The anteroposterior (AP) portions of the distal femur are generally resected externally rotated relative to the posterior condylar line (PCL) in non-navigated/measured-resection total knee arthroplasty (TKA). Some femoral sizing devices allow rotational adjustment before sizing (rotation-modified sizing devices), while others do not (rotation-unmodified sizing devices). When rotation-unmodified sizing devices are used, the stylus attached to the sizing device identifies the proximal/anterior flange cut assuming the femoral component is aligned parallel to the PCL. However, if the AP cutting device is externally rotated then unexpected notching of the anterior cortex can occur.
We simulated TKA surgery in 100 Japanese varus osteoarthritic knees using three-dimensional planning software. The femoral component was aligned parallel to the PCL and the AP component position was determined so proximal anterior flange was just touching the anterior cortex. Next, the femoral component was externally rotated parallel to the surgical epicondylar axis (SEA). If the proximal anterior flange penetrated femoral bone, the length and thickness of anterior notching were measured.
The proximal anterior flange rested on the anterior cortex or penetrated the femoral bone in all knees. There was a strong positive correlation between PCA and length of anterior notching. Anterior notching more than 10mm length or one-millimeter thickness occurred in most cases (82.4% and 70.6%, respectively) where the posterior condylar angle (PCA) was more than four degrees.
Anterior femoral notching is influenced by the design of the femoral sizing guide and subsequent rotation of cutting blocks, especially in knees with a large PCA.