Publications

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.

Keywords: posterior crossbite, unilateral posterior crossbite, masticatory muscles, surface electromyography, myotonometry, muscle asymmetry, orthodontics

Growing patients with unilateral posterior crossbite and functional mandibular deviation exhibited specific, condition-dependent alterations in masticatory muscle activity and viscoelastic properties, rather than a consistent or generalized neuromuscular asymmetry.

Myotonometric findings revealed limited differences between groups, suggesting that muscle mechanical properties are only moderately affected and may reflect localized adaptations. Electromyographic results demonstrated that muscle asymmetries were not evident during maximum voluntary contraction in intercuspation but emerged under specific functional conditions, such as rest, cotton roll clenching, and mastication. In particular, increased activation of the crossbite-side masseter under loading conditions suggests a functional adaptation to occlusal imbalance.

These findings indicate that unilateral posterior crossbite in growing patients is associated with subtle neuromuscular adaptations rather than marked dysfunction. From a clinical perspective, this supports the concept that functional alterations may precede or contribute to structural changes during growth, although their magnitude appears limited.

Further longitudinal and interventional studies are needed to determine whether these neuromuscular adaptations persist over time and to clarify their clinical relevance in relation to craniofacial development and the timing of orthodontic treatment.