Hyperdivergent skeletal class II malocclusion with anterior open bite is challenging because of the high occurrence of post-operative relapse [2]. Accordingly, this was an indication for BTX-A therapy. In this case, BTX-A injection into the anterior belly of the digastric muscle seemed to prevent post-operative open bite. The patient’s overbite was successfully maintained for 15 months post-operatively. The SNB angle and mandibular plane angle were also stable post-operatively (Fig. 2). Considering that BTX-A injection was simple and the procedure has a low rate of complications, BTX-A injection into the anterior belly of the digastric muscle may be considered an additional procedure for the prevention of post-operative anterior open bite [15].
As the mandible is a floating bone suspended by the peroral muscle group, the relative position of the mandible is determined by the balance among the groups [16]. Class II open-bite patients show a small volume of mouth closing muscles and well-developed suprahyoid muscle groups [17]. Ramus surgery for the counterclockwise rotation of the mandible increases the tension on the suprahyoid muscle groups [18]. This tension is considered a major etiologic factor for post-operative relapse [18, 19]. The relapse rate of class II open bite after orthognathic surgery varies considerably from 1.5 to 42.9% [20,21,22,23,24]. Actual amount of relapse may depend on the orthodontic treatment, fixation method, intermaxillary fixation period, osteotomy design, and additional therapy such as myotomy [3,4,5,6]. Some procedures such as the fixation method and the intermaxillary fixation period are designed to resist muscle power [4,5,6]. Posterior impaction of LeFort I osteotomy during surgery reduces the amount of mandibular counterclockwise rotation [25]. As relapse is associated with the correction amount, reduced counterclockwise rotation may be helpful for preventing post-operative relapse [25].
Procedures such as myotomy are designed to reduce muscle power [3]. The rationale for myotomy is similar to BTX-A injection. According to animal study, suprahyoid myotomy group shows less skeletal relapse compared to non myotomy group at 2 years post-operatively [26]. In this paper, pulling action of the suprahyoid musculature is a major risk causing factor in class II open-bite patients [26]. However, a multi-institutional study of 87 individuals did not prove the effectiveness of the suprahyoid myotomy in preventing skeletal relapse [27].
As a complication, anterior open bite has been frequently observed after bilateral mandibular angle fracture [28]. Bilateral mandibular angle fractures result in discontinuity between the mouth opening muscles and the mouth closing muscles [11]. The muscles responsible for opening the mouth are mainly attached to the mandibular anterior region and those for closing the mouth to the mandibular ramus [11]. Accordingly, the influence of the opening muscles is dominant in the mandibular anterior area [11]. BTX-A injection into the anterior belly of the digastric muscle could treat patients with anterior open bite after an open reduction in the bilateral mandibular angle fractures [11]. Radiofrequency therapy for the correction of post-traumatic open bite has similar mechanisms to BTX-A injection [28].
Based on these observations, BTX-A injection into the anterior belly of the digastric muscle was used for the positional stability of a class II open-bite patient who received orthognathic surgery. This study has limitations. First, this was a single case observation. For the stable occlusion after surgery, the importance of post-operative orthodontic treatment should not be ignored. The effect of a BTX-A single injection was difficult to quantify because of other contributing factors. Accordingly, a large-scale prospective study should be conducted for definite conclusions. In addition, there was no experimental support for this protocol. In order to embody the preciseness, the study should be designed and relevant data should be given to prove experimental conclusions. Second, the toxin dosage is very important to avoid any potential complications [29]. The optimal dosage should be tailored using follow-up research. However, BTX-A injection into the anterior belly of the digastric muscle is relatively safe and inexpensive compared to suprahyoid myotomy [15].
