HIPMATCH: A 2D-3D MATCHING SOFTWARE – ACCURATE AND RELIABLE (2010)

The long-term result of a total hip arthroplasty (THA) strongly depends on the correct component positioning of the acetabular cup and stem. To measure cup orientation out of a postoperative anteroposterior (AP) pelvic radiograph is highly inaccurate due to the wide variation of individual pelvic tilt and rotation. The goal of this study was to develop and validate a 2D-3D matching software (HipMatch) that allows matching a postoperative AP pelvic radiograph with a preoperative CT to accurately measure cup orientation corrected for individual pelvic orientation. The software is based on a spline-based multi-resolution 2D-3D image registration algorithm and a Markov random field theory based on similarity measurement. Based on a cone projection (imitating the path of the x-rays), the software is able to match the three-dimensional CT-based data set with the contours of the projected pelvis on the AP pelvic radiograph. This gives the possibility to correct the measured cup orientation (inclination and anteversion) by measuring it according to an anatomical defined coordinate system (anterior pelvic plane). The validation of the software consisted of accuracy, reproducibility and observer reliability measurements using cadaver and clinical data. For the cadaver validation 10 human pelves (20 hips) were used. From each pelvis 2 CT scans, one with and one without an inserted cup were acquired. The CT scan with the cup was used as the ground truth. With the cup inserted 4 AP pelvic radiographs with the pelvis in an unknown arbitrary position during acquisition were performed resulting in 80 measurements for accuracy. These measurements were performed by 2 observers at 2 different occasions resulting in a total of 320 measurements for reproducibility and observer reliability. The intraclass correlation coefficient (ICC) was used for quantification of reproducibility and observer reliability and the Bland-Altman analysis was used to detect systemic errors. The clinical validation included 33 patients with a pre- and a postoperative CT and 49 patients with only a postoperative CT in addition to the postoperative radiographs. In the cases with only a postoperative CT, for the 2D-3D matching the postoperative CT after manual excision of the cup from the CT slice sticks was used. In all cases the postoperative CT was used as the ground truth. For each patient all the available postoperative radiographs were used resulting in 236 measurements of accuracy. In the cadaver validation the cup orientation ranged from 34° – 57° for the inclination and from 1° – 24° for the anteversion measured on the CT. The accuracy showed a mean difference for the inclination of 0.9° ± 1.6° (−3.2° – 4.0°) and of 1.2 ± 2.4° (−5.3° – 5.6°) for the anteversion. The ICC for the reproducibility ranged from 0.96 to 0.99 and for the interobserver reliability from 0.95 to 0.98. No relevant systematic error was detected. In the clinical validation the cup orientation measured on the postoperative CT ranged for the inclination from 22° – 57° and for the anteversion from 7° – 35°. In the clinical setup the accuracy showed a mean difference for inclination of 1.8° ± 1.6° (−4.0° – 5.3°) and of −1.1° ± 2.9° (−5.9° – 5.7°) for the anteversion. The 2D-3D matching technique showed a good accuracy and a very good reproducibility and observer reliability. This technique allows to measure the exact cup orientation out of an AP pelvic radiograph with the help of a preoperative CT and to correct the parameters for the individual pelvic position. Therefore this software is a powerful tool to measure accuracy of CT-based computer-assisted cup placement in a large clinical series.