District-Level Hygiene-Therapy Workforce Density and Untreated Caries Trajectories in South African Primary Schools: A Multilevel Interrupted-Time-Series Analysis of Post-Fluoridation Access Gaps

South Africa's 2003 water fluoridation mandate was expected to reduce untreated dental caries in primary schoolchildren, yet the persistence of district-level disparities suggests that workforce distribution may condition the benefit. The framework specifies a two-level structure in which repeated cross-sectional caries observations are nested within districts, with workforce density entering as a time-varying moderator of the post-intervention slope. Identification rests on the timing of fluoridation implementation and on within-district variation in workforce density, with district-specific intercepts absorbing fixed contextual differences. A worked illustration using the DHS Program indicator 'Discontinuation due to access/availability' (FP_RDIS_W_ACC) across 28 African countries and 74 country-survey observations between 1992 and 2025 (mean 1.712, SD 1.37) demonstrates the data structure and reporting conventions the framework requires. Post-fluoridation access gaps are not a failure of water chemistry but a workforce-allocation problem, and that interrupted-time-series designs which ignore workforce density will systematically overstate fluoridation's population benefit in underserved districts.