# The Accuracy and Reliability of the Photometric Method—A New Noninvasive Tool for Assessing Frontal Lower Limb Alignment

## TL;DR

This study introduces a new noninvasive photometric method to assess lower limb alignment, finding it to be reliable and accurate compared to traditional radiographic methods.

## Contribution

The study introduces a novel photometric method using easily palpable landmarks for noninvasive lower limb alignment assessment.

## Key findings

- The photometric method showed high correlation (r = 0.97) with radiographic measurements.
- Intrarater, interrater, and test-retest reliability were all excellent (ICC > 0.97).
- The HKA angle was on average 3.9° more varus than the PKA angle.

## Abstract

Background/Objectives: The aim of this study was to establish the reliability and accuracy of a new noninvasive tool for FLLA (frontal plane lower limb alignment) assessment: a photometric method. Methods: Sixty-seven subjects (31 males, 36 females, age 11–47 years) participated in the study. Seventeen subjects with orthopedic disorders were marked with radiopaque markers over the anterior superior iliac spines and femoral condyles. One pelvis-to-floor radiograph and one photograph were taken in the same standardized standing position. The hip–knee–ankle (HKA) angle (radiography) and the pelvis–knee–ankle (PKA) angle (photography) were measured by one rater. In 50 healthy participants, anterior superior iliac spines and femoral condyles were marked, and two pelvis-to-floor photographs were taken in a standardized standing position. The PKA angle was measured two times by three raters. The accuracy of the photometric method was tested with Pearson’s correlation coefficient, simple linear regression, and Bland–Altman analysis. The reliability was tested with ICC(2,k) and Bland–Altman analysis. Results: The HKA angle was on average 3.9° more varus than the PKA angle, with a high correlation between measures (r = 0.97, p < 0.0001) and limits of agreement between −1.300 and −6.482. Intrarater (ICC(2,k) > 0.972), interrater (ICC(2,k) = 0.991), and test–retest (ICC(2,k) = 0.980) reliability were excellent. Conclusions: The photometric method is promising as a reliable and accurate noninvasive tool for assessing FLLA. Its accuracy across different study groups has yet to be confirmed in a larger cohort. The advantage of the presented photometric method is the use of the easily palpable anterior superior iliac spine as the proximal femoral axis point.