# Biophotonics in Dentistry
## 3 Applications in Dentistry
### Interferometry Techniques  
#### 3.2.1 Electronic Speckle Pattern Interferometry  
Electronic speckle pattern interferometry (ESPI) is a non-destructive optical technique developed in the 1970s that can also be used to investigate biomechanics in dentistry applications. Specular reflection fringe patterns from surfaces are recorded before and after a load is placed. The images before and after loading are then computed and combined to form a resultant image with a speckle pattern of fringes, allowing for calculation of three-dimensional distributions of stress and strain [97; 98]. A schematic of a typical experimental setup of ESPI is shown in Figure 7[99]. [PERSON], [PERSON] and [PERSON] (2004) considered the advantages of ESPI in terms of its capability to take high-sensitivity measurements of surfaces in a non-contact manner [100]. Figure 7: Schematic of experimental setup of ESPI. Non-destructive characterization of resin-based filling materials using electronic speckle pattern interferometry. Adapted with permission from Ref. [100]. 2019, Elsevier. Applications in dentistry have included comparisons between different restorative materials, including gold, ceramic, composite resin or amalgam inlays used to fill dental cavities. The effect of the type of material on deformation of teeth when subject to loading was investigated, with results showing deformation varied depending on the type of material. Another advantageous use of ESPI in dentistry includes the development of a non-destructive test to characterise the modulus of resin-based inlays [100]. The study was significant, as such a non-destructive test would allow time-dependent effects to be determined using the same specimens, thus saving time and material. In 2009, the ESPI technique was used to compare deformation when a tooth-mandible complex was placed under load compared to an isolated tooth [101]. Minipig molars were used, and the study demonstrated that teeth bend in the direction of the load. Other uses of ESPI include the study of dentin compression in water, where the Young's modulus of root dentin under compression was obtained. Results showed the location of the root dentin had a significant effect on the stiffness value, with root dentin from interproximal locations found to have a modulus of 21.3 GPa, whereas root dentin from buccal and lingual locations had a modulus of 15 GPa. In 2012, ESPI was used by [PERSON] et al. as a means of gaining a greater understanding of the structure and mechanical behaviour of the dentin-enamel junction and comparing it against the dentin-ceramic junction of a crown [102]. Figure 8 shows the natural sample under loading, and Figure 8A shows the initial sample under white light. Figure 8B-D shows the typical fringes when load is applied on the sample, and the dentin-enamel junction is clearly visible (represented by red arrows). The junction becomes visible at a load of around 35.5 N.  
In comparison, Figure 9 shows the crown under loading. The junction becomes visible under a load of 38 N and is a clear, continuous, visible line. The study was described as significant, as it demonstrates that the biomechanical behaviour of a prosthetic crown closely resembles that of a natural junction. Figure 8: Natural tooth sample (**A**) under white light, (**B**) under a force of 35.5 N, (**C**) under a force of 40.23 N and (**D**) under a force of 82.54 N. Comparative mechanical behaviour of dentin enamel and dentin ceramic junctions assessed by speckle interferometry (SI). Adapted with permission from Ref. [102]. 2019, Elsevier. Figure 9: Prosthetic crown sample (**A**) under white light, (**B**) under a force of 36.78 N, (**C**) under a force of 77.56 N and (**D**) under a force of 64.09 N. Comparative mechanical behaviour of dentin enamel and dentin ceramic junctions assessed by speckle interferometry (SI). Adapted with permission from Ref. [103]. 2019, Elsevier. Thermally induced deformation of human dentine was also investigated by [PERSON] et al. using ESPI [97].