Biomedical Engineering Reference
In-Depth Information
Chapter 11
The Perfect Prosthesis?
In Chap. 2, we started with the case study of a cemented Charnley prosthesis, a supe-
rior design for it is still in use in only slightly modified form and served as template
for numerous other designs. However, the stainless steel stem finally failed and even
after substitution of the originally used alloy by other better-performing one as well
as the introduction of a modular design, the prosthesis was not ideal yet, witness
the abundance of numerous subsequent designs. Was the Charnley prosthesis after
all not that ideal? A number of examples can be cited, where the femoral cortex
is weakened by bone resorption, to be attributed to the phenomenon called
stress
shielding
. The E-modulus of stainless steel is about twenty times higher than corti-
cal bone and so is the ratio of stiffness between femur and prosthesis. Consequence
is that stresses are unequally distributed over the femur walls, i.e., that
stress
on one
part of the femur wall is
shielded
from expressing a similar stress on other parts of
the femur wall (cortex, cortical bone).
By the patient's body weight, the prosthesis is tilted as shown in Fig.
11.1
(
varus
tilt) with the consequence that voids are created and a dynamic pressure difference is
built up by which 'peri-joint' fluid, occasionally containing wear debris, is pumped
into the void. The destructive action of wear debris is already discussed in Chaps. 2
and 4.
These facts must have been the concern of Robert Mathys Sr., when he developed
his isoelastic design in the late 1960s (animal experiments [352]; in current human
arthroplasty since 1977 [353], [354, pp. 591-596]).
1
11.1
The Isoelastic Prosthesis
The outcome of the experimental campaign of Mathys was a modular prosthesis
composed of a polyacetal body (stem), reinforced by a metal core (stainless steel
316L or Ti6Al4V) with tapered top, to which either a stainless steel, ceramic or
1
Robert Mathys Foundation, CH-2544 Bettlach, Switzerland.
