中国组织工程研究与临床康复 第 14 卷 第 13 期 2010–03–26 出版
Journal of Clinical Rehabilitative Tissue Engineering Research March 26, 2010 Vol.14, No.13
ISSN 1673-8225 CN 21-1539/R CODEN: ZLKHAH 2462
1Military Orthopedic
Center, Urumqi
General Hospital of
Lanzhou Military
Area Command of
Chinese PLA,
Urumqi 830000,
Xinjiang Uygur
Autonomous Region,
China; 2Department
of Precision
Instrument, Tsinghua
University, Beijing
100084, China
Shi Zhen-man, Chief
physician, Military
Orthopedic Center,
Urumqi General
Hospital of Lanzhou
Military Area
Command of
Chinese PLA,
Urumqi 830000,
Xinjiang Uygur
Autonomous Region,
China
gszlch@yahoo.com.
cn
Supported by: A
Grant from Tenth
Five-Year Plan of
Chinese PLA, No.
04Z04*
Received: 2009-10-13
Accepted: 2009-12-15
(20090412003/G)
Shi ZM, Shi J, Wang
X, Guo SZ, Wu Y,
Peng J. Finite
element mechanical
analysis on fracture
hip supporting joint
for treatment of
femoral neck
fracture. Zhongguo
Zuzhi Gongcheng
Yanjiu yu Linchuang
Kangfu. 2010;14(13):
2462-2466.
[http://www.crter.cn
http://en.zglckf.com]
Finite element mechanical analysis on fracture hip
supporting joint for treatment of femoral neck fracture*
Shi Zhen-man1, Shi Jiang1, Wang Xin1, Guo Shu-zhang1, Wu Yue2, Peng Jiang1
Abstract
BACKGROUND: For treatment of femoral neck fracture, all therapies with the exception of joint replacement encounter the
problems including slow healing, poor prognosis, various complications, and unable to bear weight for long time. Fracture hip
supporting joint (FHSJ) is a novel unlimited hip support implement that possesses the double functions of fracture fixation and joint
supporting and can be used to prevent and treat the complications of femoral neck fracture in young people.
OBJECTIVE: To investigate the mechanical effects of FHSJ on treatment of femoral neck fracture.
METHODS: Three types of two-dimensional finite element models were constructed by AutoCAD: normal hip (group A), femoral
neck fracture fixed with two screws (group B), and femoral neck fracture fixed with two screws and FHSJ (group C). The grids of
two-dimensional four nodal point elements were divided by ANSYS (PLANE82). Under the identical condition, the calculations
were performed respectively.
RESULTS AND CONCLUSION: The stress peak value of femoral head weight-bearing zone was 1.029 and 1.63 MPa in group
A and group B, respectively, and that in the group C was 0.1-0.4 MPa. The stress peak value of the screws was 37.186 and
7.474 MPa in the group B and group C, respectively. These results indicate that FHSJ installation based on fixation of multiple
screws could promote the recovery of femoral head and neck, which exhibits promising prospects in treatment of femoral neck
fracture in young people.
INTRODUCTION
Fracture of the femoral head is a severe, relatively
uncommon injury; typically, it occurs following
traumatic posterior dislocation of the hip joint.
Diagnosis is aided by a complete history, physical
examination, and imaging, including computed
tomography. Treatment consists of urgent closed
reduction of the dislocated hip followed by nonsurgical
or surgical management of any associated fractures.
Complications associated with fracture of the femoral
head and subsequent treatment include osteonecrosis,
posttraumatic osteoarthritis, and heterotopic. For
treatment of femoral neck fracture, all therapies with
the exception of joint replacement encounter the
problems including slow healing, poor prognosis,
various complications, and unable to bear weight for
long time[1].
Femoral neck fracture poses a significant health care
problem all over the world, with an annual incidence of
approximately 1.7 million, and the number of femoral
neck fracture is expected to triple in the next 50 years.
Although the fixation methods reform over 60 years,
the nonunion rate still remains at approximately 10%,
and the rates of femoral head necrosis and collapse
are not decreased. One-year disability rates currently
range from 14%-36%[2]. Caring for these patients
represents a major global economic burden. Surgical
options for the management of femoral neck fractures
are closely linked to individual patient factors and to
the location and degree of fracture displacement.
