ASTM F2118

ASTM F2118 provides a standardized method for evaluating the long-term fatigue behavior of spinal implant devices—specifically those that are subjected to constant deflection, such as intervertebral disc replacements, spinal rods, and dynamic stabilization devices. The test is designed to simulate the bending fatigue that occurs in vivo from repeated flexion-extension cycles in the human spine.

Purpose and Scope

This test method evaluates fatigue durability under constant strain amplitude, rather than load-controlled fatigue. It is ideal for:

• Dynamic stabilization implants

• Flexible spinal rods or rods with elastic components

• Devices subject to long-term flexural loading in service

Key outcomes of ASTM F2118 testing:

• Number of cycles to failure or runout

• Characterization of fatigue failure modes

• Identification of crack initiation and propagation regions

The test is especially useful for preclinical validation and design comparison of spinal implants.

Testing Equipment

• Fatigue Testing Machine:
  A servo-hydraulic or electromagnetic system capable of applying cyclic loading in displacement control, maintaining constant deflection amplitude at high frequencies (e.g., 1–5 Hz).

• Deflection Fixtures or Custom Bending Jigs:
  Designed to hold the test specimen in a 3-point or 4-point bending configuration, with defined span lengths and rigid supports. Custom grips or fixtures may be needed depending on the implant shape.

• Displacement Control System:
  To maintain consistent amplitude of deflection throughout the test (e.g., ±1 mm or other set value).

• Environmental Chamber (Optional):
  To simulate body temperature and fluid exposure, such as immersion in saline at 37°C.

• High-Cycle Data Recorder:
  For recording total number of cycles to failure or runout (usually 5 million cycles is a standard benchmark for runout).

Specimen Requirements

• Specimen Type:
  Usually an assembled implant device or a component, such as a spinal rod segment, artificial disc, or dynamic connector.

• Quantity:
  Minimum of six specimens per design is recommended to provide statistical confidence.

• Preconditioning:
  May include sterilization or soaking in saline if intended to simulate physiological conditions.

Test Procedure

1. Install the Specimen:
   Mount the device securely in the deflection fixture ensuring the correct span, alignment, and boundary conditions.

2. Apply Constant Deflection:

   • Deflection amplitude is based on expected in vivo strains or manufacturer-specified values.

   • Typical deflections range between 1 mm to 3 mm depending on the device and geometry.

3. Start Cyclic Testing:

   • Apply cyclic loading at the defined amplitude and frequency (usually 1–5 Hz).

   • Continue until failure occurs or 5 million cycles is reached (whichever comes first).

4. Monitor the Test:

   • Periodically inspect for signs of surface cracking, loosening, or deformation.

   • Maintain environmental conditions if using a saline bath or chamber.

Failure Criteria and Evaluation

Failure is generally defined as:

• Fracture or crack initiation in any component

• Permanent deformation that exceeds specified tolerances

• Loss of mechanical function (e.g., stiffness reduction, joint loosening)

Post-test evaluation may include:

• Visual inspection

• Microscopy (optical or SEM)

• X-ray or CT imaging

• Cross-sectioning for crack origin identification

Reporting Requirements

A comprehensive ASTM F2118 test report should include:

• Complete description of the device tested (materials, design, dimensions)

• Deflection amplitude and span lengths used

• Loading frequency and number of cycles applied

• Failure mode and location (if applicable)

• Number of cycles to failure or runout

• Photos of the setup and failure region

• Environmental conditions (if applicable)

• Any deviations from the standard

Common Applications

ASTM F2118 is widely used by spinal implant manufacturers and biomechanical testing labs for:

• Product development and validation

• Regulatory submissions (FDA, CE Marking)

• Comparative studies between implant designs

• Fatigue safety margin verification

Related and Similar Standards

If ASTM F2118 does not fully apply to your test scenario, these related standards may be useful:

• ASTM F2077 – Static and dynamic mechanical testing of intervertebral body fusion devices
  (Broader than F2118, includes compression, shear, and torsion fatigue)

• ASTM F1717 – Static and fatigue testing of spinal implant assemblies using a vertebrectomy model
  (Focuses on worst-case loading conditions for fusion systems)

• ASTM F1798 – Evaluating the static and fatigue properties of spinal implant components
  (Includes testing of pedicle screws, rods, and connections)

• ASTM F2346 – Fatigue testing of total intervertebral disc prostheses
  (Specifically designed for disc replacements under flexion-extension cycles)

• ISO 12189 – Fatigue testing of spinal implant assemblies under flexion-extension loading
  (An international standard similar to F2118 with emphasis on displacement-controlled testing)

Summary

ASTM F2118 is a specialized fatigue test method for evaluating bending durability under constant deflection—ideal for spinal implant components that undergo flexural fatigue during their service life. It ensures product safety and performance across millions of cycles simulating real-world loading conditions.

By replicating in vivo strain conditions, ASTM F2118 helps manufacturers identify design flaws, improve implant longevity, and meet regulatory requirements. It complements standards like ASTM F2077 and F1717 for a complete mechanical testing profile.