of the head of the stapes but substantially larger

154thanthevibrationoftheincuslongprocessintherange15–24kHz.Thus,theflexibility of the incus pedicle seems to boost the bandwidth of middle ear transmission.

1. Introduction

Thisstudywaspartlymotivatedbyfindingsthatcontradictedthecommonlyheldnotionthatthe“tympanicmembrane,theossiclesandtheirsupport- ingligamentsplayalargeroleinbandpasslimitingthemiddle-earfunction … determining the band-pass shape of the auditory threshold function” [1]. Those findings led to the conclusion that the velocity magnitudes of middle-ear vibration in several species are relatively uniform over a wide frequency range and thus do not restrict the bandwidth of hearing [2–5].

Seeking further support for that conclusion, we are re-examining ossicular vibrationsinthemiddleearofthechinchilla,aspecieswitharelativelynarrowhearingbandwidth,similartothatofhumans.Wepreviouslymeasured middle-ear vibrations in chinchilla [6] but did not report on responses at highfrequenciescomparabletotheupperlimitsofhearingbecauseoflimi- tationsinthevibration-recordingmethodology(theMössbauertechnique) and in the bandwidth of the acoustic-stimulus system. The present measurements take advantage of an acoustic-stimulus system with wider bandwidth [3] and better vibration-recording apparatus (laser velocimeters) [7]. This study was also motivated by findings in guinea pig and chinchilla suggesting that the magnitudes of stapes vibrations may exceed those of incus vibrations (Figs. 7 of [8] and 6 of [5]).

2. Methods

Middle-ear measurements were carried out in deeply anesthetized chinchillas (Chinchilla lanigera). Stimuli were tones produced by a wide-band acoustic-stimulus system [3]. The vibratory responses to tones of the long process of the incus just peripheral to the pedicle, the lenticular plate of the incus, and the head of the stapes were recorded using laser velocimeters (Dantec or Polytec) coupled to a compound microscope.

3. Results

The magnitude of vibration velocity at the head of the stapes was relatively constant (~ 0.3 mm/s/Pa) up to 24 kHz (Fig. 1A). It decreased at a rate of ~ –20dB/oct at frequencies higher than 30 kHz. Phase lag relative to pressure intheexternalearcanal(Fig.1B)increasedapproximatelylinearlybetween 0 and 20 kHz, with a slope equivalent to a pure delay of 76 μs. The phase

slope was somewhat steeper at frequencies higher than 20 kHz. Both the 155 large bandwidth and the delay of the middle-ear responses appear to origi-

nate principally at the tympanic membrane, since they are present in the vibrationsoftheincusperipheraltoitspedicle.Thesefeatures,however,are refined by a wide peak in the magnitude of the transfer function across the incus pedicle, i.e., between the long process and the lenticular plate, which is accompanied by a large phase lag. The magnitude peak boosts vibrations by about 16 dB at 25 kHz, further flattening the magnitude middle-ear transfer function. The additional phase lag extends the range of constant delay to about 20 kHz. The vibration of the head of the stapes is similar to that of the incus lenticular plate.

Acknowledgments

We were supported by Grant DC-00419 from the National Institutes of Health (USA).

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