Reducing residual forces in spinal fusion using a custom-built rod bending machine

Background and Objective: As part of spinal fusion surgery, shaping the rod implant to align with the anatomy is a tedious, error-prone and time-consuming manual process. Inadequately contoured rod implants introduce stress on the screw-bone interface of the pedicle screws, potentially leading to screw loosening or even pull-out.Methods: We propose the first fully automated solution to the rod bending problem by leveraging the advantages of augmented reality and robotics. Augmented reality not only enables the surgeons to intra-operatively digitize the screw positions but also provides a human-computer interface to the wirelessly integrated custom-built rod bending machine. Furthermore, we introduce custom-built test rigs to quantify per screw absolute tensile/compressive residual forces on the screw-bone interface. Beside residual forces, we have evaluated the required bending times and reducer engagements, and compared our method to the freehand gold standard.Results: We achieved a significant reduction of the average absolute residual forces from Image 1 for the freehand gold standard to Image 2 (p=0.0015) using the bending machine. Moreover, our bending machine reduced the average time to instrumentation per screw from Image 3 to Image 4. Reducer engagements per rod were significantly decreased from an average of 1.00±1.14 to 0.11±0.32 (p=0.0037).Conclusion: The combination of augmented reality and robotics has the potential to improve surgical outcome while minimizing the dependency on individual surgeon skill and dexterity.