Dear long bone modellers/testers,

We have run some simple sensitivity studies on an intact femur FE model,
which show that stress magnitudes at the hip are insensitive to moving the
encastre boundary condition from the knee to approximately halfway up the
femoral diaphysis. This conclusion is applicable for static and low-speed
dynamic (e.g. normal gait) loads.

However, at high speed dynamic (e.g. impact, maybe running) loads, the
natural frequencies of the bone/tissue system will become important, and
more sophisticated representations of bone support and damping will be
required to achieve "accurate" results. (For a free-standing femur
constrained at the knee, we calculated a fundamental frequency of around
80Hz; this will be reduced by the added mass of surrounding soft tissues.)

Adding a stemmed hip prosthesis obviously changes things a bit - the
fundamental frequency is likely to be increased slightly and the encastre
boundary condition must be distant from the stem tip.

I hope that this data is useful to some of you.


Stuart Kelly MEng MSc CEng MIMechE
WS Atkins Consultants
220 Aztec West
Bristol BS32 4SY

Tel: +44 (0)1454 628780
Fax: +44 (0)1454 616480

-----Original Message-----
From: Marco Viceconti [mailto:viceconti@TECNO.IOR.IT]
Sent: 16 January 2002 15:49
Subject: Re: [BIOMCH-L] Bionet: boundary conditions in bone stress analysis

Dear All:
Until recently, most bone stress analysis studies modelled muscle and
ligament actions with concentrated forces. Furthermore, in the great part
of them the epiphysis of interest is loaded with the joint force and
eventually with one or two major muscle forces (in the hip typically the
glutei), while the opposite epiphysis is rigidly constrained. Such
simplifications were frequently made without ant further explanation. In a
few cases, methodological studies (i.e. Cristofolini, L., Viceconti, M.,
Toni, A., Giunti, A., 1995. Influence of thigh muscles on the axial strains
in a proximal femur during early stance in gait. J Biomech 28, 617-24;
Stolk, J., Verdonschot, N., Huiskes, R., 2001. Hip-joint and
abductor-muscle forces adequately represent in vivo loading of a cemented
total hip reconstruction. J Biomech 34, 917-26) confirmed that such
simplifications were admissible for a given modelling purpose.

More recently however, important studies were published, in which
significant differences were observed when multiple muscle groups were
neglected. The authors of these studies recommend extensive muscle
modelling (Duda, G. N., Heller, M., Albinger, J., Schulz, O., Schneider,
E., Claes, L., 1998. Influence of muscle forces on femoral strain
distribution. J Biomech 31, 841-6). In a study still to be published Polgar
et al. found that even distributing the muscle force over the whole
insertion area, significantly change the results.

All these studies are of extreme interest, and I welcome the new articles
classification recently introduced by the Editorial board of the Journal of
Biomechanics, aimed also to acknowledge the importance of methodological
studies. However, I start to see in conferences a wrong trend: all models
that do not account in detail muscles are criticised a priori. The older
colleagues remember very well the time when only 2D FEA models were
solvable. When 3D models started to show up some studies proved that in
many cases the use of a 3D model was mandatory. However, after that for a
while it was VERY difficult to publish any paper based on a 2D model, even
if in that particular case the simplification would had made perfectly
sense. My concern is that this trend is starting to appears also muscle

We should not forget that in most cases muscle forces can only be roughly
estimated. Thus, adding more muscle forces to our model we may add more
noise than information. In particular, on the basis of my experience I

- Models aimed to compute bone-implant relative micromotion may neglect
most muscle forces. Due to the significant difference of stiffness between
a metallic implant and the bone tissue, the only relevant boundary
condition is the force directly acting on the implant (i.e. the joint
force). Thanks to telemetry studies we have good measurements of these
joint forces.

- On the contrary, bone-implant contact pressure is quite sensitive to the
local deformations produced by muscle forces, which should thus included.
However, for the same reason, such forces should be distributed over the
whole insertion areas.

- Models aimed to predict the stresses in the bone surrounding a short-stem
implant should model only the muscles acting on that region. For hip
prostheses, I propose that joint force and glutei actions are sufficient.
This may not be true for long stems such as those used in revision surgery.
The abductor force should be computed with a simple equilibrium scheme a la
Pawels (but in 3D) and not derived from complex musculo-skeletal models,
which contain muscles groups that are not present in our model.

- Being valid the St. Venant theorem, if we are studying the stresses in
the hip region we can place a rigid constrain at the knee.

Laboratorio di Tecnologia Medica tel. 39-051-6366865
Istituti Ortopedici Rizzoli fax.
via di barbiano 1/10, 40136 - Bologna, Italy

Tiger! Tiger! Burning bright in the forest of the night,
what immortal hand or eye could frame thy fearful symmetry?
Opinions expressed here do not necessarly reflect those of my employer

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