F Standard Test Methods for Spinal Implant Constructs in a Vertebrectomy Model static test~ 21, $ Standard + Redline PDF Bundle. ASTM F 'Standard Test Methods for Spinal Implant Constructs in a The Instron® spinal fixture is designed to meet ASTM F requirements. PDF | Preclinical evaluation of spinal implants is a necessary step to Definition of the parametric model describing ASTM F standard .
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ADMET offers multiple mechanical force testing systems configured to meet and exceed the ASTM test standards for spinal implant constructs. ASTM F (11), and ASTM F (12) for spinal instrumentation have been proved to be relevant with their experimental set ups, loading, and test frequency . Home; ASTM F standard by ASTM International, 06/01/ View all Printed Edition + PDF;; Immediate download; $; Add to Cart.
Contacts were defined between the springs and two rigid planes hard behaviour, friction coefficient: 0. In order to investigate the preload effect due to springs precompression, another model describing ISO experimental set-up was built ISO without precompression. The model was prepared as already described, but modifying only the initial distance 13 To read the published version of this paper feel free to contact luigi. To have a preliminary validation of the FE models describing ISO set-ups ISO and ISO without precompression , the overall stiffnesses of the assembled constructs K were calculated as the average slope of the force-deflection curves between and N and then compared to their experimental values.
Their values were than expressed as mean value and standard deviation of the considered nodal values.
To quantify the global prediction accuracy, a linear regression between the predicted and the experimental strains was performed, the determination coefficient R2 , slope and intercept were calculated; the root mean square error RMSE of FE predictions against the experimental strain measurements were also calculated.
RESULTS Despite the initial transient due to a progressive increase in the contact area between components, the experimental force-displacement curves under compression of the springs, as well as the unassembled and assembled construct demonstrate a constant slope.
The FE simulations do not catch this aspect, because all components are already in contact at the beginning of each analysis and the curves are perfectly linear. Stiffness The compressive axial stiffness k measured for each spring was The mean value In constrained compression, due to the addition of the PE test blocks in series with the three springs, KU is lower than the simple parallel of three springs When using spherical joints unconstrained compression KU is even lower The overall stiffness of the assembled construct KA according to ISO , with or without precompression demonstrate comparable values, while the FE models underestimate the experimental values Table 1.
Principal strains The rod strain measurements demonstrate a linear increase with the applied load. The values obtained using ISO procedure highlighted the important contribution due to the precompression Figure 4.
Just after this step and tightening of the screw-rod interconnection mechanisms, spring release produces a non negligible strain on the rods with the posterior and anterior aspects of the rods loaded in compression and tension, respectively. This is mainly due to the load necessary to achieve the recommended 1 mm precompression 1 of the unassembled constructs, which is As the vertical load increases, the initial extension is gradually compensated and reversed when the peak load is reached: at N the construct is flexed with the posterior and anterior surfaces of the rods loaded in tension and compression, respectively Figure 4.
The load applied in ISO configuration without precompression during assembly is one order of magnitude lower than considering precompression Moreover, upon loading, the spinal rod undergoes immediately flexion, achieving higher strains. At N, the posterior and anterior parts of the rods are subjected to tension and compression, respectively. As concern the repeatability of the experimental measurements, the SG rosettes show some differences.
With regards to the effect of precompression, the strains achieved in ISO procedure were found to be significantly different than in ISO procedure without 16 To read the published version of this paper feel free to contact luigi.
The FE model of the assembled construct according to ISO correctly predicts the force needed for spring precompression More detailed regression data are reported in Table 2. In regard to the effective stiffness of the unassembled construct KU , our study demonstrates that the effective axial stiffness strictly depends on the loading mode i. Of course the addition of the PE test blocks in series with the three springs reduces the axial stiffness of the construct with respect to the simple parallel of springs; additionally, the effective axial stiffness of the anterior support may be even 17 To read the published version of this paper feel free to contact luigi.
However, our results fall within the lower range of more specific data dealing with the compressive behaviour of healthy, degenerated and treated IVDs and FSUs Table 3. This difference may be due to the simplified design assumed for the pedicular screw 23 , which has slightly lower diameter with respect to the one used during experiments: such an aspect will need further investigation to confirm this hypothesis.
