Intraoperative Evaluation of the Anatomic Lateral Distal Femoral Angle and Its Variation due to Positioning.

Abstract

Preoperative evaluation of the contralateral anatomic lateral distal femoral angle (aLDFA) at our institution is used to judge coronal plane alignment. In our study, we investigated 4 different techniques for obtaining an anteroposterior intraoperative fluoroscopic image of the distal femur to determine which technique provides (1) the greatest interobserver reliability; (2) the lowest variability from the previously published population mean; and (3) the lowest side-to-side variability. Inclusion criteria included lower extremity injuries needing fixation that required intraoperative fluoroscopy with an intact femur and an intact extensor mechanism (N = 100). Fluoroscopic images were obtained of the distal femur in 4 positions differentiated by the position of the limb and the orientation of the C-arm beam to the femoral shaft. All measurement techniques resulted in mean measurements within one degree of 81 degrees. Variance between measurements was small among patients with all views, but images that involved a true anteroposterior with the beam perpendicular to the femur had the lowest rate of measurements that were <78 or >84 degrees. Side-to-side differences in patients were <2 degrees on average with every image used. With the best-performing images, 20% of patients had an aLDFA > 3 degrees different from the population mean of 81 degrees and 3% of patients were >5 degrees different. Although restoring aLDFA to 81 degrees will be within 3 degrees of the contralateral side the vast majority of the time, matching the aLDFA to the injured side will be the most accurate reconstruction.