School-based dental hygiene therapy and hearing-loss-related caries in South African deaf learners: a difference-in-differences evaluation of sign-language-adapted oral health instruction

Deaf learners in South Africa face elevated caries risk linked to limited access to oral health instruction in South African Sign Language (SASL), yet school-based dental hygiene therapy programmes rarely adapt content for hearing loss. The primary outcome was decayed, missing and filled permanent teeth (DMFT) over 24 months. Secondary outcomes included plaque index, gingival index, and caregiver-reported oral health-related quality of life. The analytic sample comprised 312 learners aged 8–18 years. The intervention was associated with a reduction in DMFT increment of 0.71 teeth (95% CI 0.28–1.14; p = 0.001) relative to comparison schools, after adjustment for baseline DMFT, age, sex, and school clustering. Plaque index fell by 0.34 (95% CI 0.19–0.49; p < 0.001) and gingival index by 0.27 (95% CI 0.11–0.43; p = 0.001). The findings indicate that sign-language-adapted instruction delivered by dental hygienists can reduce caries increment in deaf learners, and that hearing loss should be treated as an oral health equity determinant rather than a background characteristic.