[Bony sacroiliac corridor : A virtual volume model for the accurate insertion of transarticular screws.]

BACKGROUND: Minimally invasive sacroiliac (SI) screw fixation carries a high risk for implant malposition. Only idealised shape conceptions of the safe bony corridor exist. METHODS: Two SI corridor models were generated based on a 3D CT reconstruction of a human pelvis. Therefore two penetration depths of the screws into the sacrum were defined. RESULTS: By inserting screws into the centre of the first sacral body an osseous volume of 121 cm(3) and an iliac entrance area of 53 cm(2) were utilizable. Screw positioning beyond the opposite sacral isthmus leads to a reduction of the bony volume to 72 cm(3) (60%) and a decrease of the iliac screw entrance to 20 cm(2) (38%). CONCLUSION: The computed realistic 3D models provide exact references to confining bone structures for safe screw positions. The implementation of a software algorithm for fully automated calculation of such volumes based on fluoroscopic or CT images could enhance the performance of computer-assisted navigation systems.