The effects on the mandibular condyle of Botox injection into the masseter are not transient

Objectives To evaluate whether the effects on the mandibular condylar cartilage (MCC) and subchondral bone are transient of botulinum neurotoxin (Botox) injection into the masseter muscle. Methods Botox (0.3 U) was injected into the right masseter of 6-week-old female mice (C57BL/6; n = 16). In addition, 16 mice were used as control and received no injections. Experimental and matching control mice were killed 4 or 8 weeks after the single Botox injection. Mandibles and mandibular condyles were analyzed by means of microscopic computed tomography (microCT) and histology. Sagittal sections of condyles were stained for tartrate-resistant acid phosphatase (TRAP), toluidine blue, 5-ethynyl-2′-deoxyuridine (EdU), and terminal deoxynucleotide transferase–mediated dUTP nick-end labeling. Results Bone volume fraction was significantly decreased on the subchondral bone of the Botox-injected side, compared with the control side and control mice, 4 and 8 weeks after injection. Furthermore, histologic analysis revealed decrease in mineralization, cartilage thickness, TRAP activity, and EdU-positive cells in the MCC of the Botox-injected side 4 and 8 weeks after injection. Conclusions The effects on the MCC and subchondral bone of Botox injection into the masseter muscle persisted for 8 weeks after injection and were not considered to be transient.