Is There a Safe Zone for Lateral Border Fixation of Mandibular Angle Fractures?

Abstract

Study Design: Cross-sectional study design. Objective: There are multiple accepted treatment options for internal fixation of mandibular angle fractures. The purpose of this study was to determine if there is a safe zone for lateral border fixation of mandibular angle fractures. Methods: One hundred coronal images of facial computed tomography (CT) scans were reviewed on patients between the ages of 18 to 48. Measurements were taken in the area of the second and third molar region related to the inferior border to the superior extent of the inferior alveolar canal and apex of the second molar root, along with buccal cortical measurements to the inferior alveolar canal and apical third of the second molar root. Results: The average measurement of the inferior border in the second molar area to the inferior alveolar canal and apex of the root was 1.12 cm (0.70–1.77) and 1.39 cm (0.91–2.30), respectively. The average measurement of the inferior border of the third molar to the inferior alveolar canal was 1.26 cm (0.78–1.83). The average measurement of the buccal cortex of the second molar to the inferior alveolar canal and apical one-third of the root was 0.64 cm (0.34–1.25) and 0.59 cm (0.33–0.98), respectively. The average measurement of the third molar buccal cortex to the inferior alveolar canal was 0.45 cm (0.18–0.98). Conclusions: In the area of the second molar region, there is no ubiquitous safe zone for screw placement, cortical bone thickness is more critical than vertical placement of the fixation plate and screws. In the third molar region, cortical bone thickness and vertical orientation may provide a safe zone for screw placement.

1. Introduction

Trauma to the mandible with resultant fracture of the angle region is a common injury seen in the maxillofacial trauma practice. There are several treatment options ranging from closed reduction with intermaxillary fixation to extraoral open reduction with internal fixation utilizing a reconstruction plate. There has been much debate about the ideal method of fixation. Originally developed during the 1970’s by the AO/Association for the Study of Internal Fixation (AO/ASIF), techniques were pioneered using dynamic compression plating to provide absolute rigid fixation. Champy proved that absolute rigid fixation was not paramount for bone healing and a single noncompression miniplate placed along the ideal line of osteosynthesis was adequate for bone healing. [1] Ellis has shown that a single fixation plate provides adequate fixation and has the lowest rate of complications. [2] Current plate options most used for the treatment of mandibular angle fractures via an intraoral approach include: (1) Single miniplate placed along the external oblique ridge on the proximal segment, extending anteriorly along the lateral border on the distal segment. (2) Single miniplate placed along the lateral border on both the proximal and distal segment. (3) Single 3D strut plate placed along the lateral border along the proximal and distal segment. It is frequently necessary to place hardware in the area of the second and third molar when repairing fractures of the mandibular angle. [3] In turn, there are internal structures at risk for damage by placement of the fixation screw, if not in an ideal position or proper length. These structures include the roots of the second molar (and third molar if present) and the inferior alveolar nerve canal. The position of these structures is variable between patients.

However, we are hoping to find an ideal “safe zone” for fixation screw position and length during internal fixation of mandibular angle fractures.

2. Methods

After institutional review board approval, 100 patients between age 18 to 48 with coronal images of facial computed tomography (CT) scans taken in the facility scanner only were reviewed. The area of study was without fracture and the structures of study were easily identifiable. Scans were not distorted and patients who appeared rotated when scanned were not included. Only patients who had second molars present that were not mesially or distally inclined were included. A radiology image computer was used to measure 6 anatomic distances in each patient: In the second molar area—A. The inferior border to the superior extent of the inferior alveolar canal (Figure 1). B. The inferior border to the apex of the root tip (Figure 2). C. The buccal cortex to the inferior alveolar canal (Figure 3). D. The buccal cortex to the apical third of the root (Figure 4). In the third molar region—E. The inferior border to the superior extent of the inferior alveolar canal (Figure 5). F. The buccal cortex to the inferior alveolar canal (Figure 6). Statistical analysis was completed for each of the 6 measurement categories.

3. Results

A total of 100 coronal CT scans with an age range of 18 to 48, with an average age of 28 were analyzed. The measurements recorded for each of the 6 categories with the range of distance and the average distance measured summarized in

Table 1. Average Measurements and Ranges for Each Anatomic Area in the Second and Third Molar Region.

Abbreviations: BC, Buccal Cortex; IAC, Inferior Alveolar Canal; IB, Inferior Border.

