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Background: Photon-counting computed tomography (PCCT) is an emerging technique under evaluation for temporal bone imaging. Its capacity for radiation dose reduction while maintaining diagnostic image quality (IQ) requires further study.

Purpose: To evaluate how PCCT compares to energy-integrating CT (EICT) in temporal bone imaging in terms of IQ and visualization of anatomical landmarks, at a clinically relevant, reduced radiation dose.

Material and Methods: An anthropomorphic phantom with exchangeable fresh frozen human temporal bone specimens was scanned. Nine specimens were scanned on a PCCT system with clinical protocol settings, followed by stepwise dose reductions for one specimen. Ten specimens were scanned on a EICT system with clinical protocol settings. Three readers assessed overall IQ and visualization of 10 important anatomical landmarks on a 4-point scale.

Results: PCCT was preferred by readers for most landmarks and ranked higher in overall IQ compared with EICT. At clinical protocol, PCCT provided statistically significant better results for all landmarks and overall IQ compared to EICT with dose reduction of almost 70%. With stepwise dose reduction PCCT maintained good to excellent IQ for most landmarks, suboptimal for the ossicular chain landmarks at lower doses, and overall poor image quality at the lowest dose level. PCCT at clinical and several reduced dose levels demonstrated statistically significant difference in overall image quality and visualization of landmarks, compared to EICT (P <0.05).

Conclusion: PCCT provides improved diagnostic image quality compared to EICT at clinical protocols, with lowered radiation dose, indicating its suitability for temporal bone imaging.

To develop a modular anthropomorphic phantom to evaluate the performance of radiological techniques for detecting pathologies in the temporal bone region. A phantom was constructed using a human skull, temporal bone specimen, and 3D-printed contour of a human skull. The human skull was embedded in tissue-equivalent plastic, with a cavity to hold the plastic jars containing the exchangeable freshly frozen human temporal bones. Subsequently, stepwise introduction and examination of different clinicopathological scenarios were conducted. Radiological images were nearly identical to those acquired from patients using computed tomography (CT) and cone beam computed tomography (CBCT). The radiological attenuation of polyurethane plastic (PUR) and alginate were similar to those of the soft tissues of living human patients. The mean Hounsfield unit values of the CT slices representing tissue at the brain and temporal bone level were 184 and 171 in the phantom and patient groups, respectively. The modular phantom developed in this study can evaluate radiological techniques and diagnostic possibilities without exposing patients to radiation. To our knowledge, no such modular phantom has been reported in the literature or made available commercially.

The middle ear muscles have vital roles, yet their precise function in hearing and protection remains unclear. To better understand the function of these muscles in humans, the morphology, fiber composition, and metabolic properties of nine tensor tympani and eight stapedius muscles were analyzed with immunohistochemical, enzyme-histochemical, biochemical, and morphometric techniques. Human orofacial, jaw, extraocular, and limb muscles were used as references. The immunohistochemical analysis showed that the stapedius and tensor tympani muscles were markedly dominated by fibers expressing fast contracting myosin heavy chain MyHC-2A and MyHC-2X (79 ± 6% vs. 86 ± 9%, respectively, p = 0.04). In fact, the middle ear muscles had one of the highest proportions of MyHC-2 fibers ever reported for human muscles. Interestingly, the biochemical analysis revealed a MyHC isoform of unknown identity in both the stapedius and tensor tympani muscles. Muscle fibers containing two or more MyHC isoforms were relatively frequently observed in both muscles. A proportion of these hybrid fibers expressed a developmental MyHC isoform that is normally absent in adult human limb muscles. The middle ear muscles differed from orofacial, jaw, and limb muscles by having significantly smaller fibers (220 vs. 360 μm², respectively) and significantly higher variability in fiber size, capillarization per fiber area, mitochondrial oxidative activity, and density of nerve fascicles. Muscle spindles were observed in the tensor tympani muscle but not in the stapedius muscle. We conclude that the middle ear muscles have a highly specialized muscle morphology, fiber composition, and metabolic properties that generally showed more similarities to orofacial than jaw and limb muscles. Although the muscle fiber characteristics in the tensor tympani and stapedius muscles suggest a capacity for fast, fine-tuned, and sustainable contractions, their difference in proprioceptive control reflects different functions in hearing and protection of the inner ear.

Background

The middle ear transmits and amplifies sound vibrations from the tympanic membrane via three ossicles to the inner ear. Moreover, it contains two muscles, the stapedius muscle (SM) which protects the inner ear from loud noise, and the tensor tympani (TT) whose function is still debated. The majority of hearing loss caused by disruption of the ossicular chain is a result of chronic otitis media and cholesteatoma. Variations in pathology, surgical skill and individual healing conditions make objective evaluation of ossicular replacement prosthesis in vivo difficult. Prosthesis development and the investigation of trauma mechanisms are affected by the same challenges. With few changes postmortem, the temporal bone (TB) is suitable for studies of middle ear mechanics and allows a controlled environment. Equally important, it allows theories to be tested without patient risk. In this thesis we used human TBs to find factors associated with optimal sound transfer in the two types of ossicular replacement prostheses. Furthermore, we investigated the mechanism and forces involved in rare cases of isolated malleus fractures. We also investigated the morphology, fibre phenotype composition and vascularization of the human middle ear muscles in order to better understand their roles.

