ORIGNAL RESEARCH

How Partial Skull Defect Affects
Vulnerability of the Skull in Traumatic
Situations: A Biomechanical Study
Tomohisa Nagasao, MD1; Tomoki Miyanagi, MD1; Motoki Tamai, MD1; Asako Hatano,
MD, PhD2; Yoshiaki Sakamoto, MD, PhD3; Naoki Takano, PhD4

Abstract
Background. Part of the skull can be lost due to neurosurgical diseases or trauma. Skulls with partial defects can develop
different fracture patterns from those of intact skulls. This study aims to clarify the differences.
Methods. A 3-dimensional skull model was produced by referring to the computer-tomography data of a 23-year-old intact
male volunteer. We defined the model as Intact Model. Another model was produced by removing part of the frontal bone,which was defined as Defect Model. Dynamic simulations of impacts were performed varying the site and direction of impact.
Fracture patterns caused by the impacts were calculated using dynamic analysis software (LS-DYNA; Livermore Software
Technology Corp.) and were compared between the intact model and defect model.
Results. When Defect Model was impacted, fracture involved wider areas than when Intact Model was impacted. This finding
was observed not only when Defect Model was impacted on its defect side but also when it was impacted on its intact side.
Conclusions. When a skull carrying a defect on one side is impacted, serious fracture occurs even when the non-defect side is
impacted, meaning that a skull with a defect is vulnerable to impacts on the non-defect side. This finding should be taken into
consideration in deciding indications of skull defect reconstruction.

Key Words: defect; skull; finite element; simulation; fracture; trauma

Introduction
Partial cranial defects can occur when part of the skull
is damaged in trauma situations.1,2 To decompress increased
intracranial pressure due to brain tumors or intracranial hemorrhage,3,4 bone pieces are temporarily removed. Though theremoved pieces are usually replaced in their original positions, thepieces sometimes develop infection and are therefore discarded,5causing partial defect of the skull (Figure 1). Partial defects ofskulls can be restored with artificial bone,6-8 titanium plates,9-11
and autogenous tissues.12,13
However, patients with partial defects do not necessarily
wish to have the defects restored; some patients have received
multiple surgeries and are often frustrated by more surgery. In
deciding whether to perform skull reconstruction surgery, merits
and demerits of the reconstruction surgery must be considered.
One possible demerit is potential increase in the vulnerability ofeplasty

the skull in trauma situations. Intuitively, it seems possible thatwhen impacted, a skull that has a defect will be damaged more
seriously than an intact skull. However, no evidence has been
provided to this stipulation, so far. The present study performs
dynamic simulations by using 3-dimensional computer models
to investigate how the presence of a partial skull defect affects
the biomechanical vulnerability of the skull.

Methods and Materials
This study was conducted under the approval of the research
ethics committee of Kagawa University (Approval Number:
H29-170).
Production of 3-dimensional computer models of the skull
Computed tomography (CT) data of a 23-year-old male volunteer, who received CT examination to screen for brain injury
1Nagasao, et al.

PARTIAL SKULL DEFECT AFFECTS VULNERABILITY OF THE SKULL

after striking head during a traffic accident, were used to produce 3-dimensional
computer skull models. The examination revealed damage to neither his skull
nor his brain, and his skull showed nocongenital deformity. The CT data weretransferred to preprocessor software(Scan IP; Simpleware Ltd.) as DICOM(Digital Imaging and Communications in
Medicine) files and then transformed to
STL (Standard Triangulated Language)
Figure 1. Female pafiles, forming a 3-dimensional simulation
tient with partial defectmodel of the skull. This model was definedon the right side of skull.
as Intact Model. Simulating decompression craniectomy, part of the bone wasremoved from the left anterior-lateral region of Intact Modelby using 3-dimensional graphic software (Geomagic Freeform;
3D SYSTEMS).
The model with the defect was defined as Defect Model. By
using the preprocessor software (Scan IP) again, Intact Modeland Defect Model were transformed to finite element models
consisting of 2,230,000 and 1,820,000 tetrahedrally shaped 10node elements, respectively. Specific density of the skull bone
was calculated by translating Housefield gray scale14; Young’s
modulus was calculated based on the equation of Kopperdahl {E= -34.7 + 3230 QCT}, where E and QCT mean Young’s modulus(in MPa) and CT density, respectively.15 The calculated material
properties were assigned to the finite element models.
Dynamic Simulation of Impact Application
Modelling of Striking Tool
Simulating a lead ball with a diameter of 25 mm, a sphereshaped finite element model was produced. The specific gravity
and Young’s modulus of lead were assigned to the ball.
Conditions of Impact Application
The junction between the frontal bone and the zygoma wasdefined as anterior-lateral (AL) point. The horizontal planecrossing the AL point, the sagittal plane crossing the tragus, andthe coronal plane crossing the tragus intersect at a point. That
point was defined as temporal-lateral (TL) point.
Intact Model and Defect Model were impacted with the ballmodel in the following conditions (Figure 2):
Condition 1: Intact Model impacted at right AL point
Condition 2: Defect Model impacted at left AL point
Condition 3: Defect Model impacted at right AL point
Condition 4: Intact Model impacted at right TL point
Condition 5: Defect Model impacted at left TL point
Condition 6: Defect Model impacted at right TL point
Velocity and Direction of Impact
The lead ball model was initially placed 5 mm away from theskull models. Then the ball model was launched at a velocity of
10 m/s. For each of the 6 impact application patterns, the ball
was launched varying the direction in 3 ways: at 30° of elevation
2

PARTIAL SKULL DEFECT AFFECTS VULNERABILITY OF THE SKULL

Nagasao, et al.

