Abstract
Mandibular asymmetry has negative impacts on maxillofacial aesthetics and psychological wellbeing. The present study was aimed to investigate the effects of unilateral injection of botulinum toxin type A (BTX-A) into the masseter muscle on mandibular symmetry.
Forty Wistar rats (4-week-old) were divided into 4 groups (n=10) with BTX-A injected into the right masseter when suited: Control, Group 1 (1U BTX-A), Group 2 (3U BTX-A) and Group 3 (1U BTX-A for 3 times). Cone-beam computerized tomography (CBCT) scans were taken before the injection (T0) and at 2-week (T1), 4-week (T2), and 6-week (T3) after the injection. Histological and immunohistochemical staining were done for the condylar cartilage. RNA sequencing and RT-qPCR were used to detect gene expression in the angular process.
In Groups 2 and 3, the right “angular process length” and the “ramus height” were reduced since T2, resulting in the mandibular midline deviated to the right side; the right condylar cartilage had reduced thickness and decreased expression of RUNX2, SOX9 and COL II (P<0.05 for all). Totally 261 genes were differentially expressed (256 down-regulated) in the angular process at 3 days post BTX-A injection, and the calcium signaling pathway was unveiled through the KEGG pathway enrichment analysis. Furthermore, the expression of TRPC1, Wnt5a, CaMKII, Ctnnb1 and RUNX2 was significantly down-regulated at 1 day and 3 days post the injection.
Unilateral injection of BTX-A into the masseter muscle in adolescent rats induces mandibular asymmetry by suppressing the angular process growth on the injected side.
•Unilateral BTX-A injection in the masseter muscle in adolescent rats results in mandibular asymmetry with the mandibular midline deviated to the injection side.•The angular process growth is suppressed on the injection side, partially by restraint of the calcium signaling pathway.•These findings are coincident with the assumption that unilateral mastication in adolescents might lead to mandibular asymmetry.•Unilateral BTX-A injection in the masseter muscle might have the potential for improving mandibular asymmetry in adolescents.