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

Background/purpose: 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.

Methods: 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.

Results: 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.

Conclusion: 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.