ASTM F2694-16 Standard Practice for Functional and Wear Evaluation of Motion-Preserving Lumbar Total Facet Prostheses

The ASTM F2694-16 Standard Practice for Functional and Wear Assessment of Motion-Preserving Total Facet Prostheses of the Lumbar Spine , developed by the American Society for Testing and Materials (ASTM) , describes a procedure for guidance on functional, kinematic, and wear testing of motion-preserving total facet prostheses of the lumbar spine. These implants are designed to provide motion and support to functional spinal units by replacing the natural facets.

This test method is not designed to address the bone-implant interface or the static properties of the prosthetic components. Fatigue properties are included, but only as a byproduct of cyclic wear testing under facet loading and therefore are not addressed in the process of establishing a typical stress-life characterization. Additionally, the biocompatibility of the materials used in the total facet prosthesis is not addressed in this application.

For the purposes of this standard, the following terms and definitions apply (all functional and kinematic test terminology is consistent with the referenced standards unless otherwise stated):

Mechanical failure is failure associated with a defect in the material, such as a fatigue crack or bond between materials. This may or may not also result in functional failure.
Exhaustion (cycles) is the maximum number of cycles that a test must be performed if functional failure has not yet occurred.
Coordinate systems/axes: the spherical XYZ orthogonal axes are defined following the right-handed Cartesian coordinate system where the XY plane is parallel to and coplanar with the superior end plate of the lower vertebral body. The spherical axes are fixed relative to the lower vertebral body, which in this application is also assumed fixed relative to the frame of the testing machine. The origin is the center of the global coordinate system located at the posteromedial position of the superior end plate of the lower vertebral body. The positive X axis shall be oriented anteriorly with respect to the initial unloaded position of the specimen. The positive Y axis shall be oriented laterally (to the left) with respect to the initial unloaded position of the specimen. The positive Z axis shall be oriented superiorly with respect to the initial unloaded position of the specimen.
Fluid absorption refers to the liquid absorbed by the device material during testing or while implanted in vivo.
Functional failure is permanent deformation or wear that renders the total facet prosthesis assembly ineffective or unable to perform its intended function.
The net volumetric wear rate during the cycle interval is the bulk density of the wear material.
A total facet prosthesis is a non-biological construct that aims to restore the support and motion of the natural vertebral facet joint.
The kinematic profile is the relative motion between adjacent vertebral bodies that the total facet prosthesis undergoes while being tested.
The load profile refers to the load to which the device is subjected when tested under a defined kinematic profile or the load to which the total facet prosthesis is subjected if tested under load control.
The radius of gyration is the distance between the center of rotation for a given movement and the functional position of the total facet prosthesis, i.e. the load-bearing contact point.
The weight of the soak control sample is the weight at the beginning and at the end of the cycle interval.
The weight of the wear sample is the weight at the beginning and at the end of the cycle interval.
The net wear (weight loss) of the wear sample was corrected for liquid absorption at the end of the cycle range.
Wear is the progressive loss of material from devices or device components as a result of surface relative motion with another body, as measured by the change in mass of the total facet prosthesis or components of the total facet prosthesis.

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