International Journal of Advanced Robotic Systems
The Kinematic and Static Analysis
of the Tibio-femoral Joint Based
on a Novel Spatial Mechanism
Regular Paper
Yonggang Xu1, Rongying Huang1,* and Qiang Xu1
1 School of Mechanical Engineering and Automation, Beihang University, Beijing, China
* Corresponding author E-mail: buaahry@sina.com.cn
Received 17 Aug 2012; Accepted 31 Aug 2012
DOI: 10.5772/52943
© 2012 Xu et al.; licensee InTech. This is an open access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract To reveal the characteristics of knee movement
and tibio‐femoral joint contact force, a novel single
degree of freedom spatial mechanism is built to
simulate the joint kinematics based on a three
dimensional model of the human knee. The length
changes of the three ligaments can be obtained by
establishing and solving the kinematics spiral function.
Based on this mechanism, a static model is built where
linear springs are used to model the ligaments and
whose stiffness coefficients are obtained by the finite
element method. The main strength of the proposed
model is that it associates the knee’s flexion motion with
internal/external rotation of the tibia based on the
isometricity of the anterior cruciate ligament. This offers
an efficient method to model and analyse the changes of
ligament lengths and static kinematics after ligament
reconstruction, which is crucial in designing knee
recovery and rehabilitation equipment.
Keywords Spatial mechanism, Knee, Ligament, Tibio‐
femoral joint
1. Introduction
The knee is the largest and the most complex joint in the
human body, whose function is providing the
fundamental support for body movements. However,
due to direct exposure, the joint components such as the
anterior cruciate ligament (ACL), are easily injured which
might lead to joint instability and movement changing,
and even disability. The structure/kinematics
characteristics of the knee are a key problem in knee joint
rehabilitation or reconstruction surgery.
Recently, the analysis of the movement of knee joint has
become a hot topic. Due to the knee’s complexity, the
mechanism theory is gradually used to simulate the joint
motion. O’Connor [1] proposed a planar four‐bar linkage
in sagittal plane to analyse the knee movement, where the
anterior cruciate ligament (ACL) and the posterior
cruciate ligament (PCL) were assumed to be isometric
and their positions on the tibia and femur were joint
centres. It had significant value in analysing the knee
motion and the interaction forces between the ligament
and muscle, and also developing a machine to restore
1Yonggang Xu, Rongying Huang and Qiang Xu:
The Kinematic and Static Analysis of the Tibia-Femoral Joint Based on a Novel Spatial Mechanism
www.intechopen.com
ARTICLE
www.intechopen.com Int J Adv Robotic Sy, 2012, Vol. 9, 205:2012
knee functions. Later, Wilson and O’Connor [2] proposed
a novel equivalent spatial mechanism with a single degree
of freedom. The anterior cruciate ligament (ACL), posterior
cruciate ligament (PCL) and medial collateral ligament
(MCL) were considered as links, each ligament connected
to the tibia and femur with a spherical and a universal
joint. The medial and lateral condyles of femur are
modelled as spheres and the tibia condyles as planar
surfaces, the contact between the femur and tibia condyle
are single point of frictionless contacts. In [3‐6], Wilson et
al. improved the tibial condyle planes in the ESM‐1
mechanism into more anatomically‐shaped surfaces by
substituting the femur and tibia in ESM‐1 with two spheres
(ESM‐2) and general shape surfaces (ESM‐3). The results of
the models were in better agreement with experiments
when more anatomically‐shaped surfaces were applied.
Knowing the kinematics of the tibio‐femoral joint is of
importance to analysing the joint contact force and
decreases the possibility of ligament, menisci and
articular cartilage trauma. The paper presents a novel
spatial mechanism to simulate the knee kinematics and
quasi‐static analysis after ACL reconstruction.
2. Material and Methods
2.1 Construct the spatial mechanism of tibio‐femoral joint
2.1.1 Extract the characteristic structure and feature
points of the tibio‐femoral joint
The three dimensional model of the tibio‐femoral joint was
built in MIMICS software based on the computer
tomography (CT) and magnetic resonance images (MRI)
from a volunteer. The surface processing was done in
Geomagic Studio software including Boolean operations,
geometry registering, image registration and NURBS
surfaces generating. Then a three dimensional model of the
tibio‐femoral joint (Fig. 1) was obtained including femur,
tibia, fibular and articular cartilages, and four ligaments
which are the anterior cruciate ligament (ACL), the
posterior cruciate ligament (PCL), the medial collateral
ligament (MCL) and the lateral collateral ligament (LCL).
