# Biophotonics in Dentistry
## 3 Applications in Dentistry
### Photoelasticity  
Photoelasticity is an optical characteristic that has been used for many years in experimental stress analysis. Most transparent materials exhibit a temporary birefringence property when exposed to strain and loading due to changes in molecular orientation distribution [84]. This can be utilised to study biomechanics within dentistry and various types of orthodontic movements or structural design. A model was constructed using a transparent 'photoelastic' material, and the magnitude and direction of occlusal forces was replicated to mimic those experienced in physiological conditions. When these models were placed under stress, they exhibited birefringence, where normally incident light rays split into two parts with two refractive indices and velocities directly related to the stress at that point [85]. Analysis of the birefringence using a polariscope allows for calculation of shear stress. Plane polariscopes and circular polariscopes can be used to analyse light fringes with circular polariscopes, providing the advantage of only showing isochromatic and not isoclinic light rays. The optical technique has been applied in dentistry for many years, dating back to 1935, when it was used to study effects of orthodontic movements on the dental alveoli [86]. It has since been used in various studies of dental biomechanics, including investigating the relationship between dental caries and stress concentrations in 1966 and two-dimensional experimental stress analysis of inays in 1967, which improved cavity design by lowering stress concentrations [87]. In 1971, photoelasticity was used by [PERSON] and [PERSON] as a means of determining the stress distribution on a gold dental bridge, with the study revealing that highest stress concentrations occurred at soldered joints [88]. [PERSON], [PERSON] and [PERSON] (1975) later utilised the optical technique to develop a photoelastic model, allowing the technique to be used to study root tipping [89]. Photoelasticity has also allowed studies comparing different endodontic dowels, such as the effect of length, diameter and other design parameters on stress distributions [90]. In 2000, the photoelasticity technique was used to analyse in vivo stress and strain distributions in human dental supporting structures from the tooth root to the alveolar bones [91]. An epoxy resin model (Figure 5) was used to represent the tooth and alveolar bone, and a diffuse-light polariscope was used to observe the fringe patterns when loaded with forces applied at different axes to represent forces applied under physiological conditions. The resultant fringe patterns are seen in Figure 6. Most of the stresses are distributed along the cervical and middle third of the root and supporting bone. The study highlighted the role of the periodontitis in distribution of stresses and bone remodelling. [PERSON] and [PERSON] (2001) further investigated the stress distributions between the tooth and bone interface using photoelasticity (Pakistan _et al._, 2017). Their study considered some limitations of the use of the photoelasticity technique in its past applications in dentistry. One such limitation is that in order to generate sufficient fringes to analyse forces much larger than  
Figure 5: Model of tooth and supporting bone for photoelastic stress analysis. Strain gauge and photoelastic analysis of in vivo strain and in vitro stress distribution in human dental supporting structures. Adapted with permission from Ref. [92]. 2019, Elsevier. Figure 6: (**a**) Fringe pattern showing stress distribution in a tooth with normal supporting bone at a load of 225 N along its long axis. (**b**) Fringe patterns showing stress distribution when load is directed at (**A**) \(0^{\circ}\) (**B**) \(30^{\circ}\) and (**C**) \(60^{\circ}\) to the long axis. Strain gauge and photoelastic analysis of in vivo strain and in vitro stress distribution in human dental supporting structures. Adapted with permission from Ref. [92]. 2019, Elsevier. normal, physiological occlusal forces must be applied.