The purpose of this study was to assess the frequency of irreversible lingual nerve (LN) injury in patients undergoing sagittal ramus osteotomies (SRO) with bicortical screw fixation. A retrospective cohort study of patients treated by a single surgeon was performed (follow-up 2–11 years). The sample consisted of a series of subjects with a bimaxillary dentofacial deformity (DFD). The SRO and bicortical screw fixation techniques were consistent. The primary outcome variable was the prevalence of irreversible LN injury. Two hundred sixty-two subjects undergoing 523 SROs with bicortical screw fixation met the inclusion criteria. Average age at operation was 25 years (range 13–63 years) and there were 134 females (51%). The majority of SROs were fixated with three bicortical screws (92%). Simultaneous third molar removal was done in 209 of the 523 SROs (40%). For primary mandibular deficiency subjects ( n = 40), the mean mandibular advancement was 11.0 mm (range 5–17 mm), with 42.5% undergoing counter-clockwise rotation. In the study group ( n = 523 SRO’s) there was one irreversible LN injury (<1%). This study confirmed a lack of association of LN injury at the time of SRO with sex, age at operation, simultaneous removal of a third molar, use of bicortical screw fixation, pattern of DFD, and extent of mandibular advancement.
The frequency of injury to the lingual nerve (LN) during a sagittal ramus osteotomy (SRO) of the mandible is not precisely known, but it is assumed to be rare. A literature search to clarify the prevalence of this injury provided limited insight. Jacks et al. reported a retrospective analysis of LN sensory change after SRO fixation with either intraosseous wires or bicortical screws. They used a patient response-based questionnaire for the analysis of sensory change; however, only 43% of the subjects returned the questionnaire. Nineteen percent of respondents reported a LN sensory change, of whom 69% reported resolution within 1 year. The study found no statistical relationship between LN sensory change and the patient’s age, sex, extent of mandibular repositioning (setback vs. advancement), type of fixation (wires vs. bicortical screws), or the removal of a third molar. Becelli et al. completed a retrospective analysis of complications after SRO and bicortical screw fixation. All cases were mandibular setback for class III malocclusion. Three of the 482 SROs sustained immediate postoperative ‘LN dysfunction’. Unfortunately, no neurosensory testing was carried out and the occurrence of long-term dysesthesia was not reported. Bouwman et al. reported on subjects who underwent SRO with bicortical screw fixation. They documented ‘four patients with LN dysesthesia’. No neurosensory testing was carried out and the occurrence of long-term dysesthesia was not reported. The majority of the medical literature concerning LN injuries has focused on either isolated case reports or case series reporting the management of these cases once the damage had been done. Published case series have generally not separated LN and inferior alveolar nerve injuries and have combined a spectrum of aetiologies, including SRO, third molar removal, trauma, or tumour. Consensus concerning the risk factors for LN injury during SRO and the best treatment protocols has yet to be reached.
When a LN injury occurs in association with SRO, it may be at the time of incision placement, during sub-periosteal dissection along the medial ramus, or when using a rotary drill or reciprocating saw during the medial ramus cortical cut. It may also occur with the use of electrocautery, during placement of screw fixation, during wound closure, or from stretching or compression when the mandible is repositioned. Pogrel completed a cadaver study demonstrating that different modalities (e.g., scalpel laceration, fissure bur injury, or retraction stretch injury) produce different types of LN injury. The recovery of a damaged nerve is believed to be strongly influenced by the type of injury sustained. Unfortunately, studies have indicated that a majority of LN injuries are unknown to the surgeon at the time of operation. To date, limited information is available to evaluate the relationship between specific operative, anatomic, and demographic risk factors and the occurrence of LN injury at the time of SRO.
The purpose of this retrospective cohort study was to evaluate the incidence of irreversible LN injury in association with SRO and bicortical screw fixation. It was hypothesized that when using consistent osteotomy and fixation techniques, the occurrence of LN injury would be rare. It was also speculated that the simultaneous removal of a third molar in the line of the SRO or the extent of mandibular advancement would not increase the risk of LN injury. The primary aim of the study was to document the incidence of irreversible LN injury when using consistent SRO and bicortical screw fixation techniques. It was also planned to document any variation in the occurrence of LN injury with regards to simultaneous removal of a third molar, the pattern of the dentofacial deformity (DFD), subject age at operation, sex, and the extent of mandibular advancement.
Materials and methods
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.
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 ).
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.
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.
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.