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

THE MECHANICAL ENVIRONMENT IN THE DISTAL FEMUR:
INFLUENCE OF BOUNDARY CONDITIONS ON FE MODELS
1. ABSTRACT
N. Conlisk 1 , P. Pankaj 2 and C.R. Howie 3
Finite element (FE) modelling of the musculoskeletal structure requires appropriate
input of the geometry of the system being considered, material properties of different
components, loading regimes and boundary conditions. This study compares the effect
of two different boundary conditions (BCs) on the mechanical environment in the
femur: Commonly used restrained BCs; and BCs that incorporate muscles for pre and
post total knee arthroplasty (TKA) situations. In the first set of BCs investigated the
displacements at the mid-shaft of the femur were restrained in all directions. In the
second set of BCs no restraints were applied directly to the femur itself; instead muscle
and ligament structures were attached to support the femur. Muscle points that did not
lie on the femur were assumed to be restrained at their point of insertion in space.
Loading data corresponding to a single legged stance was obtained from telemetric
implant studies and applied to the femur as distributed pressure loads.
The restrained femur was found to produce significantly higher stresses along the shaft
towards the fixed boundary in comparison to the much more uniformly distributed
pattern of stress observed for the femur supported by muscle and ligaments. However it
was also found that the stress distribution across the distal femur for both models was
very similar, suggesting that the mechanical environment close to the point of load
application is minimally influenced by boundary conditions at some distance. Post TKA
investigations of long stemmed implants with a 4mm lateral offset revealed that fixed
boundary conditions may lead to an over estimation of end of stem stress and that for
instances where the mechanical environment of the whole femur is of interest
appropriate modelling of musculoskeletal contributions must be considered to produce
results more representative of the in-vivo situation.
2. INTRODUCTION
The use of finite element (FE) models as a tool to investigate complex clinical scenarios
and critical cases is becoming more wide spread. These models provide information
which cannot easily be obtained from a lab or clinical research investigation. A common
limitation in some previous FE studies of the lower limb relates to the applied boundary
conditions of the system. In particular many studies on the knee pre and post
1
PhD student, Edinburgh Orthopaedic Engineering Centre, RM 3.13, A.G.B Building, The Kings
Buildings, The University of Edinburgh, EH9 3JL, Edinburgh, UK
2
Senior Lecturer, School of Engineering, The Kings Buildings, The University of Edinburgh, EH9 3JL,
Edinburgh, UK
3
Consultant Orthopaedic Surgeon, Edinburgh Orthopaedic Engineering Centre, The University of
Edinburgh, EH16 4SA, Edinburgh, UK