Nail biting (NB) is a repetitive and uncontrolled parafunctional activity that can affect oral health by altering tooth shape and intraoral position. Nowadays, there is not enough scientific evidence about the impact of NB on teeth; therefore, this study aimed to evaluate the effects of NB on the length, width and inclination of upper central incisors. This retrospective study involved 76 patients, 40 males and 36 females, with a mean age of 10.6 ± 0.3 years. Digital scans of the maxillary arch of each patient were recorded. Next, the length, width and inclination of upper central incisors used and not used for NB were measured. Finally, data were analysed with a paired t-test. Statistical analysis showed statistically significant differences in the length and inclination of upper central incisors used for NB compared with those of upper central incisors not used for NB, while the width did not show a significant difference. There were relevant changes in upper central incisors subjected to NB, demonstrating that NB impairs the shape, morphology and inclination of teeth. Therefore, because of the potentially negative consequences of NB, it is recommended that NB not be underestimated.

Background/Objectives: Orthodontic treatment with a Rapid Maxillary Expander (RME) induces stress on the mid-palatal suture and involves the surrounding craniofacial sutures, leading to significant variations in bone turnover markers (BTMs). The aim of this study was to evaluate whether monitoring biomarkers of bone resorption and deposition in saliva could provide indications for assessing RME treatment effectiveness in pediatric patients. Materials and Methods: The study was conducted at the Dental Clinic of Foggia in collaboration with the Clinical Pathology Unit of the Policlinico-Bari from January 2023 to September 2025. Salivary samples were collected using cotton swabs (SALIVETTE^®, SARSTEDT, Nümbrecht-Elsenroth, Germany) from 47 patients (aged 9–13 years) presenting with Class I dental and skeletal relationships and transverse maxillary deficiency. Samples were collected at five time points: before RME application (t0), and 15 (t1), 30 (t2), 45 (t3), and 60 (t4) days after the start of expansion. For pairwise comparisons of concentrations at different time points the Wilcoxon signed-rank test was used. Analysis of Variance (ANOVA) and Ordinary Least Squares (OLS) multiple regression were used to analyze the relationships between different biomarker concentrations over time. Spearman correlation was used to assess relationships between BTM concentrations at the different time points. Results: The Wilcoxon test showed that the medians of the differences for P1NP measurements were statistically significant at all time points, except between t0 and t1. Differences in PTHrP concentrations were significant, except for the comparison of t2 and t3 relative to t4. No statistically significant differences were observed for TRAcP across any measurement times. ANOVA yielded a high F-value for P1NP (F = 3.6128; p = 0.0128), indicating significant variation, whereas the F-ratios for PTHrP (F = 1.3329; p = 0.2736) and TRAcP (F = 1.3915; p = 0.2534) were close to 1, suggesting non-significant variability. Spearman correlation indicated that P1NP showed the strongest relationships with temporal variables. Discussion and Conclusions: Rapid maxillary expansion results in a significant increase in P1NP levels, reflecting collagen deposition and the onset of bone formation. This preliminary study on saliva highlights how biochemical changes can support clinicians in monitoring and validating the effectiveness of treatment.