Anthropometric measurements of the femur change with component positioning in total knee arthroplasty.

BACKGROUND: To analyze aspect ratio (AP size/ (ML) size) of osteoarthritic knees at four different areas of the femur and to observe if proximalization of the femoral cut would change the ML size as well as confirm that external rotation increases the measurements for the AP dimensions of the femur.

METHOD: From the available MyKnee database (Medacta International, Castel San Pietro, Switzerland) 1030 patients were randomly selected within 20° of deformity consisting of 400 men with a mean (SD) age of 67.5 (9) years and 630 women with a mean (SD) age of 69 (10) years (p<0.0001). A specific software program was developed to measure AP and ML dimensions of the femur on CT-scans for (3D) planning in four areas. The AP femoral size was measured with neutral axial rotation following the epicondylar axis and without accepting anterolateral notching.

RESULTS: Proximalization of the femur resulted in no changes except for a larger ML3 area in men. Increased axial rotation increased the AP dimensions for the same femur by a mean (SD) 2.5 (1) mm for males and females.

CONCLUSIONS: The crucial area for overhang of the femoral component is the anterior region (ML1) with an aspect ratio of about ±, but with an important range. Proximalization of the femoral cut is not accompanied by narrowing of the anterior femur but ML widening of the more posterior femur in men. Increased external rotation leads to a measurement of bigger AP size leading to an AP versus ML mismatch and change in aspect ratio.