The proportion of cross-sectioned dentin tubules with subsurface occlusion ( occluded to a mean of @ @ m , range : @ to @ m below the surface ) for the @ % strontium group on days @ and @ ( pre-acid ) was @ % ( SD : @ % , @ % confidence interval -LSB- CI -RSB- = @ % to @ % ) and on days @ and @ ( post-acid ) was @ % ( SD : @ % , CI = @ % to @ % ) .