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Objectives. When performing middle ear operations, such as ossiculoplasty or stapes surgery, patients and surgeons expectan improvement in air conduction (AC) hearing, but generally not in bone conduction (BC). However, BC improve-ment has often been observed after surgery, and the present study investigated this phenomenon. Methods. We reviewed the preoperative and postoperative surgical outcomes of 583 patients who underwent middle earsurgery. BC improvement was defined as a BC threshold decrease of >15 dB at two or more frequencies. Subjects ingroup A underwent staged ossiculoplasty after canal wall up mastoidectomy (CWUM), group B underwent stagedossiculoplasty after canal wall down mastoidectomy (CWDM), group C underwent ossiculoplasty only (thus, theyhad no prior history of CWUM or CWDM), and group D received stapes surgery. We created a hypothetical circuitmodel to explain this phenomenon. Results. BC improvement was detected in 12.8% of group A, 9.1% of group B, and 8.5% of group C. The improvementwas more pronounced in group D (27.0%). A larger gain in AC hearing was weakly correlated with greater BC im-provement (Pearson’s r =0.395 in group A, P <0.001; r =0.375 in group B, P <0.001; r =0.296 in group C, P <0.001;r =0.422 in group D, P =0.009). Notably, patients with otosclerosis even experienced postoperative BC improvementsas large as 10.0 dB, from a mean value of 30.3 dB (standard error [SE], 3.2) preoperatively to 20.3 dB (SE, 3.2) post-operatively, at 1,000 Hz, as well as an improvement of 9.2 dB at 2,000 Hz, from 37.8 dB (SE, 2.6) to 28.6 dB (SE, 3.1). Conclusion. BC improvement may be explained by a hypothetical circuit model applying the third window theory. Sur-geons should keep in mind the possibility of BC improvement when making a management plan.
