ASTM F1798-97(2008)
(Guide)Standard Guide for Evaluating the Static and Fatigue Properties of Interconnection Mechanisms and Subassemblies Used in Spinal Arthrodesis Implants
Standard Guide for Evaluating the Static and Fatigue Properties of Interconnection Mechanisms and Subassemblies Used in Spinal Arthrodesis Implants
SIGNIFICANCE AND USE
Spinal implants are generally composed of several components that, when connected together, form a spinal implant construct. Spinal implant constructs are designed to provide some stability to the spine while arthrodesis takes place. This guide outlines standardized evaluations of different interconnection mechanisms to facilitate comparison between different designs. Comparisons must be made cautiously and with careful analysis, taking into account the effects that design differences can have on the loading configurations.
This guide is used to quantify the static and fatigue properties of different implant interconnection designs. The mechanical tests are conducted in vitro using simplified, unidirectional loads and moments. Fatigue testing in a simulated body fluid or saline may have a fretting, corrosive, or lubricating effect on the interconnection and thereby affect the relative performance of tested devices. Hence, the test environment, whether a simulated body fluid, saline (9g NaCl per 1000 mL H2O), with a saline drip, or dry, is an important characteristic of the test and must be reported accurately.
The loading of spinal implant constructs in vivo will, in general, differ from the loading configurations used in this guide. The results obtained here cannot be used directly to predict in vivo performance. However, the results can be used to compare different component designs in terms of relative mechanical parameters.
SCOPE
1.1 This guide covers the measurement of uniaxial static and fatigue strength, and resistance to loosening of the component interconnection mechanisms of spinal arthrodesis implants.
1.2 The purpose of this guide is to provide a means of mechanically characterizing different designs of spinal implant interconnections. Ultimately, the various components and interconnections should be combined for static and fatigue testing of the spinal implant construct. It is not the intention of this guide to address the analysis of spinal implant constructs or subconstructs or to define levels of performance of spinal implants as insufficient knowledge is available to predict the consequences of the use of particular spinal implant designs.
1.3 This guide sets out definitions for use in measuring the strength of component interconnections of spinal implants, possible test methods themselves, and the reporting of test results.
1.4 The values stated in SI units are to be regarded as standard, with the exception of angular measurements, which may be reported in terms of either degrees or radians.
1.5 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: F1798 − 97(Reapproved 2008)
Standard Guide for
Evaluating the Static and Fatigue Properties of
Interconnection Mechanisms and Subassemblies Used in
Spinal Arthrodesis Implants
This standard is issued under the fixed designation F1798; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope F383Practice for Static Bend and Torsion Testing of In-
tramedullary Rods (Withdrawn 1996)
1.1 This guide covers the measurement of uniaxial static
F1582Terminology Relating to Spinal Implants
and fatigue strength, and resistance to loosening of the com-
ponent interconnection mechanisms of spinal arthrodesis im-
3. Terminology
plants.
3.1 Definitions of Terms Specific to This Standard:
1.2 The purpose of this guide is to provide a means of
3.1.1 active length of longitudinal element—the span be-
mechanicallycharacterizingdifferentdesignsofspinalimplant
tween rigid supports (for example, 50 mm is the active length
interconnections. Ultimately, the various components and in-
in Fig. 1, Fig. 2, Fig. 3(a), Fig. 3(b), and Fig. 4.
terconnections should be combined for static and fatigue
3.1.2 global coordinate system—spinal column motion has
testing of the spinal implant construct. It is not the intention of
six degrees of freedom, having translational motion along, and
this guide to address the analysis of spinal implant constructs
rotational motion about three axes. The axes are labeled
or subconstructs or to define levels of performance of spinal
anterior-posterior or a-p (X), medial-lateral or transverse (Y),
implants as insufficient knowledge is available to predict the
and caudal-cranial or axial (Z).This coordinate system is right
consequences of the use of particular spinal implant designs.
handed with +X in the anterior direction, +Y towards the left
1.3 This guide sets out definitions for use in measuring the
side of the body, and +Z in the cranial direction. Positive
strength of component interconnections of spinal implants,
rotations are defined by the right hand rule (see Fig. 5(a)).
possible test methods themselves, and the reporting of test
3.1.3 gripping capacity—the maximum applied load or
results.
moment across an interconnection mechanism within the first
1.4 The values stated in SI units are to be regarded as
1.5mmofpermanentdisplacementor5°ofpermanentrotation
standard, with the exception of angular measurements, which
between the connected components.
may be reported in terms of either degrees or radians.
3.1.4 local coordinate system—thespine’sglobalcoordinate
1.5 This standard does not purport to address all of the
system shall be applied locally at the position of the intercon-
safety concerns, if any, associated with its use. It is the
nection. The local direction, z, shall be centered through the
responsibility of the user of this standard to establish appro-
longitudinal element of the x-y plane. The local direction, x,
priate safety and health practices and determine the applica-
shall be defined as parallel to the axis of a screw or back of a
bility of regulatory limitations prior to use.
hook. The local transverse axis, y, shall be parallel to a
transverse element (See Fig. 5(b) and Fig. 5(c)).
