A simple and convenient method guide to determine the magnification of digital X-rays for preoperative planning in total hip arthroplasty

Scaling of anteroposterior digital pelvic X-rays with variable magnification is the premise for accurate preoperative planning of total hip replacement with digital templating. Conn's method of placing a marker of known diameter beside the thigh at the level of the femur has been reproduced in many studies and confirmed as one of the most accurate methods. But in our experience, it is inconvenient for radiographers and is not well tolerated by some patients. We modified this method by placing a coin on the radiograph plate. One hundred patients who had undergone hip replacement were enrolled in the study and randomly divided into two groups. The actual diameter of the prosthesis head was taken as the gold standard for assessment of the magnification of the coin in Group A. The coin was within a mean of 117.95% (range 114.37–122.02%) of magnification for male, and 111.71% (range 114.37–120.93%) for female patients. The variation was small and limited, and had no correlation with body shape parameters (i.e. height, weight, BMI). Subsequently, the magnification of the coin was used to correct the measuring scale of the X-rays of the other 50 patients (Group B). Bias did not exceed 1.96 mm during measurement of the prosthesis with a diameter of less than 56 mm, and a range of absolute error of measurements of 56–66 mm (standard deviation, SD, 0.04–3.95 mm). Furthermore, in order to confirm the expressiveness of the modified method, CT scans of another 50 patients were randomly selected. The distance between the rotation center of the hip and the table, which is acknowledged to be a factor which influences magnification of the coin, changed little in response to body shape. Variation in magnification was caused by variation in distance between the rotation center of the hip and the table. The minimal change in distance for patients with different body shape led to easier and more convenient examination, and increased the feasibility of our modified coin method, except in cases where implantation of a very large-sized prosthesis is necessary.