Moreover, these results are comparable with the experimental data already reported in literature for in vitro tests under axial compression The comparison between strains measurements and FE predictions highlighted some differences which may be also due to simplifications of the numerical model e. In addition, the observed 18 To read the published version of this paper feel free to contact luigi.
SG location on the spinal rod , inaccuracies in rods positioning on consecutive tests, tolerances in the mechanical characteristics of the calibrated springs i. However, both methods shed light in understanding how the initial precompression produces non negligible preload on the implant before testing Figure 4.
In particular, just after precompression, the assembled constructs turns out to be extended, so that it is necessary to apply a certain amount of load in order to compensate it and achieve a flexion of the construct. Despite few authors used ISO for the numerical evaluation of spinal implants, only La Barbera and colleagues 19 started including the precompression also during simulations of a 2 FSUs assembled construct.
In the current study, we used a similar approach, but studying a relatively more simple single FSU construct instrumented with four SG rosettes applied both on the anterior and posterior aspects of the spinal rods. The slightly higher percentage values found in a previous study 19 may be roughly related to the higher load shared by the instrumentation rod diameter 6. As concern clinical practice, Rohlmann and colleagues demonstrated that the internal loads on a rigid spinal stabilization device primarily depend on the surgical procedure adopted during implantation.
Distraction involves high bending moments on the spinal rods; conversely after compression, the spinal rod can be subjected to a significant preload which superposes to the loads due to body weight and muscle forces. This result is in contrast with data reported by Rohlmann and colleagues, who measured mainly a compressive axial load with a superimposed bending moment in flexion , possibly supporting our idea that the initial precompression step in ISO procedure is needed only for practical reasons.
Given that the effective free length of the calibrated springs may vary within a tolerance of 1 mm around its nominal value, it may guarantee the stability of the overall construct and to avoid any movement of the springs within their housings on the test blocks.
While acknowledging the importance of the experimental approach to provide data for model validation and for the preclinical characterization of the mechanical reliability of spinal implants, it is anyway important to recognize the importance of our model in supporting the design of new implants.
In fact, the validated procedure here proposed may serve as a tool to design new implants in a framework which is close to the one where it will be finally tested experimentally, but repeatable, being insensitive to experimental uncertainties. Therefore, such numerical approach could provide a reliable 20 To read the published version of this paper feel free to contact luigi. Moreover, the FE model could be furtherly extended to quantitatively study the load-sharing mechanism of a variety of implant stiffness, predicting the internal load supported by the posterior stabilization device for any given anterior support stiffness.
The implications of these approaches have already been discussed LaBarbera The initial 1 mm spring precompression significantly affects the strain values on the spinal rods: precompression produces an extension of the assembled construct, thus reducing the effective strain i. This aspect should be kept in mind by any designer while planning the mechanical fatigue testing of any implant according to ISO procedure.
Implants for surgery - Mechanical testing of implantable spinal devices - Fatigue test method for spinal implant assemblies using an anterior support. Standard test methods for spinal implant constructs in a vertebrectomy model. Static and cyclical biomechanical analysis of pedicle screw spinal constructs. Spine ; 18 12 Biomechanical evaluation and comparison of polyetheretherketone rod system to traditional titanium rod fixation. Spine J. Comparative effectiveness of PEEK rods versus titanium alloy rods in lumbar fusion: A preliminary report.
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The inverse effects of load transfer and load sharing on axial compressive stiffness. Spine J ; 1 5 —9. The biomechanical significance of anterior column support in a simulated single-level spinal fusion.
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Thoracic human vertebrae. DOI: No further reproductions authorized.
F 11a FIG. The ultimate load should be a function of the device and not of the load cell or testing machine. The anterior-posterior axis is X with positive being anterior. The medial-lateral axis is Y with left being positive when viewed posteriorly.
The superiorinferior axis is Z with superior being positive. F 11a 3. The insertion points shown in Figs. In situations where the design of the spinal implant assembly or the manufacturers surgical instructions for installation dictate otherwise, the attachment points may deviate from these dimensions.
Each type of anchor has an intended method of application to the spine. Spinal implant assemblies are developed for specic spinal locations such as the anterior cervical spine or the posterior thoracolumbar, lumbar, and lumbosacral spine. A cervical construct is secured with a 9.