Figure 3. Fractures occurring in Condition 1. Damage of the bone is localized near the impacted sites. The 3 arrows indicate the direction in which
the ball is launched.

Figure 7. Fractures occurring in Condition 5. Wide areas are destroyed. The
3 arrows indicate the direction in which the ball is launched.

Figure 4. Fractures occurring in Condition 2. Wide areas of bone on thedefect side are damaged. The 3 arrows indicate the direction in which theball is launched.

Figure 8. Fractures occurring in Condition 6. Compared with the equivalent
impact pattern in Intact Model, wider areas of the skull are damaged. Difference is most evident when the impact is horizontally directed (Center).
The 3 arrows indicate the direction in which the ball is launched.

Figure 2. Conditions of impact application. The 6 ellipses indicate the skull
viewed from the upper side. The anterior sides of the skulls correspond to
the lower side of the figure. The dark-colored quadrants indicate defects.
In each of these conditions, impact was applied in 3 different directions (at30° in elevation; horizontally; at 30° downward)(meaning upward), horizontally (meaning parallel to Frankfort
Plane), and at 30° of depression (meaning downward).
Calculation
Dynamic calculation was performed under the above-statedconditions with LS-DYNA, and fracture patterns resulting fromthe impact were calculated. Transient analysis was used in thecalculation, wherein an element disappears when the strain
occurring on it exceeds its breakdown threshold. The yield
threshold of the bone was assumed to be 150 MPa.16
Evaluation
After the conditions of fracture caused by impacts were
calculated, the positions and degrees of fracture were evaluated.

(Figure 8), fracture involves wider areas than in Condition 4(Figure 6). These findings indicate that when a skull with a defectis impacted, even when the non-defect side is impacted, more
serious fracture occurs than when a normal skull is impacted.

Discussion

Results
Fractures occurring in the 6 conditions are shown in Figures
3-8. The findings are summarized as follows.

Figure 5. Fractures occurring in Condition 3. Wider areas of the skull are
destroyed than in the equivalent condition in Intact Model. The 3 arrows
indicate the direction in which the ball is launched.

When the defect side of a skull with a defect is impacted,
serious fractures occur.
In Condition 1, fracture localizes near the impact area (Figure 3).
In Condition 2, in contrast, fracture extends to wide areas beyond
the impacted site (Figure 4). Similarly, in Condition 5 (Figure 7),
fracture extends to a much wider area than in Condition 4(Figure 6).
These findings indicate that when a skull with a defect is
impacted on its defect side, serious fractures occur.
When the non-defect side of a skull is impacted, more serious
fracture occurs than when an intact skull is impacted.
In Condition 3 (Figure 5), fracture occurs across wider
areas than in Condition 1 (Figure 3). Similarly in Condition 6
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Figure 6. Fractures occurring in Condition 4. When the impact is applied
upward, serious fracture occurs (Left). When impact is applied horizontally(Center) or downward (Right), fracture is localized. The 3 arrows indicate
the direction in which the ball is launched.

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Partial defects of the skull can be caused by injury or bydecompression of intracranial pressure for the treatment of
intracranial hemorrhage or brain tumors. We raised the clinicalquestion of whether or not the presence of partial skull defects
increases the vulnerability of the skull in trauma situations.
This question is important from a clinical viewpoint because
patients who have skull defects may fall and hit their heads while
walking or be involved in traffic accidents due to impaired motor
functions. In addition to this clinical interest, we conducted thepresent study to clarify the relationship between partial skull
defect and the vulnerability of the skull.
Methodologically, the simplest way to evaluate the vulnerabilityof skulls is to strike actual skulls. However, accurate replicationof experimental conditions is difficult in this method. Becauseof the structural complexity of the skull, subtle differencesin striking sites and directions can greatly affect subsequent
fracture patterns. Furthermore, use of skulls for experimental
purposes raises controversy from ethical standpoints. Therefore,3PARTIAL SKULL DEFECT AFFECTS VULNERABILITY OF THE SKULL

Nagasao, et al.

we conducted computer simulation rather than using actual
skulls in this study.
Computer simulation with the finite element method is anestablished method in craniofacial research and is used to clarify the occurrence mechanisms of orbital floor fractures17 andfrontal bone fractures18,19 and in the evaluation of operationalrisks in Le Fort 3 osteotomy.20 Computer simulation allows us toapply impact at specific sites of the skull with specific directions.
Moreover, computer simulation on virtual skulls is not ethicallycontroversial. Because of these advantages, computer simulation
was used in the present study.

