What are the bias, imprecision, and limits of agreement for finding the flexion–extension plane of the knee with five tibial reference lines?

• Kinematically aligned TKA sets the tibial component parallel to the F–E plane.
• Four tibial reference lines are biased from the knee's flexion–extension plane.
• Five tibial reference lines are imprecise with a 4° to 6° SD of malrotation.
• Other tibial reference lines should be considered in kinematically aligned TKA.

BackgroundInternal–external (I–E) malrotation of the tibial component is associated with poor function after total knee arthroplasty (TKA). Kinematically aligned (KA) TKA uses a functionally defined flexion–extension (F–E) tibial reference line, which is parallel to the F–E plane of the extended knee, to set I–E rotation of the tibial component.MethodsSixty-two, three-dimensional bone models of normal knees were analyzed. We computed the bias (mean), imprecision (± standard deviation), and limits of agreement (mean ± 2 standard deviations) of the angle between five anatomically defined tibial reference lines used in mechanically aligned (MA) TKA and the F–E tibial reference line (+ external).ResultsThe following are the bias, imprecision, and limits of agreement of the angle between the F–E tibial reference line and 1) the tibial reference lines connecting the medial border (− 2° ± 6°, − 14° to 10°), medial 1/3 (6° ± 6°, − 6° to 18°), and the most anterior point of the tibial tubercle (9° ± 4°, − 1° to 17°) with the center of the posterior cruciate ligament, and 2) the tibial reference lines perpendicular to the posterior condylar axis of the tibia (− 3° ± 4°, − 11° to 5°), and a line connecting the centers of the tibial condyles (1° ± 4°, − 7° to 9°).Clinical relevanceBased on these in vitro findings, it might be prudent to reconsider setting the I–E rotation of the tibial component to tibial reference lines that have bias, imprecision, and limits of agreement that fall outside the − 7° to 10° range associated with high function after KA TKA.