Anatomic vs functional length. (1) Anatomical femoral length: distance between the centre of the femoral head (a) and the centre of the trochlea (b). (2) Anatomical tibial length: distance between the centre of the tibial spine (intercondylar eminence) (c) and the centre of the ankle joint (d). (3) Functional length: distance between the centre of the femoral head to the centre of the ankle joint (AD). (4) Anatomical length: sum of the anatomical femoral and tibial lengths (AB + CD)