According to the observations by Amis [7], the fibres
within ACL could be considered isometric during the
knee flexion. In the proposed model, the ACL is
represented by a link ‐ the starting and ending points of
the link is the intersection points between ligaments and
bones as depicted as points, the distance between the two
points is the length of ACL. As in [8], the posterior
femoral condyles were close to spheres, the two circular
posterior femoral condyles are modelled as two spheres,
the axis penetrating through the two spherical centres
(called the flexion facet centres, FFC) was defined as a
rotation feature line of the femur. The tibio‐femoral
contact point was defined as the feature point on the tibia
medial condyle.
Figure 1. The feature elements on the tibio‐femoral joint
2.1.2 Spheres fitting on circular posterior femoral condyles
After constructing the spatial mechanism of the tibio‐
femoral joint, regression algorithms are used to solve the
rotation line of the femur. Let 0 0 0 0( , , )P x y z be the centre
of the sphere, r be the radius of the sphere, the spherical
equation can be expressed as:
2 2 2 20 0 0( ) ( ) ( )x x y y z z r (1)
Twenty points on the posterior femoral condyles’ surface
were selected, of which the position vector is defined as
( , , )i i i iA x y z , ( 1,2, ,20)i . In line with the least
squares principle, the residual error denoted by f could be
expressed as:
2 2 2 2 20 0 0
1
[( ) ( ) ( ) ]
n
i i i
i
f x x y y z z r
(2)
simplified as:
20 0 0
1
[ 2 2 2 ]
n
i i i i
i
f x x y y z z R b
(3)
where R and ib is expressed as:
2 2 2 2
0 0 0
2 2 2
i i i i
R x y z r
b x y z
the position of sphere centres, as well as the radius of
each sphere can be obtained as the least squares solution
of equation (3). The results show that the radius of the
medial condyle fit sphere is 21.34 mm and that of the
lateral condyle is 23.04 mm, which is in agreement with
the results in [7] (the radius of the medial sphere is about
22 mm and that of the lateral sphere 24 mm).
2.1.3 Build the spatial mechanism of the tibio‐femoral joint
1) Feature elements of the ligaments
In the normal tibio‐femoral joint, the ACL is modelled as
a link which is connected to the femur and tibia by
2 Int J Adv Robotic Sy, 2012, Vol. 9, 205:2012 www.intechopen.com
spherical (S) and universal (U) joints respectively. The
other three ligaments, MCL, LCL and PCL, are modelled
as two links, each of which is connected by a prismatic (P)
joint, and each two link is connected to the femur and
tibia by spherical (S) and universal (U) joints respectively.
2) Feature elements of the femur and tibia
Both the femur and tibia are defined as circular planes.
The revolute (R) joint on the femur is defined by the
rotation feature line. Another revolute (R) joint on the
tibia is set by the axis across the contact point and
perpendicular to the tibia condyle.
Figure 2 The spatial mechanism with global frame
The spatial mechanism was constructed as presented in
Fig. 2. However, both the femur and tibia have the
relative rotation to the global frame. In order to simplify
the process of solving the kinematics equations, an
equivalent link was defined to maintain the same motion
when the global frame was built on the tibia (tibial fixed
frame). The final spatial mechanism is shown in Fig.3.
Figure 3 The spatial mechanism with tibial fixed frame.
Feature point x/mm y/mm z/mm
D (starting point of ACL
on femur) 86.61 93.76 0.79
C (Ending point of ACL
on tibia) 82.23 80.54 ‐24.17
A (contact point on tibia
medial condyle) 112.21 81.26 ‐29.56
B (centre of medial
condyle sphere) 116.28 88.78 ‐3.80
Bʹ (centre of lateral
condyle sphere) 64.06 97.93 ‐2.64
F (starting point of PCL
on femur) 103.75 95.19 ‐7.42
E (ending point of PCL
on tibia) 88.49 114.55 ‐35.49
M (starting point of
MCL on femur) 132.94 85.50 ‐4.79
N (ending point of MCL
on tibia) 125.01 82.72 ‐45.56
H (starting point of LCL
on femur) 50.86 106.67 1.68
G (ending point of
MCL on fibular) 35.17 119.18 ‐65.77
Table. 1 Coordinates of feature points measured in global fixed
frame XYZ.
