THE INFLUENCE OF MECHANICAL VERSUS KINEMATIC ALIGNMENT ON KNEE DESIGN PERFORMANCE
DURING WALKING GAIT
We utilized KneeSIM, a computational kinematic knee simulation software to evaluate the long
term outcomes of Stryker Triathlon CR components during the activity of walking gait.
KneeSIM results in measurements of component motions and patella tracking that are associated
with patient satisfaction, and contact forces and articular wear paths that are associated
with implant longevity.
Two surgical procedures were followed, the classic Mechanical Alignment and the more
recently performed Kinematic Alignment.
Mechanical alignment, on the left, places the components perpendicular to the vertical
mechanical axis while kinematic alignment, on the right, places the components parallel
to the transverse flexion extension axis which follows the inclination of the natural joint
line in the AP view.
These two approaches result in different bone resections, placing the TKA components in
two different orientations, mechanical aligned in blue, kinematically aligned in orange.
Two different KneeSIM models were created, one for each case.
Different reference axes are used in mechanical versus kinematic alignment.
A vertical midplane axis perpendicular to the floor is moved to coincide with the geometric
center of the ankle joint.
The mechanical axis is typically three degrees from the vertical axis.
The mechanical axis is difficult to directly visualize in the surgical theater, however,
there are many methods of indirectly determining its location.
One of these, the transepicondylar axis, determines the placement of saw guides that create bone
resection planes that are perpendicular to the mechanical axis.
Kinematic alignment seeks to restore the joint line to the individual patient’s pre-diseased
state, referencing the flexion-extension axis.
This imaginary line passes through the geometric centers of each posterior femoral condyle.
Equal resection, allowing for cartilage wear and saw kerf, of the distal femur and posterior
condyles ensure that the resection surfaces are parallel to the flexion-extension axis.
When the thickness of the implanted femoral component matches the resected bone fragments,
the patient’s estimated healthy cartilaginous surface anatomy would be restored.
The proximal tibial bone is resected to restore the individual patient’s original varus
and posterior slope.
The axial rotation of the tibial component aims to closely match the axial rotation of
the femoral component.
The kinematic alignment flexion-extension reference axis and bone resection planes (in orange)
may be compared to those of the classic mechanical alignment surgical procedure (in blue).
AP views and axial views are provided for clarity.
On average, the kinematically aligned femoral component is 6 degrees more flexed, 3 degrees
more valgus and 4 degrees more internally rotated than the mechanically aligned femoral component.
There were minimal differences between the mechanical and kinematic aligned results of
both femoral and patellar component motions, contact articulations and forces.
Average absolute differences over the stance phase of walking gait were less than 2 mm,
2 degrees and 200 Newtons for all results recorded.
The motion of the kinematic aligned femoral component (in orange) can be observed during
walking gait.
The orange jack is rigidly attached to it, allowing a simplified representation of its
motion for easier comparison to the mechanically aligned femoral component jack (in blue).
The progression of tibial-femoral contact areas in both the medial and lateral compartments
can also be compared, kinematic alignment in orange, mechanical alignment in blue.
No functional difference was found between mechanical and kinematic alignment surgical
procedures during the walking gait cycle with Triathlon CR TKA components.
Other product designs that are asymmetrical, single radius or offer greater articular conformity
than the Triathlon CR design may yield different results.
Thank you for your attention!
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