Transmitted-light plethysmography (TLP) is an optical technique to detect circulatory changes in pulp tissue. It has been reported as overcoming the disadvantages of electrical pulp testing in terms of objectivity and invasiveness. In a previous study, we reported the applicability of the TLP system with a 525 nm light-emitting diode (LED) to traumatized young permanent and deciduous teeth. The system works by detecting the pulsation intensity of light transmitted through a tooth and visualizing the pulpal blood flow synchronized with the heartbeat. During the clinical follow-up, we found that TLP pulse amplitudes and pulse shape characteristics change gradually due to the pathological condition. However, the origins of tooth plethysmograms were not fully understood because of the structural specificity of teeth. In this study, multiwavelength optical plethysmography was used to examine the light transmission properties of teeth to clarify the effects of physiological parameters such as the blood volume and SO2 on the pulsation of TLP pulse waves. The results indicated that the optical process in teeth mainly depended on the blood cell concentration (Hctp), light attenuation by the hard tissues, and the light source wavelength. Clinically, it was assumed that the changes in blood vessel density, pulpal blood volume, and pulp chamber size associated with subjects' ages or pathological conditions might affect TLP pulse waves. We also demonstrated the applicability of pulse oximetry to dental pulp using an in vitro blood circulation model. The LEDs with 470 nm and 590 nm wavelengths showed high sensitivity to the blood SO2 level. R values calculated from pulsatile/nonpulsatile component ratios of two different wavelengths were significantly correlated with the pulpal SO2, indicating the possibility of pulpal blood SO2 measurement. The present results indicate that TLP pulse waves potentially provide additional information to diagnose the circulatory status of the pulp in relation to pathological conditions of the dental pulp.
