Digital Planning Software Fails to Reflect Stem Torsion on Plain Radiographs after Total Hip Arthroplasty

Purpose: The purpose of this study was to evaluate the validity of commercially available planning software on plain radiographs after THA compared to CT scans as the gold standard. Patients and Methods: In a prospective clinical study, anteroposterior (AP) radiographs and three-dimensional CT scans (3D-CT) were obtained for 121 patients, who underwent minimally invasive, cementless THA with a straight tapered stem, in a lateral decubitus position. For measuring SV, we used digital planning software (TraumaCad 2.0, BrainLAB Feldkirchen, Germany). Two independent raters repeated the analysis after a six-week interval. Radiological measurements were compared with 3D-CT measurements by an independent, blinded external institute. This investigation was approved by the local ethics commission (no. 10 - 121-0263) and is a secondary analysis of a larger project (DRKS00 000 739, German Clinical Trials Register May-02 - 2011). Results: The radiograph measurements showed very high intra-and interrater agreement. The intra- class correlation (ICC) of the intrarater agreement was 0.97 for rater 1 and 0.98 for rater 2. The intrarater reliability was 0.99 using the mean values of both rater measurements. The mean difference between the average radiograph measurement and the 3D-CT-based measurement was 0.41 degrees (SD 11.24 degrees) (range: -33.85 degrees-22.50 degrees; 95 % limits of agreement: - 21.63 - 22.45), but there was no correlation found between both methods. Conclusion: Measuring stem version with the help of commercially available digital planning software on plain radiographs after THA has high intra- and interrater reliability but clinically inacceptable validity and reliability when compared to 3D-CT scans.