Component Rotation in Kinematic Alignment: It’s Just Plane Simple
At ICJR’s Pan Pacific Orthopaedic Congress, Dr. Stephen Howell discussed the concept of kinematic alignment in total knee arthroplasty. Below is the abstract of his presentation.
By Stephen M. Howell, MD
Incorrect internal-external rotation of the femoral and tibial components contributes to patients being dissatisfied with their knee function after total knee arthroplasty (TKA). 
Two millimeters or more of instability can occur in a compartment in 0° and 90° of flexion if the knee was mechanically aligned, meaning the femoral component rotation was set to reference lines parallel to the anteroposterior axis of the trochlear groove, parallel to the transepicondylar axis, or 3° externally rotated to the posterior condylar line of the femur.
In this situation, instability is uncorrectable by a collateral ligament release in up to 80% of knees. 
Kinematically aligned TKA has gained interest because function, coronal limb and knee alignment, contact kinematics, and implant survivorship at 3 years are better than that seen with a mechanically aligned TKA. [3-6]
The goal of a kinematically aligned TKA is to set the anteroposterior axis of the femoral component and tibial component parallel to the sagittal kinematic plane. [2,4] Because the sagittal kinematic plane is perpendicular to the 2 parallel transverse axes in the femur that determine the flexion and extension path of the tibia and patella, setting the components parallel to this plane:
- Avoids instability in a compartment between 0° and 90°
- Minimizes the need for ligament release
- Maintains stability in the tibiofemoral and patellofemoral joint [2,4,7-10]
A kinematically aligned TKA sets the rotation of the femoral component by removing cartilage and bone from the posterior femoral condyles, after correcting for wear and kerf that equals the thickness of the posterior condyles of the femoral component. 
The rotation of the tibial component is set by positioning the anteroposterior axis of the tibial trial parallel to the major axis of the nearly elliptical boundary of the articular surface of the lateral tibial condyle. [6,12]
A study of 71 consecutive patients showed the range of rotation from the sagittal kinematic was -3° internal to 2° external for the femoral component, and -11° internal to 12° external for the tibial component. The average Oxford knee score was 42, and the WOMAC score was 89.
Kinematically aligned TKA performed with generic instruments achieves a range or rotation of the femoral and tibial components from the sagittal kinematic plane that results in high function scores.
Dr. Howell’s presentation can be found here.
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