Novel synthetic vertebrae provide realistic haptics for pedicle screw placement

Publication, 2017


M. Hollensteiner, P. Augat, D. Fürst, B. Esterer, S. Gabauer, K. Püschel, F. Schrödl, A. Schrempf - Novel synthetic vertebrae provide realistic haptics for pedicle screw placement - Proceedings of the PMU Science Get Together 2017, Nürnberg, Nürnberg, Germany, 2017


Objective During vertebral surgery, misplaced pedicle screws can harm vital neural and vascular structures1. Haptic distinction between cortical and cancellous bone structures is therefore essential for correct screw placement. This tactile experience during pedicle screw placement can be obtained by training on human or animal specimens albeit expensive or ethically questionable2. A novel training option, a hybrid simulator for minimally invasive spine surgeries was already developed. It provides a detailed visualization of the surgical field, allows the usage of real surgical instruments and physical anatomical models3. In this study, novel synthetic vertebrae were evaluated within this hybrid simulator to provide realistic haptics for the training of pedicle screw placement. Methods Synthetic vertebrae made of calcium powder-based composites and imitating both, cancellous and cortical bone, were custom made. Mechanical properties of synthetic surrogates were validated for pedicle screw placement and cement augmentation and were compared with those obtained on human vertebrae. Therefore, 4.5x30mm screws (ulrich medical GmbH, Ulm, Germany) were inserted into the probes with a custom-made, automated screw driver with a speed of one RPM and a constant feeding force of 40N. During screw insertion, the torques and screw entry path were continuously recorded. Results The examined data did not show normal distribution. Thus, Mann-Whitney-U-tests were used to detect differences between human and artificial vertebrae. In human vertebrae the pedicle screw torque measurements resulted in mean torque slopes of 82±33Nm/m. Calcium carbonate-based materials achieved lower torques than the human bone whereas calcium phosphate-based bone surrogates showed comparable results. A further differentiation of the calcium phosphate-based vertebrae revealed, that synthetic vertebrae with lower amounts of blowing agent, achieved suitable torques (83±28Nm/m) in comparison to the human reference (p=0.387). Cement application and subsequent fluoroscopy images confirmed that the cancellous core of the synthetic vertebrae is open-celled and thus enables cement augmentation. Conclusion The measured torques of the pedicle screw placement revealed comparable haptics for calcium phosphate-based bone surrogates in comparison to human specimens. The artificial bone samples mimicked the tactile properties of human bone during pedicle screw placement and enabled cement augmentation. In conclusion, our findings suggest that the synthetic vertebrae are suitable for surgical training within a hybrid surgical simulator.