Dental hygiene therapy workforce maldistribution and preventable extraction rates in Senegalese health districts: a multilevel analysis of staffing-mix thresholds

Senegal's oral health system relies on a thin and geographically uneven dental hygiene therapy workforce, yet the relationship between staffing mix and avoidable tooth loss has not been quantified at the health-district level. The framework treats the ratio of dental hygiene therapists to dentists as a district-level exposure, preventable extractions as a count outcome, and incorporates random intercepts for region and random slopes for workforce density. Using a replication-based calibration with parameter values drawn from published Senegalese and West African oral health workforce estimates, the analysis derives threshold values at which the marginal effect of staffing mix on preventable extractions changes sign. The results indicate that a therapist-to-dentist ratio of approximately 1.5 therapists per dentist marks an inflection point below which extraction rates accelerate, and that this threshold varies systematically with district remoteness and population density. Workforce maldistribution should be understood as a threshold phenomenon rather than a linear gradient, with implications for task-shifting policy and district-level resource allocation in Senegal and comparable West African settings.