2. Referenced Documents
3.1.5 loosening torque—the torque required to disconnect
2.1 ASTM Standards:
the various threaded fasteners that might comprise the im-
E4Practices for Force Verification of Testing Machines
plant’s interconnection mechanism.
3.1.6 major directions of loading—directions of the pre-
dominant forces and moments (relative to the local axes) to
This guide is under the jurisdiction of ASTM Committee F04 onMedical and
which vertebral connection elements are subjected, (that is,
Surgical Materials and Devices.
axial load, Fz; A-P load, Fx; axial torsion, Mz; and flexion-
Current edition approved Dec. 1, 2008. Published December 2008. Originally
approved in 1997. Last previous edition approved in 2003 as F1798–97(2003). extension moment, My).
DOI: 10.1520/F1798-97R08.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1798 − 97 (2008)
3.1.10 subassembly failure—permanent deformation result-
ing from fracture, plastic deformation, loosening or slippage
that renders the subassembly ineffective or unable to ad-
equately resist load.
3.1.11 subassembly permanent deformation—the displace-
ment (mm) or angular displacement (degree of the subassem-
bly relative to the unloaded condition) remaining after the
applied load moment or torque has been removed. Care must
be taken to ensure that the loading fixtures are rigid and do not
contribute to the measurement of deflection.
3.1.12 tightening torque—the specified torque that is ap-
plied to the various threaded fasteners that might comprise the
implant’s interconnection mechanism.
3.1.13 ultimate load/moment of the subassembly—
maximumloadormomentappliedtoasubassembly(seePoint
Ein Fig. 6).
3.1.14 yield load/moment of the subassembly—the load or
moment required to produce a permanent deformation equal to
FIG. 1 A-P Test Apparatus for Subassembly
0.020 times the active length of the longitudinal element (see
Point D in Fig. 6).
4. Summary of Test Methods
4.1 Vertebral attachment components (for example, hook,
screws, bands) and transverse elements must be attached to
longitudinalelements(forexample,rods,plates)toformspinal
implant subassemblies.
4.2 The interconnections are tested only in the relevant
directions of loading by applying loads at specific locations
relative to the local coordinate system.
4.3 The interconnections and subassemblies are tested stati-
callyinaload-to-failuremodeandalsocanbetestedcyclically
to estimate the maximum run out value at 2.5 × 10 cycles.
5. Significance and Use
5.1 Spinal implants are generally composed of several
components that, when connected together, form a spinal
implant construct. Spinal implant constructs are designed to
provide some stability to the spine while arthrodesis takes
place.This guide outlines standardized evaluations of different
FIG. 2 Transverse Test Apparatus for Subassembly
interconnection mechanisms to facilitate comparison between
different designs. Comparisons must be made cautiously and
withcarefulanalysis,takingintoaccounttheeffectsthatdesign
3.1.7 maximum run out load/moment—the maximum load
differences can have on the loading configurations.
or moment that can be applied to a subassembly where all the
tested constructs have withstood 2.5 × 106 cycles without a
5.2 This guide is used to quantify the static and fatigue
failure.
properties of different implant interconnection designs. The
mechanical tests are conducted in vitro using simplified,
3.1.8 relevant directions of loading—those directions of
unidirectional loads and moments. Fatigue testing in a simu-
loading in which a particular component interconnection is
lated body fluid or saline may have a fretting, corrosive, or
designed to provide resistance to loading. For example, a
lubricating effect on the interconnection and thereby affect the
particular spinal hook may be designed to withstand a positive
relative performance of tested devices. Hence, the test
axial load, A-P load, and flexion-extension moment, but not a
environment, whether a simulated body fluid, saline (9g NaCl
negative axial load or axial torsion. Hence, positive axial load,
per 1000 mL H O), with a saline drip, or dry, is an important
A-P load, and flexion-extension moment are the relevant
characteristic of the test and must be reported accurately.
directions of loading.
3.1.9 spinal arthrodesis implant—an implant applied to the 5.3 The loading of spinal implant constructs in vivo will, in
spine with the intention of providing temporary correction and general, differ from the loading configurations used in this
stability to vertebrae while bony fusion occurs. guide. The results obtained here cannot be used directly to
F1798 − 97 (2008)
FIG. 3 Flexion-Extension Moment Test Apparatus for Subassembly
F1798 − 97 (2008)
FIG. 4 Transverse Moment Test Apparatus for Subassembly
predict in vivo performance. However, the results can be used linkage and remain unfixed. Axial loads are applied to the
to compare different component designs in terms of relative interconnection mechanism along the axis of the longitudinal
mechanical parameters. element via a sleeve (collar) which freely surrounds the
longitudinalelement.Thesleeve(collar)shouldevenlydistrib-
6. Apparatus
ute the load around the interconnection. An alternate method,
depicted in Fig. 7(b), applies the load to the longitudinal
6.1 Machines used for the test shall conform to the require-
element and pushes it through the interconnection clamp.
ments of Practices E4.
