2023 ISAKOS Biennial Congress ePoster
Patellar Medial-lateral Position can be Used to Correct the Effect of Leg Rotation on Preoperative Planning in Total Knee Arthroplasty for Varus Knees
Shinichiro Nakamura, MD, PhD, Kyoto JAPAN
Shota Takemoto, MD, Kyoto JAPAN
Shinichi Kuriyama, MD, PhD, Kyoto JAPAN
Kohei Nishitani, MD, PhD, Kyoto JAPAN
Hiromu Ito, MD, PhD, Kyoto JAPAN
Mutsumi Watanabe, MD, PhD, Shimane JAPAN
Young Dong Song, MD, PhD, New York UNITED STATES
Shuichi Matsuda, MD, PhD, Kyoto JAPAN
Kyoto University, Kyoto, Kyoto, JAPAN
FDA Status Cleared
Summary
When the angle between the mechanical and anatomical axes of the femur is measured in a long leg radiograph, the method to correct the angle according to the patellar ML position can be used to reduce the measurement error, which reflects the proper angle in the true AP view.
ePosters will be available shortly before Congress
Abstract
Background
A long leg radiograph remains a gold standard during the preoperative planning for the femoral resection, and the angle between the mechanical and anatomical axes is applied to the cutting jig. Lower limb malrotation can be observed in long leg radiographs, affecting the measurement of the angle between the mechanical and anatomical axes. The purposes were to analyze the effect of limb rotation and to evaluate the accuracy of the corrected angle between the mechanical and anatomical axes based on the patellar medial-lateral (ML) position. The hypothesis was that the correction of the angle between the mechanical and anatomical axes according to the patellar ML position can reduce the error from the angle in the true antero-posterior (AP) view in most of the knees.
Materials And Methods
A total of 100 consecutive knees with varus deformity undergoing primary total knee arthroplasty were included. Computed tomography images were digitally reconstructed in the neutral position, and internally and externally rotated at 10° and 20° relative to the mechanical axis of the femur, connecting the center of the femoral head and knee joint, respectively. The patellar ML position relative to the medial (0%) and lateral (100%) epicondyles and the angle between the mechanical and anatomical axes of the femur were measured. The angle between the mechanical and anatomical axes of the femur was measured in each digitally reconstructed image. The corrected angle between the mechanical and anatomical axes was calculated using the averaged translational ratio.
Results
In the neutral position, the patellar center position was 56.1% (standard deviation [SD] = 4.7%), which was 31.4% (SD = 7.2%) and 80.2% (SD = 5.4%) in the 20° internal and external rotation, respectively. The angle between the mechanical and anatomical axes was 2.6° (SD = 2.0°) and 8.1° (SD = 2.1°) in the 20° internal and external rotation, respectively. On average, if the patellar center shifted 10%, the change of the angle between the mechanical and anatomical axes of the femur was 1.13°. The following equation was applied to calculate the corrected angle between the mechanical and anatomical axes of the femur.
z = (56.1 - x) × 0.113 + y
(x: patellar ML position (%), y: measured angle (degrees), z: corrected angle (degrees))
In the 20° external and internal rotations, approximately 40% of the knees showed a discrepancy of >3°. Applying the corrected angle, a discrepancy from the neutral position decreased.
Conclusion
The patellar ML center and the angle between the mechanical and anatomical axes are altered with limb rotation. When the angle between the mechanical and anatomical axes of the femur is measured in a long leg radiograph, the method to correct the angle according to the patellar ML position can be used to reduce the measurement error, which reflects the proper angle in the true AP view.