ABSTRACT
Background
Conductive hearing loss requires the surgeon to evaluate ossicular movement to plan and execute surgery. An objective assessment of the ossicular chain with laser vibrometry would likely improve the results of ossiculoplasty.
Methods
Minimally invasive intraoperative laser vibrometry (MIVIB), a fast and accurate technique, using a continuous time‐domain signal that stimulates the Med‐El FMT transducer attached to the malleus in a temporal bone model, was used to evaluate artificial fixation of the ossicular chain.
Results
Through this analysis method, differing incus velocity responses were seen between simulated single and multiple ossicle fixations in frequencies above 1000 Hz (</> −25 dB respectively). The incus–umbo velocity ratio was shown to detect stapes fixation in both multiple and single ossicle fixations (> −20 and > −10 dB, respectively) highlighting the importance of determining between single and multiple ossicle fixations first.
Conclusion
MIVIB provides an objective assessment of the ossicular chain to identify specific fixations and therefore assist the surgeon in improving surgical outcomes.
An objective assessment of the ossicular chain with laser vibrometry would likely improve the result of ossiculoplasty. MIVIB provides an objective assessment of the ossicular chain to identify specific fixations and therefore assist the surgeon in improving surgical outcomes.
1. Introduction
A significant number of adults suffer from a conductive hearing loss which, in the presence of an intact tympanic membrane (TM), is most likely due to a pathology of the ossicular chain. Although both audiometry and wide band tympanometry can inform the surgeon [1], one is often required to elevate the TM for accurate determination of type and location of ossicular fixation in the middle ear (ME). Such an approach allows a surgeon to visually inspect the ME components and manually palpate each ossicle to search for the location of fixation. This approach is reliant on the fact that the surgeon has experience with a variety of fixations, ideally in large volumes, during surgical training. Moreover, little objective feedback is given to the surgeon during the operation to assess the effectiveness of the manipulations with respect to ossicular functioning. An objective measurement of the movement of the ossicular chain is therefore an advantage to provide the surgeon with information about the location and significance of the fixation.
The application of laser vibrometry to measure the movement of the ossicular chain in both an intact TM [2] and after myringectomy [3] has been shown. Based on these works, we have developed a technique named minimally invasive intraoperative laser vibrometry (MIVIB) [4, 5]. Using a clinically approved floating mass transducer (FMT, Med‐El, Innsbruck, Austria) the ossicular chain is mechanically vibrated by sending a sine‐wave voltage signal through an audio interface to the FMT. By connecting the FMT to the umbo after lifting the TM, an objective vibration measurement is made by the surgeon without the need for manual palpation or extensive opening of the ME attic. The vibrations are recorded using a laser doppler vibrometer (LDV) at two different locations along the ossicular chain.
In the previous work we performed fixation of the stapes [4] after myringectomy or fixation of the malleus and incus [5] through a middle fossa approach. The current work focuses on individual ossicular fixations performed through an atticotomy, as well as fixation of multiple ossicular bones at once. The changes in the malleus and incus velocity after these fixations could then be quantified. Based on this, a clinical protocol for intraoperative use is presented. Moreover, a multisine approach is also presented, which allows for the reconstruction of the frequency response in the ME within 1 s of measurement.
2. Methods
Eighteen fresh‐frozen human temporal bones (TBs), defrosted at room temperature, were examined with a surgical microscope where pathology in either the external auditory canal, TM, or ME was ruled out. The measurement setup previously reported in [4, 5] consisted of an LDV (Polytec model OFV‐534) coupled to a surgical microscope (OPMI Sensera/S7, Carl Zeiss, Jena, Germany). Small pieces of reflective tape were attached to the measurement points to improve the strength of the reflected laser vibrometer signal. The protocol has been extensively described in [4, 5] and a summary is reported here with changes as specified.
The pinna and cartilaginous external auditory canal were surgically removed. A joystick‐controlled mirror was used to position the vibrometer's laser beam on the umbo (spot size 200 μm at a distance of 30 cm) as previously described [3].
