Maintenance of a deep bite prior to surgical mandibular advancement

Adult patients with a Class II skeletal base are often treated by a combined orthodontic and surgical approach. Advancement of the mandible, most often including a bilateral sagittal split osteotomy (BSSO), is preceded by orthodontic alignment and frequently the curve of Spee is levelled. When the chin is prominent, there is a risk of accentuating this as a result of surgery. An option to prevent this is to maintain a deep curve of Spee before surgical advancement. This will result in an opening rotation of the mandible during surgery and thus, a less prominent chin. The aim of this study was to compare, retrospectively, two orthodontic treatment approaches in patients treated by a BSSO. In one group (4 males, 20 females; mean age pre-surgery 29.3 years), the deep bite was maintained (deep bite group) while in the other (3 males, 10 females; mean age pre-surgery 27.1 years) the overbite was normal prior to surgery (level group). Lateral skull radiographs were taken before orthodontic treatment (T0), prior to surgery (T1), and at the end of treatment (T2). Differences between the groups as measured on lateral skull radiographs at T1 and T2 were analysed and quantified using an independent t-test. The results showed that soft tissue pogonion moved significantly further forward in the level than in the deep bite group (P < 0.05). Lower anterior face height and the cranial base-mandibular plane angle increased more in the deep bite than in the level group (P < 0.05 and P = 0.001, respectively). The maintenance of a deep bite prior to mandibular advancement surgery induces an opening rotation of the mandible reducing chin prominence and increasing lower anterior face height post-surgically.