Dental hygiene therapy scope-of-practice reform and untreated caries in Nigerian primary care: a computed control analysis of regulatory expansion thresholds

Nigeria's oral health workforce is dominated by dentists concentrated in urban tertiary facilities, while dental hygienists and therapists remain restricted in the procedures they may perform independently in primary care. The design treats scope-of-practice reform as a staggered, multi-state policy shock and constructs donor pools from Nigerian states that did not reform, weighting pre-reform caries and workforce indicators to approximate each treated state's counterfactual trajectory. Using the DHS Program indicator 'Percentage receiving injection given by health care provider' (FP_SPUS_W_SPH) as a calibration benchmark for facility-level service delivery across 14 African countries surveyed between 2021 and 2025, the paper demonstrates the donor-pool construction and placebo-inference machinery on a real, verified indicator with a mean of 91.964 and a standard deviation of 12.468. The design specifies three regulatory thresholds — delegated diagnosis and local anaesthesia, delegated excavation and restoration, and delegated extraction of deciduous teeth — and argues that only the second threshold is likely to produce a caries-specific effect large enough to separate from secular trends. The contribution is a transferable identification strategy for oral health policy evaluation in data-sparse settings, not a claim about realised Nigerian reform effects.