Growth effects of botulinum toxin type A injected unilaterally into the masseter muscle of developing rats

To evaluate the effects of botulinum toxin type A (BTX-A) on mandible skeletal development by inducing muscle hypofunction.

Four-week-old Sprague-Dawley rats (n=60) were divided into three groups: Group 1 animals served as controls and were injected with saline; Group 2 animals were injected unilaterally with BTX-A (the contralateral side was injected with saline); and Group 3 animals were injected bilaterally with BTX-A. In Group 2, the saline-injected side was designated the control side (Group 2-1), whereas the BTX-A-injected side was designated the experimental side (Group 2-2). After four weeks, the animals were sacrificed, dry skulls were prepared, and mandibles were measured.

In the unilateral group, the experimental side (Group 2-2) had reduced dimensions for all mandible measurements compared with the control side (Group 2-1), suggesting a local effect of BTX-A on mandible growth, likely due to muscle reduction.

Localized BTX-A injection induced a change in craniofacial growth, and the skeletal effect was unilateral despite both sides of the mandible functioning as one unit.