THE EFFECTS OF DIFFERENT CRANIAL SIZE ON MECHANICAL
PROPERTIES OF CRANIAL SUTURE IN RAT AND SAME-AGED MICE
Yii-Der Wu1, Chi-Hui Chien1,* Yuh J. Chao2 and Thaiping Chen1,3
Department of Mechanical and Electro-Mechanical Engineering,
National Sun Yat-Sen University, Kaohsiung, Taiwan 804, R. O. C.
2
Department of Mechanical Engineering, University of South Carolina,
300 Main Street, Columbia, SC 29208, U.S.A.
3
Department of Electrical Engineering
Fortune Institute of Technology, Kaohsiung, Taiwan, 83160, R. O. C.
*[email protected]
1
ABSTRACT
The objective of this study was to determine the mechanical properties on different size of cranial sutures in rat and
same-aged mice. Ten sagittal sutures were harvested from 4-month-old Lewis rats and C57BL/6 mice. The
specimens, kept moist, were mounted fresh and distracted until rupture. Load-displacement curve was constructed.
The stiffness, Young’s modulus, fracture stress and fracture energy were calculated. Moreover, fracture mechanism,
SEM images, AFM images are also discussed. The discuss of different cranial size on mechanical properties of
cranial suture in rat and same-aged mices was also provided. The results show difference between the two groups
with higher values from rat. It shows the higher mechanicl properties occurred mainly on the large type of build in our
mammal system.
Introduction
“Cranial suture” is the soft tissue [1-5] connecting the cranial bones forming the skull of mammals and is responsible
for regulating the growth of cranial bones. Abnormal growth of brain/skull in infants results in craniosynostosis.
Furthermore, the frequency of head injuries among humans has inspired numerous investigations of the mechanical
properties of cranium. This work focused on the mechanical characteristics of the cranial suture in Lewis rat and
C57BL/6 mice, where the cranial size in rat is larger than in mice (as shown in Figure 1).
Mechanical properties of cranial sutures in mammalian skulls have been studied by Jaslow [6]. He showed that
sutures were not as strong in bending as bone. The load-displacement characteristics of Neonatal rat cranial sutures
have been studied by McLaughlin et al. [7]. Their study provides data regarding the basic mechanical behavior of
neonatal cranial sutures in mammalian system. Infant skull and suture properties have been studied by Margulies and
Thibault [8]. They have shown that the elastic modulus and the rupture modulus of infant cranial bone and suture
increase significantly with loading rate but do not approach adult values. And the energy absorbed to failure in each
of the pediatric tissues does not change significantly with loading rate.
The objective of this study was to determine the mechanical properties of cranial sutures in different mammals.
Ten sagittal sutures were harvested from 4-month-old female rats and mice. The specimens, kept moist, were
mounted fresh and distract at 5 µm/sec until rupture. Load-displacement curve were constructed.
Figure 1. (Right) Lewis rat skull (Left) C57BL/6 mouse skull
Materials and Methods
Lewis rats and C57BL/6 mice were obtained from the Department of Cell and Molecular Biology at Medical
College of Georgia. Ten Lewis rats (female) and C57BL/6 mice (female) aged 4 months were deeply anesthetized
with isofluorane in a chamber then euthanasized by cervical dislocation. The skull specimens were stored in DMEM
and kept in 5°C before testing. The bone-suture-bone specimens were obtained from sagittal suture as shown in
Figure 2. Wet bone-suture-bone samples were tested in the air less than 5 min from the time taken out of the
container to fracture to maintain hydration throughout the tests.
P Frontal
Coronal
Coronal
Bone-suture-bone specimen
Sagittal
Lambdoid
Lambdoid
Figure 2. The bone-suture-bone specimen as harvested from the skull
Applying the elastic formula for tensile test configuration and using the slope of the linear region BC (as shown
in Figure 3) from the load-displacement curve, the stiffness of the sample can be calculated by [9]
S=
∆P
∆d
∆P
and
where
(1)
∆d are the load level and displacement from B to C (the linear part), respectively.
Using the slope of the linear region BC from the stress-strain curve (as shown in Figure 3), the Young’s modulus,
E, of the sample is written by [10]
E=
∆σ
∆ε
(2)
where E is the slope of BC section from the stress-strain curve shown in Fig. 1, ∆σand ∆ε are the stress level
and strain from B to C, respectively.
If one assumes the elastic response until fracture, the maximum stress or the fracture strength, σc , can then be
calculated by
σc =
Pmax
A
(3)
where Pmax is the maximum load from the test and A is the cross-sectional area.
The area under the complete load-displacement curve yields the total energy to break the sample. And, the area
under the curve from A to D gives the energy to fracture (initiation).
S t res s (MP a)
0.8
D
0.6
Maximum load
C
0.4
B
Yielding point
Elastic region
0.2
0
E
A
0
5
10
Strain
FIGURE 3. A typical stress-strain curve of suture specimen in C57BL/6 mice.
