Digital magnification error when templating total hip replacement?

Background: Digital templating has become an accepted method of pre-operative planning for total hip replacement (THR). Templating software permits digital templating in a complete digital environment. Accurate templating requires knowledge of the true magnification of the radiograph. Aims and objectives: The aim of this study was to compare the measured magnification factor from using an external calibration marker (ECM) in a standardized radiology protocol, to the true magnification derived from using a prosthetic femoral head of known diameter. Materials and Methods: All patients awaiting staged bilateral THR between 2012 and 2017 were identified. Their radiographs were reviewed, and included in the study if an ECM was present with THR prosthesis in situ. The radiographs were assessed for scaling using two methods, a built-in software method using the ECM, and a manual method, using the implanted femoral head. Magnification factor derived from both methods were compared. Statistics: Statistical analysis was performed using paired Student’s t-test for matched samples. Results: A total of 64 radiographs were included. The two methods of scaling produced consistently different magnification factors. Using the ECM, the mean measured magnification was 113.9% (SD 5.4%). The mean true magnification using the implanted femoral head was 119.6% (SD 2.1%). The mean difference was 6.9% (SD 5.7%, range of -18% to 17.1%, p<0.05). Conclusion: In conclusion, a single external calibration marker will have significant error. In clinical practice, the ECM consistently underestimated the magnification of templating radiographs. In patients with an implanted hip replacement, we recommend using the femoral head prosthesis for radiograph scaling.