Note:
4-bar knees inherently are very stable due to the geometry built into each
design. This is commonly referred to as the Instant Knee Center (IKC). The IKC
point when doing bench alignment, will fall behind the traditional TKA line that we
will reference. (Fig. 2a,2b) Tg line in Fig. 3 is ideal placement, but in certain
instances, it may be necessary to accommodate placement anteriorly (0 to
10mm). The Tg line is referencing a moving A/P weight bearing line, so it could
be slightly anterior or neutral.
a) With prosthesis assembled, taking into account hip flexion contractures,
abduction, Line Of Progression, and toe out (Fig.2a), the TKA plumb line should
pass through the knee center (center ot proximal/anterior pivot Fig.2a, 2b) and in
front of the K point (IKC).
NOTE: Take into account shoe heel height, and add 3mm safety factor.
b) Ideally, the pylon connecting the knee and foot should end up vertical. There
may be a variance due to the foot alignment recommendations. In this case, the
maximum anterior tilt of the pylon should not exceed 3 degrees, and it may be
necessary to utilize 1222T offset tube clamp adapter.
c) With prosthesis donned, the weight line should pass through the centerline of
the knee in the Coronal or M/L plane (Fig. 2c). Excessive outset or inset will put
undue stress on the knee joint.
d) With prosthesis donned, the weight line for Sagittal or A/P plane should have
the plumb line passing ideally through the knee center (proximal anterior pivot),
and be perpendicular to the ground. (Fig. 2a, 2b)
7.1 BENCH ALIGNMENT:
It is not recommended to have alignment posterior to the reference line, as
it could cause knee instability!
