**1. Introduction**

Dental caries is one of the most common chronic diseases in people worldwide because of its high incidence and the wide range of the affected population [1]. Caries is a disease of dental hard tissue caused by oral microorganisms. Normally the teeth are in a dynamic balance of alternating demineralization and remineralization, in which demineralization is the dissolution of dental minerals in the presence of acid, from which inorganic ions such as calcium and phosphate are removed, and remineralization refers to the reprecipitation and crystallization of minerals in tissues that have been demineralized [2]. If demineralization continues, caries will occur. Early enamel caries involves the demineralization of the tooth surface, and does not show substantial defects. There is no significant difference between sound enamel and early enamel caries by visual observation. At this time, it is possible to promote the repair of carious tissues through non-destructive treatment of remineralization and interrupt caries progression. However, further irreversible dental hard tissue loss will occur without interventional treatment, and then only traumatic treatment can be performed [3]. Therefore, the detection of early caries is of great importance.

Common clinical diagnostic methods mainly include visual inspection and X-ray radiography. Visual inspection lacks objectivity and has limited accuracy, while X-ray radiography can only detect severe caries lesions [4,5]. These conventional methods cannot detect early caries, which makes the early diagnosis of caries difficult and delays the best time for treatment [6]. In recent years, optical-based caries detection methods, such as Raman spectroscopy [7,8], quantitative light-induced fluorescence (QLF) [9,10] and fiberoptic transillumination [11,12], have overcome some disadvantages of traditional methods,

**Citation:** Shi, B.; Niu, J.; Zhou, X.; Dong, X. Quantitative Assessment Methods of Early Enamel Caries with Optical Coherence Tomography: A Review. *Appl. Sci.* **2022**, *12*, 8780. https://doi.org/10.3390/ app12178780

Academic Editor: Nunzio Cennamo

Received: 28 July 2022 Accepted: 30 August 2022 Published: 31 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

but they do not show the internal microstructural features of teeth and cannot quantify dynamic changes in early stages of enamel lesions [13].

Optical Coherence Tomography (OCT) is a noninvasive imaging method with high resolution and high sensitivity, regarded as an "optical biopsy" [14,15]. It has a wide range of medical applications, including ophthalmology [16,17], dentistry [18,19], dermatology [20,21], and gastroenterology [22,23]. For dental caries, OCT can detect tiny, demineralized areas on the tooth surface and inside, overcoming the disadvantages of other optical detection methods [24]. In dentistry, much research has demonstrated that OCT can image dental tissue clearly and be applied to the quantitative assessment of early enamel caries [25–28]. Mineral loss is the gold standard in cariology research that determines the degree of demineralization of the lesion. Only when the quantitative index of enamel caries obtained by OCT has a definite correlation with the enamel mineral loss, the severity of early enamel caries can be quantified more objectively and accurately.

To advance the application of OCT for early caries detection, a standardized and validated method to achieve quantitative detection of early caries is required. The purpose of this paper is to present an overview of the principles, methods and applications for the quantitative evaluation of early enamel caries by OCT and to provide a basis for the establishment of a uniform clinical standard in the future. This paper first discusses the optical properties of teeth and their changes with tooth demineralization, secondly introduces methods of artificial caries preparation, and then summarizes current research advances in evaluation of early enamel lesions using different quantitative indicators and discusses the limitations of the methods.