For free‐field measurements, a speaker positioned at 20 cm away from the TB was used to generate the acoustic stimulus during measurements with an intact TM. Sound pressure in the cavity was generated using a horn driver speaker (TOA, TU‐650, Surrey, UK). The sound pressure at the level of the TM was measured using a probe microphone (Bruel & Kjaer, type 4182, Nærum, Denmark), connected to a 25 mm long rigid tube where the most distal part of the tube was placed close to the TM [4]. The range of 0.25–4 kHz was chosen because previous measurements had shown maximal sensitivity of the instrument in this region. With the TM intact, the umbo vibration response was measured using acoustic stimulation at 90 dB SPL measuring 50 periods for each frequency.
Since the FMT device mechanically stimulates the ossicles, a way to define an input voltage which results in a relevant ossicular vibration for each frequency is needed. The most straightforward approach is pure sine wave excitation [4]. In this study, a differing approach was explored using a signal playing which contains all frequency information simultaneously. A continuous time‐domain signal was constructed containing 16 logarithmically spaced frequencies per octave, resulting in a total of 65 frequencies within the range of 0.25–4 kHz. Using the Fourier transform, the 64 vibration amplitudes contained within the signal can be reconstructed after a measurement of just 1 s.
A custom‐built clamp and an otologic drill (W&H Dentalwerk, Bürmoos, Austria) were used to perform an atticotomy without affecting the TM and so as to have exactly the same angle of the TB to the laser beam as with previous measurements. After completely elevating the TM (myringectomy), reflective patches were placed on the umbo, the lateral surface of the distal (inferior) end of the incus long process (LP) and on the bony ear canal (as a negative control). As previously in [4, 5], an FMT with a suitable audio connector (Med‐El, Innsbruck, Austria) was attached to the manubrium of the malleus using a left Incus‐LP coupler that was manipulated so that the transducer was positioned in such a way that the longitudinal axis of the transducer body was parallel to the piston axis of the stapes. Measurements were then made at the three locations discussed above. Baseline ME transfer function is similar to that of an intact membrane, as shown in [4]. The technique also delivers the curves of phase between stimulation and motion, but as the aim is to see how loudness transmission is altered by fixation, these data are not presented.
Subsequently, the mobility of the ossicles was reduced by applying a few drops of glass ionomer luting cement (GC, Tokyo, Japan). The fixation was confirmed both visually and by manual palpation before measurement (Data S1). Of the 18 bones assessed in this study, fixation was performed of the stapes in four, malleus in four, incus in six, incus‐malleus in nine, stapes‐malleus in seven, malleus‐incus‐stapes in seven TBs.
3. Results
3.1. Single Ossicle Fixations of the Stapes, Malleus, and Incus
After atticotomy and myringectomy, an FMT was placed on the malleus manubrium using a modified left‐LP coupler. Dental cement was then used to perform single ossicle fixations in 18 TBs. Stapes, malleus, or incus fixations alone resulted in less than −25 dB incus velocity change at all frequencies above 1000 Hz. Below 1000 Hz, malleus or incus fixation alone resulted in less than −25 dB incus velocity change in all frequencies. This was not the case in stapes fixation alone.
Stapes fixation (Figure 1) alone results in a significant decrease of incus velocity at all frequencies, with the greatest velocity decrease (> 25 dB) in frequencies lower than 1000 Hz. The average incus velocity decrease was −35.2 ± 4.9 dB in frequencies < 1000 Hz and −19.7 ± 1.9 dB in frequencies > 1000 Hz. Umbo velocity was also decreased in frequencies lower than 1000 Hz. This resulted in an incus–umbo velocity ratio of at least −10 dB in all frequencies (−22.2 ± 5.1 dB averaged over all frequencies).
FIGURE 1.
Effect of stapes fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones) and stapes fixations are in color (four temporal bones). Mean with standard deviation is shown and data for individual bones are reported in the Supporting Information (Figure S3). The velocity noise floor of the LDV signal is −80 dB (Re. 1 mm/s).
Malleus fixation (Figure 2) alone results in a remarkable reduction of the umbo velocity in all frequencies, but most importantly, in contrast to stapes fixation alone, in frequencies greater than 500 Hz. The average velocity change across all frequencies at the Umbo is 13.1 ± 2.8 dB and at the incus 15.5 ± 2.9 dB. This results in little change of the incus–umbo velocity ratio.
