Significance and Use
5.1 This guide can be used to develop test parameters for evaluating fatigue and wear behavior of IVD prostheses under impingement loading. It must be recognized, however, that there are likely many possible impingement conditions for a given IVD prosthesis.
5.2 The user should attempt to determine the clinically relevant and geometrically possible impingement conditions and dictated by the design and impingement wear test parameters that may result in wear and fatigue damage for the IVD prosthesis. The user should also attempt to select the device size which will represent a worst case for the impingement conditions and parameters selected.
5.3 The user should reference and utilize existing sources of information to identify the impingement test parameters that produce the clinically relevant impingement wear and damage for their IVD prosthesis. Prior clinical experience with the device design may aid in the development of impingement test parameters through analysis of device retrievals and radiographs. However, prior clinical experience with the IVD being tested should not be considered as a prerequisite for performing impingement testing.
5.4 This guide details a three-step process for assessing device impingement under a selected set of conditions:
5.4.1 The user selects previously identified impingement conditions, one at a time, or clinically observed conditions.
5.4.2 The user selects the worst-case size of device to apply the selected conditions.
5.4.3 Solid modeling and the quasistatic test method should be employed to assess the impingement condition and determine the impingement test parameters – most importantly, the angular displacement limits to be used in the impingement wear test.
5.4.4 The impingement wear test is then conducted using the impingement test parameters.
5.5 This guide serves to evaluate devices with various designs, materials (i.e., metal-on-metal versus polymer-on-polymer), and stiffness in the impingement region using the same axial force and angular displacement control.
5.5.1 In the case where the device has no limit in a given direction or does not allow motion in a given direction, a rationale for excluding that condition should be provided (e.g., intended design or function of the device).
5.6 Impingement occurs over a range between an initial and an ultimate angle rather than at a discrete angle and location because both design (e.g., mobile bearings) and material combinations (e.g., inclusion of polymeric materials) may lead to compliance, deformation and wear, which in turn may lead to a change in the angular displacement at which contact occurs over the course of the test. A range of angular displacement is therefore prescribed to ensure that the impingement region is fully loaded during each impingement cycle.
5.7 The suggested test parameters in Table 1 have been provided with the objective of minimizing Mode I wear at the bearing surface while providing sufficient motion to fully offload the bearing surface for each cycle. Given that the intended function of the devices is typically to articulate, it may be impossible to fully eliminate Mode 1 wear at the intended bearing interface.
5.8 The point of impingement (POI) is a simplification for the purpose of determining an impingement moment arm and thus calculating the theoretical ultimate moment (Mt). Mt may be useful for comparing device designs.
5.9 The contribution of axial rotation to impingement damage is still under-studied. However, retrieval analysis has provided evidence that it may contribute to impingement damage. Many total disc replacements are unconstrained in axial rotation. Therefore, unlike flexion-extension or lateral bending where a moment versus angular displacement response can be readily developed, axial rotation will have a near-zero moment response. The axial rotation parameters provided in Section 15 are based on the Mode 1 wear test methods and should be assessed and altered if justification (e.g., wear patterns from retrievals, scientific literature, etc.) exists.
Scope
1.1 This standard is intended to provide guidance on the evaluation of wear and fatigue characteristics of total disc prostheses under cyclic impingement conditions.
1.2 This guide describes impingement testing of devices with articulating components. The user is cautioned that the methods described herein are intended to produce an impingement condition which may or may not be indicative of clinical performance and which may or may not be consistent with the intended use of the device, and that this should be considered when interpreting the data. Clinically, total disc prostheses should always be implanted per labeling and the manufacturer’s instructions for use.
1.3 Impingement has been observed in retrievals among several total disc prosthesis designs; however, impingement is not necessarily associated with device or clinical failure. It is the intent of this guide to investigate possible impingement-induced wear and mechanical failure modes associated with device design, as well as potential mechanical failure modes associated with clinical events such as subsidence, malpositioning, and improper implant sizing. Note that mechanical failure may or may not be associated with functional failure.
1.4 It is recommended that the user define the bearing and non-bearing features of the intervertebral disc (IVD) prosthesis and evaluate the performance of the IVD prosthesis under Mode 1 wear by using Guide F2423 or ISO 18192-1 prior to use of this guide. This standard is not intended to provide guidance on Mode I testing.
1.5 The goal of this guide is to evaluate impingement in IVD prostheses regardless of the intended region of the spine (cervical or lumbar), material or material combinations (ceramic, metal, polymer), and bearing type (fixed or mobile).
1.6 It is the intent of this guide to enable comparison of IVD prostheses with regard to wear and fatigue characteristics when tested under the specified conditions.
1.7 The values stated in SI units are to be regarded as the standard with the exception of angular measurements which should be reported in degrees.
1.8 The use of this standard may involve the operation of potentially hazardous equipment. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