Experimental Observation
Figure 3 shows a typical stress-strain curve of bone-suture-bone sample. It was converted from the loaddisplacement curve at a speed of 5 µm/sec until rupture using the cross sectional area and width of the suture. The
curve can be divided into four major regions. The initial part AB is non-linear which could be due to the wetness and
settling of the test sample to have good contact with the support. BC part is linear. CD becomes non-linear again
which demonstrates the non-linear behavior of the bone structure/material after yielding. Point D has the maximum
load and, as in typical brittle fracture, fracture initiation is assumed to occur at this point.
The schematic of the specimen under tension and SEM image of the bone-suture-bone specimen are shown in
Figure 4 and figure 5, respectively. The AFM images of both bone and suture surface are shown in Figure 6(a) and
Figure 6(b), respectively. In Figure 6(a), the AFM image shows the bone surface with some small cavities but flatter
than the suture, Figure 6(b). On the suture surface as shown in Figure 6(b) there are some rough tissue fibers
present.
Figure 7 shows the fracture process of the bone-suture-bone specimen during a tensile test. Figure 7(a) is before
fracture corresponding to AD part in Figure 3. Fig. 7(b) at or immediately after the fracture point (Point D). Finally,
Fig. 7(c) shows the soft tissue extension mechanism (DE part) after the initial fracture. Figure 7 also clearly shows
that the deformation of the suture-bone-suture sample is pre-dominantly from the cranial suture and not from the
bone. This observation is quite reasonable because the cranial bone is much more rigid than the cranial suture.
Bone
Suture
Bone
FIGURE 4. Two solid squares are the locations for AFM imaging
FIGURE 5. SEM image of bone-suture-bone specimen in C57BL/6 mice
(a)
(b)
Figure 6. (a) AFM images from the surface of cranial bone (b) AFM images from the suture surface in C57BL/6 mice
(a)
(b)
(c)
Figure 7. Fracture process of the bone-suture-bone specimen during a tensile test (a) before fracture (b) fracturing
followed by (c) soft tissue extension
Results
Using equations (1) to (3) and the experimental data, results of cross-sectional area, fracture strength, fracture
energy, stiffness and Young’s modulus for Lewis rat and C57BL/6 mice are presented in Table 1. The mean value ±
standard difference of fracture strength, fracture energy and Young’s modulus in Lewis rats are determined as 2.00 ±
0.77 MPa, 1.54 ± 0. 47 mJ and 2.35 ± 0.86 MPa, respectively. Similarly, The mean value ± standard difference of
fracture strength, fracture energy and Young’s modulus in C57BL/6 mice are determined as 0.62 ± 0.05 MPa, 0.10 ±
0. 07 mJ and 0.58 ± 0.22 MPa, respectively. These values can be regarded as the properties of the cranial suture of
the C57BL/6 mice although the specimens were bone-suture-bone as shown in Figure 2.
The geometric size, Young’s modulus, fracture strength and fracture energy were calculated and listed in Table 1.
The volume (width*length*depth) of skull in Lewis rat is about 6 times than in C57BL/6 mice.The value of Young’s
modulus in Lewis rat is about 4.05 times than in C57BL/6 mice, the value of Fracture energy in Lewis rat is about
15.4 times than in C57BL/6 mice and the value of fracture strength in Lewis rat is about 3.23 times than in C57BL/6
mice. The size effect of different type of build in mechanical behaviors of cranial sutures under tensile loading shows
all difference between the two groups with higher values from the large size animal (Lewis rat).
TABLE 1. Size and mechanical properties of cranial sutures from Lewis rat and C57BL/6 mice.
Material
Cranial
width/mm
Cranial
length/mm
Cranial
depth/mm
Young’s
modulus/MPa
Fracture energy/
mJ
Fracture
strength/MPa
Rat
~13.5
~23.5
~1.7
2.35 ± 0.86
1.54 ± 0. 47
2.00 ± 0.77
Mice
~10.0
~15.0
~0.6
0.58 ± 0.22
0.10 ± 0. 07
0.62 ± 0.05
Conclusions
Mechanical properties of cranial suture in Lewis rat and C57BL/6 mice were studied and provided from this study.
The fracture strength, fracture energy, stiffness and Young’s modulus were measured or calculated from the test data.
Moreover, fracture mechanism, SEM images, AFM images and the effect of different size animals are also discussed.
The results show difference between the two groups with higher values from rat. In conclusion, it shows the higher
mechanicl properties occurred mainly on the large type of build in our mammal system. Besides, These mechanical
properties are also useful for further understanding of the biomechanical behavior in human cranium.
Acknowledgements
Financial support for Yii-Der Wu’s in the Department of Mechanical Engineering at the University of South Carolina
was provided by grand 094-2917-I-110-002 from the National Science Council in Taiwan, R.O.C.
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