FIGURE 2.
Effect of malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones) and malleus fixations are in color (four temporal bones). Mean with standard deviation is shown and data for individual bones are reported in the Supporting Information (Figure S4). The velocity noise floor of the LDV signal is −80 dB (Re. 1 mm/s).
Incus fixation (Figure 3) alone results in no significant effect on the umbo velocity, incus velocity, or incus–umbo velocity ratios.
FIGURE 3.
Effect of incus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones) and incus fixations are in color (six temporal bones). Mean with standard deviation is shown and data for individual bones are reported in the Supporting Information (Figure S5). The velocity noise floor of the LDV signal is −80 dB (Re. 1 mm/s).
3.2. Multiple Ossicular Fixations of the Malleus‐Incus, Malleus‐Stapes, or All Three Bones
When more than one ossicle was fixated in this model, independent of which bones were fixated, a minimum of 25 dB reduction in incus velocity was seen in all frequencies. This is in contrast to the individual bone fixations where less than 25 dB reduction in velocity was seen in all frequencies, where the exception to this is seen in stapes fixation in frequencies < 1000 Hz. These fixations were further divided into two groups: fixations involving the stapes and at least one other bone, and malleus‐incus fixations. The clinical implications are whether one could perform a partial ossicular replacement prosthesis (PORP) on a mobile stapes without opening the attic or that a procedure on the stapes, such as stapedotomy, would be required.
Incus‐malleus fixation (Figure 4) leads to a reduction in umbo velocity in base frequencies < 500 Hz and a significant reduction of −20 dB in all frequencies of incus velocity. This leads to an incus–umbo velocity ratio change of −20 dB in frequencies above 500 Hz. This is in contrast to when the stapes is fixated (malleus‐stapes fixation [Figure 5] and malleus‐incus‐stapes fixation [Figure 6]) where > −20 dB incus–umbo velocity ratios are seen between 500 and 2000 Hz. Therefore, the greater the incus–umbo velocity ratio is at these frequencies, the more likely the stapes is fixated.
FIGURE 4.
Effect of Incus‐malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones) and incus‐malleus fixations are in color (nine temporal bones). Mean with standard deviation is shown and data for individual bones are reported in the Supporting Information (Figure S6). The velocity noise floor of the LDV signal is −80 dB (Re. 1 mm/s).
FIGURE 5.
Effect of stapes‐malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones) and fixations are in color (seven temporal bones). Mean with standard deviation is shown and data for individual bones are reported in the Supporting Information (Figure S7). The velocity noise floor of the LDV signal is −80 dB (Re. 1 mm/s).
FIGURE 6.
Effect of malleus‐incus‐stapes (complete) fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones) and fixations are in color (seven temporal bones). Mean with standard deviation is shown and data for individual bones are reported in the Supporting Information (Figure S8). The velocity noise floor of the LDV signal is −80 dB (Re. 1 mm/s).
4. Discussion
Fixation of the ossicular chain can be diagnosed through clinical examination (reduced movement of the malleus shaft with pneumatic otoscopy), pure tone audiometry (shape of the air conduction curve compared to bone conduction curve) or, more recently, wideband tympanometry [6, 7]. However, when utilizing these methods, the location of the fixation in the ossicular chain is difficult to define. It is essential to know where the ossicular chain is fixated so that a surgeon who is performing an ossiculoplasty can improve hearing to better than 30 dBHL (air conduction) or within 15 dB of the contralateral ear (Belfast rule of thumb) [8]. MIVIB offers an alternative objective intraoperative approach to determine the location of these fixations and therefore assist the surgeon in improving hearing outcomes.
Through the use of the MIVIB protocol (Figure 7), the surgeon can quickly identify whether there is a fixation of single or multiple bones, and whether the stapes itself is fixated. This is crucial information that a surgeon requires to perform a surgery that will fulfill the Belfast rule of thumb and therefore result in improved patient satisfaction. It is also important to avoid an unnecessarily extended operation. If the results show a fixation of either the malleus or incus alone, or both together, and that there is a mobile stapes, then the surgeon could place a titanium PORP prosthesis without removing the lateral ossicles. It is estimated that the MIVIB recording would add no more than 10 min to the operation; however, this has not been assessed directly in this study. In surgery, a reflective surface for the laser signal as well as line of sight may be limitations and require further research.
