ASTM F1798-97(2003)
(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 so that comparison between different designs is facilitated. 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 lubrication 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.
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 2003)
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 F 1798; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 739 Practice for Statistical Analysis of Linear or Linear-
ized Stress-Life (S-N) and Strain-Life (e-N) Fatigue Data
1.1 This guide covers the measurement of uniaxial static
E 1150 Definitions of Terms Relating to Fatigue
and fatigue strength, and resistance to loosening of the com-
F 383 Practice for Static Bend and Torsion Testing of
ponent interconnection mechanisms of spinal arthrodesis im-
Intramedullary Rods
plants.
F 1582 Terminology Relating to Spinal Implants
1.2 The purpose of this guide is to provide a means of
mechanically characterizing different designs of spinal implant
3. Terminology
interconnections. Ultimately, the various components and in-
3.1 Definitions of Terms Specific to This Standard:
terconnections should be combined for static and fatigue
3.1.1 active length of longitudinal element—the span be-
testing of the spinal implant construct. It is not the intention of
tween rigid supports (for example, 50 mm is the active length
this guide to address the analysis of spinal implant constructs
in Fig. 1, Fig. 2, Fig. 3(a), Fig. 3(b), and Fig. 4.
or subconstructs or to define levels of performance of spinal
3.1.2 global coordinate system—spinal column motion has
implants as insufficient knowledge is available to predict the
six degrees of freedom, having translational motion along, and
consequences of the use of particular spinal implant designs.
rotational motion about three axes. The axes are labeled
1.3 This guide sets out definitions for use in measuring the
anterior-posterior or a-p (X), medial-lateral or transverse (Y),
strength of component interconnections of spinal implants,
and caudal-cranial or axial (Z). This coordinate system is right
possible test methods themselves, and the reporting of test
handed with +X in the anterior direction, +Y towards the left
results.
side of the body, and +Z in the cranial direction. Positive
1.4 The values stated in SI units are to be regarded as
rotations are defined by the right hand rule (See Fig. 5(a)).
standard.
3.1.3 gripping capacity—the maximum applied load or
1.5 This standard does not purport to address all of the
moment across an interconnection mechanism within the first
safety concerns, if any, associated with its use. It is the
1.5 mm of permanent displacement or 5° of permanent rotation
responsibility of the user of this standard to establish appro-
between the connected components.
priate safety and health practices and determine the applica-
3.1.4 local coordinate system—the spine’s global coordi-
bility of regulatory limitations prior to use.
nate system shall be applied locally at the position of the
2. Referenced Documents interconnection. The local direction, z, shall be centered
2 through the longitudinal element of the x-y plane. The local
2.1 ASTM Standards:
direction, x, shall be defined parallel to the axis of a screw or
E 4 Practices for Force Verification of Testing Machines
back of a hook.The local transverse axis, y, shall be parallel to
E 6 Terminology Relating to Methods of Mechanical Test-
a transverse element (See Fig. 5(b) and Fig. 5(c)).
ing
3.1.5 loosening torque—the torque required to disconnect
E 468 Practice for the Presentation of Constant Amplitude
the various threaded fasteners that might comprise the im-
Fatigue Test Results from Metallic Materials
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
Surgical Materials and Devices and is the direct responsibility of Subcommittee
which vertebral connection elements are subjected, (that is,
F04.25 on Spinal Devices .
axial load, Fz; A-P load, Fx; axial torsion, Mz; and flexion-
Current edition approved Nov. 1, 2003. Published November 2003. Originally
extension moment, My).
approved in 1997. Last previous edition approved in 1997 as F 1798 – 97.
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contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
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F1798–97 (2003)
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 insure 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—
maximum load or moment applied to a subassembly (see Point
E in Fig. 6).
3.1.14 yield load/moment of the subassembly—the load or
moment required to produce a permanent deformation equal to
0.020 times the active length of the longitudinal element (see
FIG. 1 A-P Test Apparatus for Subassembly
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
longitudinal elements (for example, rods, plates) to form spinal
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-
cally in a load to failure mode and also can be tested cyclically
to estimate the maximum run out value at 2.5 3 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
interconnection mechanisms so that comparison between dif-
FIG. 2 Transverse Test Apparatus for Subassembly
ferent designs is facilitated. Comparisons must be made
cautiously and with careful analysis, taking into account the
effects that design differences can have on the loading configu-
3.1.7 maximum run out load/moment—the maximum load
rations.
or moment that can be applied to a subassembly where all the
5.2 This guide is used to quantify the static and fatigue
tested constructs have withstood 2.5 3 106 cycles without a
properties of different implant interconnection designs. The
failure.
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
lubrication 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 envi-
axial load, A-P load, and flexion-extension moment, but not a
ronment, whether a simulated body fluid, saline (9g NaCl per
negative axial load or axial torsion. Hence, positive axial load, 1000 mL H O), with a saline drip, or dry, is an important
A-Pload,andflexion-extensionmomentarerelevantdirections
characteristic of the test and must be reported accurately.
of loading.
