- Determine Appropriate AP Component Size:
- most instrument systems incorporate cutting block that is placed on end of femur & rotated to appropriate orientation;
- prior to inserting the AP sizing guide, remove any prominent meniscal remnants so that the posterior "fins" of the sizing guide can fully be seated;
- if there is any question that the sizing guide is not seating down properly, then perform the
proximal tibial cut first and then reapply the femoral cutting jig;
- when applying the femoral sizing stylus, but sure to remove all synovium between the stylus and the anterior femoral surface;
- remember that most systems offer sizes that go up in 4 mm increments, and therefore a small amount of synovium entrapped between the stylus
and bone can very easily lead to oversizing the femoral component;
- distal femoral sizing guide is then selected (from template);
- if there is a significant mis-match between the templated size and the size determined from the sizing jig, remeasure the sizing jig, and make sure that it is fully seated;
-
management of femoral / component mismatch
- if the proposed femoral cut lands between available sizes, then it will be necessary to choose the lesser
size and then to translate the component anteriorly to the point that the femoral cortex is not notched;
- for example, if the optimal size for the femoral component is a 3 1/2, then choose a size 3 and move the component
midway between the standard positions for a size 3 and a size 4 femoral components;
- Rotational Alignment of AP Cutting Guide:
-
AP axis method:
- posterior femoral condyles as a rotational guide:
- axial alignment of the
femoral component either is determined by posterior femoral condyles
or is adjusted to provide equal
collateral ligament tension w/ knee in flexion;
- 3 deg external rotation is typically utilized;
- some systems currently call for externally rotating
femoral component 3-4 deg relative to posterior femoral
condyles to aid in patellar tracking;
-
pitfalls:
-
femoral component is also commonly placed in excessive internal rotation, which sign affects patellar tracking;
- obviously, significant internal rotation should be avoid to prevent increasing the
Q angle;
- in some cases, there will be a larger AP size of lateral femoral condyle than medial condyle, and in this situation it may not be possible
to externally rotate the femoral cutting guide inorder to avoid undercutting anterior cortex;
Assessing rotational alignment in total knee arthroplasty.
Malrotation associated with implant alignment technique in total knee arthroplasty.
Knee stiffness on extension caused by an oversized femoral component after total knee arthroplasty.
A report of two cases and a review of the literature.
Varus tibial joint line obliquity: a potential cause of femoral component malrotation.
