Abstract
Introduction
Intramedullary femoral alignment guide is mostly used in total knee arthroplasty (TKA). Accurate preoperative plan is critical to get good alignments when we use intramedullary femoral guide, because the center of femoral head cannot be looked directly during operation. Commonly, the planning is carried out using preoperative anteroposterior radiographs of the femur. The angles formed between mechanical axes of the femur and distal femoral anatomic axes (AMA) are measured as reference angles of resection of distal femur, and the entry points of intramedullary femoral guide are also planned. The purpose of this study is to investigate the influence of femoral position on radiographic planning in TKA.
Materials and Methods
We examined 20 knees of 20 female patients who received TKA. Fourteen patients suffered from primary osteoarthritis of the knees, and 6 suffered from rheumatoid arthritis. Fifteen patients have varus knee deformities and 5 patients have valgus knee deformities. Long leg computed topography scans were performed in all cases before operations, and all images were stored in DICOM file format. The analyses were performed with computer software (3D template, JMM, Osaka, Japan) using DICOM formatted data. The planes containing the center of femoral head and transepicondylar axes were defined as reference planes, and the reference planes were fixed all through analyses. At first, to assess the influence of femoral rotation, the femur was rotated from 30 degrees external rotation to 30 degrees internal rotation in 5 degrees increments in full extension. After that, to examine the influence of knee flexion, the knee was bended from full extension to 30 degrees flexion in 5 degrees increments in neutral rotation. Reconstructed coronal planes parallel to the reference planes were made, the angles between mechanical axes of the femur and distal femoral anatomic axes (AMA) and the distance from entry points to the center of femoral intercondylar notch were measured in each position. The distal anatomic axes were made by connecting the center of femoral canal at 8 centimeters proximal to joint line and that at 16 centimeters proximal to joint line. The entry points of intramedullary femoral guide were defined the points where distal anatomic axes meets intercondylar notch.
Results
The mean AMA was 6.8 degrees in neutral position, 7.1 degrees in 10 degrees external rotation, 7.3 in 20 degrees external rotation, 7.2 in 30 degrees external rotation, 6.2 in 10 degrees internal rotation, 5.5 in 20 degrees internal rotation, 4.6 in 30 degrees internal rotation, 6.9 in 10 degrees flexion, 7.2 in 20 degrees flexion, 7.8 in 30 degrees flexion, respectively. The entry points moved 3.9 millimeters laterally in 20 degrees external rotation and 3.6 millimeters medially in 20 degrees internal rotation relative to neutral position.
Discussion and Conclusion
Femoral position affected on preoperative planning using anteroposterior radiographs. It is important to get a correct anteroposerior view of total femur, because the femur was easy to rotate in knee disorders.
