Mandibular loading and bone quality following masseter Botox® injection

Objective: To evaluate functional loading and bone quality of the mandible following injection of botulinum neurotoxin type A (BoNT/A) in the masseter. Methods: Forty adult female New Zealand white rabbits received injection of either BoNT/A or saline into one masseter muscle. Animals were terminated at 4 and 12 weeks. In a terminal procedure, gages were applied to the condylar necks and the alveolar processes, and strain was recorded during awake feeding. After termination, the muscles of mastication were weighed and bone specimens were scanned with microCT. Results: Saline controls showed no side differences. At 4 weeks the BoNT/A-injected masseter was 18% and the medial pterygoid 10% smaller than the contralateral side (p≤0.001). At 12 weeks, the injected side masseter and medial pterygoid were 7% and 4% smaller, indicating partial recovery. Compressive strain at the condylar neck was re-oriented (p=0.005) and tended to be low on the injected side at 4 weeks but not at 12 weeks. There were no side differences in alveolar strain, but BoNT/A animals in general had lower strain than saline animals (p=0.14-0.006). Bone density of the injected side condyle was 9% lower than the non-injection side at 4 weeks (p<0.0005), but only 2% lower at 12 weeks (NS). Alveolar bone was also affected (5% less at 4 weeks, p=0.06) but did not recover (4% less at 12 weeks, p=0.01). Conclusion: Loss of masseter function following BoNT/A injection was not compensated by the medial pterygoid, which also lost mass. Muscle recovery was incomplete at 12 weeks. Low strains on the alveolar bone and injected side condyle of BoNT/A animals reflect the decrease of muscle force. Loss of bone was likely due to underloading. The faster recovery of bone density in the condyle may reflect the capacity of this tissue to generate new bone. Supported by NIDCR DE018142.