Biomedical Engineering Reference
In-Depth Information
A comparison of contact stresses for such articulations for the effects of greater
congruity and greater congruent flexion range is given in Fig. 6.102 [123].
20
COMPRESSION FORCE 2200N
B-P
LCS
15
MANUFACTURERS RECOMMENDED LIMIT
10
5
0
0°
15°
30°
45°
60°
90°
105°
FLEXION ANGLE
Fig. 6.102
Contact Stress Comparison
It may be seen that stresses in the B-P are substantially lower than the LCS at
all flexion angles.
c)
Tibial Components
The B-P Tibial Platform (Fig. 6.103) is quite similar to the latest LCS version and
is anatomically shaped. It differs from the LCS in that it contains a “Stop Pin” on
its superior surface. This pin engages a slot or hole in the inferior surface of the
Bearing to limit bearing rotation to prescribed limits.
Rotational dislocation of the rotating platform in the New Jersey LCS rotating
platform knee is a significant complication (1.2% in the two PMA clinical trials).
By proper attention to the maintenance of collateral ligament tension during
implantation the rate of such dislocation can be kept acceptably low.
Nevertheless, due to the absence of the cruciate ligaments, the principal anterior-
posterior (A-P) and medial-lateral (M-L) stabilizers of the knee, the potential for
such dislocation remains.
This instability characteristic is illustrated in Fig. 6.104. Due to the combined
effects of an A-P shearing load, distraction of one of the condylar compartments,
and a lax collateral ligament associated with the distracted compartment, the
rotating bearing can be forced to rotate to a dislocated position. Only ligament
tension sufficient to prevent the femoral condyle on the distracted side from
climbing over the lip of the bearing can prevent such dislocation.
