Biomedical Engineering Reference
In-Depth Information
FIGURE 10-15
Range of Otto Bock
passive knee
prostheses.
(a) Mechanical.
(b) Fluid controlled.
(Courtesy of Otto
Bock, with
permission.)
The displacement of a four-bar linkage can be described in a closed mathematical
form depending only on the lengths and locations of the four bars. The point where the
linkages cross defines the instantaneous hinge axis. It is therefore possible to develop a
linkage that follows the path described by equation (10.11) with a high degree of accuracy
(Forner-Cordero, Pons et al., 2008).
A number of manufacturers have developed passive knee prosthetics to minimize
the issues described in the previous section. The first designs used constant-friction and
friction-brake mechanisms developed as a result of research conducted after WWI. They
include the Otto Bock 3R22 and 3R15 designs shown in Figure 10-15. They were superior
to the old peg leg but were not effective for working on uneven surfaces or at a range or
speeds (Torrealba, Fernandez-Lopez et al., 2008).
In the 1950s, Hans Mauch introduced the fluid-controlled prosthesis, and many pros-
thetic knees were produced with Mauch
®
cylinders. The introduction of fluid control,
which accommodated variable torques during the gait cycle, improved both swing control
and stability during the stance phase. Typical examples are the Gaitmaster from Ossur
and the Otto Bock 3R80 and 3R92, which are equipped with innovative, load-dependent
brake mechanisms. The brake responds to a heavy heel load and stabilizes the prosthesis
and maintains a high level of security during the stance phase. Adjustable stance flexion
acts as a shock absorber and reduces the amount of stress on the body. A forefoot load
automatically turns off the brake, which facilitates a graceful transition to the swing phase.
The friction brake knee joint reduces the requirement for unnatural movements like hip
hike. The 3R92 has a progressive pneumatic swing phase controller.
Another lightweight knee, shown in Figure 10-16, is the Aulie 802. It is made from
nylon and stainless steel for use in harsh environments and is apparently the only knee
prosthetic that can be used in the water. It includes an adjustable hydraulic control that is
both light and simple. The adjustment clamp controls fluid flow by deforming the walls
of the cylinder, while a stainless steel spring provides extension assist.
Most of these general-purpose knees come with standard fittings and are therefore
interchangeable. They weigh between 600 g and 1 kg, can generally carry loads of 100
kg and more, and cost thousands of dollars. At the other extreme are the JaipurKnee and
the LEGS M1 knees, both of which were developed by university students in the United
States for the Third World.
