Proper alignment of total knee arthroplasty (TKA) components is critical for the success and longevity of the prosthesis. It has been well documented in literature that malalignment can cause aseptic loosening, ligament instability, patellar maltracking, decreased range of motion and increased wear of the implant.

The flexion-extension axis and the way in which it is defined is contributing to this malalignment. The objective of this research was to compare and contrast axes used clinically for femoral implant positioning in total knee arthroplasty (TKA). Through this comparison, the goal was to ascertain which axis best represents the true functional flexion-extension axis of the knee and therefore the ideal axis to be used in TKA alignment of single radius of curvature implants. The four axes investigated include:​ the transepicondylar (TEA), posterior condylar (PCA), cylindrical (CA), and Whiteside’s line (also known as anteroposterior).

Kinematic data for all six degrees of freedom were collected on nine cadaveric knees for both the transepicondylar and cylindrical axes. The relative position of all four axes were also determined on a CT based model for each of the nine knees. Each axis was then compared in the frontal and transverse planes and the angular variance was measured.

Analysis of the collected kinematic data provides evidence that the CA most closely represents the natural axis of rotation of the knee. When comparing the angles of the other three axes relative to the CA, all had values greater than three degrees. Since a malalignment larger than three degrees has been show to double to contact pressure on the medial compartment, the clinical use o f the TEA, PCA and Whiteside’s line could cause premature wear and the ultimate demise o f the implant. Placement of an implant relative to a surrogate axis other than the CA may only increase these attributes. Proper alignment of implants could restore natural knee kinematics, decrease the amount of soft tissue balancing necessary, and decrease wear, thereby improving implant longevity.