Cartilage Conduction Hearing and Its Clinical Application

Abstract

Cartilage conduction (CC) is a form of conduction that allows a relatively loud sound to be audible when a transducer is placed on the aural cartilage. The CC transmission mechanism has gradually been elucidated, allowing for the development of CC hearing aids (CC-HAs), which are clinically available in Japan. However, CC is still not fully understood. This review summarizes previous CC reports to facilitate its understanding. Concerning the transmission mechanism, the sound pressure level in the ear canal was found to increase when the transducer was attached to the aural cartilage, compared to an unattached condition. Further, inserting an earplug and injecting water into the ear canal shifted the CC threshold, indicating the considerable influence of cartilage-air conduction on the transmission. In CC, the aural cartilage resembles the movable plate of a vibration speaker. This unique transduction mechanism is responsible for the CC characteristics. In terms of clinical applications, CC-HAs are a good option for patients with aural atresia, despite inferior signal transmission compared to bone conduction in bony atretic ears. The advantages of CC, namely comfort, stable fixation, esthetics, and non-invasiveness, facilitate its clinical use.

Keywords: airborne sound; aural atresia; bone conduction; bone-anchored hearing aid; cartilage conduction; conductive hearing loss; hearing aid.

Figure 1

Possible cartilage conduction pathways. (Figure 1 was originally presented in Nishimura et al. 2015, Figure 1 [11]).

Figure 2

Sound pressure level (SPL) in the canal when the transducer is attached to the tragus (—) and unattached (- - -). The black, dark gray, and gray lines indicate input voltages of 2.0, 1.0, 0.5 V, respectively. (Figure 2 was originally presented in Shimokura et al. 2014, Figure 6 [13]).

Figure 3

Effects of ear canal water injection on the transmission pathways. (A) The water stays within the bony portion of the ear canal, interrupting direct- and cartilage–air conduction. (B) The water enters the cartilaginous portion of the ear canal, avoiding an impedance mismatch between air and water in the cartilage-AC pathway. (C) The water level exceeds the ear canal, allowing for direct water vibrations. (Figure 3 was originally presented in Nishimura et al. 2015, Figure 1 [11]).

Figure 4

Cartilage conduction hearing aids (HB-J1CC, Rion Co Ltd., Kokubunji, Tokyo, Japan) have three transducer types: (A) simple-attachment, (B) ear-chip attachment, and (C) ear-chip embedded.

Figure 5

Examples of ears with and without cartilage conduction hearing aid (CC-HA). Some patients wear CC-HA in the same manner as conventional behind-the-ear hearing aids (A). For other patients, double-sided tape is needed for fixation of the hearing aids (B). (Figure 5 was originally presented in Nishimura et al. 2018, Figure 1 [40]).