To prevent and treat the complications of femoral neck
fracture in young people, we developed fracture hip
supporting joint (FHSJ), a novel unlimited hip
supporting implement that possesses the double
functions of fracture fixation and joint supporting, and
performed finite element mechanical analysis,
hopefully investigating the underlying mechanisms of
action and providing theoretical evidence for clinical
application.
MATERIALS AND METHODS
Materials
Self-made FHSJ of stainless steel; cancellous bone
screw with a diameter of 4.62 mm; anterioposterior
X-ray image of the hip.
Model establishment[3]
Three types of two-dimensional finite element model
were established according to anterioposterior X-ray
image of the hip by AutoCAD. Group A: the normal
hip; group B: femoral neck fracture fixed with two
screws; group C: femoral neck fracture fixed with
two screws and FHSJ. The models included pelvis
and proximal femur, which have identical sizes. The
diameter of femoral head bony part was 45 mm, the
thickness of articular cartilage surface was 2.0 mm,
and the thickness of sub-cartilaginous bone plate was
0.6 mm. The stress distribution on coronal plane of
the femoral head spherical center was simulated.
The pelvic anteversion angle was neglected, which
was vertical to the horizontal plane. The Pauwel’s
angle of femoral neck fracture was 70° and the two
screws were upper-below paralleled through
alignment, which had 30°included angle with
horizontal plane. The assisting acetabulum was
fixed in the iliac bone of acetabular superior border
to broaden the acetabulum. The assisting femoral
head was fixed in the femur with plate and screws,
thus the non-weight-bearing surface of femoral
head became weight-bearing surface. At the same
time, the assisting acetabulum and assisting
femoral head constituted the assisting hip joint
(Figure 1).
Shi ZM, et al. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture
ISSN 1673-8225 CN 21-1539/R CODEN: ZLKHAH 2463
www.CRTER.org
Unit type and material characteristics
The AutoCAD models established were loaded into ANSYS10.0
software (ANSYS, USA). The grids of two-dimensional four
nodal point elements (Figure 2) were divided by ANSYS
(PLANE 82).
Total 14 regions were given and each region was assumed to
be homogeneous, isotropic and linearly elastic material. Elastic
modulus and Poisson’s ration are given in Table 1.
Hypothesis and calculation
Before calculation, the hypothesis of reduced fracture and being
contacted each other, which was permitted by the finite element
calculation and did not impact the final conclusion. There were
two contact pairs being defined: between cortical bones and
between cancellous bones. The influence of screw type was
still neglected. If the condition that FHSJ, screw, and bone
were firmly fixed, without moving each other but with sufficient
screw strength to avoid fragmentation was provided, then
walking, one foot standing, and suffering force in static state,
were simulated. The element of link10 was selected to simulate
the muscle tensile force which suffered from tensile force but no
pressure force. The abductor tensile force (2 000 N) and
adductor tensile force (100 N) as element incipient variable
value were loaded. X and Y degree of freedom catenated with
PLANE82 element corresponding nodal point degree of
freedom, Z degree of freedom was designated as zero. The
boundary condition was according to the method of Wei HW,
body weight 600 N was loaded onto two keypoints at the right
superior angle; the red downward arrow respectively indicated
300 N. In the model, the red upward arrow represented the
self-weight of model. In the ANSYS, it was force of inertia
(Figure 3).
Under the identical condition, the calculations were performed
respectively for each fixed model.
a: Normal hip b: Fixed with two screws
c: Fixed with two screws and fractured hip supporting joint
Figure 1 Three types of two-dimensional finite element
models
a: Normal hip b: Fixed with two screws
c: Fixated with two screws and fractured hip supporting joint
Figure 2 Three types of the two-dimensional finite element
model grids division
Table 1 Material characteristics
Region
Elastic
modulus (MPa)
Poisson’s
ratio
Cortical bone
Subchondral plate
Cancellous bone
Articular cartilage
Stainless less
Pelvis
Neck of femur
Femur
Acetabulum
Femoral head
Pelvis
Femoral head
Neck of femur
Femur
Acetabulum
Femoral head
Screw
Supporting joint of
fractured hip
17 000
2 000
17 000
700
1 100
600
600
1 000
600
15
15
210 000
210 000
0.3
0.3
0.28
0.3
0.3
0.3
0.3
0.3
0.3
0.45
0.45
0.3
0.3
Figure 3 Simulation of the weight loading with single leg
Shi ZM, et al. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture
P.O. Box 1200, Shenyang 110004 cn.zglckf.com 2464
www.CRTER.org
RESULTS AND ANALYSIS
Results
The Von mises stress distribution peak value (MPa) of three
finite element models is displayed in Table 2.
