Accurate and early detection of carious lesions is of paramount importance in treatment decision making and in implementing preventive strategies[1]. However, early detection of proximal caries is difficult as contact points impede direct visualization of initial lesions[2]. Especially in primary teeth, thinner enamel and dentine thickness, less mineralization and wider contact points all attribute to rapid progression of proximal caries that are left undetected until marginal ridges breakdown and become clinically cavitated[3-6].

Conventional diagnostic methods for proximal carious lesions in primary molars include visual-tactile inspection and bitewing radiography. Although these methods provide good overall performance with high specificity, visual inspection has relatively low sensitivity (0.274 - 0.543)[7,8]. Bitewing radiography is often used in conjunction with visual-tactile inspection, but American Dental Association (ADA) guideline restricts its use for screening purposes as it inevitably exposes patients to ionizing radiation[9].

Quantitative light-induced fluorescence (QLF) technology is a non-invasive, optical method for detection and quantitative assessment of carious lesions[10]. It utilizes 405 nm visible blue light irradiated on tooth surface and measures fluorescence loss and red autofluorescence emitted as a result of mineral loss and bacterial metabolites on carious lesions. Since it was first introduced in 1980s, it has undergone series of developments with improved filters and capturing methods for better clinical applications[11]. The pen-type QLF device has been used as an adjunct method for caries detection but there are still no studies that assess the clinical performance of the device in detecting proximal caries in primary dentition.

The aim of this in vivo study was to evaluate the screening performance of the pen-type QLF device in detecting proximal carious lesions in primary molars using fluorescence loss, red autofluorescence and a simplified QLF score for proximal caries suggested by Kim et al.[12], compared to bitewing radiography.

In this study, total of 60 participants aged 3 - 10 years, with mean age of 5.35 were included (boys : n = 32, girls : n = 28). Three hundred forty-four approximal surfaces were evaluated. In total, 92 (26.7%) maxillary first primary molar, 83 (24.1%) maxillary second primary molar, 78 (22.7%) mandibular first primary molar, 91 (26.4%) mandibular second primary molar surfaces were selected.

The pen-type QLF device used in this study, introduced in 2015, is relatively new in the market and this is the first study to assess its clinical performance in detecting proximal caries in primary dentition. Over the past years, various techniques including electrical conductance, fiber optic transillumination and DIAGNOdent have been developed in search for a more sensitive and accurate caries detection tool[13]. Sensitivities and specificities of these tools show large variations according to a systematic review and most of the results were from in vitro studies on permanent teeth[13]. Primary teeth, however, have different characteristics to permanent teeth requiring new caries detection tools to be tested on primary teeth for validation[14]. In this study, AUROC values for detecting dentine level caries (R_0-2/R_3-5) were 0.750 - 0.794 using QLF parameters, which suggest fair clinical performance, whereas AUROC values for detecting enamel level caries (R₀/R_1-5) were lower. In a previous study that assessed screening performance of QLF technology in detecting proximal caries in permanent dentition[12], AUROC values were higher (0.860 - 0.902). However, direct comparison cannot be made as the device used in the study on permanent dentition was QLF-Digital Biluminator 2+ (QLF-D, Inspektor Research Systems BV, Amsterdam, the Netherlands), which is different to the pen-type QLF device used in this study, though they both utilize QLF technology. Also, cut-off value of △F to detect dentine level caries found in this study was -6.15, which is lower than -12.4, the cut-off value suggested for permanent teeth[12]. This difference may be explained by some characteristics of primary teeth, having thinner enamel and dentine layer with lower degree of mineralization than permanent teeth[3,4]. Further evaluation is necessary to determine whether these differences arise from the differences in characteristics of permanent and primary dentition or due to inherent differences of devices.

