Introduction Temporary paralysis of the masseter muscle using botulinum toxin is a common treatment for temporomandibular disorders, bruxism, and muscle hypertrophy. Loss of masseter force is associated with decreased mandibular mineral density. Our objectives were (1) to establish whether bone loss at the mandibular condyle is regionally specific, and (2) to ascertain whether the treatment affects the condylar cartilage. Methods Young adult female rabbits received a unilateral masseter injection of botulinum neurotoxin serotype A (BoNT/A, n=31), saline (n=19) or no injection (n=3) and were also injected with bromodeoxyuridine (BrdU), a replication marker. Termination occurred 4 or 12 weeks following treatment. Condyles were processed by paraffin histology. Cortical thickness, cartilage thickness and trabecular bone areal density were measured, and replicating cells were counted after BrdU reaction. Results BoNT/A rabbits exhibited a high frequency of defects in the condylar bone surface, occurring equally on injected and uninjected sides. Bone loss was seen only on the side of the BoNT/A injection. Cortical as well as trabecular bone was severely affected. The midcondylar region lost the most bone. Recovery at 12 weeks was insignificant. Condylar cartilage thickness showed no treatment effect but did increase with time. Numbers of proliferating cells were similar in treatment groups, but BoNT/A animals showed more side asymmetry in association with the condylar defects. Conclusion Bone loss may be a risk factor for the use of botulinum toxin in jaw muscles.

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