Pitfalls in determining knee alignment: a radiographic cadaver study

Full-length anteroposterior (AP) radiographs provide the basis for evaluating leg alignment. A cadaver study was performed to determine the effect of knee flexion and hip rotation on projected angles on full-length AP radiographs. The outcomes were verified mathematically. The results of the cadaver study were similar to the mathematical results. Flexion of the knee without rotation of the lower extremity has little effect on angles as projected on full-length AP radiographs. Rotation of the lower extremity without flexion of the knee also has little effect. Simultaneous flexion of the knee and rotation of the leg, however, cause large changes in projected angles. Full-length radiographs can be taken without fluoroscopic control as long as the knee can be fully extended. In flexion contracture, a full-length AP radiograph taken under lateral fluoroscopic control is necessary to obtain accurate determination of the mechanical axis.