warfare-related skull injury, skulls are often reconstructed insecondary treatments.23,24
Despite the importance of secondary cranial reconstruction,
patients with skull defects often remain undecided whether or
not to receive skull reconstruction surgery because rejection of
the surgery is not fatal. To support the decision-making processof these patients, detailed information should be provided regarding the advantages and disadvantages of skull reconstruction.
Although accurate understanding of the vulnerability of thedefective skull should be a part of such information, little suchinformation has been available.
The lack of objective information can mislead patients. For
instance, some patients might think, “I have a defect in myskull. I know the brain might be seriously damaged if the skull
around the defect is struck. However, if only I take care not to
expose the defective side, I’ll be safe,” and refuse to receive skullreconstruction. However, the second finding shows that even if
the intact side of the head is struck, serious fractures can occur.
Proof of this phenomenon might encourage patients to undergo
restoration surgery of the skull.
Patients with partial skull defects usually protect their heads
with special helmets. The second finding provides a guiding
concept for designing such protection. Intuitively, it appears thatmere coverage of the area overlying the skull defect is enoughto protect the brain. However, the second finding demonstrates
that impact application on the non-defect side of the skull cancause serious brain damage. Therefore, protection should bedesigned so that it protects not only the defective side of theskull but also the intact side. Thus, taking the second finding
into consideration in designing head protection can avoid theoccurrence of serious brain damage.

Limitations
In the present study, models simulating merely the bone partof the head were produced. However, the actual head contains
soft tissues such as the scalp, fat, and muscles. These soft tissues
can function as shock absorbers when the head is impacted.
Accordingly, fractures occurring on actual heads can be lessserious than those on bare skulls. It is expected that the generalfindings with skull models―that more serious fractures canoccur in defective skulls than in intact skulls—also holdrue withactual heads. However, inclusion of soft tissues in the simulation
models would make the results more persuasive. Models for
future studies will attempt to incorporate these considerations.
The findings of the present study are summarized as 2 propositions. First, when a defective skull is impacted on the side of thedefect, serious fractures occur. Second, even when the non-defectside of a defective skull is impacted, fracture becomes more serious than when the equivalent site of an intact skull is impacted.
The first finding confirms what people would intuitively
surmise. When the head of a patient who has a defect is impacted
on the side of the defect, a wide area of the skull is destroyed,
potentially accompanied by serious damage of the brain. Patientsshould take care to protect the defective sides in their daily lives.
The second finding is important for patients making decisions
on whether or not to receive skull reconstruction surgery. For
patients who have life-threatening diseases such as malignant
tumors, the decision of whether or not to receive the surgicaltreatment is not difficult because the rejection of treatment
results in death.
When the defect of the skull remains unrepaired, the patient
can occasionally develop headache or depression. These symptoms,
called sinking skin flap syndrome or trephined syndrome, arecaused by unstable intracranial pressure due to a lack of brain
protection. Reconstruction is effective to prevent this complication,
so it is preferable that the skull be reconstructed.21,22 Besides theprevention of skin flap syndrome, the skull should be repaired
to avoid serious brain injury. The brain under the skull defectcan be seriously damaged if the defective part is impacted in
accidents or other trauma situations. Hence, in cases where
the defects were not reconstructed in emergency care, such as
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PARTIAL SKULL DEFECT AFFECTS VULNERABILITY OF THE SKULL

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Affiliations: 1Department of Plastic and Reconstructive Surgery, Faculty of Medicine/Graduate
School of Medicine, Kagawa University, Japan; 2Department of Plastic and Reconstructive
Surgery, Sanno Hospital, Japan; 3Department of Plastic and Reconstructive Surgery, Facultyof Medicine, Keio University, Tokyo, Japan; 4Department of Mechanical Engineering, Keio
University, Tokyo, Japan.
Correspondence: Tomohisa Nagasao, MD; nagasao@med.kagawa-u.ac.jp
Disclosures: The authors report no competing or financial interests. Part of the present
study was supported by a Grant-in-Aid for Scientific Research (C: 18K12034) provided by the
Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.

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Conclusions
This study demonstrates the influence of partial skull defects
on the mechanical vulnerability of the skull. Three-dimensional
computer models of the skull were produced, and dynamic impacts were applied to them. Observation of subsequent fracture
patterns was performed. The essential finding is that when askull with a partial defect is impacted, even when it is impacted
on the contralateral side of the defect, more serious fractures
occur than when an intact skull is impacted. This finding should
be referred to in planning skull reconstruction surgery and/or
in designing head protection.

Acknowledgments
The authors extend their gratitude to Shingo KAGAWA andMiki MIYAZAKI at JSOL CO. (Tokyo, Japan) for their technicalsupport in finite element analyses in the present study.
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