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ORIGINAL ARTICLE

Frontal Bone Fracture Patterns Suggesting Involvementof Optic Canal Damage
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Koji Imajo, DDS, Tomohisa Nagasao, MD, PhD,y Niyazi Aizezi, MD,yTadaaki Morotomi, MD, PhD,z Motoki Tamai, MD, PhD,y and Minoru Miyake, DDS, PhDPurpose: Fracture of the frontal bone can be accompanied bydamage to the optic canal. The present study uses finite element
analysis to identify fracture patterns, suggesting the involvement of
the optic canal.
Methods: Ten finite-element skull models were generated fromcomputer tomography data of 10 persons. Then, dynamic analyses
simulating collision of a 2-cm-radius brass ball to 6 regions on thefrontal bone in the 10 models were performed. Fracture patterns
presented by the frontal bone in the 60 experiments were observed,
and all those involving the optic canal were selected.
Commonalities of the selected fracture patterns were identified.
Results: Fracture of the optic canal was observed in 9 of the 60
patients. In all 9 patients, fracture existed on the anterior andposterior walls of the frontal sinus and on the superior orbital wall.
Conclusion: When the anterior and posterior walls of the frontal
sinus and the superior orbital wall are all broken, the optic canal ishighly likely to be involved in the damage. When this pattern isobserved in emergency examination, preventive decompression of

From the Department of Dentistry and Oral Surgery; yDepartment of
Plastic and Reconstructive Surgery, Graduate School of Medicine,
Kagawa University; and zDepartment of Plastic and Reconstructive
Surgery, Graduate School of Medicine, Kindai University, Ikenobe,
Japan.
Received March 11, 2018.
Accepted for publication July 12, 2018.
AQ3 Address correspondence and reprint requests to Tomohisa Nagasao, MD,
PhD, Department of Plastic and Reconstructive Surgery, Faculty of
Medicine, Graduate School of Medicine, Kagawa University, Kagawa
Prefecture, Kida County, Miki-Cho Ikenobe 1750-1, Japan;
E-mail: nagasao@med.kagawa-u.ac.jp
This study was approved by the institutional review board of Kagawa
University.
The finite element models used in the present study are preserved in digitalformats. So the analyses conducted in the study are reproducible.
Part of the present study was supported by a Grant-in-Aid for Scientific
Research (C: 15K01292) provided by the Ministry of Education, Culture,
Sports, Science and Technology of the Japanese Government.
AQ4 The authors report no conflicts of interest.
Authors’ contribution: KI did part of the analyses. TN directed the study andwrote the manuscript. NA corrected CT data for model production. TM
provided clinical advice about the meaning of the study. MT reviewed thearticle. MM provided advice from the standpoint of a dentist.
Example of analysis result showing progress of fracture in response to
impact on the IC (Inferior-Central) region (Supplemental Digital Content, http://links.lww.com/SCS/A359).
Supplemental digital contents are available for this article. Direct URL
citations appear in the printed text and are provided in the HTML and
PDF versions of this article on the journal’s Web site (www.jcraniofacialsurgery.com).
Copyright # 2018 by Mutaz B. Habal, MD
ISSN: 1049-2275
DOI: 10.1097/SCS.0000000000004926

the optic nerve should be considered to avoid potential occurrenceof blindness.
Key Words: Blindness, finite element analysis, fracture, frontal
bone, optic canal, orbit, simulation(J Craniofac Surg 2018;00: 00–00)

W

hen the frontal region is strongly struck, the frontal bone canfracture.1– 5 The anatomical structure of the frontal bonepresents inhomogeneity according to location.6 Some parts of thefrontal bone are thick, and others are thin. Some parts are adjacent to
the frontal sinus, and others are not. Because of this structural
unevenness, the frontal bone presents various fracture patterns,
depending on which part is struck.
Among various fracture patterns, those involving the optic canalneed to be treated with great care,7 because fracture of the optic
canal can cause edema of the optic nerve and subsequent blindness.
The present study, using simulation with computer models, aims toidentify the patterns of frontal bone fracture wherein the involvement of the optic canal should be suspected.

