Planning the Surgical Correction of Spinal Deformities: Toward the Identification of the Biomechanical Principles by Means of Numerical Simulation

The set of surgical devices and techniques to perform spine deformity correction has widened dramatically. Nevertheless, the rate of complications due to mechanical failure remains rather high. Indeed, basic research about the principles of deformity correction and the optimal surgical strategies (i.e. the choice of the fusion length, the most appropriate instrumentation, the degree of tolerable correction) did not progress as much as the techniques. In this work, a software approach for the biomechanical simulation of the correction of patient-specific spinal deformities aimed to the identification of its biomechanical principles is presented. The method is based on three dimensional reconstructions of the spinal anatomy obtained from biplanar radiographic images. A user-friendly graphical interface allows for the planning of the deformity correction and to simulate the instrumentation. Robust meshing of the instrumented spine is provided by using consolidated computational geometry and meshing libraries. Based on finite element simulation, the program predicts the loads acting in the instrumentation as well as in the biological tissues. A simple test case (reduction of a low grade spondylolisthesis at L3-L4) was simulated as a proof-of-concept. Despite the limitations of this approach, the preliminary outcome is promising and encourages a wide effort towards its refinement

A comparative analysis of a disposable and a reusable pedicle screw instrument kit for lumbar arthrodesis: integrating HTA and MCDA

Objective: Lumbar arthrodesis is a common surgical technique that consists of the fixation of one or more motion segments with pedicle screws and rods. However, spinal surgery using these techniques is expensive and has a significant impact on the budgets of hospitals and Healthcare Systems. While reusable and disposable instruments for laparoscopic interventions have been studied in literature, no specific information exists regarding instrument kits for lumbar arthrodesis. The aim of the present study was to perform a complete health technology assessment comparing a disposable instrument kit for lumbar arthrodesis (innovative device) with the standard reusable instrument. Methods: A prospective and observational study was implemented, by means of investigation of administrative records of patients undergoing a lumbar arthrodesis surgical procedure. The evaluation was conducted in 2013, over a 12- month time horizon, considering all the procedures carried out using the two technologies. A complete health technology assessment and a multi-criteria decision analysis approach were implemented in order to compare the two alternative technologies. Economic impact (with the implementation of an activity based costing approach), social, ethical, organisational, and technology-related aspects were taken into account. Results: Although the cost analysis produced similar results in the comparison of the two technologies (total cost equal to € 4,279.1 and € 4,242.6 for reusable instrument kit and the disposable one respectively), a significant difference between the two instrument kits was noted, in particular concerning the organisational impact and the patient safety. Conclusions: The replacement of a reusable instrument kit for lumbar arthrodesis, with a disposable one, could improve the management of this kind of devices in hospital settings

Anterior support reduces the stresses on the posterior instrumentation after pedicle subtraction osteotomy: a finite-element study

Study design: The investigation was based on finite-element simulations. Objective: Pedicle subtraction osteotomy (PSO) is an effective but technical demanding surgical technique, associated with a high risk of rod failure. The present study aims at investigating the role of the anterior support in combination with PSO, with a numerical comparative analysis. Methods: An osteotomy was simulated at the L3 level of a lumbosacral spine. An implantation of various combinations of devices for the anterior (1 or 2 cages of different material) and posterior stabilization (1 or 2 rods) was then performed. ROM, loads, and stresses acting on the rods were calculated. Results: A 4â\u80\u938% reduction of the ROM was obtained introducing one or two cages in the instrumented model. However, the anterior support had only a minor influence on the ROM. The load on the posterior instrumentation decreased up to 8% using one cage and about 15% with two anterior devices. A 20â\u80\u9330% reduction of the stresses on the rods was calculated inserting one cage and up to 50% using two cages. Following the introduction of the anterior support, the greatest stress reduction was observed in the model having two cages and spinal fixators with two rods. Conclusions: The use of cages is crucial to ensure anterior support and decrease loads and stresses on the posterior instrumentation

Spinal rods contouring: an experimental and finite element study to control fatigue

French bender is the clinical gold standard for spinal rod contouring. Despite it allows the surgeon in achieving any desired shape, it is believed to weaken the implants, finally promoting fatigue failure. The current study proposes a new method combining non-linear FE models and experimental tests to better understand the role of residual stresses resulting from contouring. Learning how to control this phenomenon may contribute in reducing the high failure rate met during clinical use, as well as improving the usage of current implants.status: accepte

Instrumentation failure following pedicle subtraction osteotomy: the role of rod material, diameter, and multi-rod constructs

Purpose: Pedicle subtraction osteotomy (PSO) has a complication rate noticeably higher than other corrective surgical techniques used for the treatment of spinal sagittal imbalance. In particular, rod breakage and pseudoarthrosis remain burning issues of this technique. Goal of this study was to investigate the biomechanical performance of several hardware constructs. Methods: The study was performed using two validated finite element models of the lumbosacral spine (L1–S1) incorporating a PSO on L3 and L4, respectively. Both models were instrumented two levels above and below the osteotomy site. Different combinations of materials (Ti6Al4V and Cr–Co) and device configurations (bilateral single vs. double rod, rod diameters of 5 and 6 mm) were investigated. The loading was represented considering a force of 500 N (imposed along the spinal curvature and connecting the vertebral bodies) and pure moments of 7.5 Nm in flexion-extension, lateral bending and axial rotation. The results were evaluated in terms of range of motion (ROM), load, and stresses acting on the instrumentation. Results: A comparable ROM was found for all the models. The simulations showed a different behavior of the devices: increasing the stiffness an 8–19% increase of the load was calculated on the rod. However, the stress on the instrumentation resulted higher on Cr–Co devices and on smaller rods. The highest stress reduction (up to 50%) was ensured using double rod constructs. Conclusions: The bilateral double parallel rods configuration resulted the best to reduce the stresses on the spinal fixators at the osteotomy site. However, the high loads acting on the rods with respect to the physiologic condition could slow down the bone healing at the osteotomy site