Discussion

This study compared pre- and post-operative frequency-specific hearing outcomes in patients with chronic otitis media. As anticipated, all frequencies of the group showed significant post-operative improvement in hearing in the air-conduction threshold and ABG tests. The gain is most prominent at low and middle frequencies. This is consistent with previous studies.^{Reference Şevik Eliçora, Erdem, Dinç, Damar and Bişkin4} The normal middle-ear pressure gain (a result of ossicular coupling) is frequency-dependent.^{Reference Merchant, Ravicz, Puria, Voss, Whittemore, Peake and Rosowski5} Choi et al. noted that the mean gain decreases 6 dB or less per octave for frequencies above 1000 Hz.^{Reference Choi, Lee, Chang, Yeo, Yoon and Jun3} The low- and middle-frequency improvements may represent a normalisation of the ossicular coupling effect following reconstructive surgery. In addition to air-conduction threshold elevation, some patients with chronic otitis media also have bone-conduction threshold elevation.

An elevation in bone-conduction threshold can result from tympanosclerosis, stapes fixation, round window membrane alteration, tympanic membrane perforation, and ossicular chain disruption or fixation.^{Reference Yoshida, Miyamoto and Takahashi6} The current study found that, with the exception of 250 and 4000 Hz, bone-conduction threshold significantly improved in the whole cohort. The ossicular chain has a large resonance effect at 500–2000 Hz. Weakness or disappearance of resonance caused by destruction of the ossicular chain can lead to bone-conduction threshold changes at 500–2000 Hz, and bone-conduction threshold can be improved when the continuity and resonance of the ossicular chain are restored through reconstructive surgery.^{Reference Merchant, Ravicz, Puria, Voss, Whittemore, Peake and Rosowski5–Reference Tüz, Doğru, Uygur and Gedikli7}

At all frequencies except 2000 Hz, the hearing improvement of patients who underwent tympanoplasty was better than that of patients who underwent canal wall down tympanoplasty in this study. The difference in efficacy between tympanoplasty and canal wall down tympanoplasty typically has been ascribed to poor Eustachian tube function and greater disease severity in patients requiring canal wall down tympanoplasty.^{Reference Lee, Hong, Hong, Cho and Chung8} Furthermore, Quaranta et al. claimed that an intact posterior canal wall was considered a predictor of the better hearing result in ossiculoplasty.^{Reference Goldenberg and Emmet9}

Hearing improvement at 2000 Hz was significantly better in the canal wall down tympanoplasty group than in the tympanoplasty group in our investigation. This was consistent with results in research by Şevik Eliçora et al.^{Reference Şevik Eliçora, Erdem, Dinç, Damar and Bişkin4} It is perhaps relevant that the intact ossicular chain has the greatest effect on hearing at 2000 Hz.^{Reference Quaranta, Taliente, Coppola and Salonna10} The bone-conduction threshold change results were superior in the canal wall down tympanoplasty group compared with the tympanoplasty group at 250–2000 Hz. The canal wall down tympanoplasty group had poorer post-operative bone-conduction threshold outcomes compared to the tympanoplasty group at 4000 Hz. Excessive ossicular chain movement, vibration or noise from drilling, or other inner-ear disruption are all associated with high-frequency sensorineural hearing loss (i.e. strong suction or thermal damage).^{Reference Dinç, Damar, Erdem, Eliçora, Akyıldız and Kumbul11–Reference Gerber, Mason and Lambert13} Canal wall down tympanoplasty generally has more drilling time in the middle ear near the stapes and oval window, and the bone-conduction threshold seems to be more susceptible to its adverse effect at 4000 Hz.

Surprisingly, the TORP group had the best hearing improvement, followed by the PORP group and the None group at all frequencies except 1000 Hz. The None group was closer to normal hearing to begin with and had less room for improvement. Most studies assumed that PORP was associated with superior post-operative hearing outcomes when compared with TORP.^{Reference Choi, Lee, Chang, Yeo, Yoon and Jun3} They suggested that the stapes suprastructure plays an important role in hearing,^{Reference Badr-El-Dine, James, Panetti, Marchioni, Presutti and Nogueira14} and the stability between TORP and stapes floor is inadequate and prone to displacement, resulting in the interruption of TORP and stapes footplate connection. But in their studies, post-operative audiometry tests were generally conducted about three months after surgery^{Reference Choi, Lee, Chang, Yeo, Yoon and Jun3}.

It is our opinion that the residual stapes in subsequent recovery may continue to be disrupted and experience adhesion with the surrounding tissue. Furthermore, ossicular coupling of TORP may be superior to that of PORP.^{Reference House and Teufert15} Cadaveric temporal bone studies support this viewpoint and considered that TORP placement to a stapes footplate offers acoustic advantages over other ossiculoplasty techniques. The reduced middle-ear pressure can be restored after ossiculoplasty. Therefore, the results suggest that air-conduction threshold changes occurred principally at lower and middle frequencies, with no substantial changes at higher frequencies. Tonndorf et al. thought that the influence of ossicular chain mechanics on bone-conduction threshold is expressed at most as improvement for the frequency of 2000 Hz, which is caused by the reduction or elimination of resonance within the ossicular chain.^{Reference Baker, O'Connell, Nguyen and Lambert16} Similarly, we found that three groups all showed significant improvement at 1000–2000 Hz.

This study is limited by its retrospective nature and small sample size. Other confounding factors, such as infection degree, operator experience and surgery duration, could influence study results. In addition, hearing data at 6000 Hz were lacking in this study. Although 6000 Hz has little effect on the verbal communication, the relevant changes still need to be studied further.