Halitosis, Oral Health-Related Quality of Life, and Active Dental Treatment: A Prospective Observational Comparative Study Across Periodontal, Prosthodontic, and Orthodontic Modalities

Background and Objectives: Halitosis is a prevalent oral concern that meaningfully affects oral health-related quality of life (OHRQoL), yet how active dental treatment is associated with short-term changes in the objective–subjective halitosis–QoL nexus remains poorly quantified. Interpretation is complicated by the multifactorial nature of malodor and by baseline differences between patients selected for different dental procedures. We compared changes in volatile sulfur compound (VSC) emissions, organoleptic ratings, tongue-coating burden, and OHIP-14 across three contrasting treatment modalities and explored whether VSC change statistically accounted for OHRQoL change. Methods: In a non-randomized prospective comparative study, 119 adults (18–67 y) commencing one of three procedures were assessed at baseline and at 8 weeks: scaling and root planing (Group A, n = 42), fixed prosthodontic rehabilitation (Group B, n = 38), or fixed orthodontic appliance bonding (Group C, n = 39). Outcomes included Halimeter® VSC (ppb), Rosenberg organoleptic score (0–5), Winkel tongue-coating index (TCI), self-perceived halitosis, and OHIP-14 total and seven-domain scores. Mixed-design ANOVA, ANCOVA, prespecified multivariable regression, mediation (5000 bootstrap resamples), receiver operating characteristic analysis, and four-class latent class analysis were performed. A sensitivity-analysis framework including expanded covariate adjustment, propensity-score overlap weighting, and baseline-severity strata was also applied to address residual baseline imbalance. Secondary mediation, ROC, and latent-class analyses were considered exploratory. Results: At 8 weeks, VSCs fell by 116.4 ± 38.7 ppb in Group A and 35.4 ± 29.1 ppb in Group B but rose by 34.3 ± 28.6 ppb in Group C (p < 0.001). OHIP-14 improved by 10.3 and 4.9 points in A and B and worsened by 3.7 in C (p < 0.001). ΔVSC correlated with ΔOHIP-14 (ρ = 0.51, p < 0.001) and most strongly with the psychological discomfort domain (ρ = 0.58). VSC change mediated 35.1% of the periodontal-versus-orthodontic association on QoL (indirect β = −4.7; 95% CI −6.3 to −3.1). Because VSC and OHIP-14 changes were measured over the same interval, mediation was interpreted cautiously. A ΔVSC threshold of −63 ppb predicted clinically meaningful OHIP-14 improvement (AUC = 0.81). Latent class analysis identified four distinct responder phenotypes. The cutoff and responder classes were internally derived and require external validation. Sensitivity analyses preserved the direction of the primary contrasts, but residual confounding remains possible. Conclusions: Treatment modality was associated with the direction and magnitude of halitosis and QoL change, with orthodontic patients constituting a vulnerable subgroup. Targeted oral-hygiene reinforcement during fixed-appliance therapy is warranted.