An Assessment of Knee Flexion in Lateral Knee X-rays

Purpose: Patient positioning plays a crucial role in the field of radiology. Lateral knee x-rays are a type of image that often has incorrect positioning of the angle of knee flexion. The ideal range is between 20-30 degrees. The goal of this study was to assess the angle of knee flexion at two different locations in a single hospital system while determining if several variables influence the angle.

Methods: This study is a retrospective chart review that assessed the angle of knee flexion in patients 18 years or older that underwent a lateral-mediolateral knee x-ray taken at an urgent care center and a general diagnostic center of a hospital within the same system between March 1 and December 1, 2021. Variables including age, sex, BMI, technologist, and location were collected from these patients’ charts and evaluated. MRI information was gathered for patients who underwent an MRI within 30 days of a lateral knee x-ray. The research team assessed effusions reported on x-ray compared to effusions reported on MRI for these patients.

Results: Among patients included in the study (n=665) the average angle of knee flexion was 51.28 degrees. Age, sex, BMI, and location were not significantly associated with the mean angle of knee flexion with p-values of 0.63, 0.13, 0.55, and 0.15 respectively. The radiology technologist taking the image did have an association with the angle of knee flexion with a p- value of 0.001. Differences in the mean angle of knee flexion between the groups of x-rays with effusions reported compared to the groups of x-rays where effusions were not reported but found on MRI resulted in a p-value of 0.83.

Conclusions: The technologist taking the image was the only variable of this study that had a significant difference in mean angle of knee flexion. Additional studies are needed to determine what technologist factors are most important in determining the angle of knee flexion. Using MRI information to evaluate if effusions were not reported due to the angle of knee flexion was limited in this study due to small sample size.