Residual Limb Model for Osteointegration - Computational Biomechanics of the Musculoskeletal System - page 163

14 Residual Limb Model

for Osteointegration

Winson C.C. Lee

ContentS
Summary ........................................................................................................................................ 163
14.1 Introduction .......................................................................................................................... 164
14.2 Model Development .............................................................................................................. 165
14.2.1 Geometry .................................................................................................................. 165
14.2.2 Mesh .......................................................................................................................... 165
14.2.3 Material Properties and Contact Simulation ............................................................. 166
14.2.4 Load Applications: Weight-Bearing Exercise and Level Walking ........................... 166
14.2.5 Parametric Analysis Investigating the Possibility of Bone Mechanical Failure ....... 166
14.3 Model Findings ..................................................................................................................... 167
14.3.1 Loading A ................................................................................................................. 167
14.3.2 Loading B .................................................................................................................. 167
14.3.3 Loading C .................................................................................................................. 168
14.3.4 Effects of Various Parameters ................................................................................... 168
14.4 Applications .......................................................................................................................... 169
References ...................................................................................................................................... 170

Summary
Direct anchoring of a lower-limb prosthesis to the bone (osseointegration) has been shown to solve
some common problems associated with conventional socket prostheses. During the rehabilitation
phase, amputees fitted with osseointegrated implants apply static loading against the abutment on
a weighing scale to prepare the bone to tolerate the forces likely to be experienced during walking.
However, the weighing scale measures only the vertical force. Moments and other directions of
forces, which can affect the bone-implant interface stresses and the rehabilitation outcome, are not
measured. When the amputee starts to walk, in addition, there is a risk of bone mechanical failure.
This chapter illustrates the development of a finite element (FE) model to study the stresses in the
bone and at the bone-implant interface.
Bone-implant interface stresses were compared under three loading conditions: (1) vertical force
corresponding to the load clinically prescribed in a weight-bearing exercise; (2) loads applied on
the three axes, corresponding to the “true” load measured by a triaxial load transducer during the
same exercise; and (3) loads experienced during independent level walking. An additional paramet-
ric analysis was performed to predict the bone structural integrity under different conditions. The
model revealed that the weighing scale in fact applied much greater and less uniform stresses on the
bone than expected. During walking, high stress occurred at a location different from that experi-
enced during the weight-bearing exercise. In addition, with a bone loss of 40% and the application
of a push-off loading at four times its normal force, bone failure would occur. These findings imply
that triaxial loading should be monitored during weight-bearing exercises and carefully prescribed.

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