5.3 The loading of spinal implant constructs in vivo will, in
3.1.9 spinal arthrodesis implant—an implant applied to the general, differ from the loading configurations used in this
spine with the specific intention of providing temporary guide. The results obtained here cannot be used directly to
correction and stability to vertebrae while bony fusion occurs. predict in vivo performance. However, the results can be used
F1798–97 (2003)
FIG. 3 Flexion-Extension Moment Test Apparatus for Subassembly
F1798–97 (2003)
FIG. 4 Transverse Moment Test Apparatus for Subassembly
to compare different component designs in terms of relative element via a sleeve (collar) which freely surrounds the
mechanical parameters. longitudinal element.The sleeve (collar) should evenly distrib-
ute the load around the interconnection. An alternate method,
6. Apparatus
depicted in Fig. 7(b), applies the load to the longitudinal
element and pushes it through the interconnection clamp.
6.1 Machines used for the test shall conform to the require-
ments of Practices E 4. 6.3 TheapparatusforA-P(x)mechanicalpropertymeasure-
6.2 The apparatus for axial (z) gripping capacity measure- ments of a subassembly is depicted in Fig. 1. Both ends of the
ments of an interconnection mechanism is depicted in Fig. longitudinal element shall be clamped rigidly, with the inter-
7(a). One end of the longitudinal element shall be clamped connection centered on a 50 mm section of the longitudinal
rigidly, leaving 5 mm exposed between the interconnection element. The local origin of the interconnection mechanism
mechanismandthetestmachinebase.Asectionoflongitudinal shall be centered between the mounts. Loads are applied to the
element at least 5 mm shall extend beyond the interconnection interconnection (perpendicular to the longitudinal element) via
linkage and remain unfixed. Axial loads are applied to the a clamp on the hook, screw, or band. The load should be
interconnection mechanism along the axis of the longitudinal centered through the local x coordinate axis.
F1798–97 (2003)
FIG. 5 Coordinate System
F1798–97 (2003)
FIG. 6 Load/Displacement Curve
6.4 The apparatus for transverse (y) mechanical property 6.5 The apparatus for flexion-extension moment (My) me-
measurements of a subassembly is depicted in Fig. 2. Both chanical property measurements of a subassembly is depicted
ends of the longitudinal element shall be clamped rigidly, with in Fig. 3. Both ends of the longitudinal element shall be
the interconnection centered on a 50 mm section of the clamped rigidly, with the interconnection centered on a 50 mm
longitudinal element. The local origin of the interconnection section of the longitudinal element. The local origin of the
mechanism shall be centered between the mounts. Loads are interconnection mechanism shall be centered between the
applied to the interconnection (perpendicular to the longitudi- mounts. Loads are applied to the interconnection (parallel to
nal element) via a clamp on the transverse connector. The load the longitudinal element). For spinal hooks, the load shall be
should be centered through the local y coordinate axis. appliedviaacylindersetinthehooknotch,Fig.3(a).Forother
F1798–97 (2003)
FIG. 7 Axial Gripping Capacity Test Apparatus
elements (screws) the load shall be applied 25 mm from the run down, half-interval approach with one specimen per run
local z axis, Fig. 3(b). down interval or half-interval and three consecutive specimens
6.6 The apparatus for transverse moment (Mx) mechanical showing run out to 2.5 3 10 cycles. Alternative methods for
property measurements of a subassembly is depicted in Fig. 4. determining the starting point of the fatigue curve are the
As in the previous test, 6.5, both ends of the longitudinal run-up method or choosing 75 % of the ultimate static load or
element shall be clamped rigidly, with the interconnection moment.
centered on a 50 mm section of the longitudinal element. The
8. Procedure for Measuring Static Mechanical Properties
localoriginoftheinterconnectionmechanismshallbecentered
between the mounts. Loads are applied to the interconnection 8.1 Measure the tightening torques for any set screws or
nuts which are incorporated into the interconnection linkage.
(parallel to the longitudinal element), 25 mm from the z axis.
6.7 The apparatus for axial torque (Mz) gripping capacity 8.2 Apply all tightening, crimping, or locking mechanisms
as specified by the manufacturer.
measurements of an interconnection mechanism is depicted in
Fig. 8(a) and is similar to that described in 6.2 with the 8.3 The recommended maximum rate for applying a load is
20 N/s (or 25 mm/min) and is 25 N-m/min (or 25 °/min) for
exception that the axial torque is applied via notches in the
sleeve that surrounds the longitudinal element. An alternative applying a moment or torque. Since rate is machine and
software dependent, it may be necessary to run the tests slower
method is to hold the interconnection rigidly and apply the
torsional force to the longitudinal element as shown in Fig. to achieve accurate data.
8.4 StaticA-Pload (Fx), transverse load (Fy), axial gripping
8(b). A third alternative is to apply the torque via a force
applied to a moment arm as shown in Fig. 8(c), but this capacity (Fz), and transverse moment (Mx), flexion-extension
moment (My), and axial torque (Mz) shall be measured using
alternative may introduce an additional variable of bending of
the anchor component. In any case, care must be taken to the apparatus described in 6.1-6.7.
8.5 Loads and moments in only the relevant directions of
evaluate and minimize the affect of the torsional properties of
the longitudinal element on the results. loading need be measured.
8.6 After each load or moment measurement, loosening
7. Sampling
torque shall be measured (if applicable).
7.1 The samples tested shall be previously unused parts
9. Procedure for the Measurement of Fatigue Run Out
only, and shall not be re-tested.
9.1 Measure the tightening torques for any set screws or
7.2 The test constructs shall be labeled and maintained
nuts that are incorporated
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