Study design and sample

To address the research objectives, a retrospective cohort study was designed and implemented. The study sample was derived from an index group of patients treated by one surgeon (JCP) in a private practice setting, with surgery carried out at a single hospital between 2004 and 2013. The index group of subjects all had a bimaxillary developmental DFD also involving the chin, as well as symptomatic chronic obstructive nasal breathing. The subjects then underwent surgery that at a minimum included Le Fort I osteotomy, bilateral SROs of the mandible, osseous genioplasty, septoplasty (submucosal resection), and reduction of the inferior turbinates.

Subjects were excluded if their jaw deformity was syndromic, cleft-related, previously operated upon, post-traumatic, or tumour-related. Patients not residing in North America were also excluded, as long-term follow-up was geographically inconsistent. Other exclusions for semi elective orthognathic surgery included subjects using nicotine products for at least 3 weeks prior to surgery, those currently taking bisphosphonate medication, immunosuppressed individuals, and those with poorly controlled diabetes. All subjects in the study were confirmed to be cardiovascularly stable without pulmonary disease, renal disease, or a known coagulation disorder. Each subject underwent lateral cephalometric and panoramic radiography within 2 months before surgery and at approximately 5 weeks after surgery. The postoperative panoramic radiograph was reviewed specifically for the placement of SRO fixation, removal of the third molar, unexpected fractures of the mandible or dental injury, and the identification of foreign bodies. The range of follow-up was 2–11 years.

Predictor variables

The predictor variables studied for LN injury were grouped into the following categories: demographic, anatomic, and operative.

The demographic variables collected were age at the time of operation and sex.

With regard to anatomic variables, the first anatomic variable was the presence or absence of a third molar at the osteotomy site. If present, the third molar was further classified into one of three categories: fully impacted (no exposure through the oral mucosa), fully erupted (full crown exposure through the oral mucosa), or partially erupted (partial eruption of the crown through the oral mucosa). The second anatomic variable was the pattern of the developmental DFD. At presentation, each subject was classified into one of six abnormal jaw growth patterns: primary mandibular deficiency, maxillary deficiency with relative mandibular excess, asymmetric mandibular excess, short face, long face, and bimaxillary dental protrusion. In addition, there was an atypical category that included DFDs strongly influenced by parafunctional habits such as thumb sucking.

Operative variables included SRO design, method of osteotomy fixation, method of third molar removal, and the extent of mandibular advancement and pitch orientation change.

The SRO design was consistent for all study subjects. The cortical osteotomies included connected medial, lateral, and vertical cuts. The medial osteotomy was designed to be ‘low and short’ to limit the occurrence of a ‘bad’ split. The lateral osteotomy extended anteriorly for a varied length depending on the planned mandibular movement. Greater advancements required a greater anterior extension prior to creating the vertical osteotomy. This modification was made to ensure maximum bone contact after the distal mandible was repositioned into its new location. The connection between the lateral and vertical osteotomies was rounded to limit unwanted fractures. The vertical osteotomy was bevelled through the buccal cortex and carried just through the inferior border for the same reason. After splitting of the ramus osteotomy, the inferior alveolar nerve was visualized in the surgical field. The LN location was assumed, but it was not dissected and not typically visualized ( Fig. 1 ).

Intraoperative view of a completed (left) sagittal ramus osteotomy. Separation of the osteotomy segments demonstrates the inferior alveolar nerve and the lingual nerve. The lingual nerve is not typically visualized in the surgical field during this procedure.

The SRO was planned to be fixated with three bicortical screws that were 2.3 mm in diameter (Stryker Corporation, Kalamazoo, MI, USA) ( Fig. 2 ). The length of the screws varied from 14 mm to 18 mm according to the width of the overlapping medial and lateral cortices of the mandible. To provide effective rigid fixation, the bicortical screws extended through the lingual cortex ( Fig. 3 ). The drill holes and screws were placed through a transbuccal trocar (Stryker Corporation). The trocar included a depth gauge component to determine the necessary length of the screws. This was accomplished without additional sub-periosteal dissection and instrument protection along the lingual cortex of the distal segment. The surgeon used proprioception to stop the drill once it had perforated through the lingual cortex, to limit injury to the soft tissues within the floor of the mouth.

The proximal segment of the mandible is seated with the condyle in the terminal hinge position. Bicortical titanium screws (2.3 mm in diameter and 14–16 mm in length) are generally sufficient to passively secure the proximal and distal segments together, achieving rigid fixation (taken from Posnick (chapter 15, Figure 15-44)).

A dry skull is used to demonstrate a completed sagittal ramus osteotomy (left and right sides). Three titanium screws (2.3 mm in diameter) have been placed across the buccal and lingual cortices to achieve rigid fixation. (A) Three bicortical screws are in place demonstrating extension of the screw tips past the lingual cortex. (B) The bicortical screws are in place but with greater than ideal extension of the screw tips past the lingual cortex. This may cause irritation of the tongue during function and possibly leave the lingual nerve susceptible to injury.

If extraction of a mandibular third molar was required, it was the senior author’s (JCP) preference to remove it at the time of orthognathic surgery. If the mandibular third molar was fully impacted, it was removed through the osteotomy site after splitting the ramus. If the mandibular third molar was partially or fully erupted, it was removed after the incision and soft tissue dissection but prior to splitting of the mandible to limit unintended fracture of the lingual plate.

Subjects with a primary mandibular deficiency pattern of DFD underwent the most significant advancements. Therefore, the extent of mandibular advancement in these subjects was studied as a risk factor for LN injury. The mandibular advancement was measured in millimetres (at the incisors), and the pitch orientation change (clockwise, counter-clockwise, neutral) was recorded.

Outcome variable

The primary outcome variable studied was the occurrence of irreversible LN injury. Prior to orthognathic correction, the surgeon made specific inquiries about each subject’s facial sensibility. If the history indicated abnormal sensation of the facial soft tissues, surface of the tongue, gingiva, or roof of the mouth, then a focused physical examination and neurosensory testing of the area in question was conducted. After surgery, all subjects were followed at weekly intervals for the first 5 weeks by the operating surgeon. Significant return of oro-motor function was expected to have occurred at the 2-month postoperative follow-up visit. A negative history of sensory loss over the dorsum of the tongue was taken as confirmation that a significant LN injury was not present. If the history indicated abnormal sensation over the dorsum of the tongue or diminished sense of taste, then a focused physical examination and initial neurosensory testing were conducted. Neurosensory testing for taste, light touch (cotton swab), and pain (pin prick) over the dorsum of the tongue was performed to confirm the subjective history of LN injury. If the neurosensory testing was abnormal, then referral to a peripheral nerve specialist for an in-depth evaluation and treatment was made.

Collection, management, and analysis of data

The data were abstracted from each subject’s hospital and outpatient medical records and recorded on a standardized data collection form by one researcher/observer (EC). The data were entered into a database using Microsoft Access (Microsoft Inc., Redmond, WA, USA). The data were transferred to a software package for statistical analysis (SPSS 10.0; SPSS Inc., Chicago, IL, USA). Descriptive statistics were computed for all study variables. No subjects were lost to follow-up and no records or data points were missing for any of the subjects for any parameters in the study.