Despite the evident advantages of surgery-first orthognathic surgery, it lacks worldwide popularity. One of the challenges for the surgery-first approach is the surgical occlusion setup. For example, in classic (i.e., orthodontics-first) approach pre-surgical orthodontic treatment brings maxillary and mandibular teeth into ideal relationships to their individual underlying skeletal bases,21,22 so the surgical occlusion is very close to the final occlusion (i.e., ideal occlusion) after complete pre-surgical orthodontic treatment. Thus, in orthodontics-first approach, surgical occlusion ideally is set as normal overjet and overbite,21,23,24 Class I canine and molar relationship,21-26 tooth-to-tooth contacts,22,26,27 and no occlusal interference.22,24,27 In contrast, in surgery-first approach, dental decompensation is deferred after surgery, so the surgical occlusion is different from the final occlusion. The surgical occlusion setup serves to foresee the tooth movements necessary to achieve an ideal occlusion after post-surgical orthodontic treatment. Thus, the occlusion is set as treatable malocclusion. Accurate surgical occlusion setup is important so surgery-first approach is not suggested to be managed by an orthodontist with limited experience in orthognathic surgery.8,9,18
This systematic review provided inconclusive evidence about the guidelines of surgical occlusion setup for Class III malocclusion using the surgery-first approach. The reasons for the discordant results are due to the heterogeneity of the samples in the selected studies, as shown by the variety of the deformity types, surgical design, and orthodontic mechanisms or techniques used in post-surgical orthodontics (Table 1), as well as the methodological deficiencies (Table 2). These are described in more details as below.
In the sagittal dimension, seven studies used first molars as a guide for antero-posterior dental position7,8,10,14-16,18 as incisors cannot be used as a guide in the sagittal dimension (i.e., incomplete horizontal skeletal correction) in contrast to classic surgical-orthodontic treatment, in which incisor decompensation is performed before surgery. Therefore, when nonextraction was performed in the lower arch the molar relationship could be either Class I when clockwise rotation of the palatal plane was used for upper incisor decompensation7,10,14,19 or Class II when upper premolar extraction10,19 or distalization of the maxillary dentition with Skeletal Anchorage System (SAS)5,6,16 was used for upper incisor decompensation.
Because arch coordination is deferred after surgery with the surgery-first approach, the occlusion setup in the transverse dimension often poses a significant challenge. Four studies used little or no posterior crossbite as a guide for transverse dental position10,13,18,19 because their philosophy was to achieve stable posterior occlusion and limit the post-surgical orthodontics to antero-posterior dental movement only. In other words, surgical occlusion setup should at most require only antero-posterior dental adjustment with minimal transverse or vertical dental movement.28 Therefore, severe arch incoordination, either skeletal or dental origin, are corrected by maxillary segmental surgery. On the other hand, maxillary segmental surgery is only indicated for severe skeletal crossbite (i.e, skeletal origin) in one study.8 In contrast to previous philosophy, posterior dental crossbite, either complete or incomplete, or mild skeletal crossbite are corrected by bending of orthodontic archwire, inter- or intra-arch elastics, transpalatal arch or lingual arch after surgery (i.e., the orthodontic way).
In the vertical dimension, only two studies8,19 mentioned deep overbite or posterior open bite as a guide for supero-inferior dental position in order to compensate for the space for dental alignment, and arch leveling and coordination after surgery. The posterior open bite is easier to correct than anterior open bite after surgery; therefore, surgical occlusion setup with anterior open bite should be avoided. The posterior open bite is also helpful for correction of posterior cross bite from buccoversion of maxillary molars (i.e., dental origin), which is quite common in Class III malocclusion, due to unlocked occlusion. On the other hand, one study suggested anterior open bite in order to achieve stable posterior occlusion.5
The need for stable surgical occlusion was emphasized in eight studies.5-7,13,14,17,19,20 However, definition of stable occlusion varied between studies; four studies defined stable occlusion as at least 3-point contact,13,17,19,20 whereas another four defined it as stable posterior occlusion.5-7,14 To prevent unstable jaw position from occlusal instability after surgery, six studies used an occlusal splint,5,13,15-17,20 another 2 studies used a chin cap,10,19 and the other four studies used intermaxillary elastics.8,14,15,17