Bimaxillary and dentoalveolar protrusion is characterized by protrusive lip and teeth, inability of lip closure, gummy smile, abnormal mentalis activity, etc1,2. It can cause diastema, periodontal problems, and various skeletal dishar monies later in adults3. Conventionally, bimaxillary protrusion has been treated by orthodontic treatment after the extraction of the first or second premolars. Note, however, that this method has resulted in complications such as root resorption, linguoversion, and/or incomplete retraction of anterior teeth, bony dehiscence or fenestration, and increased exposure of gummy tissue3; thus requiring surgery for the treatment of bimaxillary protrusion to get more esthetic and functionally stable results4.

The anterior segmental osteotomy of the maxilla was first described by Cohn-Stock in 1921 and modified by Wassmund, Wunderer, and Epker et al.5. Nonetheless, this technique has been applied to limited cases because its indication was not correct and many complications occur such as necrosis of the repositioned anterior segment, devitalization of anterior teeth, limited movement of segment, and unpredictable soft tissue changes.

Reported in 1996 and published in 2008, the modified anterior segmental osteotomy technique (Park's method) has been used to prevent these complications. In this surgical technique, horizontal vestibular incision is performed from canine to canine; no other (palatal) incision is required. Vertical osteotomy is done on the extraction site. Bicortical horizontal osteotomy between the canine root tip and the aperture piriformis is then carried out to connect the right and left vertical osteotomies6,7. After repositioning the segment, it is fixed by semi-rigid fixation. Note, however, that the proper position and adequate number of miniplates have yet to be determined.

This study sought to evaluate stress distribution according to the fixation methods after anterior segmental osteotomy and suggest the proper position of the miniplates.

Anterior segmental osteotomy is indicated in the case wherein substantial movement of the anterior segment of the maxilla is required but orthodontic tooth repositioning alone is not sufficient in terms of objective and subjective factors such as the amount of tooth movement, periodontal conditions, patient's age, total treatment time, etc4,6,7,12. The advantages of this surgical technique include the more stable results, easier setback of segment, short treatment time, and no intermaxillary fixation periods. Note, however, that it has disadvantages, i.e., possibility of devitalization of anterior tooth, unpredictable soft tissue changes, no movement of posterior segment of the maxilla, and surgery itself6,7. Generally, the anterior segment is fixed by a semi-rigid method, allowing opening the mouth as soon as possible. Its merits include keeping the airway space safe and eating and cleansing the mouth immediately after operation. If the metal plates are poorly manipulated, however, the anterior segment may be displaced13.

FEA is widely applied in the engineering and aerospace industries and can also be used to address physical problems in medical science. It is a highly reliable, accurate technique used to analyze the stress and strain distribution on the anatomic structure under force14,15. This study sought to find the most stable fixation method after anterior segmental osteotomy on the maxilla. Thus, the amount of stress on each type of metal plates was measured using FEA. Vollmer et al.16 reported that FEA simulations were highly related to in vitro values using the mandibular model. FEA is known to be very useful in evaluating the stress and strain patterns in facial bone with its various functions and anatomies17,18. Alberts et al.19 studied the osseous strain pattern of the maxilla after Le fort I osteotomy and suggested that FEA had more advantages than the synthetic model analysis. Note, however, that Armstrong et al.20 cited its limitations based on an in vitro experiment.

In this study, we hypothesized that the maxilla is of a homogenous, isotropic, and linearly elastic structure although it has various biomechanical characteristics. In addition, masticatory force was presumed to be constant even if Song et al.21 reported that masticatory force decreased immediately after surgery but increased during the recovery phase.

Stress distribution pattern is important because abnormal stress can resorb the bone gradually and loosen the screws by mastication. Preventing such unwanted loosening of plate and screws requires a more stable fixation method. This study showed that more stress occurred on Types 1 and 2 plates and screws. This may be caused by excessive bending, suggesting that bending of the plates is important in stress distribution. Stress was also observed to be in high concentration on screw holes near the bending curvature. Note, however, that stress on the bone around the screws was similar for all types of plates and less than that of plates or screws. The deflection of metal plates was not different among Types 1, 3, and 4. Based on these results, we assume that Type 3 may be a more stable method than the others. When applying Type 3 is difficult because of the anatomical limitation or variations of the maxilla, however, Type 4 may be useful.

Based on this study, the Type 3 (L-type) plate can be concluded to be the most stable for the fixation of the anterior segment of the maxilla. Note, however, that it has certain limitations, whereas FEA has various advantages such as restricted application of masticatory force. More clinical variations should be considered.