Abstract
The inferior alveolar nerve (IAN) is vulnerable to injury from mandible fractures and surgical procedures so anatomical variations of IAN are important. Postoperative sensory alteration of the lip and chin region is high after mandibular orthognathic surgery. The incidence of IAN paresthesia following sagittal split ramus osteotomy (SSRO) ranges from 54% to 86% at 4–8 days, 41 to 75% at 1 month, 33 to 66% at 3 months, 17 to 58% at 6 months and 15 to 33% at 1 year postoperatively. This study determined the anatomical position of the mandibular canal in relation to cortical bone and molar teeth in Chinese using archived CT records. The mandibular canal was the farthest from the buccal cortex at the second molar region (mean 6.79 mm; minimum distance 4.80 mm). The anatomical location of the mandibular canal in local Chinese compares with studies on Asian cadavers. The mandible body was thickest in the region of the second molar (11.9 mm). The vertical buccal cut for SSRO should be in the region of the mandibular second molar where the bone is thickest and the mandibular canal is furthest from the buccal cortex. The safe depth for the vertical buccal cut is 4.8 mm (minimum horizontal distance).
The inferior alveolar nerve (IAN) is of clinical significance because of its vulnerability to injury from mandible fractures, and surgical procedures including third molar surgery, implant surgery, orthognathic surgery, surgery for pathology and endodontic therapy. Anatomical variations of the IAN are therefore of significance to the clinician.
Olivier found that the neurovascular bundle divided soon after its entry into the mandibular canal in almost 34% of mandibles . C arter & K een noted a single neurovascular bundle in 7 of 8 dissected mandibles . N ortje ‘s radiographic study demonstrated posterior duplication or division of the mandibular canal in only 0.96% of 3612 cases . Controversies exist regarding the tubular nature of the IAN canal (mandibular canal). C ryer described the mandibular canal as a cribriform tube with a porous nature .
The mandibular canal loses its tubular form at or around the molar region . Others claim that the mandibular canal extends as a tube beyond the molar region, at least up to the mental foramen . Olivier found that the mandibular canal courses lingual to the roots of the second and third molars, adjacent to the roots of the first molar and lateral to the roots of the premolars . In the area of the mandibular foramen, the IAN was found to occupy almost the entire cancellous space between the lingual and buccal cortical plates and always maintained close relation to the lingual plate . As it approached the mental foramen the mandibular canal turns sharply from medial to lateral towards the foramen.
Current information on the course of the IAN is derived from studies performed on cadaveric mandibles , and to a lesser extent from studies using computed tomography (CT) to determine the position of the mandibular canal in patients undergoing orthognathic surgery . The spatial location of the mandibular canal has only been loosely correlated to the mandibular and mental foramina and to the lower molar teeth as reference points . CT studies used different planes for assessment, compared with studies using cadaveric mandibles . This makes it difficult to compare the dimensional differences of the mandibular canal in relation to the buccal, lingual and inferior cortices at different locations of the mandible body between studies. The race of the cadavers from whom mandible specimens were obtained was omitted in several studies , whilst the gender and age were not stated in two studies .
Clinical significance
The likelihood of postoperative sensory alteration of the lip and chin region is particularly high after mandibular orthognathic surgery . The incidence of IAN paresthesia in prospective studies on IAN sensory alteration confirmed with neurosensory testing following sagittal split ramus osteotomy (SSRO) alone has been reported to range from 54% to 86% at 4–8 days, 41% to 75% at 1 month, 33% to 66% at 3 months, 17% to 58% at 6 months and 15% to 33% at 1 year postoperatively . The upper limit of this range of incidence of IAN paresthesia was associated with SSRO using rigid fixation with screws or plates, whilst the lower limit of this incidence range derived from studies of SSRO using wire fixation. The incidence of IAN paresthesia following intraoral vertical ramus osteotomy (IVRO) is lower than that of SSRO but IVRO cannot be used to correct mandibular retrognathism. The incidence of mental nerve paresthesia following genioplasty is lower; the long-term incidence is estimated to be around 5–10% .
