Anatomic Referencing of the Distal Femur in Total Knee Arthroplasty: The Impact of Orientation on Femoral Component Size.

ABSTRACT

BACKGROUND: Anatomic referencing in total knee arthroplasty places the femoral component flush to the anterior cortex while maintaining posterior condylar offset (PCO). The intent of this study was to evaluate how component position influences the femoral component size. METHODS: Digital surface models were created using 446 femora from an established computed tomography database. Virtual bone resections, component sizing, and component placement were performed assuming neutral (0°) flexion and neutral (3°) rotation relative to the posterior condyles. The appropriately sized femoral component, which had 2 mm of incremental size, was placed flush with the anterior cortex for optimal restoration of the PCO. Sizing and placement were repeated using 3 and 6° flexion and 0, 5, and 7° external rotation (ER). RESULTS: At 0° flexion, decreasing ER from 3 to 0° resulted in an average decreased anterior-posterior height (APH) of 1.9 mm, corresponding to a component size decrease of 1 for 88% of patients. At 7° ER, component size increased by an average of 2.5 mm, corresponding to a size increase for 80% of patients. Flexing the femoral component to 3° with ER at 3° resulted in a decrease in APH of 2.2 mm (1 size decrease in 93% of patients). At 3° flexion and 3° ER, 86% had the same component size as at 0° flexion and 0° ER. Increasing ER at 3° flexion increased APH by 1.2 mm at 5° and 3.1 mm at 7° on average, relative to 3° ER. Increasing flexion from 3 to 6° extended this effect. CONCLUSIONS: Flexion decreases the APH when the ER is held constant. The ER of the femoral component increases the APH across all tested flexion angles, causing an increase in the ideal femoral component size to maintain PCO. With anatomic referencing, alterations in femoral component positioning and subsequent changes in component size can be accounted for.