Unilateral injection of botulinum toxin type A into the masseter muscle induces mandibular asymmetry in adolescent rats by suppressing the angular process growth.

[INTRODUCTION] Mandibular asymmetry has negative impacts on maxillofacial aesthetics and psychological well-being. This study investigated the effects of unilateral injection of botulinum toxin type A (BTX-A) into the masseter muscle on mandibular symmetry.

[METHODS] Forty Wistar rats (4-week-old) were divided into 4 groups (n = 10): control, group 1 (1U BTX-A), group 2 (3U BTX-A), and group 3 (1U BTX-A for 3 times). BTX-A was injected into the right masseter of treatment groups. Cone-beam computerized tomography scans were taken before the injection and then at 2 weeks, 4 weeks, and 6 weeks after injection. Histologic and immunohistochemical staining were done for the condylar cartilage. RNA sequencing and quantitative reverse transcription polymerase chain reaction were used to detect gene expression in the angular process.

[RESULTS] In Groups 2 and 3, the right angular process length and the ramus height were reduced 4 weeks after injection, resulting in the mandibular midline deviating to the right side; the right condylar cartilage had reduced thickness and decreased expression of RUNX2, SOX9, and COL II (P <0.05). Two hundred sixty-one genes were differentially expressed (256 downregulated) in the angular process at 3 days post-BTX-A injection, and the calcium signaling pathway was unveiled through the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. Furthermore, TRPC1, Wnt5a, CaMKII, Ctnnb1, and RUNX2 expression were significantly downregulated at 1 and 3 days postinjection.

[CONCLUSIONS] 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.