4The 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 following tasks:

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:

Dimensional Restrictions

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.

Application/Installation Requirements

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 .

High Purity/Reproducibility

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.

Elastic Modulus

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

to the more specific constraint

 Eq. 2

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, namely

such that the interfacial stiffness is very close to that of the spongy bone that it is replacing.

Strength

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).

Fatigue Resistance

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) per day.

Fracture Toughness

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

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.

Wear Resistance

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 surface.

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.

This page is maintained by Prof. Stacy Gleixner.  
Questions or problems please send email to gleixner@email.sjsu.edu or call (408)924-4051. 
The page was last updated 09/27/00 .

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