Calibration Marker Position in Digital Templating of Total Hip Arthroplasty.

BACKGROUND: We report a mathematical method to assess the vertical and horizontal positions of spherical radiopaque objects of known size in conventional radiographs. METHODS: The reliability and validity of the method were tested in an experimental setting and applied to 100 anteroposterior pelvic radiographs with external calibration markers and unilateral total hip arthroplasty (THA). RESULTS: We found excellent reliabilities; intraclass correlation coefficients for interobserver and intraobserver reliabilities were 0.999-1.000 (P = .000). The mean normal height of THA was 198 mm (range: 142-243 mm, standard deviation: 18 mm) above the detector. Vertical and horizontal external marker positions differed significantly from the true hip center (THA; P < .001 and P = .017). CONCLUSION: This method could enhance patient safety by enabling automated detection of malpositioned calibration markers by templating software.

Calibration Markers for Digital Templating in Total Hip Arthroplasty.

Digital templating with external calibration markers is the standard method for planning total hip arthroplasty. We determined the geometrical basis of the magnification effect, compared magnification with external and internal calibration markers, and examined the influence on magnification of the position of the calibration markers, patient weight, and body mass index (BMI). A formula was derived to calculate magnification with internal and external calibration markers, informed by 100 digital radiographs of the pelvis. Intraclass correlations between the measured and calculated values and the strength of relationships between magnification, position and distance of calibration markers and height, weight, and BMI were sought. There was a weak correlation between magnification of internal and external calibration markers (r = 0.297-0.361; p < 0.01). Intraclass correlations were 0.882-1.000 (p = 0.000) for all parameters. There were also weak correlations between magnification of internal and external calibration markers and weight and BMI (r = 0.420, p = 0.000; r = 0.428, p = 0.000, respectively). The correlation between external and internal calibration markers was poor, indicating the need for more accurate calibration methods. While weight and BMI weakly correlated with the magnification of markers, future studies should examine this phenomenon in more detail.