Inaccuracies in the Use of Magnification Markers in Digital Hip Radiographs

With the ubiquity of digital radiographs, the use of digital templating for arthroplasty has become commonplace. Although improved accuracy with digital radiographs and magnification markers is assumed, it has not been shown. We wanted to (1) evaluate the accuracy of magnification markers in estimating the magnification of the true hip and (2) determine if the use of magnification markers improves on older techniques of assuming a magnification of 20% for all patients. Between April 2013 and September 2013 we collected 100 AP pelvis radiographs of patients who had a THA prosthesis in situ and a magnification marker placed per the manufacturer's instructions. Radiographs seen during our standard radiographic review process, which met our inclusion criteria (AP pelvic view that included a well-positioned and observed magnification marker, and a prior total hip replacement with a known femoral head size), were included in the analysis. We then used OrthoView(TM) software program to calculate magnification of the radiograph using the magnification marker (measured magnification) and the femoral head of known size (true magnification). The mean true magnification using the femoral head was 21% (SD, 2%). The mean magnification using the marker was 15% (SD, 5%). The 95% CI for the mean difference between the two measurements was 6% to 7% (p < 0.001). The use of a magnification marker to estimate magnification at the level of the hip using standard radiographic techniques was shown in this study to routinely underestimate the magnification of the radiograph using an arthroplasty femoral head of known diameter as the reference. If we assume a magnification of 20%, this more closely approximated the true magnification routinely. With this assumption, we were within 2% magnification in 64 of the 100 hips and off by 4% or more in only four hips. In contrast, using the magnification marker we were within 2% of true magnification in only 20 hips and were off by 4% or more in 59 hips. We found the use of a magnification marker with digital radiographs for preoperative templating to be generally inaccurate, with a mean error of 6% and range from -5% to 15%. Additionally, these data suggest that the use of a magnification marker while taking preoperative radiographs of the hip may be unnecessary, as simply setting the software to assume a 20% magnification actually was more accurate. Level III, diagnostic study.