Cartilage conduction (CC) is a new form of sound transmission which is induced by a transducer being placed on the aural cartilage. Although the conventional forms of sound transmission to the cochlea are classified into air or bone conduction (AC or BC), previous study demonstrates that CC is not classified into AC or BC (Laryngoscope 124: 1214–1219). Next interesting issue is whether CC is a hybrid of AC and BC. Seven volunteers with normal hearing participated in this experiment. The threshold-shifts by water injection in the ear canal were measured. AC, BC, and CC thresholds at 0.5–4 kHz were measured in the 0%-, 40%-, and 80%-water injection conditions. In addition, CC thresholds were also measured for the 20%-, 60%-, 100%-, and overflowing-water injection conditions. The contributions of the vibrations of the cartilaginous portion were evaluated by the threshold-shifts. For AC and BC, the threshold-shifts by the water injection were 22.6–53.3 dB and within 14.9 dB at the frequency of 0.5–4 kHz, respectively. For CC, when the water was filled within the bony portion, the thresholds were elevated to the same degree as AC. When the water was additionally injected to reach the cartilaginous portion, the thresholds at 0.5 and 1 kHz dramatically decreased by 27.4 and 27.5 dB, respectively. In addition, despite blocking AC by the injected water, the CC thresholds in force level were remarkably lower than those for BC. The vibration of the cartilaginous portion contributes to the sound transmission, particularly in the low frequency range. Although the airborne sound is radiated into the ear canal in both BC and CC, the mechanism underlying its generation is different between them. CC generates airborne sound in the canal more efficiently than BC. The current findings suggest that CC is not a hybrid of AC and BC.

Objectives/Hypothesis The aim of this study was to establish the sound transmission characteristics of cartilage conduction proposed by Hosoi (2004), which is available by a vibration signal delivered to the aural cartilage from a transducer. Study Design Experimental study. Method Eight volunteers with normal hearing participated. Thresholds at frequencies of 0.5, 1, 2, and 4 kHz for air conduction, bone, and cartilage conductions were measured with and without an earplug. The sound pressure levels on the eardrum at the threshold estimated with a Head and Torso Simulator were compared between air and cartilage conductions. The force levels calibrated with an artificial mastoid at the threshold were compared between bone and cartilage conductions. Results The difference in the estimated sound pressure levels on the eardrum at the thresholds between air and cartilage conductions were within 10 dB. In contrast, the force levels at the thresholds for cartilage conduction were remarkably lower than those for bone conduction. These findings suggested that sounds were probably transmitted via the eardrum for cartilage conduction. The threshold shifts by an earplug showed no significant difference between bone and cartilage conductions at 0.5 kHz. At 1 and 2 kHz, the threshold-shifts increased significantly in the order of bone, cartilage, and air conductions. These results suggested that airborne sound induced by the vibration of the cartilaginous portion of the ear canal played a significant role in sound transmission for cartilage conduction. Conclusions Cartilage conduction has different characteristics from conventional air and bone conductions. Level of Evidence N/A. Laryngoscope, 124:1214–1219, 2014

Objective: To assess the benefits of a new type of hearing aid using cartilage conduction (CC) in patients with severe conduction hearing loss and evaluate its potential for practical use. Study Design: Consecutive, prospective case series. Patients: Forty-one subjects (21 with bilateral aural atresia; 15 with unilateral aural atresia; and 5 others) participated in this study. Intervention: Fitting and gain adjustments of the CC hearing aids were performed to the ear(s) with conduction hearing loss. Main Outcome Measures: The function gains were measured. Evaluation of the measurements of speech performance-intensity functions, speech recognition scores, tolerance of environmental noise, and subject questionnaires were also performed, and judged according to the “Guidelines for the evaluation of hearing aid fitting” established by the Japan Audiological Society. Results: The thresholds were significantly improved by CC hearing aids. The functional gains for CC hearing aids were nearly equivalent to that for their previously used hearing aids. The style of the transducer fixation and the type of aural atresia had no significant influence on the functional gains. Most of the assessment results were judged to be sufficient. Before the trial, bone conduction hearing aids had been used most frequently by bilateral aural atresia subjects. However, after the trial, most subjects continued to use CC hearing aids instead of reverting back to their original device. Overall, 39 subjects continued use of the CC hearing aids. No severe adverse effects were noted in the trial. Conclusion: Cartilage conduction hearing aids could be an additional and beneficial option for severe conduction hearing loss from aural atresia.

It has been long considered that air and bone are the two major mediators that conduct sounds to the inner ear. In 2004, Hosoi found that vibration of aural cartilage, generated by placing gently a transducer on it, could create audible sound with the same level of clarity as air- and bone-conduction sound. He thus proposed the term "cartilage conduction" for this concept. This research identified a third mediator for sound conduction to the inner ear. Hosoi also proposed the development of novel communication devices, such as hearing aids, telephones, etc. using his findings. For cartilage conduction, three sound pathways can be assumed. The transducer vibration may cause airborne sound which passes into the external auditory canal through the canal entrance (direct air pathway). Alternatively, the vibration at the cartilage may generate audible sound in the external auditory canal (cartilage-air pathway), or propagate directly to the inner ear through the skull bone (cartilage-bone pathway). A series of studies has illustrated that the cartilage-air pathway is dominant for hearing sensations in listeners with normal ears. The cartilage-bone pathway works for patients with bony aural atresia. A fourth pathway, the fibrotic-tissue pathway, is considered to act in the case of fibrotic aural atresia. In this review, we summarize this series of studies and discuss the nature of cartilage conduction.