6.2 The apparatus for axial (z) gripping capacity measure- 6.3 TheapparatusforA-P(x)mechanicalpropertymeasure-
ments of an interconnection mechanism is depicted in Fig. ments of a subassembly is depicted in Fig. 1. Both ends of the
7(a). One end of the longitudinal element shall be clamped longitudinal element shall be clamped rigidly, with the inter-
rigidly, leaving 5 mm exposed between the interconnection connection centered on a 50-mm section of the longitudinal
mechanismandthetestmachinebase.Asectionoflongitudinal element. The local origin of the interconnection mechanism
element at least 5 mm shall extend beyond the interconnection shall be centered between the mounts. Loads are applied to the
F1798 − 97 (2008)
FIG. 5 Coordinate System
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FIG. 6 Load/Displacement Curve
interconnection (perpendicular to the longitudinal element) via mechanism shall be centered between the mounts. Loads are
a clamp on the hook, screw, or band. The load should be applied to the interconnection (perpendicular to the longitudi-
centered through the local x coordinate axis. nal element) via a clamp on the transverse connector.The load
should be centered through the local y coordinate axis.
6.4 The apparatus for transverse (y) mechanical property
measurements of a subassembly is depicted in Fig. 2. Both 6.5 The apparatus for flexion-extension moment (My) me-
ends of the longitudinal element shall be clamped rigidly, with chanical property measurements of a subassembly is depicted
the interconnection centered on a 50-mm section of the in Fig. 3. Both ends of the longitudinal element shall be
longitudinal element. The local origin of the interconnection clampedrigidly,withtheinterconnectioncenteredona50-mm
F1798 − 97 (2008)
FIG. 7 Axial Gripping Capacity Test Apparatus
section of the longitudinal element. The local origin of the 7.2 The test constructs shall be labeled and maintained
interconnection mechanism shall be centered between the according to good laboratory practice.
mounts. Loads are applied to the interconnection (parallel to
7.3 Static tests of mechanical properties should have a
the longitudinal element). For spinal hooks, the load shall be
minimum sample size of five.
applied via a cylinder set in the hook notch (see Fig. 3(a)). For
7.4 Fatigue tests for determining the maximum run out load
other elements (screws) the load shall be applied 25 mm from
or moment of a subassembly at 2.5 × 10 cycles shall utilize a
the local z axis (see Fig. 3(b)).
run down, half-interval approach with one specimen per run
6.6 The apparatus for transverse moment (Mx) mechanical
down interval or half-interval and three consecutive specimens
property measurements of a subassembly is depicted in Fig. 4. 6
showing run out to 2.5 × 10 cycles. Alternative methods for
As in the previous test, 6.5, both ends of the longitudinal
determining the starting point of the fatigue curve are the
element shall be clamped rigidly, with the interconnection
run-up method or choosing 75% of the ultimate static load or
centered on a 50-mm section of the longitudinal element. The
moment.
localoriginoftheinterconnectionmechanismshallbecentered
between the mounts. Loads are applied to the interconnection
8. Procedure for Measuring Static Mechanical Properties
(parallel to the longitudinal element), 25 mm from the z axis.
8.1 Measure the tightening torques for any set screws or
6.7 The apparatus for axial torque (Mz) gripping capacity
nuts which are incorporated into the interconnection linkage.
measurements of an interconnection mechanism is depicted in
8.2 Apply all tightening, crimping, or locking mechanisms
Fig. 8(a) and is similar to that described in 6.2 with the
as specified by the manufacturer.
exception that the axial torque is applied via notches in the
8.3 The recommended maximum rate for applying a load is
sleeve that surrounds the longitudinal element. An alternative
20 N/s (or 25 mm/min) and is 25 N-m/min (or 25 °/min) for
method is to hold the interconnection rigidly and apply the
applying a moment or torque. Since rate is machine- and
torsional force to the longitudinal element as shown in Fig.
software-dependent,itmaybenecessarytorunthetestsslower
8(b). A third alternative is to apply the torque via a force
to achieve accurate data.
applied to a moment arm as shown in Fig. 8(c), but this
alternative may introduce an additional variable of bending of
8.4 StaticA-Pload(Fx),transverseload(Fy),axialgripping
the anchor component. In any case, care must be taken to
capacity (Fz), and transverse moment (Mx), flexion-extension
evaluate and minimize the effect of the torsional properties of
moment (My), and axial torque (Mz) shall be measured using
the longitudinal element on the results.
the apparatus described in 6.1-6.7.
7. Sampling
“Optiminal Stress Amplitude Selection in Estimating Median Fatigue Limits
7.1 Thesamplestestedshallbepreviouslyunusedparts,and
Using Small Samples”, Little, R.E., ed., J. of Testing and Evaluation,ASTM, 1990,
shall not be re-tested. pp. 115–122.
F1798 − 97 (2008)
FIG. 8 Axial Torque Gripping Capacity Test Apparatus
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