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Thoracolumbosacral pedicle screw systems (TPSSs) are spinal implants commonly utilized to stabilize the spine as an adjunct to fusion for a variety of spinal pathologies. These systems consist of components including pedicle screws, rods, hooks, and various connectors that allow the surgeon to create constructs that can be affixed to a wide range of spinal anatomy. During the development and regulatory clearance process, TPSSs are subjected to mechanical testing such as static and dynamic compression bending per ASTM F1717, axial and torsional grip testing per ASTM F1798, and foam block pullout testing per ASTM F543. In this study, design and mechanical testing data were collected from 200 premarket notification (510(k)) submissions for TPSSs submitted to FDA between 2007 and 2018. Data were aggregated for the most commonly performed mechanical tests, and analyses were conducted to assess differences in performance based on factors such as component type, dimensions, and materials of construction. Rod material had a significant impact on construct stiffness in static compression bending testing with cobalt chromium rods being significantly stiffer than titanium rods of the same diameter. Pedicle screw type had an impact on compression bending yield strength with monoaxial screws having significantly higher yield strength as compared to polyaxial or uniplanar screws. Axial and torsional gripping capacities between components and the rods were significantly lower for cross-connectors than the other component types. The aggregated data presented here can be utilized for comparative purposes to aid in the development of future TPSSs.

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Intervertebral body fusion devices (IBFDs) are commonly used in the treatment of various spinal pathologies. Intra-operative fractures of polyether-ether-ketone (PEEK) implants have been reported in the literature and to the FDA as device-related adverse events. The device and/or implant inserter failures typically occur during device impaction into the disc space and require implant removal and replacement. These additional steps may cause further complications along with increased surgical time and cost. Currently, there are no standardized test methods that evaluate clinically relevant impaction loading conditions on IBFDs. This study aims to develop an in vitro test method that would evaluate implant resistance to failure during intra-operative impaction. To achieve this, (1) surgical implantations of IBFDs were simulated in nine lumbar cadaver specimens by three different orthopedic spine surgeons (n = 3/surgeon). Impact force and mallet speed data were acquired for each surgeon. (2) Based on the acquired surgeon data, a benchtop mechanical test setup was developed to differentiate between two TLIF IBFD designs and two inserter designs (for a total of four IBFD-inserter combinations) under impaction loading. During implant insertion, impact force measurements indicated that lumbar IBFDs are subjected to high energy forces that may exceed their mechanical strength. Our test method successfully replicated clinically-relevant loading conditions and was effective at differentiating failure parameters between different implant and inserter instrument designs. The mechanical test method developed shows promise in its ability to assess impaction resistance of IBFD/inserter designs and evaluate potential risks of device failure during intraoperative loading.

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BACKGROUND: Intervertebral body fusion devices (IBFDs) are a widely used type of spinal implant placed between two vertebral bodies to stabilize the spine for fusion in the treatment of spinal pathologies. Assessing mechanical performance of these devices is critical during the design, verification, and regulatory evaluation phases of development. While traditionally evaluated with physical bench testing, empirical assessments are at times supplemented with computational models and simulations such as finite element analysis (FEA). However, unlike many mechanical bench tests, FEA lacks standardized practices and consistency of implementation. OBJECTIVES: The objectives of this study were twofold. First, to identify IBFD 510(k) submissions containing FEA and conduct a comprehensive review of the elements provided in the FEA reports. Second, to engage with spinal device manufacturers through an anonymous survey and assess their practices for implementing FEA. METHODS: First, a retrospective analysis of 510(k) submissions for IBFDs cleared by the FDA between 2013 and 2017 was performed. The contents of FEA test reports were quantified according to FDA guidance. Second, a survey inquiring about the use of FEA was distributed to industry and academic stakeholders. The survey asked up to 20 questions relating to modeler experience and modeling practices. RESULTS: Significant gaps were present in model test reports that deemed the data unreliable and, therefore, unusable for regulatory decision-making in a high percentage of submissions. Nonetheless, the industry survey revealed most stakeholders employ FEA during device evaluation and are interested in more prescriptive guidelines for executing IBFD models. CONCLUSIONS: This study showed that while inconsistencies and gaps in FEA execution do exist within the spinal device community, the stakeholders are eager to work together in developing standardized approaches for executing computational models to support mechanical performance assessment of spinal devices in regulatory submissions.

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Lumbar intervertebral body fusion devices (L-IBFDs) are intended to provide stability to promote fusion in patients with a variety of lumbar pathologies. Different L-IBFD designs have been developed to accommodate various surgical approaches for lumbar interbody fusion procedures including anterior, lateral, posterior, and transforaminal lumbar interbody fusions (ALIF, LLIF, PLIF, and TLIF, respectively). Due to design differences, there is a potential for mechanical performance differences between ALIF, LLIF, PLIF, and TLIF devices. To evaluate this, mechanical performance and device dimension data were collected from 124 Traditional 510(k) submissions to the FDA for L-IBFDs cleared for marketing from 2007 through 2016. From these submissions, mechanical test results were aggregated for seven commonly performed tests: static and dynamic axial compression, compression-shear, and torsion testing per ASTM F2077, and subsidence testing per ASTM F2267. The Kruskal-Wallis test and Wilcoxon signed-rank test were used to determine if device type (ALIF, LLIF, PLIF, TLIF) had a significant effect on mechanical performance parameters (static testing: stiffness and yield strength; dynamic testing: runout load; subsidence testing: stiffness [Kp]). Generally, ALIFs and LLIFs were found to be stiffer, stronger, and had higher subsidence resistance than PLIF and TLIF designs. These results are likely due to the larger footprints of the ALIF and LLIF devices. The relative mechanical performance and subsidence resistance can be considered when determining the appropriate surgical approach and implant for a given patient. Overall, the mechanical performance data presented here can be utilized for future L-IBFD development and design verification.

