Voice Characteristics and Changes During Functional Appliance Therapy in Children and Adolescents with Class II Malocclusion: A Prospective Observational Study

Background/Objectives: Functional appliance therapy is performed during a period of growth and pubertal voice development. This study compared baseline voice parameters between children and adolescents with Class I and Class II malocclusion and evaluated their changes during functional appliance therapy in the Class II group. Methods: This prospective observational study included 35 patients with Class II malocclusion requiring mandibular advancement and 30 comparison participants with Class I malocclusion. Class II patients received monoblock or twin-block appliances; 31 completed treatment and were reassessed after a mean of 9.55 ± 1.75 months. Maximum phonation time, the s/z ratio, fundamental frequency, local jitter, local shimmer, and self-perceived voice handicap (Children’s Voice Handicap Index-10, CVHI-10) were assessed without an appliance in the mouth. Acoustic parameters were extracted using Praat from a stable 1 s portion in the middle of 3 s sustained /a/ samples. Baseline comparisons included all 65 participants; longitudinal comparisons were completer analyses (n = 31). Results: Baseline voice parameters and CVHI-10 scores did not differ significantly between the groups. Among Class II completers, shimmer increased from 1.99 ± 1.24% to 3.07 ± 0.99% (mean change, 1.08 percentage points; 95% CI, 0.58–1.58; p < 0.001; r = 0.67); the other voice parameters and CVHI-10 scores did not change significantly. Conclusions: Shimmer increased during functional appliance therapy, whereas the other measured voice parameters did not change significantly. Because the comparison group was not followed longitudinally and follow-up coincided with pubertal voice development, this increase represents only a temporal association that cannot be attributed to treatment or interpreted as evidence of clinically relevant vocal deterioration. Controlled studies incorporating standardized recordings, perceptual assessment, patient-reported outcomes, and longer follow-up are needed.