Longitudinal shapes of the tibia and femur are unrelated and variable

Abstract

In general practice, short films of the knee are used to assess component position and define the entry point for intramedullary femoral alignment in TKAs; however, whether it is justified to use the short film commonly used in research settings and everyday practice as a substitute for the whole leg view is controversial and needs clarification. In 138 long leg CT scanograms we measured the angle formed by the anatomic axis of the proximal fourth of the tibia and the mechanical axis of the tibia, the angle formed by the anatomic axis of the distal fourth of the femur and the mechanical axis of the femur, the "bow" of the tibia (as reflected by the offset of the anatomic axis from the center of the talus), and the "bow" of the femur (as reflected by the offset of the anatomic axis from the center of the femoral head). Because the angle formed by these axes and the bow of the tibia and femur have wide variability in females and males, a short film of the knee should not be used in place of the whole leg view when accurate assessment of component position and limb alignment is essential. A previous study of normal limbs found that only 2% of subjects have a neutral hip-knee-ankle axis, which can be explained by the wide variability of the bow in the tibia and femur and the lack of correlation between the bow of the tibia and femur in a given limb as shown in the current study.

Fig. 1A–B

The anatomic axis of the tibia (longitudinal white line) was a line joining the midpoints of the tibia at the joint line and at the junction of the proximal one-fourth and distal three-fourths of the tibia (transverse black line). The mechanical axis of the tibia was a line joining the midpoint of the tibia at the joint line and the center of the talus (longitudinal black line). The angle formed by these two lines was measured. The bow of the tibia was quantified by the offset (D) measured from a line (transverse white line) drawn perpendicular from the anatomic axis of the tibia to the center of the talus. A positive value (+) indicated a valgus tibia with the apex of the bow pointing medial and with the anatomic axis passing medial to the center of the talus. (A) The offset of the tibia with the greatest valgus bow was 3.5 cm. A negative value (–) indicated a varus tibia with the apex of the bow pointing lateral and with the anatomic axis passing lateral to the center of the talus. (B) The offset of the tibia with the greatest varus bow was −2.2 cm. The use of an extramedullary tibia guide that references the ankle would place the tibial cut in six additional degrees of varus in the valgus tibia (left) and four additional degrees of valgus in the varus tibia (right) requiring lateral and medial soft tissue release to balance the knee, respectively.

Fig. 2A–B

The anatomic axis of the femur (longitudinal white line) was a line joining the midpoints of the tibia at the joint line and at the junction of the distal one-fourth and proximal three-fourths of the femur (transverse black line). The mechanical axis of the femur was a line joining the midpoint of the femur at the joint line and the center of the femoral head (longitudinal black line). The angle formed by these two lines was measured. The bow of the femur was quantified by the offset (D) measured from a line (transverse white line) drawn perpendicular from the anatomic axis of the femur to the center of the femoral head. The larger the offset, the larger the bow with a positive value (+) indicating the anatomic axis passed lateral to the center of the femoral head and a negative value (−) indicating the anatomic axis passed medial to the center of the femoral head. (A) The offset of the femur with the greatest lateral bow was 6.4 cm. (B) The offset of the femur with the least lateral bow was −0.4 cm. Because of the variability in the lateral bow of the femur, the mechanical axis of the femur does not form a 5° to 6° angle with the anatomic axis of the distal fourth of the femur in either leg.

Fig. 3A–B

Frequency distributions of the (A) angle formed by the anatomic and mechanical axes of the tibia and (B) bow of the tibia as quantified by the offset of the anatomic axis from the center of the talus are shown. Descriptive statistics include the quantile plot and the number of patients in each column.

Fig. 4A–B

Frequency distributions of the (A) angle formed by the anatomic and mechanical axes of the femur and (B) bow of the femur as quantified by the offset of the anatomic axis of the femur from the center of the femoral head are shown. Descriptive statistics include the quantile plot and the number of patients in each column.

Fig. 5A–B

(A) The short view of the knee suggests the tibial component is malaligned in varus. (B) However, the long leg view shows the limb has a neutral hip-knee-ankle axis. The use a short knee radiograph to assess component and limb alignment is not recommended.

Fig. 6A–F

(A, D) The anatomic axis (longitudinal white lines) of the knee measured on the radiographic view of these two knees are similar; however, (B, E) the hip-knee-ankle axis of these two limbs are dissimilar. (C, F) The reason the anatomic axis of the knee does not predict the hip-knee-ankle axis of the limb is because of the wide variability in the bow of the tibia of the femur and because of the lack of correlation between the bow of a tibia and femur in a given limb.