Stress distribution on hip joint
Group A: the stress concentrated on femoral cortical bone, with
a peak value of 45.86 MPa. Group B: the stress concentrated
on screws, with a peak value of 46.175 MPa. Group C: the
stress concentrated on FHSJ, with a peak value of 158.654
MPa. The stress peak value of group C was approximately 5
times that of group A and group B(Figure 4).
Stress distribution on proximate femur
Group A: the stress was distributed from femoral head
ecto-anodic weight-bearing area to trabecular bone and
concentrated on calcar femorale via neck of femur. The stress
peak value was 6.823 MPa.
Group B and group C: the stress band was divided by bone
fracture and screw, and a twist moment formed between distal
end of fracture (calcar femorale) and screw tip. A shear force
pair formed between proximate and distal ends, and the stress
concentrated on screws. The stress peak value of screw in the
turning curve area was 37.186 MPa in the group B, 7.474 MPa
in the group C. The stress peak value of group B was
approximately 5 times that of group B (Figure 5).
Stress distribution on femoral head
Group A: the stress of femoral head ecto-anodic
weight-bearing area was 1.029 MPa. The stress was
distributed from trabecular bone and concentrated in the
junctional zone of femoral head and neck of femur, with a
peak value of 1.282 MPa. Group B and group C: the stress
band was divided by bone fracture and screw. The stress of
group B was concentrated at the femoral head weight-bearing
area which was above the upper screw tip, with a peak value of
1.63 MPa. The stress of group C was concentrated at the
medial-anodic femoral head, with a peak value of 0.759 MPa.
The stress peak value of weight-bearing area was only 0.1-0.4
MPa. The stress peak value of femoral head weight-bearing
area of group V was 1.6 times of group A and 5 times of group C
(Figure 6).
Results demonstrated that when the fractured femoral neck was
fixed by two screws, the stress was concentrated on the screws
and stress peak value of the femoral head weight-bearing area
was higher than the normal femoral head; when the fractured
femoral neck was fixed by two screws and FHSJ, the stress was
concentrated on the FHSJ, which not only decreased the stress
peak value of screw and femoral head weight-bearing area but
also made the stress peak value of femoral head moving toward
medial side, and the stress direction was in concord with the
screw way.
Table 2 Vos mises stress distribution peak value among
three groups (MPa)
Group
Calcar
femorale
A
B
C
Hip joint
Proximate femur
Proximate end of
fracture Screw
45.860
46.175
148.654
6.823
7.596
5.164
0.844
0.574
37.186
7.474
Group
Interior femo-
ral head
A
B
C
Hip joint
Femoral head
Weight-bearing
area
45.860
46.175
148.654
1.282
1.630
0.759
1.029
1.630
0.1-0.4
Group A: normal hip joint; group B: fixed with two screws; group C:
fixed with two screws and supporting joint of fractured hip
a: Normal hip b: Fixed with two screws
c: Fixated with two screws and fractured hip supporting joint
Figure 4 Von mises stress distribution on hip joint after
loading
a: Normal hip b: Fixed with two screws
c: Fixated with two screws and fractured hip supporting joint
Figure 5 Von mises equivalent stress distribution on
proximate femur among three groups
Shi ZM, et al. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture
ISSN 1673-8225 CN 21-1539/R CODEN: ZLKHAH 2465
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DISCUSSION
Pathological characteristics of femoral neck fracture
Special blood supply: There are the internal and external
capsule vascular rings in the femoral neck that form the two
class branches supplying blood to the femoral head. At the
same time, there is no periosteum in the femoral neck, and the
vascular rings and branches were tight on the bone surface.
The histological findings demonstrate that once the
displacement of fractured femoral neck damages the blood
vessel and vascular rings, the ischemic incidence rate of
femoral head would be up to 85%. Following fracture reduction,
fracture union depends on the new vessels of neck getting into
the femoral head, which contributes to femoral head surviving
and new bone formation. However, the speed of neigenesis
was slowly, and fracture union usually needs 7-8 months[4].
