Abstract
Introduction
The goal of this work is to develop a system for three-dimensional tracking of the acetabular fragment during periacetabular osteotomy (PAO) using x-ray images. For PAO, the proposed x-ray image-based navigation provides geometrical and biomechanical assessment of the acetabular fragment, which is unavailable in the conventional procedure, without disrupting surgical workflow or requiring tracking devices.
Methods
The proposed system combines preoperative planning with intraoperative tracking and near real-time automated assessment of the fragment geometry (radiographic angles) and biomechanics (contact pressure distribution over the acetabular surface). During PAO, eight fiducial beads are attached to the patient after incision and prior to performing osteotomy. Four of the beads attach to the iliac wing above the expected superior osteotomy (these are termed confidence points), and four attach on the expected fragment (denoted fragment points).
At least two x-ray images are obtained before and after osteotomy. In each set of images, image processing routines segment the fiducials and triangulate the 2D fiducial projections in 3D space. A paired-point registration between the confidence points triangulated from the two x-ray image sets aligns the imaging frames. We measured the transformation between the fragment points with respect to the confidence points to quantify the motion of the acetabular fragment. Applying an image-based 2D-3D registration to the measured acetabular transformation localises the reoriented acetabular fragment with respect to an anatomical coordinate system. We present the surgeon with visualisation and automatic estimations of radiographic angles and biomechanics of the reoriented acetabular fragment.
We conducted an experiment to evaluate feasibility and accuracy of the proposed system using a high density pelvic sawbone. Stainless steel beads were glued to the sawbone as fiducials. X-ray images were selected from cone-beam CT (CBCT) scans with an encoded motorised C-arm. Fiducial segmentation from reconstructed volumes of the CBCT scans provided a ground truth for the experiment.
Results
We used four images spaced at 45° to perform the 2D/3D registration. The measured fragment transformation errors in translation and rotation about a fixed axis when compared to the CBCT-computed transformation were 0.37°, 0.34mm for the x-ray image based approach (with 3 images spaced at 20°) and 1.49°, 4.39mm for the optical tracker.
Conclusion
We developed and evaluated x-ray image-based navigation to track the acetabular fragment in 3D Cartesian space during PAO. Capturing the fragment transformation allows automated algorithms to assess the biomechanics and geometry of the realigned acetabulum that are unavailable in 2D.
The error between the measured positions of the beads and the triangulated positions is attributed to three main sources: 1) fiducial segmentation error; 2) geometric calibration error; and 3) localisation of fiducials in volumetric reconstructions of the CBCT scans. These small reported errors suggest the procedure is a viable approach for conducting x-ray image-based navigation of PAO.
