Repeated unilateral injections of botulinum toxin in masticatory muscles in adult rats do not amplify condylar and alveolar bone loss nor modify the volume of the hypertrophic bone proliferation at enthesis

Abstract

Objectives: Botulinum toxin (BTX) induces muscle paralysis. It is used in human in masticatory muscles injections performed often repeatedly. A single BTX injection in masticatory muscles in animal induces mandibular bone loss (alveolar and condylar) with a muscle enthesis hypertrophic metaplasia. Our aim was to evaluate mandibular bone changes after unilateral repeated injections of BTX in temporal and masseter muscles in adult rats. Materials and Methods: Mature male rats were randomized into 3 groups: one, two or three injections. Each injection was performed 4 weeks after the prior injection. Each rat received injections in right masseter and temporalis muscles. The left side was the control side. Microcomputed tomography was used to perform 2D and 3D analyses. Results: Bone loss was evidenced on the right sides of alveolar and condylar bone. Alveolar bone volume increased in both control left side and injected right side whereas condylar bone volume remained constant in all groups, for both left and right sides. Enthesis bone hypertrophic metaplasias were evidenced on the BTX injected sides without any modification with the number of injections. Conclusions: BTX repeated injections in masticatory muscles lead to major mandibular condylar and alveolar bone loss that does not worsen. They lead to the occurrence of an enthesis bone proliferation that is not dependent on the number of injections. Clinical relevance: These results are an argument for the safety of BTX injections in masticatory muscles in human.