## 3 Photonic Technologies
### Optical Coherence Tomography (OCT)
#### 3.4.2 Dental Applications  
In dental science, OCT detects qualitative and quantitative morphological changes of dental hard and soft tissues, like for early diagnosis of dental diseases, such as caries, periodontal disease, and oral cancer, because of the excellent spatial resolution (\(\approx\)15 \(\upmu\)m) [102]. OCT can also be used in other types of applications [99, 100, 101], such as to evaluate the microleakage of dental restorations and endodontic fillings [103], the dental implant status [104], the integrity of dental prosthesis [105], and the surroundings of orthodontic brackets [106]. In 1991 [PERSON] _et al._[107] reported, for the first time, the development of the technique of optical coherence tomography (OCT), to measure the internal structure in biological systems and, in dentistry, a series of reports describing OCT applications appeared in 1998 [108, 109, 110], with imaging of both hard and soft oral tissues. In 1998 [PERSON] _et al._[110] used, for the first time, OCT to perform a comprehensive program of _in vivo_ and _in vitro_ structural imaging of hard and soft tissues within the oral cavity. They were able to differentiate several types of healthy oral mucosa, as well as normal and abnormal tooth structure. Polarization-sensitive optical coherence tomography (PS-OCT) was used to monitor the remineralization of carious lesions, by measuring the new amount of mineral that is deposited in the outermost layers [111]. [PERSON] _et al._[112] proposed the use of a combining optical imaging/sensing and mechanical function in an integrated dental drilling probe to show the feasibility of an optomechanical system for surgical optical OCT imaging and sensing in dental applications. The system was made by integrating micro lenses and optical fiber into a dental drill. Although they demonstrate the proof-of-concept of a dental drill, they did not succeed perfectly. More recently, [PERSON] _et al._[102] developed a swept-source optical coherence tomography (SS-OCT) system, using a fiber optic handheld scanning probe, for the diagnosis in real-time of crack and microleakage detection between dental composite resin and enamel. The system uses a 1310 nm swept-source laser as a broadband light source, which is separated into two arms (reference arm and sampling arm) and after that is recoupled in another optical coupler for interference. A balance detector is used for the detection of interference and a data acquisition card provided computer-photodetector interfacing. The input light, in the OCT portable probe, with 3 mm outer diameter, is focused by a collimator and then, by a proper scanning mechanism, the output light angle is changed in order to perform the scanning path (Figure 5). Figure 5: Schematic representation of the setup of a dental handheld probe SS-OCT, for _in vivo_ detection of crack and microleakage, between dental composite resin and enamel (adapted from [102]). The authors verified that SS-OCT can provide the diagnosis of _in vivo_ tooth microleakage and it was very reliable for the diagnosis of cracks in the enamel. However, this study provided _ex vivo_ information for large crack detection only. Further experiments must be carried out with different filling materials in order to correlate them with different image properties, such as penetration depth, image distortion, and the minimum detectable gap size. Another trend in the application of OCT is in the study of dental materials, in the inspection of crack growth, adhesion, and shrinkage.