Study Design. Experimental and computational study of posterior spinal instrumentation and growing rod constructs per ASTM F1717-15 vertebrectomy methodology for static compressive bending. Objective. Assess mechanical performance of standard fusion instrumentation and growing rod constructs. Summary of Background Data. Growing rod instrumentation utilizes fewer anchors and spans longer distances, increasing shared implant loads relative to fusion. There is a need to evaluate growing rod's mechanical performance. ASTM F1717-15 standard assesses performance of spinal instrumentation; however, effects of growing rods with side-by-side connectors have not been evaluated. Methods. Standard and growing rod constructs were tested per ASTM F1717-15 methodology; setup was modified for growing rod constructs to allow for connector offset. Three experimental groups (standard with active length 76 mm, and growing rods with active lengths 76 and 376 mm; n = 5/group) were tested; stiffness, yield load, and load at maximum displacement were calculated. Computational models were developed and used to locate stress concentrations. Results. For both constructs at 76 mm active length, growing rod stiffness (49 ± 0.8 N/mm) was significantly greater than standard (43 ± 0.4 N/mm); both were greater than growing rods at 376 mm (10 ± 0.3 N/mm). No significant difference in yield load was observed between growing rods (522 ± 12 N) and standard (457 ± 19 N) constructs of 76 mm. Growing rod constructs significantly decreased from 76 mm (522 ± 12 N) to 376 mm active length (200 ± 2 N). Maximum load of growing rods at 76 mm (1084 ± 11 N) was significantly greater than standard at 76 mm (1007 ± 7 N) and growing rods at 376 mm active length (392 ± 5 N). Simulations with active length of 76 mm were within 10% of experimental mechanical characteristics; stress concentrations were at the apex and cranial to connector–rod interaction for standard and growing rod models, respectively. Conclusion. Growing rod constructs are stronger and stiffer than spinal instrumentation constructs; with an increased length accompanied a decrease in strength. Growing rod construct stress concentration locations observed during computational simulation are consistent with clinically observed failure locations. Level of Evidence: 5
Verification, validation, and uncertainty quantification (VVUQ) can increase confidence in computational models by providing evidence that a model accurately represents the intended reality of interest. However, there are currently few examples demonstrating the application of VVUQ best practices for medical devices. Therefore, the objectives of this study were to understand the reproducibility and repeatability of experimental testing and finite element analysis (FEA), perform VVUQ activities that guide the development and refinement of a finite element model, and document best practices for future research. This study focused on experiments and simulations of three-point bend testing, which is a fundamental element of a hierarchical validation study of medical devices (e.g., spinal rod-screw systems). Experimental three-point bend testing was performed at two laboratories using medical-grade titanium (Ti-6Al-4V) spinal rods. FEA replicating the experimental test was performed by four independent institutions. Validation activities included comparing differences in mechanical properties between FEA and experimental results, where less than 10% difference was observed for all quantities of interest. Computational model uncertainties due to modeling assumptions and model input parameters were estimated using the sensitivity coefficient method. An importance factor analysis showed that rod diameter was the parameter driving uncertainty in the initial elastic region, while the material model is the primary contributor beyond this point. These results provide a proof of concept in the use of VVUQ for the use of FEA for medical device applications.
Orthopaedic impaction-instruments are used to drive implants into the bone of the patient. Pre-clinical experimental testing protocols and computer models of those are used to assess robustness and functional efficiency of such instruments. This generally involves impaction of the instrument mounted on a substrate that should represent the mechanics of the patient. In this study, the effects of the substrate on stressing of the impaction-instruments were investigated using dynamic finite element analysis. Model results were compared with experimental data from lab protocols, which have been derived to recreate the mechanics of cadaveric implantations, which represent clinical conditions.FEA models of selected experimental protocols were created in which a simplified instrument was impacted on substrates with varying material properties and boundary conditions. After impaction, the instrument settled into a modal vibration which then decayed over time. The resulting axial strain data from the computational model was compared to strain-gauge data collected from experimental measurements. Strain signal amplitude, frequency and decay were compared. The damping-ratio was derived from the decay of the strain signal.The computational model slightly over-predicted the initial experimental strain amplitudes in all cases, but the frequency of the cyclic strain signals matched. However, the model underestimated the experimentally measured rate of signal decay. Inclusion of implant seating and soft-tissue conditions had little effect on decay.Clinical failures of impaction-instruments may be related to multiple fatigue cycles for each impaction and should be modelled accurately to allow failure prediction. Any soft substrate results in an impedance mismatch at the instrument interface, which reflects the pressure wave and causes vibration with a frequency related to the speed-of-sound in the instrument, and its geometry. While this could be accurately modelled computationally, signal decay was underestimated. Further experimental quantification of energy losses will be important to understand vibration decay.
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