Predicting knee rotation by the projection overlap of the proximal fibula and tibia in long‐leg radiographs

Abstract Purpose Rotation of the lower limbs in long‐leg radiographs has a significant impact on imaging the mechanical femorotibial angle, the femoral anatomic mechanical angle, the mechanical lateral distal femoral angle (mLDFA) and the mechanical medial proximal tibial angle (mMPTA). In this study, we assessed the rotation of the lower limbs in conventional radiographs and hypothesized that the relative position of the proximal fibula to the proximal tibia on long‐leg radiographs is related to the rotation of the knee joint. Methods Radiological examinations in different rotational positions of the knee joint (incremental 40° internal to 40° external rotation) were imitated by 50 computed tomography scans (50 patients, 25 men and 25 women). The extent of the projection overlaps of the fibula, the fibular tip and the distance from the fibular tip to the lateral cortex were determined for every rotational position. Results Multiple regression analysis showed a very strong correlation between the measured fibular parameters and knee rotation between 20° of internal rotation and 40° of external rotation ( R 2 ~ 0.94, p < 0.001). By means of these results, we created a formula for predicting knee rotation: urn:x-wiley:09422056:media:ksabf00474:ksabf00474-math-0001 This strong correlation could not be found between 20° and 40° of internal rotation. Discussion Because incorrect internal and external rotation negatively influence the correct measurement of angles (mechanical femorotibial angle, femoral anatomic mechanical angle, the mLDFA and the mMPTA), long‐leg radiographs should be assessed for proper rotation angles before measurement. Using the provided formula rotation of the lower limb in weight‐bearing, long‐leg radiographs can be reliably predicted. Level of evidence Diagnostic study, Level II.