An important issue in orthognathic surgery of the mandible is the prevention of injury to the IAN, with the attendant dysfunction from sensory loss of the lower lip and chin, such as drooling, lip biting, speech difficulties and interference with psycho-social function . Knowledge of the relationship of the IAN will be particularly useful when performing SSRO for correcting mandibular prognathism, retrognathism or asymmetry.
The aim of this study was to determine the anatomical position of the mandibular canal in relation to the cortical bone and molar teeth in Chinese patients using archived CT records. To provide information on the relative distance of the IAN to the bone cuts performed during SSRO.
Materials and methods
The CT image datasets of Chinese patients who had three dimensional (3D) facial scans performed on multi-slice CT scanners at the Department of Diagnostic Radiology at the Singapore General Hospital were retrieved. The scans were screened for image quality and those with artefacts secondary to patient motion or dental artefacts were excluded.
20 patients with 40 mandible sides that fulfilled the following inclusion and exclusion criteria were selected for this study. The inclusion criteria were: Chinese males or females; aged 15–75 years; with at least the first and second molars present per mandible side. The exclusion criteria were: pathology in the mandible (e.g. cysts, tumours), displacing teeth or the IAN; osteomyelitis or osteopetrosis in the mandible; history or evidence of previous surgery in the mandible (e.g. presence of metal bone plates); asymmetry of the mandible; missing or malpositioned lower first or second molar.
The archived CT studies from 2003 to 2006 were retrieved and reviewed. The first 20 patients that fulfilled the above inclusion and exclusion criteria were selected for the study. The CT scans were examined using the 3D function of the Siemens Somatom Sensation Software Version A70A. This software allows multiplanar reconstruction of the axial 3D image dataset into the appropriate plane to measure the most direct or shortest distance of the measures shown in Fig. 1 . These measurements were taken at the second premolar, mesial root of first and second molar, and at the region where the occlusal plane intersects the ascending ramus. The slice thickness of the selected scans was 1 mm.
The three planes were oriented as shown in Fig. 2 . The axial plane was adjusted parallel to the occlusal plane of the mandibular posterior teeth on both sides and was centred at the mid-point of the distance between the two first molars. The sagittal plane was obtained by an oblique line joining the centre of the mental foramina and the most medial edge of the lingula. In the coronal view, the sagittal plane was adjusted such that it bisected the dento-alveolar complex centred at the cervix of the tooth. The coronal plane was at right angles to the oblique sagittal plane in the axial view and was adjusted to bisect the mesial root of the first molar tooth in the sagittal view.
Measurements were taken with a digital millimetre scale to one decimal place. The measurements were taken by two researchers and repeated by each after 1 month and the intraclass correlation coefficient calculated to determine the intra-rater variability.
The measurements of 40 mandible sides in 20 patients were recorded in a standard proforma and entered into a Microsoft Excel™ database. The data were analysed with the assistance of a biostatistician. The project has been approved by the Singapore General Hospital, Institutional Review Board (IRB).
Materials and methods
The CT image datasets of Chinese patients who had three dimensional (3D) facial scans performed on multi-slice CT scanners at the Department of Diagnostic Radiology at the Singapore General Hospital were retrieved. The scans were screened for image quality and those with artefacts secondary to patient motion or dental artefacts were excluded.
20 patients with 40 mandible sides that fulfilled the following inclusion and exclusion criteria were selected for this study. The inclusion criteria were: Chinese males or females; aged 15–75 years; with at least the first and second molars present per mandible side. The exclusion criteria were: pathology in the mandible (e.g. cysts, tumours), displacing teeth or the IAN; osteomyelitis or osteopetrosis in the mandible; history or evidence of previous surgery in the mandible (e.g. presence of metal bone plates); asymmetry of the mandible; missing or malpositioned lower first or second molar.
The archived CT studies from 2003 to 2006 were retrieved and reviewed. The first 20 patients that fulfilled the above inclusion and exclusion criteria were selected for the study. The CT scans were examined using the 3D function of the Siemens Somatom Sensation Software Version A70A. This software allows multiplanar reconstruction of the axial 3D image dataset into the appropriate plane to measure the most direct or shortest distance of the measures shown in Fig. 1 . These measurements were taken at the second premolar, mesial root of first and second molar, and at the region where the occlusal plane intersects the ascending ramus. The slice thickness of the selected scans was 1 mm.
