Audiological outcome after stapes surgery in relation to prosthesis type

Stapedectomy was first performed by Frederick L. Jack at the end of the nineteenth century. The technique was rediscovered by John Shea in 1956 [1], who fixed the first Teflon implant between the long process of the incus (LPI) and the oval window (OW) [2, 3]. Schuknecht was the first to fix an implant by way of crimping an intraoperatively created steel wire/fat prosthesis to the LPI [3]. Since then, a great variety of similar prostheses using different materials and fixation methods have been developed. Today, titanium CliP® Piston or crimp prostheses are most widely used, and other materials, such as platinum wire/Teflon implants less so. In the past 20 years, studies have analysed the piston diameter [4], the method for stapedotomy [5], cement fixed crimpers [6], or the different clip types [7‐11]. The few studies that have compared CliP® Piston and crimp prostheses showed no significant difference in the audiological outcome [8, 9]. CliP® Piston prostheses are fixed to the LPI dynamically and are stiff due to spring tension. Crimp prostheses are fixed statically by bending a static loop around the LPI, which provides mobility similar to the physiological ISJ, which can lead to differences in the hearing outcome [12]. It is possible that the different fixation methods result in different surgical time, because whereas the CliP® Piston is pushed onto the LPI, with crimping, the surgeon must change instrument to bend the loop using, for example, crimp forceps. Conversely only one previous study compared a nitinol CliP® Piston (n = 9) and platinum Teflon wire crimps (n = 10) prostheses performed by the same surgeon. This study did not show any differences in outcome; however, this may be due to the small number of patients included [11].

The current retrospective study was designed to compare the surgery time and the audiological outcome using the same titanium crimp and CliP® Piston prosthesis type, and with the novel aspect that the 190 included surgeries were performed by a single surgeon in an identical clinical setting. The air–bone gap (ABG) and postoperative changes in the air- and bone-conduction thresholds within the speech frequency specific changes were analysed.

The hearing outcome after stapesplasty is determined by the bone- and air-conduction thresholds and the ABG closure. With otosclerosis most of the patients show a mixed hearing loss consisting of a conductive and sensorineural component. The difference between bone-conduction and air-conduction thresholds are defined as the air–bone gap. This parameter depends directly on both thresholds. Clinically, such a hearing loss is seen in cases of chronic middle ear disease, inner ear anomalies, and especially in otosclerosis [16].

In otosclerosis a bone conduction depression at 2 kHz is well-known and was first described by Raymond Carhart in 1950. This effect is attributed to a higher impedance at the oval window niche, which is consistent with the typical intraoperative findings. According to the literature, the frequency of occurrence of otosclerosis is between 31% and 80%. A postoperative improvement of the bone conduction threshold after stapesplasty is a well-known effect and, although most commonly observed at the Carhart’s notch, can also occur at higher and lower frequencies [17‐19]. The importance of low frequencies in particular for adequate speech perception in noisy situations has been demonstrated in the literature following hybrid cochlear implantation (electro-acoustic stimulation) [20].

Our results were derived from a standardized procedure performed by one surgeon using two different prostheses. For both types we analysed the BC and the ABG at 0.5, 1, 1.5, 2, 3 and 4 kHz. In our study the BC significantly improved in all frequencies and the ABG was reduced in most of cases regardless of the prosthesis used. These positive effects are commonly observed following successful stapes surgery [21, 22].

Reduction of BC thresholds and absence of the Carhart’s notch (CN) are positive factors for postoperative hearing performance [23].

Furthermore, our results showed a significant BC improvement in all frequencies and surprisingly a significant better ABG closure at 0.5 kHz for the crimp prosthesis. Different factors such as prosthesis diameter and material, fixation at the LPI are possible factors influencing the BC shift and ABG closure.

Advantages and disadvantages of the different stapes prosthesis designs are frequently described in the literature. Nowadays, the most commonly used material is titanium, which has a low density, electrical and thermic conductivity, high stiffness and is well-tolerated within the middle ear [24, 25].

Several prosthesis types with different piston diameters and fixation methods at the LPI have been used in the past. It is well-known that the prosthesis diameter has no significant influence to the hearing outcome, and that a tight attachment to the LPI leads to a narrower postoperative AGB [12, 26]. As such, the method of prosthesis fixation at the LPI may play a major role. In a review of 17 papers reporting an ABG closure of 10 dB or less overall and 20 publications examining the mean ABG using different prostheses, the authors conclude that there is no difference in hearing outcome and no fixation method can be favoured [26]. Similar results were shown in a study comparing a CliP® Piston (n = 63) and a crimp (n = 63) prosthesis. Analysing mean values of the residual ABG, no significant differences could be demonstrated and both had good functional results [27]. In none of these studies was the ABG analysed separately for each frequency.

Nowadays the ossicular middle ear joints are considered as a mechanism not for lever induced sound amplification, but rather for realization of an adequate sound transmission under different pressure conditions of this system [25]. On the other hand, the incudostapedial joint plays an important role for middle ear sound transmission especially in the lower frequencies. An experimental fixation of the incudostapedial joint and footplate showed a decrease of sound transmission of 10–25 dB and 20–30 dB at 1 kHz and below [28]. Similar results with 3–6 dB less sound transmission were shown in an experimental study using artificial stapes fixation by cyanoacrylate adhesive [29].

In our study the prosthesis material and piston diameter for both types were identical. In consequence, systematical differences in hearing outcome can only be related to the fixation method.

Our results show that the sound transmission is significantly better at 0.5 kHz for the crimp prosthesis, which represents a clear advantage. A CliP® Piston creates a stiff connection between the LPI and the prosthesis. A crimp type is fixed with a tight bended loop that retains mobility and better mimics the ISJ, which may account for better sound transmission in the lower frequencies.

Despite these aspects many patients postoperatively need a hearing aid with an appropriate earmold fitting. From approx. 35 dB HL in the low-frequency range, an occluding earmold with a small ventilation hole (≤ 1 mm) may be necessary. This can lead to increased moisture, cerumen buildup and also to external auditory canal infections. In addition, tight occlusion causes a significant reduction in sound quality. To avoid such problems, the smallest possible air–bone gap (≤ 10 dB of 0.5 kHz) is a desirable result of stapes surgery.

The results of our study show that this goal could be reached in more than 70% of the patients using both the CliP® Piston and crimp prosthesis. In comparison, a crimp prosthesis is significantly more efficient at transduction of the important lower frequencies. Limitations to the presented study are that it was not randomised and that audiological measurements should be conducted over a longer period to assess the long term results.

In most of the cases stapes surgery leads to a hearing improvement by elevating the BC in combination with closing of the ABG between 1 and 4 kHz. Our results are consistent with the pertinent literature. Comparing the CliP® Piston and crimp prostheses the crimp showed a significant better ABG closure at 0.5 kHz. In future a long term prospective study is planned to investigate whether this has an effect on speech perception. Low frequencies are known to be important for speech perception in noise. Therefore, an improvement in low frequency thresholds may considered a clear advantage for the audiological outcome, especially in cases, where postoperatively hearing aids are indicated.