The effect of patient body mass index and sex on the magnification factor during pre-operative templating for total hip arthroplasty

Introduction: Pre-operative templating prior to hip arthroplasty has traditionally used implant-company-provided acetates, which assumed a magnification factor between 115% and 120%. In recent years, pre-operative planning has been performed with digital calibration devices, in order to calculate the magnification factor. However, these devices are not without their limitations and are not readily available at many institutions. As previous reports suggest a wide range of magnification factors, the determination of an optimal magnification factor is currently unclear. We investigated the relationship between obesity and gender on the magnification factor in order to improve the accuracy of pre-operative templating. Patients and methods: Ninety-seven consecutive pre-operative calibrated pelvic radiographs using the KingMark calibration were analyzed using the TraumaCad templating software. The magnification factor calculated by the software was considered the true magnification factor and analysis was made in order to assess the effect of sex and body mass index (BMI) on the magnification factor. A linear regression analysis was utilized to create a predictive model for optimal magnification factor value. Results: Magnification factor was significantly affected by sex (male, 120.0% vs. female 121.2%, p < 0.01) and by categorized BMI (obese 121.8% vs. non-obese 119.9%, p < 0.001). A positive linear association was found between BMI and the magnification factor (r = 0.544). The magnification factor was significantly different between the following sub-groups: obese female, non-obese female, obese male, and non-obese male (p < 0.001). When applying the model formulated by the linear regression analysis, the calculated magnification factor was within 2% of the true magnification factor for the majority of patients (n = 83, 85.6%). Conclusions: BMI and gender have a significant effect on the magnification factor. Future determination of the magnification factor should consider the influence of these variables in order to improve the accuracy of pre-operative templating in THA.