Despite the increasing number of research concerning the applications of the Laser Doppler Vibrometry (LDV) in medicine, its usefulness is still under discussion. LDV has been described as a very sensitive and noncontact method. This feature is used for measurements that assess movements of the middle ear components. The frequencies used range from 100 Hz to 25,000 Hz. This technique might be a supplementary method to impedance audiometry in middle ear testing [1,2]. In addition, it allows obtaining information about the mobility of the ossicular chain. In the literature, this is confirmed by the results of several other experiments [3-5]. However, the methodology of those measurements still presents some questions without answers. Repeatability of measurements performed during surgery, the impact of changes in the laser beam angle on results, and the necessity to use the reflective element have been still problems to be solved.

The aim of this study was to present a methodology developed in our Department for the LDV intraoperative assessment of ossicular chain reconstruction.

Four experiments were conducted. In experiment #1, #2, and #3, 10 patients were involved. Experiment #4 was performed in the laboratory. All of the enrolled patients underwent “second-look” (second stage) tympanoplasty 12 months after the first stage surgery due to otitis media with cholesteatoma. The average age of the subjects was 67.6 years (women, 69.3 years; men, 65.0 years; range, 51 to 84 years). Antromastoidectomy with posterior wide tympanotomy were performed during the “second-look” procedure. Facial nerve, chorda tympani, tympanic membrane were preserved in all cases. The surgical technique using “long columella” ossiculoplasty was performed. In this technique, ossicular prosthesis was inserted between the tympanic membrane and the stape’s footplate. Measurements of ossicular chain movements were performed using the LDV System (Polytec GmbH, Waldbronn, Germany). In this method, the Doppler effect for the light wave is used to make noncontact vibration measurements. In this technique the Doppler effect (the change in frequency of a wave reflected from the surface of interest that moves relatively to wave’s source, also called Doppler shift) in a laser beam was used to measure the displacement velocity of reflecting surface. The laser beam from LDV was directed at the surface of interest. In case of this study the surface of interest was ossicle prosthesis (Fig. 1). The vibration amplitude and frequency were extracted from the Doppler shift of the reflected laser beam frequency, and the velocity of movement was then calculated by the computer system.

The LDV System was controlled by LMS TestXpress software (LMS International, Leuven, Belgium). Acoustic waves with carrier frequencies of 500, 1,000, 2,000, and 4,000 Hz were generated by IHS System (Intelligent Hearing Systems, Miami, FL, USA) and applied to the ears using insert earphones ER2 (insert ear phones ER2, Etymotic Research Inc., Elk Grove Village, IL, USA), one frequency at a time. The stimuli were applied with the intensity of 90-dB sound pressure level (SPL) as tone bursts lasting 8 ms each and rate of 25 repetitions/sec. The stimuli set the ossicular chain in motion and its vibration was measured using LDV system.

This study is a part of a prospective project that was approved by the local Ethics Committee Review Board at the Medical University, where the project has been conducted. The project conforms with The Code of Ethics of the World Medical Association (Declaration of Helsinki).

Main goals of middle ear surgery are removal of chronic inflamed tissues and improvement of hearing as final result. The pure tone audiometry (PTA) is usually very useful in assessing the hearing after the surgery. In PTA, closure of the air-bone gap shows the improved and proper function of the ossicular prosthesis. Unfortunately, subjective hearing testing cannot be used during the surgery. For these purpose electrophysiological methods such as electrocochleography or auditory brain stem responses are of great importance and usefulness. The idea of intraoperative assessment of the ossicular chain reconstruction presents a challenge for researchers and clinicians [6-8]. Over the decades many methods have been developed to assist the surgeon in measurements of the ossicular chain mobility. Initially, a contact method using the properties of a human hair was evaluated. This method was useful only in the experimental conditions. In the 80’s and 90’s noncontacting video and piezoelectric methods were introduced for measuring the mobility of the ossicular chain [9]. LDV is another method for assessing the ossicular chain reconstruction and offers safe intraoperative measurements.

In our study in experiment #1, reproducibility of the measurements within the same patient was assessed. As we expected, there were no significant differences between the measurements (P>0.05). Responses measured at 1,000-Hz carrier frequency were more difficult to detect. In those cases, larger but still not significant differences between the measurements were observed. In our study, we used the audiological criteria for SNR. The response was recognized as present when the recorded signal amplitude was higher than the average noise level plus three standard deviations.

In experiment #2, large spread of results was obtained. In their experiment, Huber et al. [5] showed lower stapes mobility by about 5–10 dB (below 2 kHz) measured in vivo. Hydration state of the temporal bone specimen affected the LDV measurements [10,11]. However, Rosowski et al. [12] in their study did not find any statistical significance in the mobility of the tympanic membrane measured in laboratory and in the operating theater conditions. Ruggero and Temchin [13] found that differences between the measurements in vivo and in vitro resulted from postmortem decrease in muscle tension. Another explanation might be methodological differences in the measurements. In our study, after our experience in experimental measurements described in our previous study [14], middle ear acoustic conductivity was examined intraoperatively during “canal wall up” tympanoplasty. This surgical approach allows for the assessment of the ossicular chain mobility in its almost natural conditions. In Chien et al. [11]’s research, the measurements of an angle between the laser beam and the piston-like direction of stapes motion was estimated intraoperatively and was found to vary between 40° and 60°. Such differences in the measurements might be important factors adding more variations to the measurements. In our study, the maximum possible change in the angle of the laser beam not affecting the measurements (proved in experiment #4) was less than 10° and was dependent on the anatomic limitations. In our study (in experiments #1, #2, and #3), the angle change was within 3° to 5°. The other authors suggested that very small angle changes (about 1.5%) can be totally neglected. It must be, however, kept in mind that the measurement procedure we used differed from the methodology used in those other studies [15-17], therefore it would be difficult to compare the results from our study and those from other studies.

In our study, a piece of the reflective tape was attached to the ossicular prosthesis, promontory, and the round window. A percentage ratio of the reflected laser beam to the outgoing one reached 80%–100% when reflective tape was attached to the surface of interest. It was considered an indicator of correct conditions of measurement with good SNR. This stands in opposition to other authors presenting intraoperative results. They achieved a satisfactory reflection from the mucous membrane surrounding the stapes head without any reflective material. Huber et al. [18] showed that no reflective tape was required for the measurements of the tympanic membrane displacement. Chien et al. [19] showed that the reflective tape should not be used due to time required for its application/removal and the aseptic issues. In our study, we did not obtain sufficient reflection of the laser beam without the reflective tape. Thus sterile reflective tape was used during the measurements. Its application and removal took about 1 minute total.

In conclusion, LDV showed to be a valuable noncontact intraoperative tool for measurements of the middle ear conductive system mobility with a very good intraindividual repeatability. Neither a small change in the angle of the laser beam nor performing the measurements through posterior tympanotomy showed a significant influence on the results. Reflective tape was necessary to obtain good quality responses in LDV measurements.