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Cervical intervertebral body fusion devices (IBFDs) are utilized to provide stability while fusion occurs in patients with cervical pathology. For a manufacturer to market a new cervical IBFD in the United States, substantial equivalence to a cervical IBFD previously cleared by FDA must be established through the 510(k) regulatory pathway. Mechanical performance data are typically provided as part of the 510(k) process for IBFDs. We reviewed all Traditional 510(k) submissions for cervical IBFDs deemed substantially equivalent and cleared for marketing from 2007 through 2014. To reduce sources of variability in test methods and results, analysis was restricted to cervical IBFD designs without integrated fixation, coatings, or expandable features. Mechanical testing reports were analyzed and results were aggregated for seven commonly performed tests (static and dynamic axial compression, compression-shear, and torsion testing per ASTM F2077, and subsidence testing per ASTM F2267), and percentile distributions of performance measurements were calculated. Eighty-three (83) submissions met the criteria for inclusion in this analysis. The median device yield strength was 10,117N for static axial compression, 3680N for static compression-shear, and 8.6Nm for static torsion. Median runout load was 2600N for dynamic axial compression, 1400N for dynamic compression-shear, and ±1.5Nm for dynamic torsion. In subsidence testing, median block stiffness (Kp) was 424N/mm. The mechanical performance data presented here will aid in the development of future cervical IBFDs by providing a means for comparison for design verification purposes.

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OBJECTIVE Lumbar cages with integrated fixation screws offer a low-profile alternative to a standard cage with anterior supplemental fixation. However, the mechanical stability of integrated fixation cages (IFCs) compared with a cage with anterior plate fixation under fatigue loading has not been investigated. The purpose of this study was to compare the biomechanical stability of a screw-based IFC with a standard cage coupled with that of an anterior plate under fatigue loading. METHODS Eighteen functional spinal units were implanted with either a 4-screw IFC or an anterior plate and cage (AP+C) without integrated fixation. Flexibility testing was conducted in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) on intact spines, immediately after device implantation, and post-fatigue up to 20,000 cycles of FE loading. Stability parameters such as range of motion (ROM) and lax zone (LZ) for each loading mode were compared between the 2 constructs at multiple stages of testing. In addition, construct loosening was quantified by subtracting post-instrumentation ROM from post-fatigue ROM. RESULTS IFC and AP+C configurations exhibited similar stability (ROM and LZ) at every stage of testing in FE (p ≥ 0.33) and LB (p ≥ 0.23) motions. In AR, however, IFCs had decreased ROM compared with AP+C constructs at pre-fatigue (p = 0.07) and at all post-fatigue time points (p ≤ 0.05). LZ followed a trend similar to that of ROM in AR. ROM increased toward intact motion during fatigue cycling for AP+C and IFC implants. IFC specimens remained significantly (p < 0.01) more rigid than specimens in the intact condition during fatigue for each loading mode, whereas AP+C construct motion did not differ significantly (p ≥ 0.37) in FE and LB and was significantly greater (p < 0.01) in AR motion compared with intact specimens after fatigue. Weak to moderate correlations (R2 ≤ 56%) were observed between T-scores and construct loosening, with lower T-scores leading to decreased stability after fatigue testing. CONCLUSIONS These data indicate that a 4-screw IFC design provides fixation similar to that provided by an AP+C construct in FE and LB during fatigue testing and better stability in AR motion.

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BACKGROUND CONTEXT: Numerous integrated fixation cages (IFCs) have recently been introduced to the market with "zero-profile" designs that incorporate screw fixation through the vertebral endplate. It is unclear whether differences in bone quality and quantity in this insertion location may affect fixation compared with screws used in traditional anterior plate (AP) fixation. Moreover, endplate preparation for IFC implantation may affect fixation strength. PURPOSE: This study aimed to compare pullout strength of screws used in IFCs with screws used for AP implantations. STUDY DESIGN: A biomechanical cadaveric study. METHODS: T12 and L1 vertebrae from 13 human cadaver spines were prepared for pullout testing. End plates in T12 vertebrae were scraped according to surgical practice for fusion procedures. Conversely, endplates in L1 vertebrae were kept intact (unscraped). Integrated fixation cage screws were implanted at a 45° angle into the endplate and AP screws were implanted horizontally into the same vertebral body. Pullout testing was performed on all screws, and peak pullout force (PPF) and work were compared between groups to determine fixation strength. In addition, micro-CT imaging was used to assess bone quantity and quality parameters such as trabecular bone volume fraction, endplate and anterior cortex thickness at screw insertion location, endplate mineralization, and anterior cortex mineralization. RESULTS: Peak pullout force for IFC screws (176±68 N) with scraped endplates was similar (p=.26) to AP screws (192±84 N). However, PPF for IFC screws (231±75 N) with unscraped endplates was significantly greater (p<.01) than AP screws (176±50 N). Peak pullout force for IFC screws with scraped endplates was significantly lower (p=.03) than IFC screws with unscraped endplates. Scraped endplates group (0.17±0.05 mm) were thinner (p=.05) than unscraped endplates (0.21±0.06 mm) by approximately 40 µ on average. CONCLUSIONS: These data indicate that IFC and AP screws exhibited similar fixation behavior when the endplate is prepared according to common surgical practices. However, endplate scraping reduces endplate thickness by 20% on average, resulting in a decrease in fixation strength when compared with the unscraped endplates and provides bounds for IFC screw fixation strength.

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