Twati, WA and Wood, DJ and Liskiewicz, TW and Willmott, NS and Duggal, MS (2009) An evaluation of the effect of non-setting calcium hydroxide on human dentine: a pilot study. European archives of paediatric dentistry

AIM: To evaluate the effect of non-setting calcium hydroxide (NSCH) on the hardness and elastic modulus of dentine from extracted permanent premolar human teeth. METHODS: 30 freshly extracted single rooted human premolar teeth were decoronated and the roots then sectioned longitudinally into equal halves. In the experimental group a thin layer of NSCH was applied whilst the control group had no medicament. After 1, 3 and 6 months, nanoindentation was used to assess dentine hardness and the modulus of elasticity. Scanning Electron Microscopy (SEM) was used to visualize the depth of penetration of NSCH into the dentinal tubules. RESULTS: SEM images showed that there were no structural changes in the dentine slabs that had NSCH application after 1, 3 or even 6 months. However, penetration of NSCH into the dentine tubules was seen at both 3 and 6 months with a significant reduction in the hardness of dentine observed at 3 (p<0.02) and 6 months (p<0.01). The modulus of elasticity was significantly lower (p<0.01) at 6 months. CONCLUSION: It appears that there is a significant reduction in the hardness of dentine with increasing periods of calcium hydroxide application. Prolonged application of NSCH could have a detrimental effect on dentine, making the dentine more prone to fracture. Introduction At present, the management of non-vital immature incisors requires complete elimination of bacteria, necrotic pulp, tissue debris and the use of a suitable material to induce an apical hard tissue barrier in order to facilitate obturation. Calcium hydroxide (CaOH2) was introduced as early as 1920, [Hermann, 1920] and it was suggested very early on that this material mixed with camphorated parachlorophenol (CMCP) would induce a calcified barrier across the apex [Kaiser, 1964]. This material became popular when it was observed that CaOH2 also reduced bacterial contamination of the root canals [Frank, 1966]. According to Kinirons et al. [2001] the mean time taken for non-setting calcium hydroxide (NSCH) to induce an apical hard tissue barrier is 9 months. However, it has been pointed out that long-term use of NSCH in non-vital immature incisors might lead to a weakening of the dentine structure due to degradation of the proteins [Andreasen et al., 2002]. This may make the dentine more susceptible to fracture. High incidences of root fracture have been reported for non-vital immature permanent incisors that have been managed endodontically, particularly with NSCH [Cvek, 1992; Al-Jundi, 2004]. In a more recent study, Rosenberg et al. [2007] showed a significant reduction in the mean tensile fracture strength (MTFS) of dentine with prolonged use of NSCH. Given these preliminary reports it seemed prudent for dental researchers to study the effect of NSCH on the physical properties of human dentine. The aim of this in vitro pilot study, therefore, was to evaluate the effect of NSCH on the hardness and modulus of elasticity of human dentine from extracted human permanent teeth. Materials and methods Samples. Following patient informed consent, 30 extracted permanent single rooted human premolar teeth were collected. The teeth had been extracted from adolescents and young adults, aged 10-18 years of age for orthodontic reason. Each tooth was cleaned with distilled water and all remnants of soft tissue were removed with a scaler. The teeth were then stored in water with a crystal of thymol added. The teeth were decoronated and sectioned longitudinally into equal halves with a Precision Diamond Wire Saw. (Well, USA). (Figure1a). One of the pair of samples was used as a control, with no medicament, and the other was covered with a layer of NSCH (UltraCal XS, Ultradent, USA). The samples were then placed in a petri-dish alongside saline soaked gauze and incubated at 37°C. The wet gauge ensured a humid environment around the samples whilst they were in the incubator. The gauze was changed weekly. At 1, 3 and 6 months 10 paired-samples (10 experimental samples and the 10 corresponding control samples) were investigated as follows: ● Preparation of samples. 5 experimental samples plus 5 corresponding control samples were polished, etched and sputter coated with gold, ● SEM was used for examination, ● Nanoindentation. 5 experimental samples plus 5 corresponding control samples were embedded in composite resin using a custom made mould, which, because the samples were hemi-sectioned, was necessary to create a NSCH and dentine in premolars flat surface to facilitate measurement. A small hole placed in the bottom of the die enabled the composite block holding the dentine slab to be removed without any damage to the surface that was to be measured (Figure 1b). The block with an embedded sample was then cut in half transversely across the root as shown in Figure 1c. The “top” exposed surface was thus not used for measurement of hardness rather an internal surface was preferred. Hardness was assessed by Nanoindentation (Micro Material Ltd, Wrexham, UK) using 5 indentations that were spaced evenly over the surface that was to be assessed. The mean of these 5 indentations was then used as the final measurement. Calculation of Young’s Modulus of Elasticity. The value obtained from nanoindentation was used in a formula described by Oliver and Pharr [1992] to calculate the Young’s Modulus of Elasticity of Dentine. The Young’s Modulus was calculated from the following equation: