Quantifying translations in the radiohumeral joint: application of a floating axis analysis

The purpose of this study was to apply the Floating Axis analysis technique to the elbow joint, and to verify its ability to quantify clinically relevant radiohumeral translation in vitro using an electromagnetic tracking device. Of particular interest was the ability to quantify changes in anterior–posterior radial head translation, which is associated with the clinical condition of posterolateral rotatory instability of the elbow. Following the method proposed by Grood and Suntay to determine motions in the knee, an elbow coordinate system with axes representing the flexion-extension axis of the humerus, the long axis of the radius, and their mutual perpendicular, was developed. The algorithm was tested using a mechanical articulator that modeled the Floating Axis approach. Translation errors using this articulator were 0.1±0.1 mm. The algorithm was applied to kinematic data collected from 12 cadaveric elbows that underwent a pivot shift test prior and subsequent to transection of the lateral collateral ligament. Anterior–posterior radiohumeral translation increased significantly in these elbows following the ligament sectioning (p<0.0001), with the average magnitude of posterior translation increasing from 0.9 to 19.8 mm at 90° of flexion. This approach will provide valuable information related to alterations in elbow motion pathways, especially for studies aimed at quantifying changes in joint stability