Materials and Methods

Laser Doppler vibrometry (LDV) is an established method of measuring sound transfer in human TBs. We have further developed a surgical model that allows testing of a wide range of prostheses and their placements. In Paper I beneficial factors in partial ossicular replacement prostheses (PORPs) were tested. In Paper II we evaluated different types of total ossicular replacement prostheses (TORPs) including an experimental prosthesis inspired by the single ossicle system of birds. In Paper III the negative pressure trauma typically associated with isolated malleus fractures, produced by a finger being withdrawn from a wet ear canal after a shower or bath, was simulated in TBs. Based on measurement from control persons the forces involved were calculated and measured in models developed for this purpose. The force of the TT was estimated by comparing its cross-sectional area and fibre composition with those reported in published references. In Paper IV we used immunohistochemical, enzyme histochemical, biochemical and morphometric techniques on TT, SM and human orofacial and limb muscle control samples.

Results

Of the prostheses, PORPs and TORPs with lateral contact with both the tympanic membrane and the malleus handle performed best, and TORPs with distal malleus contact proved superior. Our experimental bird-type prosthesis was the most stable in such placement and performed equally to or better than other prostheses. In Paper III the application of negative pressure via the ear canal did not fracture the malleus shaft, with only a passive counterforce from support structures, although the force exceeded that required for a malleus shaft fracture. We estimate that when adding calculated counteracting forces from the TT muscle, sufficient force is generated to cause a malleus fracture. Both human middle ear muscles are predominated by fast type 2 fibres, and have rich capillarization and nerve innervation compared with limb muscles. Muscle spindles were found in the TT but not the SM.

Conclusions

Where possible, an ossicular replacement prosthesis should be placed to allow distal contact with both the TM and the malleus handle. The sound transfer capabilities combined with the stable placement of our experimental prostheses suggest room for improvement. The combination of a negative pressure created by a finger being withdrawn from a wet ear canal and a simultaneous counteracting reflexive force by the TT muscle was found to be sufficient to cause an isolated malleus fracture. The finding of muscle spindles in TT, but not in SM, suggests a difference in regulatory control; furthermore, it indicates that the TT can be activated by a sudden stretch reflex as described in the malleus fracture trauma. The human middle ear muscles have a highly specialized muscle morphology, which is more similar to orofacial than to limb muscles. The fibre phenotype composition suggests capability for fine-tuned, fast, strong and relatively sustainable contractions. Based on fibre type patterns the TT is among the fastest muscles in the human body.

BACKGROUND AND HYPOTHESIS: Isolated malleus shaft fractures are rare cases. A commonly reported cause is a finger pulled out from a wet outer ear canal after a shower or bath. The objective was to investigate experimentally the mechanism and forces needed to establish an isolated malleus shaft fracture.

METHODS: Ten fresh-frozen human temporal bones were adapted to allow visual inspection of the structures involved while negative pressure trauma was applied. Thirty malleus bones were broken and the required forces were measured. Measurements from 60 adult test subjects were used to create mathematical and physical models to calculate and measure the forces necessary for generating trauma. To calculate the maximum muscle force developed by the tensor tympani muscle, the muscle area and fiber type composition were determined.

RESULTS: The temporal bone experiments showed that applied negative pressure in a wet ear canal could not fracture the malleus shaft with only passive counterforce from supporting structures, although the forces exceeded what was required for a malleus shaft fracture. When adding calculated counteracting forces from the tensor tympani muscles, which consisted of 87% type II fibers, we estimate that a sufficient force is generated to cause a malleus fracture.

CONCLUSION: The combination of a negative pressure created by a finger pulling outward in a wet ear canal and a simultaneous counteracting reflexive force by the tensor tympani muscle were found to be sufficient to cause an isolated malleus fracture with an intact tympanic membrane.

Objective: We compared the sound transmission using different types of total ossicular replacement prostheses (TORP); we then studied the performance of a new TORP that we designed inspired by the columella, the single ossicle found in birds.

Methods: Stapedial vibrations were measured on nine freshly frozen human temporal bones with laser Doppler vibrometry. We then compared the performances of eight common TORP positions or designs as well as the new silver prototype of bird-type prosthesis, designed also according to our digital holography patterns of the human tympanic membrane (TM).

Results: The TORPs placed in lateral contact with both the TM and the malleus handle outperformed, at most frequencies, those placed only in contact with the TM.

The new bird-type prosthesis performed equally well or better than all other prostheses.

Conclusion: If the malleus handle can be retained when placing a TORP, the best sound transmission can be achieved by placing the TORP in contact with both the distal part of the malleus handle and the TM. The good performance of our bird-type prosthesis suggests that there is still room for future improvement of prosthesis design to further optimize hearing outcomes after surgery.

Objective: To compare stapes vibrations in different partial ossicular replacement prosthesis (PORP) applications.

Methods: Stapedial vibrations were measured on fresh frozen human temporal bones with laser Doppler vibrometry. Eight different types of common ossiculoplasty methods were compared regarding recovery of stapes vibrations in relation with the normal ossicular chain. The PORPs were divided into three groups: 1) PORPs with the lateral contact only with the tympanic membrane, 2) PORPs with lateral contact only to the malleus handle, and 3) PORPs with lateral contact with both the malleus handle and the tympanic membrane.

Results: The PORPs with lateral contact only to the malleus handle performed better than the PORPs with lateral contact to the tympanic membrane only at 2 kHZ, but the best recovery was found in the group with contact both to the malleus handle and the tympanic membrane.

Conclusion: The best sound transmission might be achieved by placing a PORP in contact with both the tympanic membrane and the handle of the malleus.