Total Hip Arthroplasty Templating: A Simple Method to Correct for Radiograph Magnification.

Abstract

Several factors can cause magnification errors when templating. Parallax, distance from the plate, and printing or monitoring the radiographs can lead to errors. Steel balls, 25.4 mm, underwent radiography on a standard radiograph plate in different positions from the center of the x-ray beam for evaluation of parallax and at different heights from the x-ray table for evaluation of the effects of distance on magnification. Difference in size between the center steel ball and the peripheral balls was less than 0.3 mm. As a metal ball was brought from table top to a 6-inch elevation, there was up to a 25% magnification. A simple correction factor can be calculated to correct magnification error. The diameter of the surgically removed femoral head divided by the diameter of the femoral head on the preoperative radiograph yields a correction factor. This factor multiplied by the template-determined lesser trochanter-to-center yields the length to be created during surgery. This adjusts for all causes of magnification. [Orthopedics. 2019; 42(3):e322-e325.].