1. POTENTIAL APPLICATIONS & KEY BENEFITS
OPTIMAL DESIGN OF A TRAPEZO-METACARPAL PROSTHETIC IMPLANT
D.Farhat
Service Génie Civil et Mécanique des Structures, Faculté Polytechnique, UMONS
The development of new biomaterials for orthopedic applications is one of the challenging tasks for arthroplasties today. There is an obvious need for better implants as
well as for the manufacturing of artificial tissues. Ideally, a bone implant should be such that it exhibits an identical response to loading as real bone and is also biocompatible with existing tissue. A stiff stem, which is usually made of titanium, shields the proximal bone from mechanical loading (stress shielding). On the other
hand, decreasing the stem stiffness increases the proximal interface shear stress and the risk of proximal interface failure. Therefore the purpose of this study is to solve
these conflicting requirements in order to have more uniform interface shear stress distribution and less stress shielding through the concept of functionally graded material (FGM).
OBJECTIVE
Investigating the causes of the damage and progressive pathologies
after an arthroplasty of the trapezium-metacarpal articulation (in
the basis of the thumb) and optimizing the design of the prosthetic implant.
B/ Second Approach (using a Functionally Graded Titanium
young modulus)
MATERIALS & METHODS
Based on Dr.Ledoux’s prosthesis as a model in this study and by
developing an evolutionary 3D finite element model with ABAQUS program taking into account « the stress shielding » phenomena, the bone reformation / lysis and based on the stimulus concept and the wolf’s law, the damage has been investigated as it is due to the high difference of stiffness between Titanium material of the implant and
both of cortical and trabecular bones.
FIG 5. Mathematical Optimization of The Young Modulus Distribution
CONCLUSION
The interest of an intelligent material with a functionally graded young modulus is now established. The possibilities to create such a material are :
Powder metallurgy by mixing titanium powder and less stiff inclusion before shaping the implant.
EBM process (electron beam melting) of the Arcam company and now tested by SIRIS.
These two techniques seem to be very attractive but need complementary studies of feasibility.
Industrial partners specially small and medium size companies should be welcome.
FIG 1. Dr. Ledoux’s Prosthesis
FIG 2. Finite Element Mesh
FIG 3. Bone Remodeling Law
RESULTS
It was shown that a significant improvement of the lifetime of the implanted metacarpal bone could be obtained with a special  implant made, along its length, of different slices of hypothetic homogenous titanium material with decreasing Young modulus.
Of course, the study was a first approach and additional more
advanced optimizations remain necessary to improve the actual results.
A/ First Approach (Homogeneous Titanium Prosthesis)
FIG 6. Electron Beam Melting Technology
FIG 7. Siris Prosthesis Samples
POTENTIAL APPLICATIONS & KEY BENEFITS
The concept of “an intelligent biocompatible material” in the form of a graded titanium young modulus allowing adaptation of local stiffness at any point of the implant ,was brought to light in the context of studying the trapezo-metacarpal implant, but this concept can be extended to different types of prosthesis
(hip prosthesis,…) in order to lengthen their lifetime.
FIG 4. Temporal Evolution of Bone Density