BoNT/A Masseter Muscle Injections Reduce Mandibular Condylar Bone Quality

Objectives: We investigated the effect of Botulinum neurotoxin type A (BoNT/A) masseter muscle injections upon mandibular condylar bone quality. This is important as BoNT/A injections are becoming increasingly offered by the dental profession for treatment of symptoms related to temporomandibular disorder, as well as for the cosmetic alteration of facial and periorbital muscles.
Methods: We utilized micro-computed tomography bone quality data in two experiments with a rabbit (Oryctolagus cuniculus) model. In the first experiment, individuals were unilaterally injected with either BoNT/A (n=13) or 0.9% saline (n=5) three times at 12-week intervals. In the second experiment, individuals received either a single BoNT/A (n=8) or 0.9% saline (n=8) injection bilaterally, and subsets of each group were sacrificed at either 4- or 12-weeks. 1mm (anterior-posterior) condylar cross-sections were analyzed for bone volume (BV), tissue volume (TV), and bone density (BD). Samples were considered as a whole, and in three subsections: subchondral, trabecular, and cortical.

Results: In the unilateral study, treated condyles had significantly reduced BV (29.3%), TV (21.2%), and BD (11.4%) relative to their contralateral side. For the bilateral study, BoNT/A treated individuals had a reduction of 8.7% in BD at 4-weeks. BD marginally recovered at 12-weeks (5.7% reduction). For both experiments, the trabecular region had the most dramatic reduction of BD: 37% (unilateral study); 28% at 4-weeks, and 6.4% at 12-weeks (bilateral study).
Conclusions: Overall, we observed significant alterations of condylar bone quality following BoNT/A masseter muscle injections. Consequences of reduced bone quality include osteopenia, susceptibility to fracture, and perhaps a more complicated temporomandibular disorder. Consenting patients should be made aware of possible musculoskeletal risks associated with BoNT/A injections as these treatments become more common in dental clinical practice.