Elsevier

Journal of Biomechanics

Volume 54, 21 March 2017, Pages 26-32
Journal of Biomechanics

Mechanical performance of cervical intervertebral body fusion devices: A systematic analysis of data submitted to the Food and Drug Administration

https://doi.org/10.1016/j.jbiomech.2017.01.032Get rights and content

Abstract

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 10,117 N for static axial compression, 3680 N for static compression-shear, and 8.6 N m for static torsion. Median runout load was 2600 N for dynamic axial compression, 1400 N for dynamic compression-shear, and ±1.5 N m for dynamic torsion. In subsidence testing, median block stiffness (Kp) was 424 N/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.

Introduction

Anterior cervical decompression and fusion (ACDF) is a commonly utilized surgical treatment for cervical spine pathologies including radiculopathy or myelopathy, achieving favorable clinical outcomes with minimal surgical risks (Cloward, 1958, Fraser and Härtl, 2007, Gore and Sepic, 1998). ACDF procedures commonly involve implantation of cervical intervertebral body fusion devices (IBFDs) between adjacent vertebral bodies to maintain or restore disc height while providing stability to allow for the development of a fusion mass (Hacker, 2000, Kulkarni et al., 2007, Niu et al., 2010, Thomé et al., 2004, Vavruch et al., 2002). Cervical IBFDs need to withstand the physiologic loading in the cervical spine while avoiding significant subsidence into the vertebral bodies. Thus, the mechanical strength of cervical IBFDs, as well as the propensity of the device to subside, must be evaluated prior to use in patients. ASTM F2077 Test Methods for Intervertebral Body Fusion Devices contains methods for performing static and dynamic axial compression, compression-shear, and torsion testing on IBFDs. While none of these tests in isolation replicate the complex in vivo loads in the spine, these methods allow for mechanical properties to be compared between devices under the predominant loading modes an IBFD is expected to experience in vivo. ASTM F2267 Standard Test Method for Measuring Load Induced Subsidence of Intervertebral Body Fusion Device Under Static Axial Compression contains methods to test the propensity of an IBFD to subside. These standards describe methods that allow spinal device manufacturers to compare the mechanical properties of IBFDs.

In the United States, IBFDs are regulated in FDA’s Center for Devices and Radiological Health (CDRH). IBFDs are subject to CDRH’s Pre-Market Notification (510(k)) process, which requires that a new device be deemed “substantially equivalent” to an FDA-cleared device, based on intended use and technological characteristics (U.S. FDA, 2014). FDA’s guidance document for IBFDs describes the type of information necessary for inclusion in a 510(k) submission in order to demonstrate substantial equivalence (Table 1) (U.S. FDA, 2007). Comparison of mechanical properties between the new device and a previously FDA-cleared device is an important part of substantial equivalence determinations for IBFDs. The aforementioned FDA guidance recommends that manufacturers conduct static and dynamic axial compression, compression-shear, and torsion testing per ASTM F2077, and subsidence testing per ASTM F2267 on new cervical IBFDs.

Standardized mechanical testing is thus an important element in the regulatory process for cervical IBFDs. However, publicly available mechanical performance data for cervical IBFDs are scarce. The objective of this study was to increase transparency in the FDA review process by presenting aggregated cervical IBFD mechanical testing results accumulated in 510(k) submissions. In addition, we evaluated the clinical failure modes (adverse events) for cervical IBFDs in FDA’s Medical Device Reporting (MDR) database to ensure there were no significant performance issues being reported for the devices whose mechanical test data were aggregated in this study. The data presented provides the spinal device community with a range of mechanical performance values that can be used as a means for comparison in the design verification process of new cervical IBFDs.

Section snippets

Methods

Mechanical testing data and device dimensions were retrospectively collected from Traditional2 510(k) submissions for cervical IBFDs cleared by FDA from 2007 through 2014. Only single-piece cervical IBFDs without integrated fixation, unique materials (i.e., materials other than

Device design

Table 2 shows the dimensional range offerings in the submissions for width (11–20 mm), depth (10–16 mm), height (4–14 mm), and lordosis angle (0–7 degrees). By material type, 90.4% were PEEK, and 9.6% were metallic. For device shape, 81.9% were box-shaped, 14.5% were D-shaped, and 3.6% had other single-piece designs.

The smallest footprint device (i.e., smallest width × smallest depth), from the range of sizes in a single submission, was chosen by the manufacturer for testing greater than 87% of the

Discussion

The present study is the first comprehensive analysis of mechanical testing data for FDA-cleared cervical IBFDs. These mechanical tests are important tools for spinal device manufacturers for design verification and regulatory purposes. Graham and Estes discuss the two typical sources for acceptance criteria for standardized mechanical tests: (1) test results of a comparable device, and (2) expected physiologic loads (Graham and Estes, 2009). They state that comparing test results between

Conflict of interest

The authors of this manuscript have no conflicts of interest to report.

Acknowledgements

This publication was made possible by Grant Number U01FD004979 from the FDA, which supports the UCSF-Stanford Center of Excellence in Regulatory Sciences. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the HHS or FDA. The authors would like to thank Vivek Palepu for his assistance with figures, and Katherine Kavlock, Melissa Hall, Caroline Rhim, Ronald Jean, and Mark Melkerson for their help with review.

References (11)

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First two authors contributed equally to this study.

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