2.2 Kinematics analysis of the spatial mechanism
The moving coordinate system denoted by pS is
established on the tibia where the origin point is
represented by A (contact point on medial tibia condyle),
while the static coordinate system denote by bS is
established on the femur where the origin point is
represented by B (centre of medial condyle sphere). The
two coordinate systems are parallel to each other, where
the x‐axis is parallel to the rotation feature line, y‐axis is
defined perpendicular to the coronal plane, z‐axis is
established by right hand rule. A single degree of
freedom can be obtained for the mechanism and then
screw theory is used to solve the forward kinematics. The
product of exponential formula is expressed as:
1 1 j j+1 111 11 1 1 1 1ˆ ˆˆ ˆˆ ˆ(0) (0)n n in ini ij i iji i i ist st stg e e g e e e e g (4)
where i and j are the number of branch chain and number
of joint in branch chain, ij denotes the spinor of the jth
joint in the ith branch chain, ij denotes the displacement
of the jth joint in ith branch chain, stg denotes the
configuration matrix and (0)stg represents the initial
configuration of the femur relative to the tibia.
Five motion screws were chosen in AB and CD branch
chain, expressed as:
11 11 11
12 12 12
21 21 21
22 22 22
23 23 23
( )
( )
( )
( )
( )
T
A
T
B
T
C
T
C
T
D
r
r
r
r
r
3Yonggang Xu, Rongying Huang and Qiang Xu:
The Kinematic and Static Analysis of the Tibia-Femoral Joint Based on a Novel Spatial Mechanism
www.intechopen.com
where ij represents the vector axis of the jth joint in the
ith branch chain (i=1,2; j=1,2,3), Ar , Br , Cr and Dr denote
positions of point A, B, C and D relative to space
coordinate system. So
11 11 12 12
ˆ ˆ
1 2 1
1 2 1 2 1 1
(0)
0
0 1 0 1 0 1
0 1
st st
B
B
g e e g
R R p I r
R R R R r R p
(5)
where,
11 11
1 11 11
0
0
0 0 1
c s
R s c
, 12 12ˆ2R e ,
12 12ˆ
1 ( ) Bp I e r
, 11 11c cos , 11 11s sin , ACL is
modelled as rigid link, the length denotes by , we can
get
p p
D C
p b
D st D
q q
q g q
(6)
where pDq and pCq represent the homogenous
coordinates of point D and C measured in static
coordinate system pS and bDq represents homogenous
coordinate of point D measured in moving coordinate
system bS . The equation (6) could be simplified as
1 2 2 1( )D B CR R q R r p r (7)
Now the equation (7) becomes the sub‐problem of Paden‐
Kahan 3 problem in [9], where the 11 and 1R could be
solved by the corresponding method in [9], then stg
could be solved in (5). After getting the 11 , the kinematic
characteristics of other joints could be solved by the
inverse kinematics method of the parallel robot. Finally,
the length of ligaments is represented as:
b
LCL st M N
b
PCL st F E
b
MCL st H G
l g q q
l g q q
l g q q
(8)
where bMq , bFq and bHq are the positions of points M, F
and N measured in moving coordinate system bS ; Nq ,
Eq and Gq are the positions of points N, E and G
measured in static coordinate system pS .
2.3 Static analysis of the tibio‐femoral joint after ACL
reconstruction
2.3.1 Length and equivalent elastic coefficient of ligaments
Each ligament, except ACL, is modelled as a linear
spring. The three‐dimensional model of each ligament is
meshed in ANSYS software and then analysed as shown
in Fig. 4. Ligaments are considered as isotropic material
and elastic modulus of PCL is 276 MPa, MCL is 292 MPa,
and LCL is 292 MPa, the Poisson‐ratio is 0.49 for all three
ligaments [12]. Then the elastic coefficient is calculated
using the finite element method, PCL is 192.85 N/mm,
that of MCL is 260.55 N/mm and for LCL is 29.03 N/mm
and the initial force are then calculated which are
125.01N/mm, 125.01 N/mm and 209.83 N respectively for
PCL, MCL and LCL.
Figure 4 Finite element model of ligaments. (a). PCL, (b). MCL,
(c) LCL
It is well‐recognized by most scholars that the ligaments
cannot resist compress forces. It is supposed that the
ligaments will not receive any compressive forces during
knee flexion. The minimal length of each ligament during
flexion is considered as the original length, the original
length and maximal length is obtained by the forward
kinematics solution, the initial length is measured as the
knee is at complete extension (Table 2).
