Polymers - Orthopaedic Biomaterials in Research and Practice - page 132

Sphericity

Smoothness
Hardness
Coecient of friction
Wear rate

Joint technique
Notch sensitivity
Fabrication technique
Endurance limit
Yield point
Texture

Radii
Fabrication technique

Other:
Corrosion rate
Cost
Availability

FIGUre 6.1 design requirements—proximal femoral component.

Design is a creative process and is not purely analytic. It is influenced
by temporal, cultural, historical, ethical, aesthetic, and practical factors
as well as by strict engineering considerations. Even in these engineering
considerations, a great deal of uncertainty persists. Patients' anatomy and
lifestyles are highly variable and, to a significant extent, unpredictable.
Even for “standard” conditions, only the overall outlines of the mechani-
cal, chemical, and biologic challenges are known. Designs must therefore
be forgiving during both insertion and use. They have to provide wide
ranges of component sizes, each with safety factors in the calculation of
critical requirements, to assure acceptable results in a series of patients
with defined indications treated with a relatively uniform surgical tech-
nique. (The design process, especially as it applies to development of
new materials, is considered more fully in Chapter 15.)
For these reasons, enormous variety persists in the choice of materi-
als and designs incorporating them. In femoral endoprosthetic compo-
nents alone, more than a dozen materials and over a few hundred designs
have been used, with many still in use, even without allowance for the
complexity of modern modular designs. This is how it should be, since
surgeons and designers together constantly strive for improvement in
outcome and duration of good to excellent results. However, changes
should not be made at random. The progress in total joint replacement
over the last 50+ years is striking, with good to excellent 10+ year out-
comes to be expected in 90%-95% of “typical” patients with total joint
replacement arthroplasty who received proven designs. However, what
is considered the expected outcome for patients has changed as well
owing to the changing patient demographics, the obesity epidemic, and