Necrosis of the femoral head represents a special form of
aseptic bone necrosis because it develops in the high
load-bearing region of the hip joint. The mechanisms underlying
femoral head necrosis is not precisely known. Characteristic
macroscopic findings include articular cartilage detachment in
the load-bearing zone immediately below the subchondral
osseous lamella, the disperse necrosis of the marrow space
and bone cells, and the gradual loss of normal morphology of
the femoral head[5-12].
The necrosis rate of the femoral head is 11.9%-37.4% in adults
and 21%-87.5% in children. If the necrosis occurs in 1 year
after fracture, it is usually detected at 9-12 months. The
necrosis rate is 56.8%, 96.08%, and 3.92% at 2-3 years, 5
years, and over 5 years after fracture, respectively. The
collapse rate of the femoral head is 24.09%. Time period from
initial necrosis to collapse averages 11.1 months. When
necrosis and collapse occur, the necrosis zone usually resides
in weight-bearing area of the femoral head, and when necrosis
stops, the necrosis zone is located outside of the femoral head
weight bearing area[13-14].
Special anatomy: Femoral neck fractures in the geriatric patient
continue to represent a therapeutic challenge. Despite
advances in surgical techniques and medical care, the risk of
nonunion and osteonecrosis after fixation have not changed
appreciably in the last 50 years[15]. The included angle between
the femoral neck and the femoral shaft is 130°. The curved
angle and the twist moment make femoral neck receive
complicated force. Even though the internal fixation is firm,
loosing, breakage, and bone nonunion easily occur. A union
rate of 80%-90% is described by most authors. Limb length,
rotational and angular deformities can be corrected at the same
time. A total hip replacement is probably the best option for
patients with 65-80 years old[16], but it is not suitable for young
patients[17-18]. To prevent and treat nonunion and necrosis,
effective measures must be explored, which would be able to
decrease the compressive stress of the femoral head, reduce
the complex forces of the femoral head, promote the
regeneration of new bone, and protect the long course of
treatment.
Multiple screws fixation for femoral neck fracture
Screws bearing high stress: After the fractured femoral neck is
fixed with multiple screws, the stress zone of proximate femur is
divided, the femoral head endures the downward pressure with
the upward tensile force of gluteus and the supporting force of
femoral shaft, and these forces form couple[19-20]. That is to say,
the maximum shearing force and the twist moment form at the
fracture site, and the stress is concentrated at the screws used
for fracture fixation. The stress peak value of the screws is 5
times of the proximate femur. The high stress of the screws
would lead to loosing, breakage, and bone nonunion. The
nonunion of fracture would result in a problem that newly
formed vessels at the neck hardly pass through the fracture
interspace and grow into the femoral head, which influence the
transformation and union of the femoral head.
Femoral head bearing high stress: after the fractured femoral
neck is fixed with multiple screws or total hip replacement[19-20],
the stress zone of the femoral head is divided, the stress is
concentrated on the femoral head weight-bearing area, which is
above the upper screw tip, the distance between stress surface
and stress concentrated zone of the femoral head is shortened.
However, the stress peak value is high up to 1.6 times of the
normal femoral head. If the head suffers from the weight loading,
collapsed fracture easily occurs. The femoral neck fracture was
fixed with multiple screws, which depends on the high stress
screw to retain the bone connection, but has few effects on
diminishing the complex stress of fracture zone, promoting the
regeneration of new bone, and protecting the long course of
treatment. Meanwhile, it also induces an increase in the stress
peak value of femoral head weight-bearing area higher than the
normal femoral head, which damages the femoral head.
Multiple screws and FHSJ fixation for femoral neck fracture
Stress bypass: the fractured femoral neck was fixed using
multiple screws and FHSJ, during which, a stress bypass (it
starts from the acetabulum, via the femoral head and neck, to
the femur) is established to substitute the intrinsical pathway of
stress delivery. The FHSJ suffers from high stress in articulus,
diminishes the compressive stress of the femoral head, and
a: Normal hip b: Fixed with two screws
c: Fixated with two screws and fractured hip supporting joint
Figure 6 Von mises equivalent stress distribution on
femoral head among three groups
Shi ZM, et al. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture
P.O. Box 1200, Shenyang 110004 cn.zglckf.com 2466
www.CRTER.org
decreases the shearing force and twist moment of the fracture.
The fixation using multiple screws and FHSJ makes the stress
peak value of screw being 1/5 of the fixation using simple
multiple screws, avoiding screw loosing, breakage, and
nonunion of fracture. The FHSJ protects union of fracture, i.e.,
allowing the new vessels of neck