length/mm PCL MCL LCL ACL
Minimal length 36.71 40.82 59.00 28.59
Maximal length 38.81 41.73 66.03 28.59
Initial length 37.35 41.62 66.03 28.59
Table 2 The length of ligaments
2.3.2 Static kinematics simulation of the tibio‐femoral joint
Figure 5 Model of quasi‐static kinematics
The femur is selected to analyse the force of each
ligament (Fig. 5), force and moment balance equations are
expressed as:
0F N D H BF F F F F
(9)
0BF F BN N BD D BH H Br F r F r F r F M
(10)
4 Int J Adv Robotic Sy, 2012, Vol. 9, 205:2012 www.intechopen.com
where FF
, NF
, DF
, HF
and BF
denote the forces on point
F, N, D, H and B respectively, BFr , BNr , BDr and BHr
denote the vectors of point F, N, D and H relative to point
B, BM
denotes the resultant moment on point B. We can
get
ʹ ( )( )
PCL
b
st F E
F PCL PCL
PCL
A g q q
F k l l
l
(11)
ʹ ( )( )
LCL
b
st N M
N LCL LCL
LCL
A g q q
F k l l
l
(12)
ʹ ( )( )
b
st H G
H MCL MCL MCL
MCL
A g q q
F k l l
l
(13)
0
b
st D
D
ACL
F Ag q
F
l
(14)
( )b bBi st i Br Ag q q (15)
where PCLk , MCLk and LCLk represent the elastic
coefficient of PCL, MCL and LCL. 3 3 0A I , 0F
represents the initial tension of ACL, the value could be
20 N, 40 N, 60 N or 80 N. Eliminating the effects of the
patello‐femoral joint reaction and the friction between the
femur and tibia, the vertical component of force denoted
by BF
is considered as the contact force of the tibio‐
femoral joint. Then the simulation of quasi‐static
kinematics is done in MATLAB software (Mathworks
Corp, USA) as initial tension of ACL is 20 N, 40 N, 60 N
or 80 N.
3. Results
The range of the flexion angle of normal knee (12) during
daily activities is 0°~135°, but Blankevoort’s research,
within 90° the internal/external rotation of the tibia would
not be restricted by the flexion angle of the femur, the
ligament length changes are decided by the flexion angle.
Because the calculation of the length of the ligaments is
based on the assumption that the ACL is isometric during
the knee’s flexion, considering the disparity of the
starting and ending points of ACL, five groups of data
(Table 3) are used to analyse the sensitivity of the
ligament length and femur rotation angle changes
impacted by ACL positions. The changing curves of
ligament lengths and internal/external angles with
respect to flexion angles are depicted in Figure 6. The
maximal deviation of PCL length is less than 0.28 mm, the
LCL length is less than 0.83 mm, the MCL length is less
than 0.12 mm and the degree of rotation angle is less than
3.16.
As shown in Figure 6, when the flexion angle changes
from 0° to 90°, the length of LCL decreases monotonically
indicating that the LCL is under maximum tension when
the knee is completely extended, which is in agreement
with anatomy results. Contrary to PCL, the tension on
MCL/PCL first increases/decreases then decreases/
increases which is in agreement with Blankevoort’s
results. When the flexion angle reaches 20° [10], the
degree of rotation angle gradually increases along with
the increase of the flexion angle, which is also in
agreement with Akalan’s results [11].
group
C (Ending point of ACL
on tibia)
D (Starting point of
ACL on femur)
(x, y, z) (x, y, z)
1 (82.23, 80.54, ‐24.17) (86.61, 93.77, 0.79)
2 (81.91, 82.75, ‐23.091) (87.54, 94.75, 1.91)
3 (85.24, 79.11, ‐23.38) (85.13, 96.22, ‐0.33)
4 (86.84, 79.50, ‐21.84) (84.71, 96.25, ‐2.60)
5 (86.13, 81.75, ‐21.84) (85.43, 94.50, ‐1.84)
Table.3 The starting and ending sites of the ACL
Figure 6 Feature sizes and angles as ACL position changes, (a)
degree of external/internal angle, (b) length of PCL, (c) length of
LCL, (d) length of MCL
(b)
0 20 40 60 80 100
-4
-2
0
2
4
6
8
10
Flexion angle
de
gr
ee
o
f
ex
te
rn
al
/in
te
rn
al
0 20 40 60 80 100
36.5
37
37.5
38
38.5
39
39.5
40
Flexion angle
le
ng
th
o
f
P
C
L
(a)
0 20 40 60 80 100
58
60
62
64
66
68
70
72
Flexion angle
le
ng
th
o
f
LC
L
(c)
0 20 40 60 80 100
40.8
41
41.2
41.4
41.6
41.8
Flexion angle
le
ng
th
o
f
M
C
L
(d)
5Yonggang Xu, Rongying Huang and Qiang Xu:
The Kinematic and Static Analysis of the Tibia-Femoral Joint Based on a Novel Spatial Mechanism
www.intechopen.com
Moreover, in order to analyse the sensitivity of the tibia
medial condyle contact point position, five contact points
are chosen (T