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

implantation [1-4] assume the femur to be fully restrained at some distance from the
region of interest (e.g. when investigating the knee, the proximal femur would be
restrained in all degrees of freedom). Rigid fixation of the femur is often employed in
other areas of biomechanics research such as in-vitro laboratory testing [e.g. 5].
However, this type of fixation is not very representative of the in-vivo situation and as
such is likely to introduce some level of error into the results. Recent studies suggest the
inclusion of muscle and ligament contributions to FE models can significantly influence
the patterns and levels of stress in the lower limb [6-9]. However, the complex nature of
the musculoskeletal structure and the increase in required input variables for such
models presents a number of challenges.
As such the aim of this study is to try and quantify the influence of boundary conditions
on the mechanical environment in the femur both pre and post total knee arthroplasty,
with specific focus on determining when it is necessary to incorporate more
physiological BCs to produce reliable results and when it is acceptable to use simplified
constraints without significantly impacting on the results of these analyses.
3. METHOD
The femur geometry used for this study was that of the large left fourth generation
composite femur [10]. This model is freely available in the public domain and was
reconstructed from scans of the composite biomechanical bones (Sawbones, Pacific
Research Laboratories, Vashon, WA, USA). This femur represents a good ‘average’
geometry for a normal human femur while also allowing comparison between
computational simulations and in-vitro laboratory tests. Physical implant measurements
and templates were used in conjunction with computer aided design software (Autodesk
Inventor 2010, Autodesk inc. San Rafael, California, U.S) to develop 3D models of the
femoral implants. This package was also used to incorporate surgical cuts into the
composite femur geometry to accommodate a total stabilising (TS) implant with long
offset stem (Stryker, UK Triathlon series). Surgical resections in the case of implanted
models were carried out according to the manufacturer’s surgical protocol for the
implant investigated. In the current study bone was assumed to be isotropic as in
previous studies [e.g. 3]. The properties for cancellous (Young’s modulus E = 155
MPa, Poisson’s ratio ν = 0.3) and cortical (E = 16700 MPa, ν = 0.3) bone structures
were taken from sawbones product information, and are within the range of reported
values from current literature [11, 12]. Typical properties for a cobalt chromium alloy
were used for the implant and offset adapter (E = 210000 MPa, ν = 0.3) while properties
of titanium were applied to the stem (E = 110000 MPa, ν = 0.3). Loading data
corresponding to a single legged stance were obtained from telemetric implant studies
to determine realistic in-vivo loads acting on the hip and knee joints [13]. These loads
were then applied to the femur as distributed pressure loads over realistic contact areas.
In this study the bone-implant interface was tied, modelling full osseointegration of the
implants into the bone structure. Based on convergence studies the average element size
for all FE meshes in this study was 2mm.
Two different boundary conditions were applied to the femur models. In the first the
displacements at the mid-shaft of the femur were restrained in all three directions (Fig.
1a and 1c). The second model was created with no restraints applied to the femur itself;