THE CHARACTERIZATION OF PARTICULATE DEBRIS OBTAINED FROM FAILED
Design Requirements for a Total Hip Replacement
When an engineer designs a component or process, the term
(when correctly applied) implies that she or he completes the
1) Identify a need.
2) Define the problem and subproblems that must be solved.
3) Gather background information and data.
4) Formulate objectives and criteria for solving the problem.
5) Consider all of the alternative solutions to the problem.
6) Analyze and evaluate these alternative solutions.
7) Perform tasks of decision-making and optimization.
This report will address tasks 1 through 4 completely and 5 through
7 in broad terms as they are beyond the established scope. In
this section, the requirements that must be fulfilled to design
a failure-free total hip replacement are described.
Most implant systems do not or cannot meet all of these criteria
completely, and very few systems constructed from a single material
can meet more than a few of these criteria. With this in mind,
implant manufacturers are now looking towards combining different
materials to satisfy the following requirements:
The replacement material must not occupy considerably more volume
than the tissue it is intended to replace. In the case of the
stem, it is limited to the size of the femoral cavity.
The implanted material should be readily machinable and modifiable
either before surgery by the manufacturer or during surgery by
the surgeon to fit the range of sizes required for different patients.
The device should be quickly installable with a minimum amount
of damage to the surrounding tissues or the surface of the device
The material should not leach any harmful substances even if it
fractures in service. It should also be manufactured to high purity
levels so that its composition is reproducible on a large scale
with little variation so that its potential long term effects
upon biological tissues can be predicted as well as possible.
If the production model varies considerably from the model approved
during trials, the results obtained in service may not correspond
to those anticipated from research.
Tissue Specific Properties
The stem portion of the implant should ideally exhibit anisotropic
elastic properties similar to those exhibited by bone. The stem
and socket portions should also allow for bone attachment to keep
the implant in place. There may be many other properties required
for the implanted material that strongly depend on its intended
location and function in the body.
The elastic modulus of the stem material, E, must be comparable
to the bone tissue it is intended to replace. Implants that are
too stiff can damage the neighboring tissue. Implants that are
too flexible may cause fractures in the bone or interfere with
normal bone tissue growth and repair at the bone-implant interface.
In broad terms, an elastic modulus as close as possible to that
of bone tissue is desirable. However, to be precise, other factors
must be considered. Both the bone and the stem are subject to
separate bending moments. The elastic behavior of the bone-stem
interface must also be considered. Thus, the constraint changes
from the simple
Estem = Ebone Eq.
to the more specific constraint
where M represents the bending moment of inertia, E
once again represents the elastic modulus, and I represents
the areal moment of inertia for the respective components. An
additional constraint must be placed on the interfacial modulus,
Espongy bone <<
Ecompact bone Eq. 3
such that the interfacial stiffness is very close to that of the
spongy bone that it is replacing.
Ideally, the yield strength of the implanted material will be
equal to or higher than the bony material it is intended to replace
such that a sharp blow will fracture tissue (that can heal itself)
rather than the implant (that cannot).
Within the hip joint the implanted material is subject to many
intermittent cyclic loads, sometimes up to five times the patient's
body weight (e.g., stresses of 1000 lbs/in2
many thousands of times per day). The material must therefore
exhibit a high fatigue strength. 'Man-made' materials can be considered
to be disadvantaged in comparison with living biological materials
(such as bone or cartilage) in that they are not repaired or regenerated
by the body as natural tissues are.
Other Factors Affecting Fatigue Resistance
Along with variation in the types of systems available, there
is the variation in the performance requirements of the different
human recipients. In an elderly patient, the implant may be subjected
to a maximum load of 1000 N and 100 wear cycles per day, whereas
the requirements for a young athlete may be a maximum load of
4000 N and up to 10,000 wear cycles (leg extensions and flexions)
Any material to be implanted must exhibit a high resistance to
fracture, especially those materials that will be subjected to
sudden high impact loads (e.g., the neck of the stem).
Biocompatibility is probably one of the most important requirements
of an implant material. Any degradation of the material has the
potential of causing pain, local infection, an allergic reaction,
loss of function, or even forms of cancer. Usually, a second operation
or other medical treatment is required to alleviate these symptoms.
Nearly every surface of the hip prosthesis system must be resistant
to wear. Simple sliding wear will occur at the ball and socket
surfaces, and more complex modes of wear will occur at the stem
Thus, we see the challenge: determining the structure (hence the
processing techniques) of the implant component that will provide
the desired performance and properties is a formidable task. To
perform this task, a broad range of materials options must be
considered. Table 3 provides a list of the materials properties
required for an optimum hip joint prosthesis. Table 4 lists
the service (in vivo) conditions that must be withstood
by the hip implant.
Ideal Design Requirements for Artificial Hip Implant
|Property Description||Ideal Material Properties
|Ideal Elastic Modulus ||17.1GPa in axial direction
11.5GPa in transverse direction
|Minimum Yield Strength||500MPa
|Minimum Ultimate Tens. Stren.||650 MPa
|Minimum Elongation to Fracture||8%
|Minimum Fatigue Strength||400MPa
(107 cycles, fully reversed bending)
|Other Requirements||Excellent Corrosion Resistance to provide Biocompatibility
Exceptional Wear Resistance in a corrosive media
Summary of the Service Environment for Artificial Hip
||Effect on Implant|
0-80 mmHg (depending on pH)
varies by location and nature of site
Repassivation may become more difficult as oxygen availability decreases.
Chloride ions (Cl-)
Crevice corrosion and passive film breakdown are enhanced.
Cyclic, non-uniform stresses
Loads of up to 5x body weight, frictional shear forces at entire surface of implant
Fatigue cracking, disassembly, surface damage/wear, depassivation
Strain related (piezoelectric) potentials
Voltages of up to -150 mV depending on strains
Repassivation can be hindered at lower voltages
(caused by enzyme secreting cells at or near surface of implant)
pH can vary from 8 to as low as 4 in different regions at different times
Low pH can enhance localized corrosion if depassivation occurs, may also contribute to hydrogen absorption by titanium.
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