Is There Any Difference Between a Spherical Marker and a Simple Coin for Hip Replacement Digital Planning?

Objective To evaluate the accuracy and differences between 2 types of metallic markers, sphere, and coin, for radiographic calibration in the preoperative planning of hip arthroplasty. Methods Four spherical metallic markers and four coins, both 25 mm in diameter, were placed on the greater trochanter, pubic symphysis, between the thighs, and on the table of the exam, for radiographic examination of the hip in 33 patients with hip prosthesis. The prosthesis head was used for calibration and two examiners measured the markers' image diameters, and the results were analyzed statistically. Results In the greater trochanter, the sphere and the coin were not visualized in 19 radiographs (57.6%). Between the thighs, the coin marker was not visualized in 13 radiographs (39.4%). In the greater trochanter, the 25-mm accuracy of the coin and the sphere was, respectively, between 57.1 and 63.3% and between 64.3 and 92.9%. The coin between the thighs reached 25-mm accuracy in between 50 and 60% of cases. Over the exam table, the coin and sphere markers reached, respectively, the mean diameters of 22.91 mm and 23 mm, the lowest coefficient of variation, the lowest confidence interval, and the easiest positioning. There was statistical difference between the evaluations of the markers (coin vs. sphere) in all positions ( p < 0.032), except for the exam table position ( p = 0.083). Conclusions The coin between the thighs is the best marker for radiographic calibration in the preoperative planning of hip arthroplasty, and we suggest the use of another coin on the exam table for comparison, considering the 8% reduction in relation to its real size.