ASTM F2077-18 Characterization and Fatigue of Spinal Intervertebral Body Fusion Devices

Intervertebral Body Fusion devices are used to promote arthrodesis of a spinal motion segment, following disc degeneration. The implant acts as a support of the anterior column whilst the two vertebral discs grow together and fuse the spine. Intervertebral Body Fusion is an alternative technique to that of Intervertebral Disc replacement.

The tests include:
Axial-Compression
Compression-Shear
Torsion Testing
We recommend that you review the standard to fully understand its requirements. An alternative test for intervertebral body fusion devices is specified in ASTM-F2267.

Challenges of ASTM F2077 Testing:

Machining/manufacture of test blocks to match specimen shape
Monitoring of intradiscal height during test
Assessment of mechanical failure during test
Avoidance of self-heating in test blocks
Maintaining load peaks at high frequency
Test duration for in vivo tests

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 Linear-Torsion All-Electric Dynamic Test Instruments, as well as the 8874 servohydraulic Dynamic test frame, all offer the necessary capabilities for testing to this standard with the use of specialised fixtures. Each system can be combined with a temperature controlled bath for simulation of in vivo conditions and 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.

TEST FIXTURE
For both the Axial-compression and Compression shear tests the same fixture is used. This fixture connects to the system actuator via a universal joint and features a pushrod with an additional minimal friction sphere joint at its other end. The pushrod must be at least 38cm in length between the centre of each joint so as to compensate for any misalignment in the loadstring.
For the Torsion testing, the fixture is similar, with the lower joint type being the only difference. In this case, a spherical gimbal mechanism is used so that both a compressive force and moment can be applied to the specimen simultaneously.
For each test the specimen is held between two test blocks that are machined to match the specimens form and are rigidly attached to the fixture and test frame. The test blocks for the shear tests are machined such that the sample is held at the desired angle with respect to the table plane so that a shear force is created when an axial load is applied.
For static tests, the blocks are manufactured from metal and can be re-used. For the dynamic tests, blocks are manufactured from polyacetal and are single use.