FIGURE 7.
Minimally invasive intraoperative laser vibrometry. Step 1: If there is an incus velocity of −25 dB or greater compared to normal in frequencies > 1000 Hz, then multiple ossicles are fixated. If this difference is less than −25 dB, then single ossicles are fixated. Step 2a: If multiple ossicles are fixated, one should determine if the stapes is fixated. If the malleus and incus are fixated and the stapes is mobile then the incus–umbo ratio is low < −15 dB; however, if the incus–umbo ratio is higher (> −20 dB) then it is likely that the stapes is also fixated. Step 2b: If single ossicles are fixated, one needs to identify which bone is affected. Stapes‐only fixations can be identified by an incus to umbo ratio change of > −10 in all frequencies. This is also true for multiple ossicle fixations, so it is important to complete step 1 first. The difference between the two situations (stapes or lateral chain fixations) can be confirmed by looking at umbo velocity changes at frequencies below 500 Hz, since stapes only does not affect this velocity, but malleus or incus fixations alone induce a minimum −10 dB velocity change. Incus‐only fixations can be identified in that there is no significant change in umbo or incus velocity. Malleus‐only fixations can be identified by a reduction in umbo velocity in all frequencies and a normal incus–umbo velocity ratio.
Identification of a stapes fixation is also essential to perform a successful ossiculoplasty. If multiple bones are fixated, an incus–umbo velocity ratio of > −20 dB indicates stapes fixation. If the stapes alone is fixated, the incus velocity will be reduced by less than −25 dB and the incus–umbo velocity ratio will be greater than −10 dB. However, this method does not differ between footplate or superstructure fixation of the stapes. If a stapes fixation is identified, the surgeon will still need to palpate the stapes plate to decide if a stapedotomy or TORP (total ossicular replacement prosthesis) is needed. A MIVIB recording can quickly identify the stapes as a problem so that the surgeon can focus his or her approach on widening the limbus lateral to the stapes niche and to focus the efforts on this bone. It is also very important to identify situations where the stapes and other ossicles are fixated simultaneously. Then it may be best to inform the patient and discuss other treatment options, like bone anchored hearing aids.
Another use of an MIVIB recording could be in the training of new surgeons. A surgeon is required to gain experience with many different fixations through the start of his or her career to be able to successfully assess the ossicular chain with manual palpation. The MIVIB system could be used to objectively identify the fixation so as to confirm the less experienced surgeon's manual palpation.
In our previous work [5] we reported an incus and umbo velocity change of −10 dB and no change in the incus–umbo velocity ratio when incus fixation was performed. However, this was not seen in this study. In the previous work, we utilized a middle fossa opening to the attic of the ME to perform fixations whereas in this study, we performed an atticotomy. When performing a middle fossa approach to the attic, one could detach the superior malleolar ligament, which in turn could affect the natural ossicular vibration. However, Dai et al. performed a study of superior malleolar ligament detachment where the movement of the malleus and stapes was not affected [9]. These differing approaches also led to a closed (atticotomy) or open (middle fossa) ME cavity, which could affect the inherent acoustics of this system, and therefore the effect of an atticotomy or middle fossa opening on ossicular movement is the focus of future research.
It is also interesting to note that fixation of the incus did not lead to changes in mallear velocity at the umbo or on the distal incus. The former can be explained by an intact malleus‐incus joint maintaining umbo movement. However, the lack of change in movement at the distal incus could mean that measurement at this position is more of a reflection of stapes movement and not the fulcrum effect seen with the incus. In other studies [10], stapes velocity was also measured to determine the type of fixation. This was not possible in our study as the size of the FMT limited direct sight of the laser beam against the stapes crurae (see Data S1). In the study of Nakajima et al. [10] incus velocity was not measured, which, in our study, was essential to differentiate between single and multiple bone fixations and also to generate the incus–umbo ratio.
