Does Masseter Paralysis Lead to Resorption of Alveolar Bone?

Botulinum neurotoxin type A (Botox) is a paralyzing agent of cholinergic junctions. Among its many clinical and cosmetic uses, Botox is used to reduce loading on the alveolar bone. Objectives: To determine whether mandibular alveolar bone is understrained and/or resorbed after Botox treatment using a rabbit model. Methods: Each subject, a female New Zealand White rabbit, was injected with either saline or Botox in one masseter. Alveolar bone volume and density was measured in 28 subjects for both the injected and non-injected sides using Micro CT scans. Using the three elements of rosette strain gauges bonded bilaterally on the flat surface of the alveolar bone of 11 subjects, in vivo alveolar strain was measured and principal strains were calculated. Results: Saline injection had no effect on total bone volume, and no statistically significant changes occurred between 4 and 12 weeks. Four weeks after injection of Botox, there was a reduction in total bone volume of the injected side versus the non-injected side (p=0.04). At 12 weeks the difference was no longer statistically significant (p = 0.088). Few strain data were available for 4-week Botox-injected animals, but the available data suggested that the orientation of principal strains is different from those of control animals and the strain magnitudes may have been higher. At 12 weeks, however, Botox animals showed lower tensile strain than saline animals (p<0.05), although strain angles were normalized. Conclusions: Total bone volume is decreased 4 weeks after masseter paralysis but is largely recovered by week 12. These findings explain why the mandible appeared to be overstrained at week 4 (because the less dense bone was more compliant), but it was understrained by week 12 (because the bone had recovered and the muscle was still slightly affected). Supported by NIH/NIDCR: T32 DE07132-27 and R01 DE018142.