Bone and cartilage changes in rabbit mandibular condyles after 1 injection of botulinum toxin

Abstract

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.

Decreased mandibular cortical bone quality after botulinum toxin injections in masticatory muscles in female adults

“…According to animal studies using rodents, reduced masticatory muscle thickness resulting from BTX injections in the temporalis and/or masseter muscles could lead to changes in cortical and trabecular bone structures in masticatory load-bearing areas, the TMJ condyles and alveolar bones 17,21,23,24 . Other animal studies using rabbits showed similar results 16,18 . Most studies that investigated the relationship between BTX-induced masticatory muscle atrophy and mandibular bone loss were animal studies, and only a few human studies have performed a comprehensive analysis of bone quality and muscle thickness.…”

“…Decreased masticatory muscle thickness also seemed to have direct effects on bone density of the ramus and coronoid process of the mandible [16][17][18]23,24 . Most studies about the association between BTX induced masticatory muscle atrophy and mandibular bone loss have been animal studies and sparse well-designed human studies have been carried out due to difficulties in analyzing craniofacial bone qualities in humans.…”

“…Animal studies which attempted to reveal the interactions between BTX-induced hindlimb muscle paralysis and atrophy may provide information about the effects of muscle contraction and unloading on maintaining bone structures 19,22,25 . This phenomenon also could be applied to the craniofacial area where reduced masticatory force after BTX injection may cause deterioration of the cortical and trabecular bony integrity of the condylar and alveolar bones 6,[16][17][18]20,21,23,24 . Generally 0.2-1.0 U BTX injection in per masseter muscles in rodents 6,17,20,21,23,24 and 10 U BTX injection in per masseter muscles in rabbits 16,18 could elicit mandibular bone loss.…”

“…injection could produce bone loss 6,[16][17][18][19][20][21][22][23][24][25] . Interestingly, one animal study using rodents showed that bone loss induced by intramuscular BTX injection seemed to be not solely the result of reduced mechanical loading, but bone loss could be directly affected by atrophic muscle itself 25 .…”

Repeated injections of botulinum toxin into the masseter muscle induce bony changes in human adults: A longitudinal study

“…Accordingly, several studies have investigated the effect of BoNT-A injection on bony changes in adult animals 91011. Studies on adult rabbits have reported significant bony changes associated with muscle atrophy that lasted 12 weeks after the injection of BoNT-A 1011.…”