Method for defining the horizontal plane in fluoroscopic kinematic analysis

Purpose

In vivo kinematics of skeletal joints and prostheses is affected by gravity; therefore, absolute tilt must be taken into account. However, current fluoroscopic kinematic analysis cannot measure the tilt. This study introduces and validates a method for defining the horizontal plane in fluoroscopy systems to measure the tilt of an object.

Methods

A rectangular metal plate (200 ​× ​200 ​× ​10 ​mm) with a flatness of 0.03/100 ​mm was horizontally adjusted using a leveler (sensitivity of 0.02° and accuracy of ±0.06°), and its three-dimensional coordinate was calculated from a two-dimensional fluoroscopic image. A coordinate calculation formula was developed, with precision and accuracy assessed via computer simulations. Actual fluoroscopic tests included four aspects: accuracy under the same condition; accuracy under different X-ray tube height; reproducibility when changing X-ray tube height; and reproducibility when the flat panel was tilted and returned.

Results

The theoretical measurement error indicated by the computer simulations was 0.0 ​± ​0.2° on the xy-plane and 0.0 ​± ​0.0° on the yz-plane. The actual experimental results showed that the horizontal plane tilt was measured to be −0.2 ​± ​0.1° tilt on the xy-plane and 0.3 ​± ​0.0° tilt on the yz-plane for the device used in this study. The F-test showed no significant differences between the computer simulations and the actual experiments. There were no significant differences between the four actual tests.

Conclusions

This method enables the horizontal plane to be defined and the tilt of an object to be measured in fluoroscopic kinematic analysis.