Compared to visual-tactile inspection, which is used conventionally in conjunction to bitewing radiography to detect proximal caries[8], QLF scores showed higher correlation to radiographic scores. Visual inspection, though it is the most commonly used method of caries detection with high specificity, it has limitations of low sensitivity and reproducibility[15]. Moreover, visual inspection has been reported to be influenced by discomfort reported by pediatric patients, increasing the false-positive results[16]. QLF scores for proximal caries suggested by Kim et al. simplified the QLF analysis process[12], making it possible to assess the lesion severity intuitively without the need for quantitative analysis. QLF scoring system takes both fluorescence loss and red autofluorescence into account, which can be analyzed on QLF images taken with second (QLF-D) and third generation (Qraypen C) QLF devices. Second and third generation QLF devices contain 405 nm peak wavelength of violet blue light LEDs with mounted high pass filter (> 480 nm) with a pink filter to emphasize the 630 - 640 nm band for enhanced red autofluorescence[11]. Red autofluorescence is known to be emitted by bacterial metabolites in carious lesions, such as porphyrin[17]. Previous studies have shown that red autofluorescence is increased in active carious lesions and is associated with caries progression[18,19]. However, red autofluorescence can be detected with other confounding factors, such as plaque and cracks. Hence with thorough cleaning and flossing of tooth surfaces before taking QLF images, the QLF device can be used to detect hidden proximal caries, which otherwise may have been overlooked by visual inspection only. In addition, visualization of bright red color of the hidden carious lesion may help in educating both the patients and caregivers and motivate the management.

Bitewing radiography is often used for diagnosis and as decision criteria for operative treatments in clinical settings[20], so QLF parameters and scores were compared with bitewing radiography to evaluate the clinical performance of the pen-type QLF device. However, radiographs underestimate the lesion depth and do not provide information about the presence of cavitation on the enamel nor the lesion activity[20-22], making it difficult to decide whether to remineralize the lesion or to undergo operative treatment. Temporary separations or operative interventions have been used in other clinical studies as reference standards[23,24], when validating a new diagnostic tool. However, these reference standards also have shortcomings, as temporary separation needs second visit by patients and operative interventions are invasive. Furthermore, in clinical setting, ionizing radiation limits frequent use of bitewing radiography and this method cannot be used for screening or monitoring purposes, requiring an adjunct diagnostic method. The AUROC value for QLF scoring showed fair results of 0.757 - 0.769 in detecting dentine level caries, which was higher than that of visual score (0.653). As QLF technology is non-invasive using only visible light, it can be used frequently without causing any harm to the patients. QLF images can be used as an adjunct tool for screening purposes prior to radiographic examinations and to monitor proximal caries during follow-up visits.

The QLF images used in this study were taken from the occlusal direction of primary molars. Previous in vitro studies suggested viewing from bucco-lingual direction gives more sensitive results in detecting proximal caries[25,26], as marginal ridges above proximal carious lesions mask the fluorescence detected by the QLF device[27]. Also, this viewing direction was different from the bucco-lingual viewing direction of bitewing radiography, which remains as a limitation of this study. Further studies are required to evaluate the relationship between the marginal ridges above the proximal carious lesions and detecting ability of the QLF device.

Early and accurate diagnosis of dental caries is important, especially in children with primary dentition, to spare patients from discomforts of advanced lesions and some operative procedures. The QLF technology can be used to detect and monitor approximal carious lesions that are difficult to detect with visual examinations only. In Korea, the pen-type QLF device was announced as a New Health Technology in 2018 by Ministry of Health and Welfare and its clinical use is covered by the National Health Insurance from 1st June, 2021. As the device show overall fair clinical screening performance in detecting proximal caries in primary molars, it can be used as decision criteria before prescribing bitewing radiography to minimize pediatric patients from exposure to ionizing radiation. Also, with enhanced visualization of red autofluorescence in QLF images, patient and caregiver education may be conducted effectively.

The pen-type QLF device exhibited fair clinical screening ability of detecting proximal caries in primary molars. QLF parameters showed better performance in detecting dentine level caries than enamel level caries and using QLF scores were superior in detecting proximal caries than using visual scores, when compared to bitewing radiography.