Evaluation of the effects of the surgical-orthodontics treatment of class III malocclusion on the sound

Speech is a dynamic and complex process that includes breathing, phonation, resonation, articulation, and neurological integration. While exhaling, the air coming from the lungs creates a raw sound by vibrating the vocal cords in the larynx. This raw sound is shaped in the throat, nose and mouth cavities and creates the tone that is unique to every human being. These sounds are then transformed into meaningful speech by articulation of the lips, teeth, tongue, palate and alveoli. Considering that orthognathic surgery causes changes in the orofacial structures that form the resonance system, it is expected that the functional changes due to surgery will affect the speech and the patient would need volume adjustments. Therefore, it is important to understand the possible relationships of these structures with sound production. The number of studies examining the changes caused by orthognatic surgery on the voice is very limited in the literature, most of them do not have a control group and the sample size of the studies are small. Voice records and cephalometric x-rays of 25 patients (7 males and 18 females aged between 18-34) were recorded at 2 time-points; T0: just before the operation, T1: 6 months after the operation. The sound samples were analysed in Praat version 6.0.43 (Paul Boersma Amsterdam University, Netherlands), and the x-rays were drawn manually on tracing paper. All Turkish vowels (a [a], e [ɛ], ı [ɯ], i [i], o [ɔ], ö [œ] u [u] and ü [y]) were pronounced for 3 seconds and the patients were asked to read specific words contaning these vowels. F0, F1, F2, F3, Shimmer, Jitter and NHR parameters were considered, compared pre-operatively and postoperatively, and then inter-group comparisons were made with the control group consisting of untreated 19 people. The correlation between structural changes and acoustic changes in the cephalometric x-ray was evaluated. In the orthognatic surgery group, significant differences were observed in the F1 of [e] (p=0.026), F2 (p=0.029) and Shimmer (p=0.028) of [ɯ] and F1(p=0.004) and F2 (p=0.000) of [œ] sounds, F1(p=0.018) of [y]. The post-surgery values were lower compared to those recorded before. On the other hand, the F3 of [u] sound (p=0.000) was significantly higher in comparison to the baseline values. In the comparison with the control group, significant differences were found in the amount of change of Δ F3 (p=0.028) of the o[ɔ] sound, the Δ F3 of the [u] sound (p=0.015) and the ΔF1 of the [y] sound (p=0.035). Significant differences were also found in the Δ Shimmer of [ɛ] (p=0.036), [ɯ] (p=0.022), [i] (p=0.031), [ɔ] (p=0.032), [u] (p=0.040) and [y] (p=0.034), and the ΔNHR of [ɔ] (p=0.031). The Pearson correlation analysis showed that some structural changes significantly affected some acoustic parameters; ΔF2 between ΔSNA for [ɯ] and [œ] sounds, ΔF1 between ΔHBV for [y] sound. Keywords: Orthognathic surgery, Class III Malocclusion, Voice changes, Acoustic analysis, Formant frequency