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P9. A distraction-based bench top protocol for the evaluation of growing rod concepatients

2021; Elsevier BV; Volume: 21; Issue: 9 Linguagem: Inglês

10.1016/j.spinee.2021.05.217

1878-1632

David Dick, Niloufar Shekouhi, Amey Kelkar, Jeremy Rawlinson, Derek Shaw, Vijay K. Goel,

Spinal Fractures and Fixation Techniques

BACKGROUND CONTEXT Early Onset Scoliosis (EOS) is a lateral curvature of the spine observed in children under the age of 10. The emergence of non-fusion surgical techniques, such as traditional growing rods (TGRs), has made it possible to manage the spinal deformity while allowing for the spine's natural growth. After initial implantation, patients undergo serial distraction surgeries to correct the scoliotic curvature and lengthen the growing rod hardware. Thus, at each distraction surgery, compressive loads such as upper body trunk weight along with the distraction forces and forces exerted on the implants to maintain the corrected scoliotic curvature create a complex loading condition. Therefore, understanding the effect of distraction on TGRs' biomechanics can help us to identify the potential failure mechanisms. Such understanding can be obtained by means of a suitable biomechanical testing setup that is a viable alternative to clinical studies which are very resource intensive and cost prohibitive. Recently a distraction-based protocol for TGR implants was proposed and published by authors [1] using finite element (FE) analysis. The aim of this study was to develop a bench top testing protocol to evaluate the growing rods. PURPOSE A new bench top evaluation protocol is proposed to provide a better understanding of the contributing factors to the observed clinical failure, which would potentially lead to improving clinical outcomes and patient's safety. This protocol can help justify modifications to an ASTM test standard that is clinically relevant for the evaluation of growing rods. METHODS An experimental TGR subconstruct was developed for a single sample. A total of five test blocks were used in the subconstruct to replicate four functional spine units (FSU). Four titanium pedicle screws (4.5 x 45mm) each were inserted into the top and the bottom test block. Four 5.5 mm Titanium rods were used in the test setup. The top and bottom rods on each side were interconnected using a Stainless-steel domino. Two red die springs (129N/mm) were selected from literature [1] and used at each FSU. Springs were rigidly attached to the test blocks using a clamping mechanism by means of a plate and bolt. The block moment arm was maintained at 40 mm as per ASTM-F1717 guideline. The initial active length was maintained at 193mm. The portion of the rod outside the domino were measured and 6.22 mm was marked. The set screws of the dominos were left unlocked during the distraction. The assembled subconstruct was mounted on an MTS biaxial material testing machine and a longitudinal distraction was applied until the marked position on the rods reached. Afterwards, rods were locked into the dominoes and the subconstruct was allowed to relax. Then, second active length was measured, and construct was mounted again on the MTS machine. Finally, static compression-bending was applied under displacement control at a rate of 0.2mm/sec. The yield load, stiffness, and force recorded at the end of the distraction step were recorded. The same loading conditions were replicated for the finite element model and the results obtained from the simulation were compared with the experimental data. RESULTS At the end of distraction, a 385.9 N force was measured. During the relaxation step, rods were locked and hence this force applied a compression-bending loading on the construct. Moreover, a 2.45 mm initial displacement was measured prior to compression-bending. The experimental outcomes indicated that construct's stiffness and yield load were 59.7 N/mm and 623.3 N, respectively. The FE predictions were in agreement with the experimental outcomes. FE results showed that stiffness and yield load were 61.7 N/mm and 595.9 N, respectively. The maximum von Mises stress happened adjacent to the distraction site based on the FE models. Experiment was stopped when rods touched the plastic blocks (at about 40 mm displacement). CONCLUSIONS The currently available protocol used for evaluating spinal implants (ie, "ASTM-F1717 or vertebrectomy model"), is associated with several shortcomings and hence not suitable for non-fusion devices. One of the most important limitations of the vertebrectomy model in evaluating non-fusion implants and more specifically growing rods, is the absence of an anterior support. Therefore, this guideline does not allow for simulating the physiological loading conditions such as distraction or/and follower loads in the mechanical evaluation. Recently, an in-silico modification of the ASTM-F1717 protocol was presented by authors [1] to simulate a more clinically relevant scenario in evaluating the traditional growing rods. The attachment of the springs with blocks enabled distraction of the rods and relaxation, thereafter, mimicking the clinical scenario. The proposed bench top protocol showed that the tensile load resulted from the stretched anterior support applied a bending moment on the rods which might aggravate their complications. Therefore, distraction should be included in the mechanical testing of these non-fused implants. Moreover, due to the presence of the anterior support, the construct stability increased, and fracture did not happen in the static mechanical testing, while high bending moments were observed adjacent to the axial connectors (Fig.3b). Upon removal of the load, there was some permanent deformation suggesting plastic yielding/failure. Similar to the experiment, the FE predictions indicated the distraction site as the critical stress location (Fig.4). Although this protocol considers several clinically relevant parameters in addressing the growing rod's contributing factors to failure, further investigation is needed to standardize the spring constant, initial active length, distraction force, etc. Future studies could compare various spring block attachment and investigate the most optimal approach to undertake static and dynamic test. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs. Early Onset Scoliosis (EOS) is a lateral curvature of the spine observed in children under the age of 10. The emergence of non-fusion surgical techniques, such as traditional growing rods (TGRs), has made it possible to manage the spinal deformity while allowing for the spine's natural growth. After initial implantation, patients undergo serial distraction surgeries to correct the scoliotic curvature and lengthen the growing rod hardware. Thus, at each distraction surgery, compressive loads such as upper body trunk weight along with the distraction forces and forces exerted on the implants to maintain the corrected scoliotic curvature create a complex loading condition. Therefore, understanding the effect of distraction on TGRs' biomechanics can help us to identify the potential failure mechanisms. Such understanding can be obtained by means of a suitable biomechanical testing setup that is a viable alternative to clinical studies which are very resource intensive and cost prohibitive. Recently a distraction-based protocol for TGR implants was proposed and published by authors [1] using finite element (FE) analysis. The aim of this study was to develop a bench top testing protocol to evaluate the growing rods. A new bench top evaluation protocol is proposed to provide a better understanding of the contributing factors to the observed clinical failure, which would potentially lead to improving clinical outcomes and patient's safety. This protocol can help justify modifications to an ASTM test standard that is clinically relevant for the evaluation of growing rods. An experimental TGR subconstruct was developed for a single sample. A total of five test blocks were used in the subconstruct to replicate four functional spine units (FSU). Four titanium pedicle screws (4.5 x 45mm) each were inserted into the top and the bottom test block. Four 5.5 mm Titanium rods were used in the test setup. The top and bottom rods on each side were interconnected using a Stainless-steel domino. Two red die springs (129N/mm) were selected from literature [1] and used at each FSU. Springs were rigidly attached to the test blocks using a clamping mechanism by means of a plate and bolt. The block moment arm was maintained at 40 mm as per ASTM-F1717 guideline. The initial active length was maintained at 193mm. The portion of the rod outside the domino were measured and 6.22 mm was marked. The set screws of the dominos were left unlocked during the distraction. The assembled subconstruct was mounted on an MTS biaxial material testing machine and a longitudinal distraction was applied until the marked position on the rods reached. Afterwards, rods were locked into the dominoes and the subconstruct was allowed to relax. Then, second active length was measured, and construct was mounted again on the MTS machine. Finally, static compression-bending was applied under displacement control at a rate of 0.2mm/sec. The yield load, stiffness, and force recorded at the end of the distraction step were recorded. The same loading conditions were replicated for the finite element model and the results obtained from the simulation were compared with the experimental data. At the end of distraction, a 385.9 N force was measured. During the relaxation step, rods were locked and hence this force applied a compression-bending loading on the construct. Moreover, a 2.45 mm initial displacement was measured prior to compression-bending. The experimental outcomes indicated that construct's stiffness and yield load were 59.7 N/mm and 623.3 N, respectively. The FE predictions were in agreement with the experimental outcomes. FE results showed that stiffness and yield load were 61.7 N/mm and 595.9 N, respectively. The maximum von Mises stress happened adjacent to the distraction site based on the FE models. Experiment was stopped when rods touched the plastic blocks (at about 40 mm displacement). The currently available protocol used for evaluating spinal implants (ie, "ASTM-F1717 or vertebrectomy model"), is associated with several shortcomings and hence not suitable for non-fusion devices. One of the most important limitations of the vertebrectomy model in evaluating non-fusion implants and more specifically growing rods, is the absence of an anterior support. Therefore, this guideline does not allow for simulating the physiological loading conditions such as distraction or/and follower loads in the mechanical evaluation. Recently, an in-silico modification of the ASTM-F1717 protocol was presented by authors [1] to simulate a more clinically relevant scenario in evaluating the traditional growing rods. The attachment of the springs with blocks enabled distraction of the rods and relaxation, thereafter, mimicking the clinical scenario. The proposed bench top protocol showed that the tensile load resulted from the stretched anterior support applied a bending moment on the rods which might aggravate their complications. Therefore, distraction should be included in the mechanical testing of these non-fused implants. Moreover, due to the presence of the anterior support, the construct stability increased, and fracture did not happen in the static mechanical testing, while high bending moments were observed adjacent to the axial connectors (Fig.3b). Upon removal of the load, there was some permanent deformation suggesting plastic yielding/failure. Similar to the experiment, the FE predictions indicated the distraction site as the critical stress location (Fig.4). Although this protocol considers several clinically relevant parameters in addressing the growing rod's contributing factors to failure, further investigation is needed to standardize the spring constant, initial active length, distraction force, etc. Future studies could compare various spring block attachment and investigate the most optimal approach to undertake static and dynamic test.