Guided sacroiliac joint fusion by virtual surgical planning superimposed on intraoperative fluoroscopic images using digitally reconstructed radiographs

Sacroiliac joint (SIJ) fusion is a surgical treatment for SIJ dysfunction. During surgery, implants are placed through the SIJ under fluoroscopic guidance using a minimally invasive operation technique. The procedure aims to stabilise the joint and thereby reduce pain. While placing the implants freehandedly, closely located nerve structures must be avoided to prevent complications. However, these adjacent structures are not clearly distinguishable under fluoroscopic guidance. Besides, the anatomy of the sacrum is extremely variable. This challenges safe and effective implant placement in a stable configuration. Preoperative virtual surgical planning (VSP) may help the surgeon to achieve optimal implant positioning and lower the risk of complications, operation time and radiation dose. However, to accurately recreate the VSP in the operating room is challenging since the VSP cannot be directly compared to intraoperative fluoroscopic images. Therefore, this study aimed to develop and test a method that superimposes a VSP onto intraoperative fluoroscopic images using digitally reconstructed radiographs (DRRs). To this end, an algorithm was developed using ray casting, registration and visualisation techniques. The algorithm was tested during a routine SIJ fusion surgery. The workflow is as follows. The preoperative CT scan and VSP are loaded into the software to prepare a series of low-resolution DRRs under different angles. During surgery, a lateral fluoroscopic image is loaded into the software and the optically best matching DRR is determined. This DRR is used to create a high-resolution image with overlaid VSP, showing the position of guide pins used for implant placement. The surgeon then places the pins in the same position and angle as represented on the DRR to create a comparable fluoroscopic image. The developed intraoperative workflow was found to be fast and feasible for clinical use. A postoperative placement accuracy analysis showed a mean 3D deviation at the apex of the implant of 4.6 mm, a mean angle deviation of 3.6º and a mean entry point deviation of 3.4 mm. Further research is needed to optimise the developed workflow and clinically validate the proposed method.