Methods

The method consists in using 3D models of bones, segmented from 3D preoperative image (e.g. CT or MRI scans), and 3D models of implant, and aligning them independently to X-rays by matching contours manually drawn on the X-rays and projected contours. The result gives the relative postoperative position of bone and implant. The method was tested on a phantom consisting of commonly available femoral knee implant on a physical model of a femur (Sawbones®). Result was compared to the optical scan, considered as ground truth, of the implanted saw bone. Two studies were performed: inter-operator (six operators), and intra-operator (5 tests). In addition, the inter-operator study was repeated while asking all the operators to use the same pre-drawn contours. The results are presented by calculating the distance (anterior/posterior, proximal/distal, medial/lateral) between the centers of gravity, and the angles (varus/valgus, flexion/extension, external/internal rotations) of the implants from the X-ray based method and the ground truth.

Results were also compared with the relative position of bone and implant extracted from a 3D CT postoperative image. Saw bone and implant were first segmented from this image. In order to determine the position of the implant, despite the metal artefacts in the CT images, the 3D model of the implant was registered on the segmented implant.

All processing, including segmentation, registration of X-rays, and measurements, was performed using Mimics Innovation Suite 17.0 ®.