The aim of this study was to determine the safest length of monocortical screws that can be inserted for the treatment of mandibular fractures following Champy’s technique. Fifty cone-beam computed tomography (CBCT) scans of hemi-mandibles were studied. Linear measurements were taken from the buccal cortical plate to the tooth apex, from the canine to the second premolar, and from the buccal cortical plate to the tooth apex and the inferior alveolar canal in the molar area. The minimum values of the horizontal distances both at the level of the apex and the inferior alveolar canal at the second molar were found to be 4 mm, which is greater than those of the first molar. At the canine, first premolar and second premolars, the minimum values of the horizontal distances at the level of the apex was found to be 2 mm, 2.33 mm and 2 mm, respectively. Stabilizing miniplates using 4 mm screws both at the level of the apex and the inferior alveolar canal is safe in the second molar area, anterior to this, there is a risk of injury to tooth root and inferior alveolar nerve.
Open reduction and internal fixation (ORIF) using miniplates and screws is the treatment of choice for mandibular fractures. Champy’s technique is now the most widely used method for the treatment of mandibular fractures. He demonstrated that using miniplates and screws along the ideal line of osteosynthesis provides sufficient support and stability to the bone fragments to allow immediate function.
ORIF of fractures involving the mandibular body and parasymphysis requires the placement of screws along the ideal line of osteosynthesis; this carries a risk of injury to the roots of the teeth and to branches of the mandibular nerve. Few studies have reported the complications associated with the use of monocortical screws of different lengths during fixation of the fractured mandible. Ellis reported an incidence of 1.5% root injury during fixation of symphsis, and parasymphsis fractures, concluding that there is a possibility of iatrogenic injury to the tooth roots when monocortical screws are not placed carefully. In another study, it was shown that mandibular teeth appear to be more at risk of impingement than maxillary teeth because the thick buccal plate of the bone makes identification of root contours difficult.
The purpose of this study was to determine the thickness of buccal bone at the parasymphsis and mandibular body, thereby determining the maximum length of monocortical screws that can be safely placed in these regions without injuring the tooth roots or mandibular nerve.
Materials and methods
Pretreatment cone-beam computed tomography (CBCT) scans (I-CATTM, 3-D imaging system, Imaging Sciences International Inc., Hatfield, PA, USA), taken in the Dental College, were evaluated to identify 50 consecutive patients. Inclusion criteria were any dentate adult patient who underwent a CBCT scan (0.4 voxels, 8.9 s) for evaluation of lower wisdom teeth prior to surgery. Patients who had any tooth loss, bone abnormality, bone pathology, previous orthodontic treatment or facial asymmetry were excluded. Ethical clearance for this study was obtained from the Biomedical Dental Science Department Board, College of Dentistry, University of Dammam.
Fifty CBCT scans were analyzed from 28 females and 22 males, aged 18–33 years. Measurements were taken for the following lower teeth: canine, first premolar, second premolar, first molar and second molar. One investigator marked the points used for the measurements twice. All scans were viewed using I-CAT vision (Q version 184.108.40.206, Imaging Science International, Hatfield, PA, USA) and coronal cuts were taken at a distance of 0.8 mm. A digital calibrated ruler was oriented perpendicular to the points marked on the outer buccal cortex (OBC). Using the digital ruler, the distance between points OBC 1 , which represent a point on the outer buccal cortical plate along a perpendicular line drawn from the root apex and IBC 1 , which represents a point on the inner buccal cortical plate along a perpendicular line drawn from the root apex, was measured. This represented the cortical plate thickness at the level of the apex. Similarly, measurements were taken as shown in Figs. 1 and 2 . Measurement 1, buccal cortical plate thickness at the level of the apex (OBC 1 -IBC 1) . Measurement 2, buccal bone thickness at the level of the apex (OBC 1 -AP). Measurement 3, buccal cortical plate at the level of the inferior alveolar canal (OBC 2 -IBC 2 ). Measurement 4, buccal bone thickness at the level of the inferior alveolar canal (OBC 2 -IAC).
Sample size calculation was based on a similar study, which detected thickness at the anterior area (1.78 ± 0.46 mm), and the posterior area (4.99 ± 1.08 mm). The authors anticipated the average for their population to be consistent with average recorded thickness, on 95% power, 5% level of significance, and 95% confidence level. Sample size was calculated using WHO sample size software.
Statistical package for social sciences (SPSS) version-17.0 was used for statistical analysis. The scores were presented in terms of mean and standard deviations. Independent t -test was applied to find any significant difference with regard to sex, and buccal bone measurement at different sites. P -value ≤ 0.05 was considered statistically significant.
The observed thicknesses of the buccal cortical plate and buccal bone are summarized in Tables 1 and 2 . Bone thickness increased posteriorly and inferiorly, with the exception of the second premolar.
|First molar (apex)||1.6||3.5||2.5||±0.51|
|First molar (IAC)||1.75||3.6||2.6||±0.46|
|Second molar (apex)||2||5.25||3.18||±0.76|
|Second molar (IAC)||2||4.5||2.85||±0.56|
|First molar (apex)||2||5.25||3.8||±0.81|
|First molar (IAC)||3||6.5||4.6||±0.91|
|Second molar (apex)||4||8.25||6.58||±1.27|
|Second molar (IAC)||4||7.5||5.75||±0.95|
The average thickness of the buccal cortical plate ( Table 1 ) was 1.78 ± 0.51 mm at the apex of the canine, 2.01 ± 0.49 mm at the apex of the first premolar, 1.91 ± 0.42 mm at the apex of the second premolar, 2.5 ± 0.51 mm at the apex of the first molar, 2.6 ± 0.46 mm at the inferior alveolar canal in the first molar region, 3.18 ± 0.76 mm at the apex of the second molar, and 2.85 ± 0.56 mm at the inferior alveolar canal in the second molar region. The cortical plate was thickest in the second molar at the level of the apex. The cortical plate was thinnest in the canine at the level of the apex.
The average thickness of the buccal bone ( Table 2 ) was 3.3 ± 0.84 mm at the apex of the canine, 3.7 ± 0.89 mm at the apex of the first premolar, 3.14 ± 0.81 mm at the apex of the second premolar, 3.8 ± 0.81 mm at the apex of the first molar, 4.6 ± 0.91 mm at the inferior alveolar canal in the first molar region, and 5.75 ± 0.95 mm at the inferior alveolar canal in the second molar region. The bone was thickest in the second molar region at the level of the apex; 6.5 ± 1.27 mm; and the thinnest bone was found at the level of the apex of the second premolar.
The predicted injury to the tooth apex and inferior alveolar nerve is shown in Table 3 . In this patient population, the authors found that using 4 mm screws at the level of the apex in the canine, first premolar, and second premolar would have caused injury in 30%, 18% and 38% of the patients, respectively. In the first molar region, using 4 mm screws at the level of the apex and inferior alveolar canal would have caused injury in 14% and 2% of the patients, respectively. In the second molar area, placing 4 mm screws is safe at the level of apex and the inferior alveolar canal.