Biomedical Engineering Reference
In-Depth Information
Further, it may be seen that with the exception of the Maxim knees initiation of
roll back, as illustrated in Fig. 6.36, occurs relatively late thereby losing roll back
properties in important activities such as stair climbing.
15
PFC
SIGMA
LCS-
P
S
MAEVA
IB PS II
10
5
MAXIM
0
KINEMAX
NEXGEN
-5
KINEMAX
-10
-15
-20
15°
45°
105°
120° 135°
0
30°
60° 75°
90°
FLEXION
Fig. 6.36
Roll Back in mm as a Function of Flexion Angle
Table 6.2 demonstrates that the axial rotation in all designs, other than the LCS-
PS knee, is far below the normal maximum axial rotation of at least 20º that is
needed in deep flexion [1]. Most provide marginally sufficient axial rotation to
accommodate walking. In some cases, there is insufficient axial rotation to
accommodate expected surgical misalignment.
Except for those devices such as the LCS-PS, PFC Sigma and the Maeva which
provide for axial rotation, the actual contact stresses resulting from the corner
edge contact between the post and cam resulting from axial rotation, would be
significantly higher than those estimated for line contact at zero axial rotation.
Thus, axial rotation greatly magnifies the already excessive stresses observed at
zero degrees.
Most devices do not provide significant roll-back until more than 75º of flexion
is achieved. Thus, they provide little advantage for stair climbing and descent.
It should be noted that at zero flexion the Kinemax produces an anterior
position of the femoral component on the bearing resulting in a reduction in the
quadriceps lever arm at full extension. Further this anterior positioning results in
an anterior climb of 2.3mm resulting in a reduction in the prosthetic gap on
flexion producing loosening of ligament tension.
