To what degree is digital imaging reliable? Validation of femoral neck shaft angle measurement in the era of picture archiving and communication systems.

Abstract

This study investigates the reliability of femoral neck shaft angle (NSA) measurements made with the software and images available in routine clinical practice. Using the Centricity Enterprise™ (GE Healthcare Pty Ltd Piscataway, NJ) picture archiving and communication system (PACS), the NSA of the proximal femur was measured from anteroposterior radiographs of adult hips. 3 independent observers, using a standardised technique, performed a total of 120 measurements. The Pearson's correlation coefficient for the intraobserver agreement was 0.98 (p<0.01) and for interobserver measurements 0.86 (p<0.01). Bland-Altman plots revealed the limits of intraobserver agreement to be ±2.5°, but interobserver limits of agreement to be ±6°. The intraclass correlation coefficient (ICC) was also calculated. The interobserver ICC was 0.62 (0.42-0.78, 95% confidence interval (CI); p<0.001). The intraobserver ICC was 0.98 (0.95-0.99, 95% CI; p<0.001). PACS software has many advantages, but when using systems that can display angle measurements to one-tenth of a degree caution must be exercised to ensure that reliability of these measurements is not overestimated. We found that in the context of measuring the NSA of the proximal femur the reliability of the measurement, even under the best conditions, is only ±6° for different observers.