A Comparison of Terahertz Pulsed Imaging with Surface Microhardness to Measure Mineral Changes in En

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A Comparison of Terahertz Pulsed Imaging with
Surface Microhardness to Measure Mineral Changes
in Enamel D. Churchleya, F Lipperta, Jessie
Altonb, C.Gonzalez-Cabezasc, J. Ederc
aGlaxoSmithKline Consumer Healthcare, St
Georges Avenue, Weybridge, Surrey, KT13 0DE, UK
bTeraview Limited, Platinum Building, St Johns
Innovation Park, Cambridge, CB4 0WS, UK cIndiana
University School of Dentistry, Oral Heath
Research Institute, 415 Lansing Street,
Indianapolis, Indiana 46202, USA.
Introduction Microhardness measurements are
commonly used to provide a measure of mineral
changes associated with de- remineralisation
processes.1 However, a linear relationship
between indentation length and lesion depth is
valid only in a limited range of lesion depths
and phenomena such as lesion shape and mineral
distribution might influence the indentation
length values.2 Terahertz pulsed imaging
(TPI), is an intrinsically safe, non-invasive and
non-destructive technique for characterizing
molecular structures in three-dimensions. This
technique used terahertz (THz) radiation which
encompasses frequencies between infrared and
microwaves and is defined as having a frequency
between 0.1 and 3 THz (1 THz 1012Hz),
corresponding to 3.3 100 cm-1.
Reflections occur when there is a
structural change in the tooth leading to a
change in the refractive index of the
material.3 This means THz could be used to
provide an indirect measure of mineral content
following de- and remineralisation eliminating
the need to destroy the tooth sample by
sectioning.
From the average time domain THz waveform
(collected from a 1.5 x 1.5 mm area within the
centre of the sample) the intensity of the
reflection from the air / enamel interface (AEI)
was measured. Statistical analyses were
conducted using an analysis of covariance model
SAS software, version 8.2. Where significant
differences were found, additional all pair wise
comparisons were performed using the Tukeys HSD
multiple comparisons test. Results A
representative time domain waveform from the
demineralised enamel specimen is shown in Figure
2, (A) reflection from the air / enamel
interface (AEI) (B) reflection from the lesion
/ enamel interface (LEI) (C) reflection from
the enamel / dentine junction (EDJ). From the
waveform, it is possible to calculate the lesion
depth (LD) from simple time of flight
calculations by measuring the time delay between
the AEI and LEI reflections.
.

• Objectives
• To determine the ability of TPI to measure
  mineral changes in enamel following
  remineralisation of artificially induced caries
  lesions
• To correlate the data with the SMH Recovery.
• Methods
• Forty eight artificial caries lesions were formed
  in the enamel specimens by a 48 hour immersion
  into a solution of 0.1 M lactic acid (pH 5.0)
  containing 0.2 Carbopol C907 and 50 saturated
  with hydroxyapatite. After demineralization, a
  reference area on each specimen was created by
  covering 1 mm on two opposite sides of the
  specimen surface with acid resistant varnish
  (Revlon, New York, NY). The SHM (Knoop) was
  determined from four indentations on the surface
  of each specimen (50g for 15s). Only specimens
  with an indentation length of 12020µm were
  accepted for the study.
• The specimens were randomly divided (12 specimens
  per group) into 4 treatment groups (A 10ppm
  fluoride dentifrice, B 675ppm fluoride
  dentifrice, C 1385ppm fluoride dentifrice, D
  2700ppm fluoride dentifrice). A 5050 mixture of
  pooled, human saliva and a mineral solution was
  used as the remineralisation medium in all
  treatment regimens. Fresh saliva/mineral mix was
  used each day. The cyclic treatment regimen
  consisted of a 4 h/day acid challenge in the
  lesion forming solution and four, one-minute
  treatment periods (12 dentifrice slurry). After
  treatment the specimens were placed back into the
  saliva mixture (the remineralization system).
  The regimen was repeated for 20 days. At the end
  of the study the SMH was remeasured.
• All specimens (at baseline and 20 days) were
  imaging using a TPI Imaga 1000 (TeraView Ltd.,
  UK). Each sample was positioned on at the focus
  of the beam and the optics raster scanned to
  collect a 6mm x 6mm grid of data points with data
  spacing of 50µm.

Upon treatment with the different fluoride
dentifrices, the intensity of the reflection from
the AEI increased in a dose dependant manner
(Figure 3). This was due to the increase in the
refractive index mismatch between the air and
enamel. The ?AEI was statistically significant
with the exception of 10 and 675ppm and 1385 and
2700ppm

The SMH recovery also shows a dose dependent
relationship. All differences were statistically
significant with the exception of 675 and 1385ppm
fluoride (Figure 4).