ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

analyse the effect that different attachments of the menisci have on the behaviour of
the knee using a FE model.
2. Materials and Methods
The knee model was built using bone models (femur, tibia, fibula and patella) from
MRI scans from a healthy volunteer using Simpleware© software. Using
Rhinoceros© software, the soft tissues were made up and added to the bone models.
The main ligaments of the knee [17] were included in the model: the anterior and
posterior cruciate ligaments (ACL, PCL), the medio and lateral collateral ligaments
(MCL, LCL), the patellar tendon and the quadriceps ligament (PT, QL). The MCL
was composed of two parts, a large one that was attached at the farthest regions on
both the tibia and the femur, and a short one that was attached very close to the edges
of both the femur and the tibia, to simulate the wrapping effect that the real MCL has
on the medial side, also limiting the motion on the medial meniscus. The femoral
cartilage and the menisci were also included. The ligaments were modelled as three
dimensional surfaces, the femoral cartilage was built with an approximate thickness of
2mm [20], and the menisci were made up with approximate contour curves and
thicknesses to make them fit in the tibio-femoral gap. The meshing was performed
using ANSYS ICEM CFD©. The approximate maximum element size for the bones
was 2 mm and for the soft tissues was 0.7 mm, giving a total of 88,594 elements.
The FE model of the knee was developed using FEBio software (Freeware, University
of Utah). Rigid bodies were used to simulate the femur, tibia, fibula and patella [16].
All the soft tissues were assigned isotropic elastic material properties, with material
parameters selected from analogous studies (Table 1). The long MCL had the same
material properties as the other ligaments, whereas the short MCL was assigned a
much lower Young’s modulus of 50MPa, since its objective is to constrain the
meniscal motion and to lightly stabilise the medial side of the knee making it more
robust without adding much more resistance to tensile stress compared with the long
MCL. Sliding contact interfaces were assigned between patella-femoral cartilage,
femoral cartilage-menisci and femoral cartilage-tibial plateau. The interaction
between the menisci-tibia surface was the subject of the parametric study, and three
cases were considered: a) menisci fully attached to the tibia, b) menisci partially
attached to the tibia in a section of their horns (Figure 2, left), and c) menisci attached
at a few nodes at the tips of the horns (Figure 2, right). All the sliding contacts had a
friction coefficient of zero.
Young’s
Modulus (MPa)
Poisson’s ratio
Density
(Kg/mm 3 )
Ligaments [19] 300 0.8 1.2
Femoral cartilage [20] 12 0.45 1
Menisci [16] 80 0.3 1
Table 1: Material parameters of the soft tissues
A compressive load of 1,000 N in the Z direction, and a posterior load of 1,000 N in
the X direction were applied at a node at the approximate centre of the top surface of
the femur. The tibia was fully constrained, whereas the femur was completely
unconstrained. The models were run and the simulations finished when the constraints
were not sufficient to balance the loads.