Masticatory Muscle Function in Growing Patients with Unilateral Posterior Crossbite: A Case-Control Study Combining...

Background: Unilateral posterior crossbite (UPXB) is a common malocclusion in growing patients, often associated with functional mandibular deviation and asymmetric masticatory function. However, its relationship with muscle activity remains unclear. This study aimed to evaluate masticatory muscle activity and viscoelastic properties in growing patients with UPXB compared to subjects with normal occlusion. Methods: This case-control study included 140 growing patients (60 controls and 80 with UPXB: 36 right-side and 44 left-side). Surface electromyography (sEMG) was used to assess the activity of the superficial masseter and anterior temporalis muscles during standardized static and dynamic tasks. Muscle viscoelastic properties were evaluated using myotonometry. To minimize potential bias related to craniofacial morphology, only mesofacial subjects (Ricketts' VERT -0.5 to +0.5) were included. Statistical significance was set at p < 0.05. Results: No differences were found in sex distribution, whereas the control group was slightly older than the UPXB group. Myotonometric analysis showed limited between-group differences, with isolated findings including higher stiffness of the non-crossbite anterior temporalis in left UPXB patients (p = 0.014), and minor differences in stiffness and relaxation of the left masseter between groups (p = 0.049; p = 0.045). Electromyographic results showed no significant differences during maximum voluntary contraction in intercuspation. In controls, higher activity was found in the left anterior temporalis during contraction on cotton rolls (p = 0.039) and in the right masseter at rest (p = 0.033). In left UPXB patients, the non-crossbite temporalis showed higher activity (p = 0.023). Increased activity of the crossbite-side left masseter was found in patients with UPXB during contraction and mastication (p = 0.012; p = 0.002). During mastication, both groups exhibited higher activity on the chewing side. Conclusions: UPXB is associated with specific, condition-dependent alterations in masticatory muscle activity and viscoelastic properties rather than generalized neuromuscular asymmetry. These findings suggest the presence of functional adaptations in growing patients, particularly under certain loading conditions, and highlight the importance of early diagnosis to better understand its potential impact on craniofacial development.