In vitro effects of hydroxyapatite containing toothpastes on dentin permeability after multiple applications and ageing

Abstract

This in vitro study evaluated the effect of toothpastes with different active ingredients on dentin permeability using an extended protocol including multiple applications and several thermal ageing cycles in the presence or absence of human saliva. The Null hypothesis was that dentin permeability of a hydroxyapatite containing toothpaste (BR), a potassium nitrate (SP) and an arginine and calcium carbonate (EH) containing toothpaste were similar. Dentin permeability was measured as hydraulic conductance using a commercially available capillary flow system (Flodec, Geneva) and results were expressed as % relative to matching controls. Without saliva, the ranking (best first) of dentin permeability was BR(61%) < SP(87%) < EH(118%), with saliva EH(63%) < SP(72%) < BR(88%). Saliva increased or decreased permeability dependent upon the test material. BR reduced dentin permeability significantly more in absence of saliva, with saliva EH was superior to BR. Repeated material application decreased and thermal ageing increased dentin permeability. The different tooth pastes reduced permeability differently, the best being BR without saliva, the least EH without saliva. The newly introduced test conditions (ageing, saliva, multiple applications) influenced single results significantly, and as they better simulate the in vivo situation they should be considered to be included in further in vitro permeability testing of desensitizing preparations.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1

Results for controls. Relative Hydraulic conductance rLp (%) for controls tested, without (for UC and AL) and with (for UC only) saliva for 6 measurement points. Dotted line represents baseline (t0) measurements which were set to 100% for each single dentin slice. AL was not measured at t3 and t5. Depicted are Medians and 25–75% Quantiles.

Figure 2

Overview of the results. Area coefficients (%) of materials tested without and with saliva. To gain an overview of the results, for each dentin slice the area under its rLp curve (abszissa: measurement time; ordinate: corresponding rLp value) was calculated, related to matching areas under the curves of the untreated controls (UC), and expressed as % (area coefficient; 100% ~ area under the curve of untreated control slices). Dotted and dashed lines depict the area coefficients of untreated controls (set to 100%) and AL, respectively. Depicted are Medians and 25–75% Quantiles.

Figure 3

Results (rLp) related to time points. Relative Hydraulic conductance rLp (%) for materials tested, without (left) and with (right) saliva for six measurement points. Baseline hydraulic conductance was set to 100% and visualized as dotted horizontal lines. Depicted are Medians and 25–75% Quantiles.

Figure 4

Results (ΔrLp) related to time differences. Successive differences of relative Hydraulic conductance ΔrLp (%) for materials tested, without (left) and with (right) saliva for six measurement intervals. Depicted are Medians and 25–75% Quantiles.

Figure 5

SEM, surface view of untreated controls. SEM visualization of the dentin surface of untreated controls (UC) before (t1) and after third thermal ageing period (t6), without and with saliva application at original magnifications of x 5,000 under environmental conditions.

Figure 6

SEM, surface view at t1. SEM visualization of the dentin surface after first material application and before first thermal ageing (t1) for toothpastes containing hydroxyapatite (BR), potassium nitrate (SP), and calcium carbonate and arginine (EH), without and with saliva at original magnifications of x5,000 and x20,000 under environmental conditions.

Figure 7

SEM, surface view at t2. SEM visualization of the dentin surface after first material application and after first thermal ageing period (t2) for toothpastes containing hydroxyapatite (BR), potassium nitrate (SP), and calcium carbonate and arginine (EH), without and with saliva, at original magnifications of x 5,000 and x 20,000 under environmental conditions.

Figure 8

Data treatment. Printout of original data (recorded by the Flodec software; dotted) and the corresponding linear fit (red line) for calculation of hydraulic conductance (Lp) with resulting slope b[1] and correlation coefficient r2.

Figure 9

Details of the experimental setup. (a) open permeability chamber, facing surfaces are covered with a silicone mat to hold five dentin slices. (b) detailed view of dentin slices within the silicone mats on one part of the permeability chamber. (c) Flodec apparatus connected to the closed permeability chamber. (d) complete experimental setup. (A) water column with reservoir; (B) closed permeability chamber; (C) tube for deaeration; (D) Flodec measuring unit including glass capillary with a constant inner diameter of 873 µm (details see Fig. 10); (E) dentin slice holder for thermal ageing. Copyright of the Flodec logo given to Macmillan Publishers Ltd, part of Springer Nature.

Figure 10

Flodec measuring unit in detail. The glass capillary (F) as a part of the unit and the closed permeability chamber (B; details see Fig. 9) are connected with a tube, and a dentin slice, which is inserted into the permeability chamber (B) may be permeated by opening the appropriate 3-way valve (D). Tap (C) and connected syringe are used to deareate the system. Tap (A) is the lower part of the water column (see Fig. 9(d)). Apparatus (G) is a custom made holder for material or saliva application onto dentin slices, and (H) shows the dentin slice holder for thermal ageing. Copyright of the Flodec logo given to Macmillan Publishers Ltd, part of Springer Nature.