The widespread use of portable electronic devices has increasingly promoted the prolonged maintenance of non-physiological postures, particularly anterior and downward head flexion. Therefore, this study aimed to analyze the condylar and incisor relationship displacement induced by this improper posture. A total of 20 adult subjects (9 F, 11 M; mean age 27 ± 5) were recruited at the Department of Dentistry, Vita-Salute San Raffaele University, Milan, Italy. Mandibular kinematics was recorded using JMA-Optic AG (Zebris Medical GmbH, Isny, Germany). The protocol adopted consisted of three phases: (1) Habitual occlusion with light clenching, (2) Neuromuscular rest position (RP) verified by surface electromyography (sEMG), (3) Anterior head flexion (40–60°) (HF), simulating the posture typically observed during portable digital device use. Millimetric measurements of condylar displacement from RP to HF and incisal plane changes were collected. Data were analyzed descriptively with Microsoft Excel, and inferentially with StatPlus Pro (AnalystSoft, StatPlus: mac Pro, version 8). The right condyle exhibited a mean displacement of 1.9 mm in the downward direction (p < 0.001), while the left condyle showed a downward displacement of 1.5 mm (p < 0.001). No significant difference was observed between the two sides. At the dental level, the lower incisor revealed a mean shift of 1.0 mm superiorly (p < 0.001) and 0.7 mm anteriorly (p < 0.001). The HF determines a significant condylar and incisal plane displacement, and may predispose individuals to TMJ disorders, supporting the hypothesis of an emerging cranio-cervico-mandibular condition linked to prolonged use of high-tech display terminals, here proposed as ED-TMD (Electronic Device-Induced Temporomandibular Disorder).

Surface electromyography (sEMG) of the masticatory muscles is widely employed in dentistry and gnathology for the functional assessment of the neuromuscular system [1]. The recorded signal reflects the motor unit activity, and it is influenced by multiple biological factors, including neuromuscular recruitment, functional muscle length, baseline tone, and integration with the cranio-cervical system [2]. Alongside physiological biological variability, however, a methodological component may substantially affect data quality and interpretability. Normalization through clenching on posterior cotton rolls represents the most adopted calibration approach in dental sEMG [3]. The aim is not to obtain an absolute value, but to ensure signal comparability across sessions and experimental conditions. Within this framework, calibration repeatability and signal super-imposability represent key indicators of methodological reliability [4]. This presentation proposes methodological considerations on calibration standardization, with particular emphasis on the temporal stability of the reference condition. A structured operational protocol is described, including electrode placement according to shared guidelines, control of patient posture (cranio-cervical alignment, gaze position, trunk stability), precise definition of the calibration site, and systematic documentation of the recording setup. Experimental examples demonstrate how signal superimposability seems dependent on calibration reference consistency, whereas undocumented variations in the reference calibration condition may compromise inter-session comparability, even with identical electrode placement. The ultimate objective is to offer a methodological contribution aimed at ensuring repeatable calibration over time by explicitly integrating the standardization of the calibration into the recording protocol, also considering postural characteristics of the subject [5]. Measurement quality in sEMG fundamentally relies on signal reproducibility under identical operating conditions, and on signal interpretation. Within this framework, the sEMG signal should not be interpreted as an absolute value, but rather as the outcome of the interaction between biological variability and methodological factors, the latter being controllable through rigorous protocol standardization. Although biological variability is inherently unavoidable, methodological variability remains under operator control and must be minimized. Consequently, minimizing methodological variability becomes essential for ensuring reliability, inter-session comparability, and robustness of clinical interpretation. In sEMG, the critical issue is not the absolute magnitude of the signal, but the ability to reproduce it consistently over time under stable and standardized conditions.

The masticatory cycle is a coordinated sequence of mandibular movements closely associated with masticatory muscle activity. However, the chewing pattern may be altered in temporomandibular disorders (TMDs). The aim of this study was to evaluate the masticatory cycle in patients with TMD through an integrated electromyographic and kinematic analysis, comparing the results with the correlations previously reported in the literature for healthy subjects. In this cross-sectional study, twenty-four adults diagnosed with TMD (mean age: 26 ± 6.7 years; 16 females and 8 males) were recruited at the Department of Dentistry at San Raffaele Hospital, Milan. The jaw tracking system Modjaw^® (version 3.6.4; MODJAW SAS, Villeurbanne, France) made it possible to record three-dimensional mandibular movements, including opening and closing, protrusion, lateral movements and the masticatory cycle. As for surface electromyography, the Myowise^® device (software version 1.4; Cometa S.r.l., Bareggio, Italy) was used to assess the electrical activity of the masticatory muscles during chewing. Subsequently, descriptive statistics were performed, and possible correlations between selected kinematic and electromyographic parameters were investigated. No statistically significant correlations were identified, except for a moderate negative correlation between right masticatory cycle height and right masseter muscle impact. Mandibular kinematics showed limited associations with electromyographic parameters in patients with temporomandibular disorders, differing from the correlations previously reported in healthy subjects.

