Why Evaluating Malrotation Matters
According to David Backstein, MD, Med, FRCS, there are two clear reasons for measuring and correcting malrotation after total knee arthroplasty:
- The clinical significance of malrotation has been demonstrated, showing detrimental effects on patellar tracking and tibiofemoral ligament balancing. An internally rotated tibial component and a relatively increased Q angle may cause the patella to tilt, sublux, or completely dislocate. An internal rotation on the femoral side completely changes the flexion gap, throwing off the balance of the entire knee.
- The benefits of correcting malrotation have also been demonstrated. Studies have shown that if malrotated femoral and tibial components are revised, patient satisfaction and outcomes scores improve.
At ICJR’s OrthoLIVE meeting, Dr. Backstein reviewed how to measure malrotation, using axial CT imaging according to the Berger protocol. 
- Find a cut where the medial and lateral epicondyles are visible
- Draw a line between the epicondyles.
- Draw another line parallel to the poster epicondyles of the femoral component.
- Measure the angle between the two lines.
- Based on the Berger protocol, it is considered neutral when parallel to the epicondylar axis (plus or minus 3°)
- Transpose CT scan images on top of one another.
- Make a circle or square that best fits the tibial plateau and find the geometric center.
- Scan down to find the tibial tubercle. Draw a line from that geometric center to the point on the tibial tubercle where appropriate rotation is.
- Go back up to the tibial base plate. Draw a line parallel to back of the tibial base plate and then another line perpendicular to the first line.
- Go back to the line from the geometric center to the tibial tubercle and measure the angle between the two.
Based on the Berger protocol, within 18° is considered normal.
Dr. Backstein’s presentation can be found here.
- Berger RA, Crossett LS, Jacobs JJ, Rubash HE. Malrotation causing patellofemoral complications after total knee arthroplasty. Clin Orthop Relat Res. 1998;356:144–153