Assessment of magnification of digital pelvic radiographs in total hip arthroplasty using templating software

S P White, J Bainbridge, E J Smith
Annals of the Royal College of Surgeons of England 2008, 90 (7): 592-6

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 6 mm.

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