| The Acoustic Characteristic of Titanium Total Ossicular Replacement Prostheses |
1. Introduction
Earlier research shows that 60% of the ossicular chains of the patients with middle ear disease were found to have problems (Austin, 1971). Another report on 4340 patients who had undergone middle ear surgeries shows 14.12% of these patients needed ossiculoplasties (Ogale et al. ,1997). With the development of transportation and construction industries, the number of skull injuries increases dramatically; these patients also may have conductive hearing lose due to temporal bone fracture, but they are often neglected. It is possible part of these patients mentioned above can improve their hearing through ossiculoplasty. Part of the patients with congenital middle ear abnormality or ossicular chains defect due to the excision of middle ear tumor also need have their ossicular chains reconstructed. At present the healing rate of tympanic membrane is as high as 90% (Celik et al., 2001), but only 50-70% of these patients’ hearing can satisfy their full needs (Jiang et al., 1998). The results of hearing after operation are affected by many factors such as the skills of the surgeons, the nature and extent of the disease, the function of the eustachian tube, and so on. (e.g., Dornhoffer, 1998; Harvey et al., 1999; Murphy, 2000).The shape and mass of the ossicular prostheses may also be factors that influence the transmission of sound energy.
In this experiment, we use a Mechanic Middle Ear Model (MMEM) to explore the effects of the mass and the head area of ossicular prostheses on their sound transmission function.
2. Materials and Methods
Two latex membranes were used to represent the tympanic membrane and oval window membrane, respectively; the areas of these artificial membranes were 63.62 mm2 (thickness 0.1 mm) and 7.09 mm2 (thickness 0.2 mm), respectively. The Titanium Total Ossicular Replacement Prostheses (TORP) (Wang et al., 1996) were fitted between the artificial tympanic membrane and oval window membrane during the test. Pure tone signals (audiometer AD 226, Interacoustics, Denmark) were used to stimulate the vibration of tympanic membrane. The vibration of oval window membrane was recorded by a Laser Doppler Vibrometer (Brüel & Kjær Sound & Vibration Measurement A/S // Ometron VH300+ Laser Doppler Vibrometer, Denmark) (Fig. 1). The ossicular transmission function was evaluated by comparing the vibration velocities of the artificial oval window membrane (Fig. 2). All the tests were performed in quiet condition (Fig. 3).
2.1 The evaluation of MMEM
In order to evaluate if the MMEM could represent the function of normal human middle ear, a Titanium TORP (prosthesis 5), which had the medium mass and head area, was placed in the MMEM. Pure tone signals (Frequencies 250-8000Hz) were given to stimulate the artificial tympanic membrane. The initial sound level was 100 dB HL at each frequency, if the Vibrometer could detect the vibration of the artificial oval membrane, the vibration velocity of the artificial oval membrane was recorded. The sound level was then decreased by 5 dB each time until the vibrometer could not detect the vibration of the artificial oval membrane. The sound level at this point was defined as the detection threshold of the MMEM. Through comparing the detection threshold of this MMEM with the hearing threshold of normal person, we could determine whether the MMEM could be used to evaluate the sound transmission function of TORP.
2.2 The evaluation of TORP
Two groups of TORPs were used in these tests: Area group: the prostheses 1 to 4 (head area: 3.33, 3.56, 4.83, and 5.23 mm2) with the same shape (Table 1). Mass group: the prostheses 5 to 8 (mass: 11.4, 21.2, 30.9, and 50.5 mg) with the same shape (Table 2). The sound level of the signals was 100 dB HL. The vibration velocities of artificial oval window membrane were recorded to evaluate the sound transmission functions of different prostheses.
3. Results
3.1 The detection threshold of the MMEM
According to the results listed in Table 3, the detection thresholds of the MMEM are 90, 90, 80, 60,50,50,50,50,70,70 dB HL at frequencies from 250 to 8000 Hz, respectively. After converting dB HL to dB SPL, the detection thresholds are 115.5, 101.5, 87.5, 67, 56.5, 59, 60, 59.5, 85.5, 83 dB SPL respectively. Based on this result, we can draw the detection threshold curve of the MMEM (Fig. 4). This curve shows the model is most sensitive to the frequencies between 1500～4000Hz, moderate sensitive to frequencies between 6000 Hz～ 8000 Hz and worst sensitive to frequencies below 750 Hz. This is similar to the hearing threshold curve of a normal human; therefore we can use this MMEM to evaluate the function of the prostheses.
3.2. Area group
Based on the results of the experiment, the sound transmission curves of prostheses in area group are drawn (Fig. 5). When the area ratio of head to tympanic membrane is small (Prosthesis 1 and 2 ), there is an apparent resonant peak at frequency 2000 Hz and a notch at frequency 3000 Hz in the transmission function curves. The curves are smoother when the area ratio of head to tympanic membrane is large (Prosthesis 3 and 4). The sound transmission functions of prosthesis 1 and 2 are better than prosthesis 3 and 4 at frequencies 1500 Hz～4000 Hz; But at frequencies above 6000 Hz, prosthesis 3 and 4 are better; at frequencies below 1000 Hz, it is impossible to compare their functions, this may be due to the fact that the number of prostheses is too small.
