title: Muscle weakness after repeated injection of botulinum toxin type A evaluated according to bite force measurement of human masseter muscle.
abstract: BACKGROUND
  Botulinum toxin type A (BTX-A) has been applied successfully to treat masseteric hypertrophy, but it can cause muscle weakness.
  
  
  OBJECTIVE
  To measure the change in maximum bite force (MBF) after BTX-A injection into the human masseter muscle and to evaluate the influence of a booster (repeated) injection.
  
  
  METHODS
  Thirty volunteers completed 18-week follow-up, and MBF was measured. At 18 weeks after the first injection, a booster injection was given to 14 patients, and they were followed up until 18 weeks from the booster injection.
  
  
  RESULTS
  Mean MBF was approximately 20% lower at 2 weeks than before the injection, and it recovered gradually after 4 weeks to return to the preinjection level at 12 weeks. MBF differed significantly between before the injection and at 2, 4, and 8 weeks after the injection (p<.05). In the booster injection group (n=14), MBF was markedly lower at 6 weeks (p<.05), and it recovered gradually in 12 weeks.
  
  
  CONCLUSIONS
  MBF was significantly lower after booster injection of BTX-A into the human masseter muscle, but it gradually recovered in a predictable pattern, and the degree of discomfort experienced by the subjects had little effect on normal mastication.
authors.affiliation: Temporomandibular Joint and Orofacial Pain Clinic, Department of Oral Medicine, School of Dentistry, Yonsei University, Seoul, Korea.
authors.forename: Ki Seo
authors.initials: KS
authors.lastname: Kim
authors.forename: Young Sub
authors.initials: YS
authors.lastname: Byun
authors.forename: Yong Jeong
authors.initials: YJ
authors.lastname: Kim
authors.forename: Seong Taek
authors.initials: ST
authors.lastname: Kim
mesh_terms: D000328:Adult; D001732:Bite Force; D019274:Botulinum Toxins, Type A / Q000008:administration & dosage / Q000494:pharmacology*; D005260:Female; D006801:Humans; D006984:Hypertrophy / Q000188:drug therapy; D007273:Injections, Intramuscular; D008297:Male; D008406:Masseter Muscle / Q000187:drug effects* / Q000473:pathology; D018908:Muscle Weakness / Q000139:chemically induced*; D009465:Neuromuscular Agents / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D009465:Neuromuscular Agents; D019274:Botulinum Toxins, Type A