The longitudinal direction is generally in the superior-inferior direction and, therefore, generally parallel to the z axis. A spinal implant assembly will contain anchors, interconnections, and longitudinal elements and may contain transverse elements see Fig. A specic design of test block is required for each intended spinal location and intended FIG.
The ultimate torque should be a function of the device and not of the load cell or testing machine. Summary of Test Methods 4. Alternate designs of test blocks may be used as long as equivalent performance is demonstrated. The three static mechanical tests are compression bending, tensile bending, and torsion.
The dynamic test is a compression bending fatigue test. Spinal implant assemblies will be evaluated with test congurations which simulate the clinical requirements for the intended spinal location. The intended spinal locations are both anterior see Fig. The block moment arm see 6. The cervical spine conguration see Fig. Spinal implant assemblies contain different types of anchors. For example, one assembly may include anterior vertebral body screws and rods see Fig.
The block moment arm of a test conguration will be independent of the intended method of application of a spinal implant assembly; therefore, the test data for different intended methods of application may be compared. Signicance and Use 5. Spinal implant assemblies are designed to provide some stability to the spine while arthrodesis takes place. These test methods outline standard materials and methods for the evaluation of different spinal implant assemblies so that comparison between different designs may be facilitated.
The results obtained here cannot be used directly to predict in vivo performance. The results can be used to compare different component designs in terms of the relative mechanical parameters. This test should be initially performed dry ambient room conditions for consistency. The effect of environment may be signicant. Repeating all or part of these test methods in simulated body uid, saline 9 g NaCl per mL water , a saline drip, water, or a lubricant should be considered.
The maximum recommended frequency for this type of cyclic testing should be 5 Hz. The perpendicular distance to the load direction block moment arm between the axis of a hinge pin and the anchors attachment-points to a UHMWPE block is independent of anchor-type.
The distance between the anchors attachment point to the UHMWPE block and the center of the longitudinal element is a function of the interface design between the screw, hook, wire, cable, and so forth, and the rod, plate, and so forth. Apparatus 6. Two pairs of side supports are mounted on the test machine see Fig. One pair of side supports attach to the actuator and the second to the load cell.
A mounting plate for one of the sets of side support plates should be free to rotate about the Z axis for the compression bending, tension bending and fatigue tests.
The center axis of each hinge pin should be perpendicular If screws are used to mount the spinal construct to the test blocks see Fig. If the anchors are secured into slots in the longitudinal element, then they should be centrally placed in the slots and not at either end to produce a worst-case scenario.
Spinal implant assemblies are designed for two intended spinal locations having two unique block moment arms. The two intended spinal locations are the cervical spinal implant system see Fig. The test congurations for the cervical spinal implant system have a block moment arm equal to The thoracolumbar, lumbar, and lumbosacral test congurations have a mm block moment arm.
The locations of the longitudinal elements are determined by the design of anchors and interconnections. The UHMWPE blocks are allowed to rotate around the Y-axis of the hinge pin during the compression bending, tensile bending, and fatigue tests.
Steel roll pins are placed into the modied blocks such that the outer surfaces of the roll pins are parallel to the front surfaces of the standard bilateral UHMWPE block see Fig. Hooks, wires, and cables are not fully constrained semi-rigid xation devices because they cannot transfer bending moments in the three axes.
The combination of the rotation of the modied UHM- WPE block on the hinge pin and the rotation of the hooks, wires, or cables around the steel roll pins means that the test conguration would be a mechanism.
The variation is minimized by having the hinge pins in the UHMWPE blocks rotate past the anchors as the test progresses. Position the hinge pins internal to the anchors during the tension bending test not shown.
Position the hinge pins external to the anchors during the compression bending, torsion and fatigue tests see Fig. If the longitudinal element has xed spacings and the recommended active length cannot be achieved, then select the longitudinal element that is nearest the recommended active length.
The active length should be constant for all constructs used in comparative testing. The hinge pin in the apparatus allows rotation in the X-Z plane during the static compression bending, static tension bending, and compression bending fatigue tests.
The compression bending fatigue test will use the same test conguration as static compressive bending. Sampling 7.
Implants shall not be retested. Alternate designs of test blocks which include steel roll pins see Fig. Do not disassemble the test construct after testing unless disassembly is necessary to evaluate failure surfaces, interconnections, corrosion or loosening surfaces.