3.3. Mass group
According to the transmission function curves of the mass group, the smaller the mass is, the shifting higher frequencies the peak of the curve (Fig. 6); the larger the mass is, the shifting lower frequencies the peak of the curve. This makes the small-massed prostheses function better at higher frequencies and the large-massed prostheses function better at lower frequencies. Concluding that small-massed prostheses function better overall.
3.4. The Blending of mass and area group
Research (Zhao Shouqin, Goode, 2002) showed that the slight increase of the length (0.2 mm) of the prosthesis decreased the sound transmission function of middle ear below 1. 0 kHz, but it had no effect on high frequencies (> 1.0 kHz). The difference of the length of prostheses between area group and mass group is 0.17 mm, moreover, the difference in sound transmission property is located mainly in frequencies above 1.0 kHz, so it is still feasible to compare the sound transmission properties of all these eight prostheses. The transmission curves of all these eight ossicular prostheses show that prosthesis 1 has the best sound transmission function and prosthesis 5 has the second best transmission function. (Fig. 7) Although the head area of prosthesis 4 is approximate to prosthesis 5, which has a larger mass, the sound transmission function of prosthesis 4 is worse than that of prosthesis 5.
4. Discussion
Several ways to evaluate the sound transmission properties of middle ear implants have been established. Besides computer-based simulations using acoustic and electrical analog circuits or finite element analysis, measurements can also be performed with temporal bone preparations; however a large number of parameters would influence the outcome of these measurements. Due to the simplicity of this MMEM and the stability of the material used, MMEM can provide a stable environment for the evaluation of ossicular prostheses (Meister et al., 2000).
There are only a few reports that are relevant to the relation between ossicular prosthesis head area and its sound transmission function. Morris (2004) reported that if the cartilage between ossicular prosthesis and tympanic membrane was small, the sound transmission function was best overall with slight loss in higher frequencies compared to no cartilage at all. The medium size cartilage (about size used clinically) performed similar to Partial Ossicular Replacement Prosthesis (PORP) alone, but slightly worse at high frequencies. The large cartilage performed worst overall, especially in the lower frequencies. The results of our research show that the ossicular prostheses with large head area perform the worst, especially in frequencies between 1000 ～4000 Hz.
Former researchers have studied the effect of the mass of ossicular chains in cat, human cadaver tympanic bone, and etc. Some of these researches also have used the Laser Doppler Vibrometer to measure their sound transmission properties. There are also reports about the effect of the mass of ossicular prostheses. When the mass of the incus prosthesis was increased, the vibration displacements of the stapes plate decreased in frequencies above 1000 Hz (Nishihara et al., 1994). Other researchers used electric circuit to simulate the sound transmission function of ossicular prostheses, and got the conclusion that the increased mass had no significant effect on sound transmission function (when the mass of ossicular prosthesis was increased by 16 times, the air hearing threshold only increased by 10 dB) (Rosowski et al., 1994). Recent studies indicated that additional mass added to the ossicular prostheses resulted in the resonance frequency moving towards lower frequencies while the sensitivity at the high frequency area was decreased (e.g., –6 dB at 2 kHz). The peak of the curve was also reduced (Meister et al., 2000). Research about the effect of the mass of ossicular chains showed the larger the mass of the ossicular chain, the smaller the displacement of stapes plate (Gan et al., 2001). The result of our research is consistent with those of Nishihara, Meister and Gan’s.
According to the sound transmission curves and the parameters of all the eight ossicular prostheses, when the head area of an ossicular prosthesis is given, there is an optimum mass occurring from it. Based on the results of this experiment, the optimum mass for the prosthesis with a head diameter 2.06 mm is 4.6 mg; the optimum mass for the prosthesis with a head diameter 2.56 mm is 11.4 mg. (Fig. 6)
The result of this research indicates that the ossicular prostheses with a small head area have advantage when only concerning the sound transmission function, but in clinic we should also consider the situation that the smaller head area will increase the possibility of the displacement of the ossicular prostheses, and it will also increase the possibility of perforation of the tympanic membrane. When designing the ossicular prostheses, all of these factors should be considered. The result of this research also indicates that when choosing the ossicular prosthesis during operation; both the area of the patient’s tympanic membrane and the pattern of the patient’s hearing loss should be taken into consideration. That is, if the areas of the patient’s tympanic is small, the ossicular prosthesis with a small head area should be used, and vice versa. If the patient’s hearing loss is concentrated in high frequencies, the ossicular prosthesis with a small mass should be chosen; if the patient’s hearing loss is located in low frequencies, the ossicular prosthesis with a large mass should be chosen.
References
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