METHODS
Model Production
Computer tomography (CT) data of the skulls of 10 males (mean SD: 37.5  6.7 years) were collected. The 10 patients had been
randomly selected from those who received CT examination of thebrain at Kagawa University Hospital for the screening of brain
injury after trauma. None of the 10 patients had congenital anomalyof the skull. The usage of their data for the present study was agreed
to by the patients, and the study design throughout this study wasapproved by the institutional review board of Kagawa University.
The CT data in DICOM (Digital Imaging and Communicationsin Medicine) files were transferred to preprocessor software (Scan
IP; Simpleware Ltd, Exeter, UK), where the data were transformed
to Standard Triangulated Language files, forming 3-dimensional
simulation models of the 10 skulls. The models were divided into
1,450,000 to 1,890,000 tetrahedrally shaped 10-node elements.
Appropriate specific density and Young’s modulus were assigned
to each element. Specific density was calculated by translating
Housefield’s gray scale8; Young’s modulus was calculated based on
the equation of Kopperdahl (E ¼ 34.7 þ 3230 QCT), where E and
QCT mean Young’s modulus (in megapascals) and CT density (ingrams per milliliter), respectively.9 Thus, 10 models reflecting theaccurate shapes and material properties of the original 10 skulls
were produced.

Dynamic Simulation of Impact Application
With each of the 10 models, a simulation experiment was
conducted in which the frontal bone was struck with a 2-cm-radiusbrass sphere. We defined 6 round regions on the frontal area of each

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FIGURE 3. Examples of resultant fractures in macro view. The acronym on each
picture indicates the impacted region. SL, superior-lateral region; SC, superiorcentral region; SM, superior-medial region; IL, inferior-lateral region; IC,inferior-central region; IM, inferior-medial region.

sphere to produce twice the energy of Waterhouse’s experiment.
The yield threshold of the bone was assumed to be 150 megapascals.8,11 With these experimental conditions, we impacted thefrontal bone, and calculated resultant fracture patterns with postprocessor software (Livermore Software Technology Corporation,
Livermore, CA). Examples of impact application are demonstrated
in Figure 2.
FIGURE 1. For each of the 10 finite-element skull models, 6 regions were
defined on the forehead. SL, superior-lateral region; SC, superior-central region;SM, superior-medial region; IL, inferior-lateral region; IC, inferior-central region;
AQ5 IM, inferior-medial region.

model: the superior-medial (SM) region, superior-central (SC)region, superior-lateral (SL) region, inferior-medial (IM) region,
inferior-central (IC) region, and inferior-lateral (IL) region. The
diameter of these 6 regions was defined to be 2 cm, so that thesphere could precisely strike each. These regions are demonstrated
in Figure 1.
In a previous study, Waterhouse struck the eyes of cadavers by
dropping a brass weight at the velocity of 6 m/s to produce blowout
fractures.10 The impact energy of this experimental condition is 7.4
joules. To produce fractures that could involve not only the orbital
wall but also the optic canal, more intense energy must be applied.
Hence, in the present study, we arranged the speed of the brass

Evaluation of Fracture Pattern
The above-stated simulation, striking the 6 regions of eachmodel, produced 6 resultant fractures as shown in Figure 3 (macroview) and Figure 4 (micro view). Since 6 results were obtained foreach of the 10 skull models, 60 fracture patients in total werecollected. Among the 60 patients, the patients where the optic canal
was involved in fracture were identified. For these cases, evaluation
was performed regarding the following 3 items.

Region-wise Frequency
Each model was struck at the 6 regions (Fig. 1). For each of the 6regions, the frequency of presentation of optic-canal fracture
was calculated.

Frequencies of Related Position Injury
The frequencies in which fracture involved the nasal bone,
medial orbital wall, inferior orbital wall, lateral orbital wall, superior orbital wall, anterior and posterior walls of the frontal sinus, andzygoma were evaluated.

Time-Related Development of Fracture
The LS-DYNA solver allows us to observe the time-related
progress of fracture. By using this function, how fracture initiated at
the frontal bone came to involve the optic canal was observed.

FIGURE 2. Dynamic simulation of impacting the forehead was conducted. The
left and right figure pair demonstrates the impact on the superior-central andsuperior-lateral regions, respectively.

2

FIGURE 4. Examples of resultant fractures in orbit-focus views. The acronym oneach picture indicates the impacted region. SL, superior-lateral region; SC,
superior-central region; SM, superior-medial region; IL, inferior-lateral region;IC, inferior-central region; IM, inferior-medial region.