The next logical step is to assess this system intraoperatively. Laser doppler vibrometry has already shown promise in both the assessment of conventional ossiculoplasty [11, 12] and the evaluation of 3‐D printed [13] or other novel prostheses [14] intraoperatively. It has even been used to provide real‐time information about prosthesis placement [15]. However, the referenced techniques use an external sound stimulus or a magnet that is attached with glue to the umbo of the TM, originally described in [3]. Our system provides a less invasive technique where we utilize components that are widely approved for surgery and therefore minimize the risk of introducing infection through the use of nonsterile or foreign products. An alternative method to assess the placement of ossicular prostheses that is already in clinical use is to use auditory brain stem response (ABR) or electrocochleography (ECochG) which, although accurate, is time consuming [16].
Ideally, a surgeon would not only hope to receive an objective assessment of the ossicular chain intraoperatively but even be able to assess the ossicular chain in the clinic before surgery. Wideband tympanometry, such as wideband acoustic immittance or wideband frequency impedance [1], has shown some promise in differing between ossicular fixation and separation, but is not universally adopted in the clinic, mainly due to its inability to differ between different types of fixation. TM imaging systems have also shown promise to differentiate between a fixation and separation [17], but have not been tested in humans to the authors knowledge. MIVIB can differentiate between different fixations, and in the future one would hope to be able to utilize our technique in patients with an intact TM before surgery. There are several considerations that need to be addressed before this is possible, including stimulation of an intact TM with the FMT, as well as the straight laser beam in a curved external ear canal. Some advancement in the later area has been achieved already through the use of a handheld laser doppler with curved tip [18].
5. Study Limitations
The use of the FMT in the ME reduced the visibility of the stapes and therefore laser doppler vibrometry measurements could not be performed on this ossicle as in [19] which may have provided more information to differ between fixations. We performed complete, and not partial, fixations of each bone to reduce interbone variability. Nakajima et al. [10], and our previous work [4], have shown the wide variability in ossicular velocity between partial and complete fixations and therefore a complete fixation was chosen for this study.
6. Conclusion
This study presents a protocol for the minimally invasive assessment of ossicular movement through laser vibrometry (MIVIB). Whether this system is applicable intraoperatively is the focus of future research but will likely be an important complement to the presurgical clinical and audiometric assessment of patients that is performed today.
Conflicts of Interest
The authors declare no conflicts of interest.
Supporting information
Figure S1. Experimental photos of the surgical procedure and fixations.
Figure S2. Umbo and incus velocities, and incus–umbo velocity ratio in unfixed temporal bones. Individual bones (18) are show in purple and mean with standard deviation is shown in gray.
Figure S3. Effect of stapes fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (four temporal bones).
Figure S4. Effect of malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (four temporal bones).
Figure S5. Effect of incus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (six temporal bones).
Figure S6. Effect of incus‐malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (nine temporal bones).
Figure S7. Effect of stapes‐malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (seven temporal bones).
Figure S8. Effect of complete fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (seven temporal bones).
Acknowledgments
We thank the Med‐El company for putting the floating mass transducer with a suitable audio connector at our disposal.
Funding: This work was supported by Fonds Wetenschappelijk Onderzoek, Stiftelsen Acta Oto‐Laryngologica, and the Akershus University College of Applied Science.
The first two authors contributed equally to this work.
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Associated Data
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Supplementary Materials
Figure S1. Experimental photos of the surgical procedure and fixations.
Figure S2. Umbo and incus velocities, and incus–umbo velocity ratio in unfixed temporal bones. Individual bones (18) are show in purple and mean with standard deviation is shown in gray.
Figure S3. Effect of stapes fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (four temporal bones).
Figure S4. Effect of malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (four temporal bones).
Figure S5. Effect of incus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (six temporal bones).
Figure S6. Effect of incus‐malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (nine temporal bones).
Figure S7. Effect of stapes‐malleus fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (seven temporal bones).
Figure S8. Effect of complete fixation on umbo and incus velocities, and incus–umbo velocity ratio. No fixation is in gray (18 temporal bones, mean with standard deviation) and individual bone fixations in color (seven temporal bones).