THE CHARACTERIZATION OF PARTICULATE DEBRIS OBTAINED FROM FAILED
ORTHOPEDIC IMPLANTS:
Chapter 3
3Overview
of the Materials Science of Orthopedic Implant Systems
Biomaterials can now be combined to serve as substitutes for many
entire organs or body systems (e.g., dialysis machines,
artificial hearts, polymeric skin grafts). The most common of
these body systems to be replaced are those of the skeletal joint.
Implant systems designed to replace the hip will be the focus
of this section. A schematic of a typical hip implant system and
the terms used to describe its components are given in Figure
1.
Photographs of Implant System and Human Femur. Components are labeled with the terms used to identify them in this report. An outline of the final stem position is shown on photo of the femur.
Available Implant Systems
There are many different joint replacement systems available to
the orthopedic surgeon and they are summarized in Table 2.
Each has its particular advantages, but none seem to exhibit completely
failure-free performance . The majority of hip prostheses are
fabricated from titanium and cobalt-chrome alloys owing to their
high corrosion resistance and strengths. Smooth stem systems rely
on polymethyl-methacrylate cement to remain bonded in the femoral
cavity. Porous stem systems have been partially covered (using
a sintering process) with either compressed and interlocked metal
fiber mesh or partially fused metallic spheres or beads. The ceramic
head-metal stem, hydroxylapatite (or other material) coated titanium,
and fiber reinforced composite systems appear promising with their
potentially better wear resistance, bone compatibility, and matched
stiffness, respectively. However, more testing is needed before
they can be used extensively in human patients.
One new, promising example of a new biomaterial application is
the use of a Ru-37.5Zr-12.5Pd alloy as a coating material for
a titanium stem. This would combine the low modulus of titanium
with the excellent wear and fracture resistant properties of the
Ru-37.5Zr-12.5Pd alloy. When this new alloy was subjected to pin
on disk wear tests with a PMMA pin, the alloy actually exhibited
a negative wear rate after 5 million cycles in sliding
contact. In other words, the polymer actually began to adhere
to the metallic surface. Biocompatibility studies for this new
alloy are underway, however, the high cost of the components of
this alloy may prevent it from seeing wide-spread application.
Another new (and less expensive) alloy was developed from research experience with titanium. The alloy uses 13% Nb and 13% Zr as completely biocompatible additions that serve to enhance its wear resistance while reducing its overall elastic modulus to approximately 76 GPa from Ti-6Al-4V's modulus of 114 GPa. We will see later why this reduction in elastic modulus is significant. This alloy is expected to be approved by the Food and Drug Administration (FDA) for large scale surgical use quite soon but it may be several months before it begins to replace titanium or cobalt-chrome alloy systems.
| Implant System | Advantages | Disadvantages |
| Modular
Ti6Al4V/CoCrMo
(often porous) | Easier to fit patient
Material couples used to avoid weakness of each material Low stem modulus Use of cement may be avoided | Prone to crevice/fretting corrosion at ball/neck of stem junction
Co, Cr, Mo known to be toxic in ionic form Assembly during surgery required 2 week period of no loading may be required for bone ingrowth |
| CoCrMo
(Smooth) | High wear resistance
Larger surgical tolerances (stem anchored with PMMA) | PMMA cement may fracture or cause tissue reaction
Co, Cr, Mo known to be toxic in ionic form High modulus |
| CoCrMo
(Porous) | High wear resistance
No cement required to anchor into femur | High surface area, potential for bead/mesh loss
Co, Cr, Mo known to be toxic in ionic form High modulus causes bone loss 2 week period of no loading required for bone ingrowth |
| Ti6Al4V
(Porous) | No cement required to anchor into femur
Low elastic modulus Very low toxicity | High surface area, potential for bead or mesh loss
Relatively low wear resistance 2 week period of no loading required for bone ingrowth |
| Ti6Al4V
(Smooth) | Larger surgical tolerances
Very low toxicity | Low wear resistance
PMMA cement may cause fracture or tissue reaction |
| Modular
Ti6Al4V/Al2O3 | Easier to fit patient
Alumina exhibits excellent wear and degradation resistance Titanium stem has low modulus | Leaching of Al+++
Could be prone to ball/neck crevice corrosion Assembly during surgery required |
| 316L Stainless Steel
(smooth) | Low Cost
Easy to manufacture Larger surgical tolerances Extensive research background | Excessively corrosive in some cases
Susceptible to fatigue cracking Very high modulus PMMA cement may cause fracture or tissue reaction |
| ZrO2 Coated Zr
or Oxidized Ti-13%Zr-13%Nb alloy | Low Modulus
High degree of biocompatibility High wear resistance Good coating adhesion | High cost
Difficult to fabricate Bone-ZrO2 interface not well understood |
| Diamond Like Carbon Coated/Surface Nitrided Ti6Al4V | Low Modulus
High degree of biocompatibility High wear resistance | Elastic modulus mismatch may cause delamination from in-vivo stresses
Hydrogen may weaken coating adherence in vivo |
| Ru-37.5Zr-12.5Pd Coated Ti6Al4V | Extremely wear resistant
Excellent mechanical properties Biocompatibility equal to or better than CoCrMo alloys | VERY expensive unless used only as coating
Full biocompatibility not yet determined |
| Fiber Reinforced Composites | Customizable, anisotropic properties possible to match host bone | Susceptible to swelling from joint fluids/lipids
Difficult to completely purify resins |
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