Tooth Pulp Tissue Pressure and Hydraulic Permeability

Abstract

The object of these experiments was to evaluate the hydrostatic pressure and hydraulic permeability of the tooth pulp in situ. A small hole was drilled through the enamel and dentin to the dentinal-pulpal junction in canines and third incisors of anesthetized dogs. A threaded cannula filled with mammalian Ringer's solution was screwed into the hole and connected to a microsyringe and a pressure transducer of very low compliance. Pressure was recorded when the cannula was first inserted into the hole; then small amounts of fluid were injected into the system or withdrawn from the system using the micro-syringe. The equilibrium pressure (P_eq), measured after transients had disappeared, was fairly constant for a given tooth over long periods of time. Referred to tooth level (which was approximately heart level), P_eq averaged 57 mm Hg (range 38 to 78 mm Hg) in the ten teeth tested. Superimposed upon the steady value was a random deviation (± 3 mm Hg), a hysteresis effect resulting in a higher pressure if the measurement had been preceded by injection of fluid rather than withdrawal (± 3 mm Hg), and a continuous drift with time ( + 3 mm Hg/hr). The hydrostatic permeability was calculated from the rate of fluid flow when the pressure was above or below its steady state value. When the pressure was high (fluid being forced into the pulp) flow averaged about 0.33 µl/hr-mm Hg over the range P_eq to P_eq + 20 mm Hg. When the pressure was low (fluid being withdrawn from the tooth), flow averaged about 0.14 µl/hr-mm Hg over the range P_eq to P_eq - 20 mm Hg. It is concluded that P_eq probably represents the true hydrostatic tissue pressure normally existing within the tooth pulp. This implies a high capillary pressure and thus a relatively high resistance in the venous side of the pulp circulation. The interpretation of hydraulic permeability is not certain, but this quantity probably is related to the ease with which fluid passes into or out of the pulp capillaries.