A New Angle on Bones: Robust Pose Estimation in X-Ray and Ultrasound

2026-06-03

AI Summary

This work proposes a robust algorithm for automatic estimation of skeletal angles in medical images to support clinical diagnosis and treatment planning. By integrating deep learning–based keypoint detection with robust line-fitting strategies—specifically RANSAC and the Hough transform—the method accurately extracts bone axes from both X-ray and ultrasound images and computes inter-bone angles, effectively overcoming the sensitivity to outliers that plagues conventional least-squares approaches. Evaluated on three pediatric clinical tasks, the algorithm achieves mean angular errors of 4.1°, 5.4°, and 5.51°, respectively, meeting clinically acceptable accuracy thresholds and significantly outperforming existing landmark-based methods.

Abstract

Measuring the angle between bone structures is a routine task in medical image analysis and provides a key quantitative parameter for diagnosis and treatment planning. Automated methods can reduce time and cost while improving reproducibility. In this work, we address automatic bone pose estimation using a learning-based point candidate proposal followed by a line model to extract axis parameters. Since conventional line models such as least squares are sensitive to outliers, we incorporate false-positive reduction strategies and robust fitting techniques, such as RANSAC and Hough transforms, to improve robustness. We evaluate our method on three clinically relevant paediatric angle estimation tasks: fracture fragment assessment in radiographs and ultrasound and developmental dysplasia of the hip evaluation in ultrasound using the Graf method. Our approach achieves mean errors of $4.1^\circ$, $5.4^\circ$, and $5.51^\circ$, respectively, not only remaining within the expected clinical observer variability, but also significantly outperforming landmark-based methods. Our code and annotations for fracture angle assessment in radiographs are publicly available on GitHub.

Problem

• bone pose estimation
• angle measurement
• medical image analysis
• robust fitting
• outlier sensitivity

Innovation

• robust pose estimation
• bone angle measurement
• RANSAC
• Hough transform
• learning-based point proposal