Biomedical Engineering Reference
In-Depth Information
10.3.2 Knee Joint Replacement
The prosthesis for total knee joint replacement consists of a femoral, a tibial, and/or a patellar compo-
nents. Compared to the hip joint, knee joint has a more complicated geometry and movement biome-
chanics and is not intrinsically stable. In a normal knee, the center of movement is controlled by the
geometry of the ligaments. As the knee moves, the ligaments rotate on their bony attachments and the
center of movement also moves. The eccentric movement of the knee helps distribute the load through-
out the entire joint surface (Mündermann et al. 2008).
The prostheses for total knee replacement (Figure 10.9) can be divided according to the extent they
rely on the ligaments for stability: (1) Constrained: these implants have a hinge articulation, with a
fixed axis of rotation, and are indicated when all of the ligaments are absent, for example, in recon-
structive procedures for tumor surgery. (2) Semiconstrained prosthesis: these implants control posterior
displacement of the tibia on the femur and media-lateral angulation of the knee, but rely on remaining
ligaments and joint capsule to provide the rest of the constraint. Semiconstrained prosthesis is often
used in patients with severe angular deformities of the extremities, or in those that require revision
surgery, when moderate ligamentous instability has developed. (3) Nonconstrained: these implants pro-
vide minimal or no constraint. The prosthesis that provides minimal constrain requires resection of the
posterior cruciate ligament during implantation, and the prosthetic constrain reproduces that normally
provided by this ligament. The ones that provide no constrain spare the posterior cruciate ligament.
These implants are indicated in patients who have joint degeneration with minimal or no ligamentous
instability. As the degree of constraint increases with knee replacements, the need to use of femoral and
tibial intramedullary extensions of the prosthesis is greater, since the loads normally shared with the
ligaments are then transferred to the prosthesis-bone interface.
Total knee replacements can be implanted with cement or without cement, the latter relying on
porous coating for fixation. The femoral components are typically made of Co-Cr alloy and the mono-
lithic tibial components are made of UHMWPE. In modular components, the tibial polyethylene com-
ponent assembles onto a titanium alloy tibial tray. The patellar component is made of UHMWPE, and a
titanium alloy back is added to components designed for uncemented use. The relatively small size of the
patellar component compared to the forces that travel through the extensor mechanism and the small
area of bone available for anchorage of the prosthesis make the patella vulnerable.
The wear characteristic of the surface of tibial polyethylene is different from that of acetabular com-
ponents. The point contact stress and sliding motion of the components result in delamination and
fatigue wear of the UHMWPE (Walker 2000). Presumably, because of the relatively larger particle
FIGURE 10.9
Various types of knee joints: (a) metal hinged, (b) hinged with plastic liner, (c) intramedullary fixed
semiconstrained, (d) surface replacement, (e) unicompartmental replacement, and (f) bicompartmental replacement.
