Geometry-dependent mass transfer at the clear aligner-tooth interface: a state-switched reaction-transport hypothesis for localised cariogenic selection

Clear aligners permit better plaque control than fixed appliances, yet prolonged tooth coverage may create poorly exchanged fluid spaces. This may explain why studies of saliva or pooled plaque find little overall change, whereas analyses of the inner aligner surface detect chemical or ecological shifts. We regard the seated aligner-tooth interface as transport domains that change with wear and removal and may be only partly liquid-filled. During wear, exchange depends on gap geometry, liquid occupancy and connectivity, attachment relief and transient pressure. Removal alters the boundary conditions and starts recovery. Rinsing, toothbrushing, appliance cleaning and reinsertion are treated separately because they affect dissolved species and surface biomass differently. The model combines aligner-specific seated geometry and liquid connectivity with measured exchange, irregular wear/off events and site-matched ecological outcomes. A reduced compartment model defines tracer exchange time; a spatial model represents connected pathways and concentration gradients. Partly wetted regions are described by measured occupancy and exchange conductance. We predict that local transport indices measured independently of biological outcomes will explain site-matched pH, organic-acid profiles and microbial function better than gap width, wear time and clinical covariates alone. Transient acidification, cariogenic selection, persistent ecological change and caries are separate outcomes. The hypothesis would be weakened if geometry failed to produce reproducible transport differences, if transport added no out-of-sample predictive value or if controlled alteration of connectivity produced no chemical or biological response.