## 2 Materials and Methods
### Selection of Lesions  
The vestibular surfaces of the teeth of 172 subjects (88 females and 84 males aged 24 to 70 years old), arrived from January 2017 to July 2019 at the conservative outpatient department of the University of Campania \"Luigi Vanvitelli,\" were examined by the first operator using a probe tip and a x4 magnifying lens. The tip of the probe was held perpendicularly to the tooth surface and inserted to the bottom of the gingival sulcus crossing the cement-enamel junction (CEJ). Any irregularity in the cervical enamel and at the CEJ of the teeth was considered as NCCLs. All subjects signed an Informed Consent form after verbal and written information on the study was provided.  
The procedures of this study were in accordance with the institutional and national ethical standards on human experimentation and the Helsinki Declaration of 1975, as revised in 2000, and they were approved by the Ethics Committee of AORN of Ospedali dei Colli with N. 1374.  
The first operator selected 87 adult patients (47 females and 40 males aged 24 to 70 years old) who complained of dentin hypersensitivity symptoms related to NCCLs.  
A total of 372 NCCLs were screened on the vestibular surface of the tooth (Figure 1). The NCCLs excluded from the analysis presented the following: (1) an association of the NCCLs with periodontal pockets or gingival recession; (2) an additional presence of carious lesions on the affected tooth or a pre-existing restoration; (3) evidence of cracks; (4) previous endodontic treatment; (5) primary teeth and subgingival NCCLs.  
The second researcher was employed to classify the NCCLs to be enrolled in this analysis on the basis of the pain sensation severity elicited by each NCCL using ETs: baseline (t0). A visual analog scale (VAS) was used to assess the severity of pain, enabling patients to record their perceived level of pain from 0 (the best possible pain status) to 10 (the worst possible pain status).  
Only NNCLs associated with DH scoring 6-9 on the VAS after an ET were included in this study.  
Three study groups were established by using only one lesion per patient in 3 different quadrants of the oral cavity to obtain 3 lesions per patient to be treated with individual different therapeutic protocols. Blinded to the protocol selection required for each tooth treated, the second investigator was also employed to examine the intensity of pain sensation elicited by each NCCL by an ET according to the following timeline: at treatment completion (t1), at 15 days (t2), and at 3 months (t3).  
Figure 1: A non-carious cervical lesion (NCCL).  
Allocation to the specific study group for each NCCL was achieved by the third operator using a randomization software, i.e., random allocation software [16].  
Moreover, all patients with NCCLs associated with DH underwent anamnesis with the intention of discovering and eliminating the causal factors. Instructions were provided on the adequate management of oral hygiene to carry out at home and proper dietary requirements.  
The selected teeth with NCCLs were flossed and polished before the administration of the treatment, and cotton rolls were used for isolation.