Change of distribution and timing of bite force after botulinum toxin type A injection evaluated by a computerized occlusion analysis system

Abstract

Purpose: The aim of this study was to determine the force distribution and pattern of mastication after injection of botulinum toxin type A (BTX-A) into both masseter muscles. The hypothesis to be tested was that the difference between right and left balance of occlusal force diminishes over time following BTX-A injection.

Materials and methods: Fifteen patients were submitted to BTX-A injection therapy for subjective masseter hypertrophy. A total of 25 U of BTX-A (50 U in total) was injected into two points located 1 cm apart at the center of the lower one-third of both masseter muscles. All patients were examined using the T-Scan occlusion analysis system before and 4, 8, 12, and 24 weeks after BTX-A injection.

Results: A significant change in force balance was found between the right and left sides over time and the difference between the two sides decreased with the time post-injection, reaching a minimum at 12 weeks. Comparison of the force balance between the anterior and posterior occlusions revealed no significant difference at any of the time points. The occlusion and disclusion times (right and left sides) did not differ significantly with time since BTX-A injection.

Conclusion: A decline in the difference in the clenching force between the left and right sides was found with increasing time up to 12 weeks following BTX-A injection.

Keywords: Botulinum toxin type A; bite force; dental occlusion; masseter muscle; mastication; time factors.

Fig. 1

The red boxed area shows the botulinum toxin type A injection site.

Fig. 2

Balance of clenching force in a T-Scan movie (A: divided into two parts: left and right sides, B: divided into four parts labeled in clockwise direction: anterior left, anterior right, posterior right, posterior left).

Fig. 3

(A) Occlusion time. (B) Disclusion time. (A) A1 is the point the teeth start to occlude, B1 is the point the dentition is fully occluded, and the difference of A1 and B1 is occlusion time, marked as OT. (B) C1 is the point lateral movement starts, D1 is the point the dentition is fully separated and the difference is disclusion time, marked as DT. OT, occlusion time; DT, disclusion time.

Fig. 4

Force distribution on right and left sides before BTX-A injection (A) and 12 weeks thereafter (B). (A) In the left figure, before botulinum toxin injection the distribution of the left and right force was uneven, 72.1% and 27.9%. 12 weeks post injection, (B) the right figure, the distribution evened as left 58.1% and right 27.9%. BTX-A, botulinum toxin type A.