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

CONSIDERATION OF MULTIPLE ACTIVITY LOAD CYCLES IS CRITICAL IN
BONE OPTIMISATION SIMULATION
1. ABSTRACT
D. M. Geraldes 1 and A. T. M. Phillips 1
Optimisation algorithms have recently been incorporated in finite element studies in an
attempt to simulate bone mechano-adaptation. In order to simplify these analyses, bone
is usually considered to be isotropic. This assumption does not explain the directionality
of continuum level material properties. Furthermore, simplified loading is usually
applied to these models resulting in an unrealistic driving signal for bone adaptation. A
3D optimisation algorithm where bone was modelled as a strain-adaptive continuum
with local orthotropic material properties was developed. The algorithm was applied to
a complete 3D model of the femur, where the hip, tibio-femoral and patello-femoral
joints, muscles and ligaments were included explicitly. Multiple frames from everyday
activity load cycles were modelled and their influence in the optimisation process
quantified. The impact of different frames of multiple activity cycles on bone structure
remodelling was compared, with alternative frames found to influence bone
optimisation in different regions of the femur. In addition, more physiological material
properties and directionality distributions were produced for the whole femur, when
compared to the isotropic or single load case approaches. It is hypothesised that
consideration of different instances of multiple activity load cycles is critical in order to
physiologically simulate bone optimisation for the whole femur. Creation of orthotropic
optimised constructs allows for better understanding of bone mechanical behaviour and
can impact many areas of interest in orthopaedic biomechanics, such as the study of
bone-implant interfaces or the study of the influence of certain activities in preserving
local bone material characteristics.
2. INTRODUCTION
It is hypothesised that bone adapts its architecture in response to the loading it is
subjected to, resulting in an optimized structure [1]. This optimization is driven based
on a remodelling plateau around a target strain where no bone is either gained or lost
[2]. Recently, finite element studies have incorporated remodelling algorithms in an
attempt to simulate bone’s response to altered loading conditions, such as bone-implant
interfaces [3]. In order to simplify these analyses, bone is usually considered to be
isotropic. However, this assumption does not explain the directionality of its internal
structures [4] or the orthotropic properties that have been measured in other studies [5].
The remodelling signal is dependent on bone’s loading configuration and the inclusion
of muscle forces has been shown to be a key factor when modelling the femur [6]. The
use of simplified loading, although time efficient, is considered to be unrealistic [7] and
the need for more physiological loading has been stressed [8]. A free boundary
approach to modelling the femoral construct produced more physiological stress and
strain distributions and was considered to provide a closer approximation to the in vivo
1 Structural Biomechanics Group, Dept. Civil and Environmental Engineering, Imperial College London,
SW7 2AZ London, UK. www.imperial.ac.uk/structuralbiomechanics geraldes@imperial.ac.uk