A Guide to Cyclic Fatigue Testing of Spinal Implant Constructs in accordance with ASTM F1717-18, ASTM F2706-18 and ISO 12189-8

Written by Toby Lane

A SUMMARY OF THE STANDARDS AND THE TESTS REQUIRED

During normal patient activity, spinal constructs and implant assemblies are subjected to high in vivo loading, so testing is required to avoid catastrophic failure. Spinal injuries often occur due to rotation, bending or axial loading conditions causing dislocation or fracture. Static testing is used to evaluate loads that will result in spinal fracture, whilst fatigue testing is performed to evaluate the number of cycles it takes for failure to occur when components are exposed to repeated loading at lower forces.

Fatigue, or service life testing of spinal constructs is critical as fatigue failure is more common than catastrophic failure. Loading is typically applied with a constant-amplitude, load-controlled sinusoidal waveform, running in excess of five million cycles.




ASTM F1717, Standard Test Methods for Spinal Implant Constructs in a Vertebrectomy Model and ASTM 2706, Standard Test Methods for Occipital-Cervical and Occipital-Cervical-Thoracic Spinal Implant Constructs in a Vertebrectomy Model, specify methods for both the static and fatigue testing of spinal implant assemblies. The tests include:
  • Static compressive bending
  • Static tensile bending
  • Static torsion
  • Dynamic compressive bending fatigue

ISO 12189, Implants for surgery – Mechanical testing of implantable spinal devices – Fatigue test method for spinal implant assemblies using an anterior support, species a very similar Dynamic compressive bending fatigue test.

For the majority of spinal construct testing, ultra-high molecular weight polyethylene (UHMWPE) blocks are used rather than vertebrae to eliminate variations in bone properties and geometry that may be present. We recommend that you review the standards fully to understand their full requirements.



ElectroPuls with Spinal Fixture


Challenges of Spinal Implant Testing

  • Standard cannot be completed fully without axial-torsion test system
  • Testing uses simplified load schemes to represent the complex in vivo loads that constructs will experience during normal patient activity
  • Tests must first be carried out in air before being completed in saline solution which limits the test frequency to 5 Hz
  • Clockwise and anti-clockwise torsion testing may produce different results


TEST SYSTEM

An axial-torsion system is required to encompass all test types from this standard and will allow manufacturers and researchers to conduct both static and fatigue testing on a range of implant designs with a single test system. The ElectroPuls® E3000 Linear-Torsion All-Electric Dynamic Test Instrument, ElectroPuls E10000 Linear-Torsion All-Electric Dynamic Test Instrument and 8874 servohydraulic test frame all offer the necessary capabilities for testing to this standard with the use of specialized fixtures.

Each system can be combined with a temperature controlled bath for simulation of in vivo conditions and Instron’s range of Biaxial Dynacell load cells can be mounted on the end of the moving axial-torsional actuator providing automatic compensation for errors caused by inertial loading.

Testing with WaveMatrix2 software, as well as features such as patented Stiffness Based Tuning and inertial compensation using Dynacell will optimize system response, waveform fidelity, and resolution.


ElectroPuls E3000 Biaxial Spinal Fixture

ElectroPuls E3000 Biaxial Spinal Fixture




TEST FIXTURE


2370K Spine Implant Fixture F1717
There are a wide variety of spinal assemblies that can be tested to ASTM F1717, ASTM F2706 and ISO 12189. Combinations of components are dependent on the intended spinal location and method of application to the spine. So as to keep the same fixturing for different assemblies, single-use UHMWPE blocks bespoke to each spinal assembly are used to interface with the fixturing on the test frame. The UHMWPE used must have a tensile breaking strength equal to 40 ± 3MPa to eliminate the effects of variability in bone properties and morphology. The ISO standard also recommends the use of springs to mimic the physiological behavior of discs under compressive loads.

Instron’s fixturing solution comprises of dedicated corrosion resistant adapters that can be mounted with ease with the option of attaching a saline bath to the base of the test frame for simulation of in vitro testing. When using the bath, the actuator mounted load cell utilizes Intron’s patented Dynacell technology for inertia compensation ensuring confidence in data.

The fixture design consists of two pairs of side supports, one rigidly attached, the other attached via a mounting plate that accommodates rotation around the axis of axial force. The side supports easily interface with the UHMWPE test blocks through hinge pin mechanisms. This allows the blocks to freely rotate around the hinge pins that must remain horizontal to the direction of axial force. Only for the torsion test is the mounting plate rotation restrained with aluminium blocks.

TEST PARAMETERS

For the 3 static mechanical tests outlined in ASTM F1717 and ASTM F2706, a minimum of 5 samples are required to evaluate the load required to result in spinal fracture.
  • Static compression bending: a maximum load rate of 25mm/min is used to generate a load-displacement curve. Values to be stated include the mean and standard deviation for the displacement at 2% offset yield (mm), elastic displacement (mm), compressive bending yield load (N), compressive bending stiffness (N/mm), compressive bending ultimate displacement (mm), and compressive bending ultimate load (N).
  • Static tensile bending: a maximum load rate of 25mm/min is used to generate a load-displacement curve. Values to be stated include the mean and standard deviation for the displacement at 2% offset yield (mm), elastic displacement (mm), tensile bending yield load (N), tensile bending stiffness (N/mm), tensile bending ultimate displacement (mm), and tensile bending ultimate load (N).
  • Static torsion: a maximum load rate 60º/min with zero axial load is used to generate a torque-angular displacement curve. Values to be stated include the mean and standard deviation for the angular displacement at 2% offset yield (degrees), elastic angular displacement (degrees), yield torque (N-m), and torsional stiffness (N-m/degree). angular displacement at 2% offset yield, elastic angular displacement, yield torque and torsional stiffness.

The dynamic test featured in both standards involves cyclic testing to evaluate the number of cycles for fatigue fracture to occur with a minimum of two constructs being tested.
  • Compression bending fatigue test: ASTM recommend initial fatigue loads of 75, 50 and 25% of static compression bending test with ISO suggesting 2000N as a physiologically representative value. Tests are run up to 5 million cycles with a constant load amplitude ratio maintained (≥10). Maximum frequency is 5Hz. Generated is a semi-log fatigue curve of compression bending load vs number of cycles. The test should initially be performed dry for consistency as fatigue testing in saline may cause fretting, corrosion or lubricate interconnections and affect relative performance. In order to simulate a vertebrectomy model, a large gap between the two UHMWPE test blocks is used.

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