Biomedical Engineering Reference
In-Depth Information
Fig. 2.2
Finite Element Mesh
Although strength of material methods can be used for instruments FEA is a
faster and more convenient means of stress and deflection analysis in instruments.
This is true since generally 3D solid models are produced during, and as part of,
the development of engineering drawings used in the manufacture of such parts.
These 3D models are of great importance in the development of surgical and sales
brochures and animations or other places where illustrations are used. This same
model can also be used in the CAD package in which it was created to perform a
relatively accurate FEA estimate of its behavior.
The analysis of implants is another matter. Since the implant is attached to
tissue one cannot use rigid body constraints that are normally used without loss of
significant accuracy. Further, particularly in the case of femoral stems, one wishes
to understand the effect of the applied loads and prosthetic device on the bone
onto, or into, which the device has been implanted.
Ideally then, one would create a 3D solid model of the implant attached to a 3D
model of the bone in order to perform the analysis. Such an analysis was
performed by Crowell [7] who studied an early B-P Ankle Replacement. He found
an overstressed region in the tibial plate of the device resulting in a design change
increasing it thickness.
Where bone remodeling is of concern one can then proceed with further
analysis using a bone remodeling law to alter the properties of the bone model
using the stresses found in the bone in the initial analysis. The analysis is then
repeated with the remodeled bone. The process is further repeated until the results
converge. Such a study was performed by Yau [8] who studied the interaction
between Co-Cr and Titanium alloy femoral stems and the proximal femur into
which they were implanted.
