Assessment of Magnification of Digital Pelvic Radiographs in Total Hip Arthroplasty Using Templating Software

Abstract

INTRODUCTION Templating of pelvic radiographs traditionally involved using implant-company provided acetates which assumed a magnification of 115–120%. With the introduction of digital imaging, many departments are becoming filmless. Templating software has been designed to allow on-screen templating of digital images. Knowledge of the true magnification of the image is required for accurate measurement. PATIENTS AND METHODS Fifty consecutive postoperative pelvic radiographs were analysed using templating software. The implanted component was measured using an assumed magnification factor of 115%. The template image was then reset to the known component size, and the magnification factor was adjusted until the template covered the true component. RESULTS An assumed magnification factor of 115% oversized the acetabular component by a mean of 6 mm (three component sizes) in all 50 components. The mean true magnification in our department was 127%. CONCLUSIONS Validation of the true magnification produced by a radiology department using templating software is a simple and reproducible technique. It is recommended to all departments using digital images and templating software. Assumption of a magnification factor of 115% risks oversizing components by 6mm.