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The Journal of Craniofacial Surgery  Volume 00, Number 00, Month 2018

Frontal Bone Fractures

TABLE 1. Region-wise Frequency of Optic Canal Damage
Impacted Region

Frequency of Optic Canal Involvement

Superior lateral
Superior central
Superior medial
Inferior lateral
Inferior central
Inferior medial

0
7
2
0
0
0

TABLE 2. Involvement of Surrounding Structures in Optic Canal Damage
Patients
Regions

Frequency of Optic Canal Fracture

Nasal bone
Orbital walls
Superior wallMedial wall
Inferior wall
Lateral wall
Frontal sinus
Anterior wall
Posterior wall
Zygoma

5
9
7
0
0
9
9
0

RESULTS
Frequency of Optic Canal Involvement
In the 60 patients, fracture of the optic canal occurred in
9 patients.

Region-wise Frequency of Optic Canal Fracture
In the 9 patients with optic canal fracture, the fracture occurredin 7 patients after IC region impact and in 2 patients after SM region
impact. Region-wise frequencies are shown in Table 1.

Frequencies of Fractures for Related Positions
Frequencies of fractures for the regions on and around the frontalbone are shown in Table 2. All 9 patients involving the optic canalhad fractures on the superior orbital wall and the anterior andposterior walls of the frontal sinus.

Progress of Optic Canal Involvement
In all 9 patients with optic canal involvement, fracture progressed with the same pattern. First, the frontal wall of the frontal
sinus broke; then the posterior wall broke; next, the superior orbital
wall broke; eventually, fracture reached the optic canal. Timecourse fracture development in a patient is shown in Figure 5.

DISCUSSION
The frontal bone breaks when the forehead is strongly impacted.
When the intensity of the impact exceeds a certain degree, thefracture is not localized in the frontal bone and extends to surrounding structures, including the cranial base, optic canal, and orbital
walls.1 Damage to these structures can cause liquorrhea,12 blindness,13 and diplopia,14 respectively. Among these complications,blindness in particular seriously impairs patients’ quality of life.

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2018 Mutaz B. Habal, MD

FIGURE 5. Representative pattern of fracture progress. (A) Fracture first occurs
on the anterior wall of the frontal sinus (AW). (B) Then the posterior wall of thefrontal sinus (PW) breaks. (C) The superior rim of the orbit (SR) breaks. (D)Fracture finally involves the optic canal (OC).

Blindness is caused when injury of the optic canal results insubsequent edema of the optic nerve.
Habal and Maniscalco conducted a minute anatomical study
regarding the structure of the optic nerve and classified it into 5
parts: the chiasmatic part, the intracranial part, the inter-canalicularpart, the inter-orbital part, and the interocular part.15 Habal andManiscalco showed that the optic nerve lies adjacent to the sphenoid
plate at the intracranial part, and to the optic canal at the intercanalicular part. Demonstrating that the optic nerve is tightly
surrounded by a bone wall at the inter-canalicular part, Maniscalcoand Habal state that the optic canal can be injured in the event of
trauma to the front-orbital region, referring to the potential necessity of decompression of the nerve.16
Hence, in examining trauma of the forehead, it is necessary todiagnose whether or not the optic canal is injured. Believing that
identification of fracture patterns wherein injury of the optic canal islikely to occur is useful to making more precise diagnoses, we haveundertaken the present study.
Finite-element simulation was used in the present study. Finiteelement simulation is an established technique for medical research,and is used to clarify the biomechanical behaviors of such organs as
facial bones,17 thoraces,18 and skin.19 We employed finite-element
simulation in the present study, because it guarantees accuracy ofexperimental conditions. In most previous studies regarding themechanisms of facial bone fracture, actual human skulls were
impacted, and resultant fracture patterns were observed.20–22 With
such methods, reproducibility and accuracy of the experimental
conditions cannot be guaranteed because of instability of theconditions of impact application. In addition, ethically speaking,damaging actual human skulls is unacceptable in the 21st century.
Finite-element simulation is advantageous from these standpoints,
because it not only guarantees accuracy of experimental conditions
but also has few ethical problems.
Human skulls vary in shape and hardness. Because of individual
differences, a finding with 1 skull is not necessarily true for others.
To avoid this problem and increase the generality of the findings,
we performed the same set of experiments (brass-sphere impacting
on 6 regions) with 10 skull models. Evaluation with 10 models
improves the reliability of the findings.

