Dr. Susan Herring and her colleagues presented their own research findings at the International Association for Dental Research (IADR) meeting in March of this year. Dr. Herring prepared the following lay summary for our readers:
Mandibular loading and bone quality following injection of botulinum neurotoxin type A in the masseter muscle
Rafferty K., Liu Z-J., Ye W., Slamati A. ,Gross T., Herring S., Oral Presentation at the 89th General Session & Exhibition of the IADR, San Diego, CA, March 19, 2011.
Botox works by inactivating the nerve endings that cause muscles to contract, thus paralyzing the muscle. The paralysis usually lasts for a few months, although the muscles may remain visibly small for longer times. Some providers feel that treating the jaw muscles of TMD patients with Botox could be helpful even if the muscles are not in spasm. For example, it has been argued that the jaw muscles place loads on the TMJ, and if these loads are temporarily removed, the joint might have a chance to recover.
One possible worry about unloading joints, however, is that bone strength might be lost from the unloaded area, similar to astronauts losing bone strength while in space. Because bone is rebuilt slowly, the jaw joint might actually lose needed structure.
To test this idea an animal study was carried out. In 40 adult female rabbits, one masseter muscle was injected with either saline or Botox, with a dose adjusted to be comparable to a human dose. The muscles and mandible were examined 4 weeks later, when the masseter would still have been affected by the Botox, and 12 weeks later, when the muscle was expected to have recovered.
The rabbits did not experience problems in chewing and seemed comfortable. As expected, the Botox-injected masseter muscles were atrophied at 4 weeks. They were larger at 12 weeks but still statistically smaller than the other side. From this and other evidence, the experiment was successful in unloading the jaw. However, this unloading did have adverse effects on bone content, especially for the mandibular condyle (the mandibular part of the TMJ) on the Botox-injected side. On average, 40% of bone area was lost from the head of the mandibular condyle 4 weeks after injection, and after 12 weeks the bone of the condylar head was still depleted by 22%. The bone loss occurred in the porous bone in the internal region of the condyle, which has limited capacity to regenerate once it is lost.
In conclusion, Botox in the masseter caused an osteoporotic condition in the TMJ of rabbits, raising some concern that this treatment might not be healthy for the joint in the long term.
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