Background: The widespread use of portable electronic devices may involve prolonged exposure to altered cranio-cervical postures. This study investigated whether participants classified as HT+ and HT− according to a predefined operational classification differed in masticatory muscles recruitment. Methods: This cross-sectional study included 50 healthy adults (30 F, 20 M; mean age, 27 ± 5 years) classified into two operational groups. The HT+ group (n = 25) was defined by the simultaneous presence of portable electronic device use >8 h/day and habitual marked anterior head flexion, whereas the HT− operational group (n = 25) included participants who did not simultaneously meet both criteria. Participants underwent surface electromyography of the masseter and anterior temporalis muscles during maximum voluntary clenching (MVC) in the neutral head posture and during anterior head flexion (approximately target range 40–60°) using the MyoWise device (MyoWise, Via Gran Sasso 18, 20008 Bareggio, Milan, Italy). Percentage Overlapping Coefficients, Asymmetry Index, Activity Index, Torque Index, and Impact were assessed. Between-group comparisons, within-group posture-related changes, and between-group comparisons of change scores (Δ) were performed using parametric or non-parametric tests according to data distribution. The between-group comparison of the Activity Index in the reference posture was predefined as the primary analysis, whereas secondary analyses were adjusted for multiple comparisons using the Benjamini–Hochberg false discovery rate (FDR) procedure. Statistical significance was set at p < 0.05 for the primary analysis and at FDR-adjusted q < 0.05 for secondary analyses. Results: The Activity Index was significantly more negative in HT+ than in HT− participants in both the reference posture (p < 0.001) and during anterior head flexion (p = 0.003; q = 0.032). The Activity Index decreased significantly from the neutral head posture to anterior head flexion in both groups (HT+: q = 0.025; HT−: q = 0.002), whereas the magnitude of this change did not differ significantly between groups (p = 0.101; q = 0.607). No other sEMG outcome remained significant after FDR correction. Conclusions: The HT+ operational group was associated with a distinct temporalis–masseter recruitment pattern in asymptomatic adults. The Activity Index emerged as the sEMG parameter most strongly associated with operational HT classification, indicating that the observed between-group differences primarily involved the relative recruitment balance between the jaw-elevator muscles.

Background: Digital mandibular kinematic analysis complements clinical examination by quantifying movement amplitude, symmetry, coordination, and trajectories. However, device-specific acquisition and processing characteristics may influence the measurements obtained with different jaw-tracking systems. Therefore, this study aimed to assess systematic differences in mandibular kinematic measurements and inter-system agreement among three commercially available digital jaw-tracking systems. Methods: Twenty healthy adults (12 M, 8 F; mean age, 27 ± 5 years) underwent assessment with Zebris JMA-Optic (Amann Girrbach AG, Koblach, Austria), Modjaw (Modjaw SAS, Villeurbanne, France), and Itaka (Itaka Way Med Srl, Marcon, Italy). Maximum mouth opening, protrusion, and bilateral lateral excursions were recorded in at least three valid trials, with the maximum excursion used for analysis. Differences among systems were evaluated using repeated-measures ANOVA with Bonferroni-adjusted post hoc comparisons. Overall inter-system agreement was assessed using the absolute-agreement intraclass correlation coefficient (ICC), while pairwise agreement was further characterized using Bland–Altman analysis. Proportional bias was assessed by linear regression of pairwise differences against their corresponding means. Results: Maximum mouth opening differed significantly among systems (p = 0.047; partial η² = 0.148). Modjaw recorded higher values than Itaka (mean difference, 2.49 mm; adjusted p = 0.038; Cohen’s dz = 0.62). No significant differences were found for right lateral excursion (p = 0.161), left lateral excursion (p = 0.293), or protrusion (p = 0.873). Agreement was moderate for maximum mouth opening (ICC = 0.601), right lateral excursion (ICC = 0.747), and protrusion (ICC = 0.718), but poor for left lateral excursion (ICC = 0.443). Bland–Altman analyses showed movement- and system-pair-specific variability in measurement differences, with no evidence of proportional bias. Conclusions: The three digital jaw-tracking systems showed movement-dependent differences in mandibular kinematic measurements and varying levels of inter-system agreement. These findings indicate that measurements obtained with different systems should be interpreted in the context of the specific movement assessed and the characteristics of the corresponding acquisition and processing workflows. Further studies are needed to determine the clinical relevance of the observed inter-system variability.