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Besides the accuracy of the experimental conditions, anothermethodologic advantage of finite-element simulation is that it
enables observation of the progress of the fracture. In previous
studies on actual human skulls, only the final condition of thefracture was observable; the process by which the fracture advances
was unobservable due to the brevity of the time of impact application. On the contrary, with finite-element simulation, the progress offracture can be observed at any moment (Fig. 5), by halting
calculation. This function enables observation of how fracture
initiated at the frontal bone advances to surrounding structures.
In all 9 patients where the optic canal was damaged, fracture firstoccurred at the frontal sinus, progressing to the upper orbital wall,and eventually breaking the optic canal (Fig. 6). This pattern, where
the fracture continuously progresses from the directly impacted site
to the opposite side, may appear a matter of course and unworthy of
discussion. However, this pattern is not necessarily the case withother fractures. For instance, when the region around the infraorbital foramen is impacted, the directly impacted site and theinferior orbital wall may fracture. However, the inferior orbital rim,
the structure between the 2 sites, often remains unbroken. This
characteristic fracture pattern is given the special name ‘‘blowout
fracture.’’ Before performing this study, we had expected a similar
phenomenon might occur with frontal bone fractures, presenting
discontinuity between fractures at the frontal sinus and the optic
canal, without the destruction of the upper orbital wall. However,such an enclave-like fracture pattern occurred in none of the 9patients. In all patients where the optic canal was damaged, fracture
occurred presenting a continuously progressive pattern.
In all 9 patients showing damage of the optic canal, fracture
involved the anterior/posterior walls of the frontal sinus and theupper orbital wall. Inductive evaluation of this finding allows us topropose a diagnostic protocol: ‘‘When the anterior/posterior wallsof the frontal sinus and the upper orbital wall are broken, the optic
canal is highly likely to be involved in the damage, so injury should
be suspected.’’ This diagnostic protocol is useful in screening for
optic canal injury at emergency rooms.
When the optic canal is injured, decompression of the optic
nerve should be performed as soon as possible to prevent edema of
the optic nerve and subsequent blindness. To perform this earlystage prevention, damage to the optic canal must be screened for
immediately after injury. However, identification of optic canaldamage in an emergency room is often challenging for the following 2 reasons. First, the patient is often unconscious, so physicians
cannot examine the patient’s visual function. Second, the fracture of
the optic canal is often too subtle, presenting only small slits, to befound in low resolution computer-tomographic images. Hence, inearly stage screening, the fracture can go undetected.
The given diagnostic protocol works to reduce such misjudgment. When evident fracture is observed in the previously stated 3

FIGURE 6. Schematic demonstration of fracture progress pattern.

4

structures (the anterior and posterior walls of the frontal sinus, andthe upper orbital wall), the optic canal is likely to be damaged too,even if the damage is not evident. In such situations, the optic canalshould be re-examined with computer tomography at a higher
resolution, and double checked by a team of physicians. By taking
such care, the occurrence of blindness can be prevented.
The present study focused on mechanisms of frontal bone
fracture. However, besides the frontal bone, fracture also occurs
frequently on other facial bones, such as the zygoma, upper jaw, andnasal bone. We plan to extend our study to include these bones toclarify diagnostic protocols regarding these bones. We believe
accumulation of such diagnostic protocols will greatly contributeto the accuracy of diagnosis of facial bone fractures.
Furthermore, recent advancement of microscopic and endoscopic surgical techniques enables surgeons to access the tissues
around the optic canal and the cavernous sinus more accurately andeffectively than in the past.23,24 Accordingly, the necessity ofevaluating damage to these structures in traumatic situations isincreasing. In the present study, the damage to the optic canal wasobserved at the macro level. However, since the optic canal is a
subtle anatomical structure, minor damage, even on scales <1 mm,
can cause edema of the optic nerve and impair it. Hence, we plan toexpand our studies to elucidate micro-level damage of the optic
canal in traumatic situations in our future studies.

CONCLUSION
By using finite-element simulation, the present study analyzed
various fracture patterns of the frontal bone. In 60 patients withdynamic simulation with 10 skull models, the optic canal was
injured in 9 patients. All the 9 patients had fracture of the anterior
and posterior walls of the frontal sinus and of the upper orbital wallin common. Inductive reference of this finding suggests a diagnostic
protocol: ‘‘When the anterior and posterior walls of the frontal sinus
and the upper orbital wall are broken, fracture of the optic canalshould be suspected.’’ Clinical application of this protocol inemergency diagnosis facilitates accurate screening of optic canaldamage, and contributes to prevention of possible blindness.

ACKNOWLEDGMENT
The authors extend their gratitude to Shingo Kagawa and Miki
Miyazaki at JSOL Co (Tokyo, Japan) for their technical support infinite element analyses in the present study.

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