. The range of distance from the mandibular inferior border in the second molar area to the superior extent of the inferior alveolar canal (A) was 0.70 cm to 1.77 cm, with an average distance of 1.12 cm. The range of distance from the mandibular inferior border in the second molar area to the apex of the root tip of the second molar (B) was 0.91 cm to 2.30 cm, with an average distance of 1.39 cm. The range of distance from the buccal cortex of the second molar to the inferior alveolar canal (C) was 0.34 cm to 1.25 cm, with an average distance of 0.64 cm. The range of distance from the buccal cortex of the second molar to the apical one-third of the root (D) was 0.33 cm to 0.98 cm, with an average distance of 0.59 cm. The range of distance from the mandibular inferior border in the third molar area to the superior extent of the inferior alveolar canal (E) was 0.78 cm to 1.83 cm, with an average distance of 1.26 cm. The range of distance from the buccal cortex of the third molar to the inferior alveolar canal (F) was 0.18 cm to 0.98 cm, with an average distance of 0.45 cm.

4. Discussion

Cortical bone thickness, the position of tooth roots and inferior alveolar canal have been measured previously in the mental foramen region utilizing dry human adult hemimandibles. This was done to evaluate anchorage for miniplate fixation of parasymphyseal fractures. [4] The American Academy of Oral and Maxillofacial Radiology position statement recommend cross sectional imaging be used for assessment of implantation sites. [5] Computed tomography is the main complementary exam used by the craniofacial community. It has also been used routinely to determine the position of the inferior alveolar nerve through the mandibular canal preoperatively. [6] Linear measurements obtained on reformatted spiral CT images have been shown to be accurate. [7] Cone beam computed tomography has been used to evaluate the relationship between cortical bone thickness and root morphology, demonstrating no significant difference in skeletal class 1, 2 or 3 patients. Cortical bone thickness was thicker in the mandible compared to the corresponding maxilla and thicker in the posterior region. [8,9] Generally, cortical bone thickness is noted to be thicker in adults compared to adolescents. [9]

There are multiple options for the treatment of mandibular angle fractures ranging from closed reduction to open reduction with internal fixation. Placement of the titanium fixation plate along the lateral border is a popular treatment option. Multiple plating systems are available with the thickness of plates ranging from 1.0 mm to 2.0 mm (Figure 7). One must take in account the thickness of the plate when determining the final length of screw chosen along with the length of the corresponding drill bit. The position of the inferior alveolar canal and the second molar was highly variable. The inferior border of the mandible was used as a measurement reference because it is easily identifiable radiographically and intraoperatively. The cortical bone thickness in the second molar region showed an average of 0.64 cm in the area of the inferior alveolar canal with 95% (95/100) of the values being greater than 0.40 cm. There was an average of 0.59 cm cortical bone thickness in the area of the apical one-third of the root with 94% (94/100) of the values being greater than 0.40 cm. There was no ubiquitous safe zone for fixation plate placement between the inferior alveolar canal and the apex of the second molar as the distance between the 2 areas is only 0.27 cm on average. Therefore, the vertical orientation is less critical than the cortical bone thickness in the second molar area for screw placement.

In the third molar region the average height from the inferior border of the mandible to the superior aspect of the inferior alveolar canal was 1.26 cm and the average cortical bone thickness to the inferior alveolar canal was 0.45 cm with 86% (86/100) greater that 0.30 cm and 63% (63/100) greater than 0.40 cm. Therefore, cortical bone thickness along with vertical orientation of placement (especially in patients missing a third molar) may provide a safe zone for screw placement with the goal of placement of the proximal aspect of the plate superior to the inferior alveolar canal. Stripping the periosteum along the inferior one-third of the mandible is often not necessary for plate placement and may provide benefit to prevent lateral splaying of the proximal and distal segment (Figure 8).

The fixation of non-comminuted mandibular angle fractures with a 2.0-mm curved angle 3-D strut plate is predictable. [10] However they can pose an additional risk to vital structures due to increased level of screw placement similar to a 2-plate construct. Prospective, randomized clinical trials have shown a single noncompression miniplate is an efficacious as 2 plates. [11,12]

5. Conclusions

We have analyzed and shown potential safe areas for screw fixation placement in the treatment of mandibular angle fractures utilizing open reduction and internal fixation.

Multiple surgical techniques exist as do the thickness of the associated implants utilized. In general, the use 4 mm of intrabony threads (5 mm screw if using a 1 mm thick plate) in the area of the second molar region and placement of the proximal aspect of the plate at least 1.50 cm above the inferior border in the area of the third molar should provide some margin of safety. We advocate identification and measurement of the critical structures on CT to determine the ideal screw length along with the implant and screw position for individual patients.