FEMORAL/TIBIAL COMPONENT ROTATION: TRUTH AND CONSEQUENCES

There are basically 4 ways advocated to determine the proper femoral component rotation during TKA: (1) The Trans-epicondylar Axis, (2) Perpendicular to the “Whiteside Line,” (3) Three to five degrees of external rotation off the posterior condyles, and (4) Rotation of the component to a point where there is a balanced symmetric flexion gap. This last method is the most logical and functionally, the most appropriate. Of interest is the fact that the other 3 methods often yield flexion gap symmetry, but the surgeon should not be wed to any one of these individual methods at the expense of an unbalanced knee in flexion. In correcting a varus knee, the knee is balanced first in extension by the appropriate medial release and then balanced in flexion by the appropriate rotation of the femoral component. In correcting a valgus knee, the knee can be balanced first in flexion by the femoral component rotation since balancing in extension almost never involves release of the lateral collateral ligament (LCL) but rather release of the lateral retinaculum. If a rare LCL release is anticipated for extension balancing, then it would be performed prior to determining the femoral rotation since the release may open up the lateral flexion gap to a point where even more femoral component rotation is needed to close down that lateral gap. It is important to know and accept the fact that some knees will require internal rotation of the femoral component to yield flexion gap symmetry. The classic example of this is a knee that has previously undergone a valgus tibial osteotomy that has led to a valgus tibial joint line. In such a case, if any of the first 3 methods described above is utilised for femoral component rotation, it will lead to a knee that is very unbalanced in flexion being much tighter laterally than medially. A LCL release to open the lateral gap will be needed, increasing the complexity of the case. My experience has shown that intentional internal rotation of the femoral component when required is well-tolerated and rarely causes problems with patellar tracking. It is also of interest to note that mathematical calculations reveal that internally rotating a femoral component as much as 4 degrees will displace the trochlear groove no more that 2–3 mm (depending on the FC size), an amount easily compensated for by undersizing the patellar component and shifting it medially those few mm. There are basically 3 ways to determine the proper tibial component rotation during TKA: (1) Anatomically cap the tibial cut surface with an asymmetric tibial component, (2) Align the tibial rotation relative to a fixed anatomic tibial landmark (most surgeons use this method and align relative to the medial aspect of the tibial tubercle), (3) Rotate the tibial component to a point where there is rotational congruency in extension between the femoral and tibial articulating surfaces. This third method must be used with fixed bearing arthroplasties (especially with conforming articulations) to avoid rotational incongruency between the components during weight-bearing that can create abnormal and deleterious torsional forces on posterior stabilised posts, insert tray interfaces and bone-cement interfaces. Rotating platform articulations can tolerate rotational mismatch unless it is to a point where the polyethylene insert rotates excessively and causes symptomatic soft tissue impingement.