Variation of Femoral Component Size by Reference Points and Target Alignments in Total Knee Arthroplasty

Selecting appropriately sized components is important in total knee arthroplasty because they can affect postoperative knee function and pain. This study investigated size differences of 19 different femoral component placements from the standard position by three-dimensional virtual surgery using three-dimensional bone models of 101 varus osteoarthritic knees. Distal femoral bone was cut perpendicular to the femoral mechanical axis (MA) in the coronal plane. Twenty different component placements consisting of five cutting directions (perpendicular to MA, 3° and 5° extension/flexion relative to MA in the sagittal plane), two rotational alignments (clinical and surgical epicondylar axes), and two rotational types of anterior reference guide (central and medial) were simulated. The mean anteroposterior dimension of the standard position was 55.5 mm which means that the difference compared to 19 different methods ranged from -1.2 ± 0.2 mm to 7.1 ± 1.3 mm. Multiple regression analysis revealed that flexion cutting direction, surgical epicondylar axis, and central were associated with smaller component size. In conclusion, the femoral component size can be affected easily by not only cutting direction but also the reference guide type and the target alignment. Our findings could provide surgeons with clinically useful information to fine-tune for unintended loose or tight joint gaps by adjusting the component size.