Finite Element Analysis Based Lumbosacral Revision Surgery Using an Individual Navigation Template

A revision surgery in case of a lumbosacral non union can be challenging especially if an implant related failure (e.g. a broken S1 screw) is complicating the clinical situation. Removal of the broken screw impairs the local bony enviroment jeopardizing the outcome of the revision. In the S1 segment the convergent bicortical screw trajectory provides a superior anchoring compared with any other directories (e.g. sacral ala screw) but the proper insertion of the new screws in a revision surgery can be impossible with freehand or fluoroscopy guided technique. In this paper, we present a case suffering from a lumbosacral non union complicated with a broken sacral pedicle screw what has been surgically managed by the application of a CT based 3D reconstruction method combined with finite element analysis (FEA) and computer assisted design (CAD) Method A step by step approach was developed and performed to manage the clinical problem. (1) Quantitative computed tomography (QCT) based patient specific FE model of the sacrum was created. (2) To plan the revision surgery CAD model of the pedicle screw was inserted in the sacrum model in a bicortical convergent and a monocortical divergent position. (3) According to the two screw insertion scenarios two static FEAs were performed using 500 N tensile load applied to the screw head. (4) A template with the two screw guiding structures designed to fit on the bone surface was created for the sacrum using 3D design and photoactive 3D printing technology. The final template was made by cobalt chrome. (5) The revision screw has been implanted into the biomechanically optimal position guided by the patient and condition specific template. Postoperative CT scan was used to evaluate the accuracy of the pedicle screw placement.  Based on the FEA results the modified bicortical convergent screw had better stability resulting in optimal von Misses stress distribution and less displacement compared with the monocortical divergent placement. Preoperatively the template was found to fit exactly on the printed plastic sacrum model, and screw insertion simulation was successfully performed. The design concept was proved to be accurate based on the CT scan and virtual model comparison. Intraoperatively the template also fitted on the bone surface and screw insertion was completed successfully. Postoperative CT scans confirmed that the inserted pedicle screw reached the virtually planed position.  The intraoperative pedicle screw navigation provided by a patient specific screw guiding template allows the surgeon to insert the screw into its optimal position considering the local bone material property and the challenging geometrical situation. This technology for the surgical navigation can be widely accessible in the future through dedicated knowledge providers. Its advantages compared with the conventional surgical navigation techniques are the relatively low cost, minimized intraoperative X ray exposure and the possibility for the consideration of the patient specific biomechanics. This new patient and condition specific approach can be widely used in revision spine surgeries or in challenging primary cases after its further clinical validations.